Building blocks for N-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nTs; n = 2-6). Systematic synthesis, spectroscopy, electrochemistry, and solid-state organization

Published on Web 09/14/2004

Building Blocks for N-Type Molecular and Polymeric

Electronics. Perfluoroalkyl- versus Alkyl-Functionalized

Oligothiophenes (nTs; n ) 2-6). Systematic Synthesis,

Spectroscopy, Electrochemistry, and Solid-State Organization

Antonio Facchetti, Myung-Han Yoon, Charlotte L. Stern, Geoffrey R. Hutchison,

Mark A. Ratner,* and Tobin J. Marks*

Contribution from the Department of Chemistry and the Materials Research Center,

Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113

Received February 23, 2004; E-mail: ratner@northwestern.edu; t-marks@northwestern.edu

Abstract: The synthesis, comparative physicochemical properties, and solid-state structures of five

oligothiophene (nT) series differing in substituent nature and attachment, regiochemistry, and oligothiophene

core length (n) are described. These five series include the following 25 compounds: (i) R,ω-diperfluorohexyl-

nTs 1 (DFH-nTs, n ) 2-6), (ii) â,â′-diperfluorohexyl-nTs 2 (isoDFH-nTs, n ) 2-6), (iii) R,ω-dihexyl-nTs 3

(

DH-nTs, n ) 2-6), (iv) â,â′-dihexyl-nTs 4 (isoDH-nTs, n ) 2-6), and (v) unsubstituted oligothiophenes 5

RnTs, n ) 2-6). All new compounds were characterized by elemental analysis, mass spectrometry, and

multinuclear NMR spectroscopy. To probe and address quantitatively how the chemistry and regiochemistry

of conjugated core substitution affects molecular and solid-state properties, the entire 1-5 series was

investigated by differential scanning calorimetry, thermogravimetric analysis, and optical absorption and

emission spectroscopies. Single-crystal X-ray diffraction data for several fluorocarbon-substituted oligomers

are also presented and compared. The combined analysis of these data indicates that fluorocarbon-

substituted nT molecules strongly interact in the condensed state, with unit cell level phase separation

between the aromatic core and fluorocarbon chains. Surprisingly, despite these strong intermolecular

interactions, high solid-state fluorescence efficiencies are exhibited by the fluorinated derivatives. Insight

into the solution molecular geometries and conformational behavior are obtained from analysis of optical

and variable-temperature NMR spectra. Finally, cyclic voltammetry data offer a reliable picture of frontier

MO energies, which, in combination with DFT computations, provide key information on relationships

between oligothiophene substituent effects and electronic response properties.

6

7

8

9

Introduction

nescent diodes, lasers, sensors, and photovoltaic cells, as well

1

0

as fluorescent markers for biomolecules. In addition, new

Over the last 10 years, organic-based π-conjugated materials

have proven to be important components in the scientific

development of new generations of optoelectronic devices, with

some applications now reaching commercialization. Among this

relatively new class of materials, π-conjugated thiophene-based

oligomers (nTs) and polymers (PTs) are central players because

of their chemical and electrochemical stability and ready

1

12

dendritic and crown-annelated molecular structures have

recently been described from which it is reasonable to expect

new self-assembly modalities, properties, and therefore, materi-

als performance.

1

2

(

4) (a) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-

VCH: Weinheim, Germany, 1999. (b) Polythiophenes: Electrically

ConductiVe Polymers; Schopf, G., Kossmehl, G.; Eds.; Springer, Berlin,

3,4

functionalization. Besides the classic field of conducting

1997.

4

(5) (a) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.-C.; Van Patten, P. G.;

Bao, Z. Chem. Mater. 2003, 15, 1778. (b) Mushrush, M.; Facchetti, A.;

Lefenfeld, M.; Katz, H. E.; Marks, T. J. J. Am. Chem. Soc. 2003, 125,

components, these materials are currently under intense inves-

5

tigation for applications in thin film transistors, electrolumi-

9414.

(

1) (a) Shirakawa, H Chem. Commun. 2003, 1. (b) Karl, N. Synth. Met. 2003,

(6) (a) Pasini, M.; Destri, S.; Porzio, W.; Botta, C.; Giovanella, U J. Mater.

Chem. 2003, 13, 807. (b) Suzuki, M.; Fukuyama, M.; Hori, Y.; Hotta, S.

J. Appl. Phys. 2002, 91, 5706.

(7) (a) Pisignano, D.; Anni, M.; Gigli, G.; Cingolani, R.; Zavelani-Rossi, M.;

Lanzani, G.; Barbarella, G.; Favaretto, L. Appl. Phys. Lett. 2002, 81, 3534.

(8) (a) Torsi, L.; Lovinger, A. J.; Crone, B.; Someya, T.; Dodabalapur, A.;

Katz, H. E.; Gelperin, A. J. Phys. Chem. B 2002, 106, 12563. (b) Roncali,

J. J. Mater. Chem. 1999, 9, 1875.

(9) Hara, K.; Kurashige, M.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.;

Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783.

(10) Barbarella, G. Chemistry 2002, 8, 5072.

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(12) Jousselme, B.; Blanchard, P.; Levillain, E.; Delaunay, J.; Allain, M.;

Richomme, P.; Rondeau, D.; Gallego-Planas, N.; Roncali, J. J. Am. Chem.

Soc. 2003, 125, 1363.

1

33, 649. (c) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater.

2

002, 14, 99. (d) Pron, A.; Rannou, P. Prog. Polym. Sci. 2002, 27, 135.

(

e) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (f)

Crone, B.; Dodabalapur, A.; Llin, Y.-Y.; Filas, R. W.; Bao, Z.; LaDuca,

A.; Sarpeshkar, R.; Katz, H. E. Nature 2000, 403, 521. (g) Tour, J. M.

Acc. Chem. Res. 2000, 33, 791.

2) (a) Information Display; Special OLED issue, June 2003, 19 (6). (b) Tullo,

A. H. Chem. Eng. News 2000, 26 (June 26), 20-21. (c) Visit, for instance,

the Pioneer Home Page. http://pioneerelectronics.com.

3) (a) Frere, P.; Raimundo, J.-M.; Blanchard, P.; Delaunay, J.; Richomme,

P.; Sauvajol, J.-L.; Orduna, J.; Garin, J.; Roncali, J. J. Org. Chem. 2003,

(

6

8, 5357. (b) Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.;

Sirringhaus, H. Nat. Mater. 2004, 3, 171. (c) Afzali, A.; Breen, T. L.; Kagan,

C. R. Chem. Mater. 2002, 14, 1742.

13480

9

J. AM. CHEM. SOC. 2004, 126, 13480-13501

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Part of the interest in organic (opto)electronics stems from

the facile control and flexibility that organic synthetic methodol-

ogy affords in functionalizing the (oligo, poly)thiophene core.

By judicious choice of functionality it is possible to fine-tune

key optical, electronic, and electrooptical properties.¹Electron-

donating (alkyl, alkoxy, alkylamino, etc.) or -withdrawing

sharp contrast to other classes of organic compounds where the

unique (per)fluorination-related properties such as high chemical

and biological stability, volatility, thermal stability, low coef-

ficient of friction, hydro- and lipophobicity, and permea-

bility to some gases have afforded diverse applications in

materials and medicinal chemistry and in biochemistry.²⁰It is

therefore expected that incorporation of the aforementioned

properties into π-conjugated conductors will lead to novel

materials of potential fundamental scientific and technological

interest.

3

(CHO, CN, NO2, etc.) substituents can raise or lower the

energies of the highest occupied molecular orbital (HOMO) and

the lowest unoccupied molecular orbital (LUMO) relative to

the unsubstituted system, allowing rational modulation of the

molecular “band gap”.¹⁴Appending electron-withdrawing or

In a recent communication,²¹we described a limited number

of the first end-capped fluorocarbon-functionalized oligo-

thiophenes and reported on the consequences of fluorine-for-

hydrogen substitution on the molecular and thin-film semicon-

ductor properties. That preliminary report revealed unexpected

similarities, differences, and trends within and between fluo-

roalkyl- and the corresponding, widely investigated, alkyl-

functionalized nTs and raised a number of intriguing questions.

The present contribution describes a complete account of our

studies aimed at better fundamental understanding of the

chemical, structural, and physical properties as well as solid-

state characteristics of five homologous series of thiophene-

based π-conjugated materials. The questions we address are

whether other classes of fluorocarbon-substituted oligo-

thiophenes are synthetically accessible, how the properties of

fluorocarbon-substituted oligothiophenes correlate with those

of the other classes of fluorine-free nTs, whether a straightfor-

ward model can explain the trends found, and whether potential

applications exist in the large field of π-electron organic solids.

To achieve these goals, a full series of compounds has been

synthesized and investigated, ranging from the R,ω-diperfluo-

rohexyl-substituted series 1 (DFH-nTs, n ) 2-6) and the

corresponding R,ω-dihexyl-substituted system 3 (DH-nTs, n )

2-6) to the parent unsubstituted oligothiophenes 5 (RnTs, n )

2-6). In addition, to fully address regiochemistry effects,

isomerically pure â,â′-diperfluorohexyl-substituted series 2

(isoDFH-nTs, n ) 2-6) has been synthesized for the first time

and is compared to the â,â′-dihexyl-functionalized system 4

(isoDH-nTs, n ) 2-6). These latter systems provide additional

insights into the molecular and solid-state properties of fluo-

roalkyl- versus alkyl-oligothiophenes functionalized at a different

skeletal position. Also, since the R,ω-positions are free, they

provide key building blocks for the preparation of, and are

probably better descriptors of, the properties of the correspond-

ing regioregular fluorinated polymers. In addition to the

aformentioned syntheses, the molecular properties of these

homologous and homogeneous sets of compounds are compared

and contrasted in detail by a combination of techniques,

including differential scanning calorimetry (DSC) and thermo-

gravimetric analysis (TGA), optical absorption (UV-vis),

emission (PL) spectroscopies, multinuclear NMR spectroscopy,

and cyclic voltammetry (CV). Single-crystal XRD data are

obtained for key members of the fluorinated family, and density-

functional theory (DFT) modeling provides additional insights

into experimental results. Finally, thin films are grown on glass

substrates and studied by UV-vis and PL spectroscopy. With

full understanding of 1-5 molecular physical properties and

-

donating groups onto the heterocyclic skeleton also alters the

14,15

redox and optical properties.

These modifications ultimately

affect the charge transport characteristics of the bulk solid and

define the role that the material can play in various device

configurations.

In the extensive class of functionalized thiophene-based

conductors, polythiophenes bearing fluorinated substituents have

13

been barely investigated. For synthetic reasons, most of the

structures reported to date feature an “insulating” spacer (alkyl,

O, SiMe2) between the fluorinated fragment (usually a perfluo-

roalkyl group) and the heteroaromatic core, which greatly

attenuates or even reverses the electronic effects of fluorocarbon

substitution.¹⁶The only report of a poly(3-perfluoroalkyl)-

thiophene is brief mention of the anomalously low oxidation

potential of the monomer, 3-trifluoromethylthiophene, compared

to the fluoro- and difluoromethyl analogues, and higher oligo-

mers were not described.¹⁷Leclerc et al. reported the synthesis

of regiorandom poly(3′-perfluoroalkyl-2,2′:5′,2′′-tertthiophene),

which has a higher oxidation potential than the alkyl analogue,

but without further details. Very recently, poly(3-perfluorooctyl)-

thiophene has been reported and, interestingly, exhibits solubility

in supercritical CO2.¹⁹However, synthetic details and polymer

molecular weight have not been disclosed. As far as oligo-

thiophenes are concerned, to the best of our knowledge, no

systematic study has been reported concerning homologous

series of fluorocarbon-substituted oligothiophenes with well-

defined substitution patterns, chain, and π-core lengths. This is

a clear deficiency since oligothiophenes have been shown to

be viable models for predicting and understanding the elec-

tronic structures and optical properties of the corresponding

polymers. Furthermore, the limited literature concerning (semi)-

fluorinated (oligo, poly)heteroaromatic conductors stands in

18

(

13) (a) Electronic Materials: The Oligomer Approach; M u¨ llen, K., Wegner,

G. Eds.; Wiley-VCH: New York, 1998. (b) Handbook of ConductiVe

Polymers; Skotheim, T. A., Elsenbaumer R. L., Reynolds, J. R., Eds.;

Marcel Dekker: New York, 1998; pp 325-326.

(

14) (a) Casado, J.; Pappenfus, T. M.; Miller, L. L.; Mann, K. R.; Orti, E.;

Viruela, P. M.; Pou-Amerigo, R.; Hernandez, V.; Lopez Navarrete, J. T. J.

Am. Chem. Soc. 2003, 125, 2524. (b) Moratti, S. C.; Cervini, R.; Holmes,

A. B.; Baigent, D. R.; Friend, R. H.; Greenham, N. C.; Gr u¨ ner, J.; Hamer,

P. J. Synth. Met. 1995, 71, 2117.

(

15) (a) Casado, J.; Miller, L. L.; Mann, K. R.; Pappenfus, T. M.; Higuchi, H.;

Orti, E.; Milian, B.; Pou-Amerigo, R.; Hernandez, V.; Lopez Navarrete, J.

T. J. Am. Chem. Soc. 2002, 124, 12380. (b) Lux, A.; Holmes, A. B.; Cervini,

R.; Davies, J. E.; Moratti, S. C.; Gr u¨ ner, J.; Cacialli, F.; Friend, R. H.

Synth. Met. 1997, 84, 293. (c) Br e´ das, J. AdV. Mater. 1995, 7, 263.

16) (a) Collard, D. M.; Li, L.; Hong, X. M. Polym. Mater. Sci. Eng. 2002, 86,

(

3

8. (b) Laforgue, A.; Simon, P.; Fauvarque, J.-F. Synth. Met. 2001, 123,

11. (c) Hong, X. M.; Collard, D. M. Macromolecules 2000, 33, 3502. (d)

3

Hong, X. M.; Tyson, J. C.; Collard, D. M. Macromolecules 2000, 33, 6916.

(e) Thobie-Gautier, C.; Guy, A.; Gougues, A.; Jubault, M.; Roncali, J. AdV.

Mater. 1993, 5, 637. (f) Kassmi, A. E.; Buchner, W.; Fache, F.; Lemaire,

M. J. Electroanal. Chem. 1992, 326, 357. (g) Buchner, W.; Garreau, R.;

Lemaire, M.; Roncali, J.; Garnier, F. J. Electroanal. Chem. 1990, 277, 355.

17) Ritter, S. K.; Noftle, R. E.; Ward, A. E. Chem. Mater. 1993, 5, 752.

18) Robitaille, L.; Leclerc, M. Macromolecules 1994, 27, 1847.

(

(20) Hiyama, T. Organofluorine Compounds: Chemistry and Applications;

Springer-Verlag: Berlin, 2000.

19) Li, L.; Counts, K. E.; Kurosawa, S.; Teja, A. S.; Collard, D. M. AdV. Mater.

(21) Facchetti, A.; Mushrush, M.; Katz, H. E.; Marks, T. J. AdV. Mater. 2003,

2

004, 16, 180.

15, 33.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13481

A R T I C L E S

Chart 1

Facchetti et al.

2

3o

3,3′,5,5′-tertrabromo-2,2′-bithiophene (17), 3,3′-dibromo-2,2′-bithio-

phene (18),²3-bromo-2,2′-bithiophene (22).

3o

23n

The following are new compounds. For synthetic characterization

and computational details as well as film growth methods, see

Supporting Information.

5,5′-Diperfluorohexyl-2,2′-dithiophene (DFH-2T). mp 97 °C. Anal.

Calcd for C20

H

4

F

26

S

2

: C, 29.94; H, 0.50; F, 61.57. Found: C, 29.90;

+

H, 0.57; F, 61.73. HRMS (EI, 70 eV): found m/z, 801.9336 (M ),

calcd for C20

thiophene (DFH-3T). mp 132 °C. Anal. Calcd for C24

H

4

F

26

S

2

, 801.9334. 5,5′-Diperfluorohexyl-2,2′:5′,2′′-ter-

: C, 32.59;

6 26 3

H F S

H, 0.685; F, 55.85. Found: C, 32.50; H, 0.69; F, 55.63. HRMS (EI,

+

7

5

2

0 eV): found m/z, 883.9226 (M ), calcd for C24

H

6

F

26

S

3

, 883.9217.

,5′′′-Diperfluorohexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DFH-4T). mp

13 °C. Anal. Calcd for C28

H

8

F

26

S

4

: C, 34.81; H, 0.84; F, 51.14.

+

Found: C, 34.66; H, 0.94; F, 51.06; MS (EI, 70 eV), 965.9 (M , 100%).

5,5′′′′-Diperfluorohexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquethiophene (DFH-

5

T). mp 252 °C. Anal. Calcd for C32

10 26 5

H F S : C, 36.64; H, 0.96; F,

4

7.10. Found: C, 36.71; H, 1.12; F, 47.49. 3,3′-Diperfluorohexyl-2,2′-

dithiophene (isoDFH-2T). Oil. HRMS (EI, 70 eV): found m/z,

+

801.9362, calcd for C14H F S

5 13 2

(M ), 801.9339. 3,3′′-Diperfluorohexyl-

2

,2′:5′,2′′-terthiophene (isoDFH-3T). HRMS (EI, 70 eV): found m/z,

+

calcd for C24

H

6

F

26

S

3

(M ). 3,3′′′-Diperfluorohexyl-2,2′:5′,2′′:5′′,2′′′-

quaterthiophene (isoDFH-4T). mp 145 °C. Anal. Calcd for C28

: C, 34.79; H, 0.83; F, 51.10. Found: C, 34.85; H, 0.95; F, 51.05.

,3′′′′-Diperfluorohexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquethiophene (isoD-

FH-5T). mp 160 °C. Anal. Calcd for C32 : C, 33.65; H, 0.76;

F, 47.10. Found: C, 36.64; H, 1.17; F, 47.38. 3,3′′′′′-Diperfluorohexyl-

,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene (isoDFH-6T). mp 217

C. Anal. Calcd for C36 : C, 38.24; H, 1.07; F, 43.69. Found:

8

H -

26 4

F S

3

10 26 5

H F S

2

°

12 26 6

H F S

C, 38.41; H, 1.07; F, 43.60. 3,3′′′′′-Dihexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-

qinquethiophene (iso-DH-5T). mp 77-78 °C (hexane). HRMS (EI, 70

+

eV): found m/z, 580.1415 (M ), calcd for C32

H

36

S

5

, 580.1410. Anal.

Calcd for C32

Hexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene (isoDH-6T). mp

37 °C (toluene-hexane). Anal. Calcd for C36 : C, 65.21; H, 5.78.

Found: C, 65.40; H, 5.59.

36 5

H S : C, 6616; H, 6.26. Found: C, 65.98; H, 5.96. 3,3′′′′′-

1

38 6

H S

knowledge of single-crystal geometries and organization, the

next goal is to provide additional insights into solid-state

microstructure and define a field of potential application as a

_n_e_w_c_l_a_s_s_o_f_m_a_t_e_r_i_a_l_s_._I_n_a_r_e_l_a_t_e_d_c_o_n_t_r_i_b_u_t_i_o_n_,₂2 we describe

thin film growth and microstructure properties and morphologies

of the present compounds, and electrical response in field-effect

transistor (FET) device configurations. The combined results

provide key and surprising information on the properties and

solid-state organization of fluoroalkyl-containing oligo-

thiophenes, and a coherent picture of how these properties

correlate with those of the fluorine-free analogues. Chart 1

shows the structures of 1-5.

2-(Tri-n-butylstannyl)-5-(1-perfluorohexyl)thiophene (8). Oil. HRMS/

+

EI (70 eV) Calcd for C22

H

29SSnF13: 692.0804, found: 692.0729 (M ).

5-Perfluorohexyl-2,2′-dithiophene (9). mp 51 °C. Anal. Calcd for

: C, 34.72; H, 1.04; F, 51.00. 5-(tri-n-Butylstannyl)-5′-(1-

14 5 13 2

C H F S

perfluorohexyl)-2,2′-dithiophene (11). Oil. HRMS (EI, 70 eV): found

+

m/z, 774.0680, calcd for C26

H

44

S

2

Sn (M ), 774.0682. 5-Perfluorohexyl-

2

,2′:5′,2′′-terthiophene (17). mp 158 °C (i-PrOH). Anal. Calcd for

: C, 38.17; H, 1.25; F, 43.61. Found: C, 37.95; H, 1.18; F,

4.01. HRMS (EI, 70 eV): found m/z, 565.9501, calcd for C18

(M ), 565.9502. 3,3′,5,5′-Tetrabromo-2,2′:5′,2′′-terthiophene (19). mp

156 °C (toluene). Anal. Calcd for C12 Br : C, 25.56; H, 0.72.

Found: C, 25.08; H, 0.82. 3,3′-Dibromo-2,2′:5′,2′′-terthiophene (20).

mp 106 °C. Anal. Calcd for C12 Br : C, 35.48; H, 1.49. Found:

C, 35.22; H, 1.48. 3-Perfluorohexyl-2,2′-dithiophene (23). Oil. Anal.

Calcd for C14 : C, 34.72; H, 1.04; F, 51.00. Found: C, 35.01;

18 7 13 3

C H F S

4

7 13 3

H F S

+

H

4

4 3

S

H

6

2 3

S

Experimental Section

5 13 2

H F S

The following compounds were prepared according to known

23

procedures: 5,5′′′′′-diperfluorohexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-

(

23) (a) Facchetti, A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks,

2

3a

23b

sexithiophene (DFH-6T), 5,5′-dihexyl-2,2′-dithiophene (DH-2T),

T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547. (b) Barbarella,

G.; Favaretto, G.; Sotgiu, G.; Zambianchi, L.; Pudova, O.; Bongini, A. J.

Org. Chem. 1998, 63, 5497. (c) Byron, D.; Matharu, A.; Wilson, R.; Wright,

G. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 265, 61. (d) Garnier,

F.; Hajlaoui, R.; El Kassmi, A.; Horowitz, G.; Laigre, L.; Porzio, W.;

Armanini, M.; Provasoli, F. Chem. Mater. 1998, 10, 3334. (e) Li, W.; Katz,

H. E.; Lovinger, A. J.; Laquindanum, J. G. Chem. Mater. 1999, 11, 458.

(f) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet,

B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716. (g) Kagan, J.;

Arora, S. K. Heterocycles 1983, 20, 1937. (h) Nakayama, J.; Nakamura,

Y.; Murabayashi, S.; Hoshino, M. Heterocycles 1987, 26, 939. (i) Katz,

H. E.; Torsi, L.; Dodabalapur, A. Chem. Mater. 1995, 7, 2235. (j)

Nenajdenko, V. G.; Baraznenok, I. L.; Balenkova, E. S. J. Org. Chem.

23c

5

,5′-dihexyl-2,2′:5′,2′′-terthiophene (DH-3T), 5,5′′′-dihexyl-2,2′:5′,2′′:

′′,2′′′-quaterthiophene (DH-4T),

_′_′_′_,₂_′_′_′_′_-_q_u_i_n_q_u_e_t_h_i_o_p_h_e_n_e₍_D_H_-₅_T₎_,₂3e 5,5′′′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′:

′′′,2′′′′:5′′′′,2′′′′′-sexithiophene (DH-6T), 2,2′:5′,2′′:5′′,2′′′-quater-

thiophene (r4T), 2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquethiophene (r5T),

2

3d

5,5′′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′:

2

3f

23g

23h

23i

2

,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene (r6T), 5,5′-dibromo-

,2′-bithiophene (6),²2-(1-perfluorohexyl)thiophene (7), 5-perfluo-

3j

23k

2

3a

rohexyl-5′-bromo-2,2′-dithiophene (10), 5-bromo-2,2′-bithiophene

12), 2,5-bis(tri-n-butylstannyl)thiophene (13), 5,5′-bis(tri-n-butyl-

stannyl)-2,2′-dithiophene (14), 5-bromo-2,2′:5′,2′′-terthiophene (15),

2

3l

(

1

998, 63, 6132. (k) Chen, G. J.; Tamborski, J. Fluorine Chem. 1990, 46,

137. (l) Bauerle, P.; Wurthner, F.; Gotz, G.; Effenberger, F. Synthesis 1993,

1, 1099. (m) Wei, Y.; Yang, Y.; Yeh, J.-M. Chem. Mater. 1996, 8, 2659.

23m

23n

1

(

n) Nakayama, J.; Konishi, T.; Murabayashi, S.; Hoshino, M. Heterocycles

(

22) Facchetti, A.; Mushrush, M.; Yoon, M.-H.; Hutchison, G. R.; Ratner, M.

1987, 26, 1793. (o) Yu, W.-L.; Meng, H.; Pei, J.; Huang, W.; Li, Y.; Heeger,

A. J. Macromolecules 1998, 31, 4838.

A.; Marks, T. J. J. Am. Chem. Soc., in press.

13482 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Scheme 1. Synthesis of Compounds of Family 1^a

a

(i) NBS; (ii) Cu bronze, C6F13I; (iii) BuLi, Bu3SnCl; (iv) Pd(PPh3)4; and (v) BuLi, CuCl2.

H, 1.04; F, 49.98. 5,5′′′-Bis-trimethylsilyl-3,3′′′-diperfluorohexyl-2,2′:

End-capped fluoroalkyl compounds DFH-nTs (1) were pre-

pared according to Scheme 1. The shortest oligomer DFH-2T

was prepared without thienyl-thienyl coupling, but directly via

perfluoroalkylation of dibromodithiophene 6 in ∼40% yield.

This yield is somewhat lower compared to similar reactions on

monobrominated systems (50-80%) and probably reflects the

large molar excess of perfluorohexyl iodide employed. Such a

ratio avoids formation of monofluorinated derivatives from

which DFH-2T is difficult to separate. For the synthesis of the

other DFH-nTs, the final step involves a Stille coupling reaction,

which proceeds in yields greater than 80%. An attempt to

prepare DFH-6T by Cu(II)-promoted dimerization of lithiated

5

C

′,2′′:5′′,2′′′-quaterthiophene (25). mp 173 °C. Anal. Calcd for

Si : C, 36.75; H, 2.18; F, 44.46. Found: C, 36.67; H, 2.18;

34

H

F S

24 26 4

2

F, 44.76. 3-(1-Perfluorohexyl)-5′-bromo-2,2′-dithiophene (27). Oil.

+

HRMS (EI, 70 eV): found m/z, 563.8721, calcd for C14

H BrF13S

4 2

(M ),

5

63.8709. 3-Hexyl-5′-bromo-2,2′-dithiophene (28). Oil. HRMS (EI, 70

+

2

eV): found m/z, 327.9942, calcd for C14H17BrS (M ), 327.9955.

Results

Synthesis. In contrast to alkylation reactions,²⁴there are

relatively few convenient synthetic methods for introducing

perfluoroalkyl groups on aromatic and heteroaromatic skeletons.

Among them are copper-promoted coupling reactions between

1

terthiophene 16 affords impure samples in low yield. The H

2

5a,b

perfluoroalkyl iodides and aromatic halides,

catalyzed or photoinduced

(Ph3P)3RuCl2-

NMR spectrum of a portion quenched with D2O indicates

formation of 5′′-lithum-5-bromotertthiophene in the presence

of other lithiated species. Dimerization of this mixture results

inevitably in formation of a variety of products. All of the longer

oligomers prepared via Stille coupling can be purified by

gradient sublimation. The optimization of key intermediate 9,

which affords 10 upon bromination, was performed. This system

was initially prepared in 42% overall yield, starting from

2

5c

25d,e

reactions of perfluoroalkane-

sulfonyl halides with aromatic halides, and sulfinatodehalogena-

tion.²The synthetic route chosen here involves coupling of

perfluorohexylcuprate (from perfluorohexyl iodide and copper

bronze) with the appropriate bromothiophene derivative in

DMSO. Extension of the thiophene(s) core from two to six

thiophene units is accomplished principally via Pd(0)-catalyzed

coupling of bromothiophene derivatives with stannyl (Stille

5f,g

2

3a

2

6a

_a_n_d_G_r_i_g_n_a_r_d₍_K_u_m_a_d_a_c_o_u_p_l_i_n_g₎26b thienyl re-

dithiophene. Major problems encountered are the difficult

isolation of 12 in good yield from dibrominated 6 and separation

of 9 from other fluorinated products. For this reason, a more

efficient route starts from commercially available 2-bro-

mothiophene, which can be converted to 7 in excellent yield

and easily purified by distillation. Quantitative metalation of 7

coupling)

agents.

(

24) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms, and

Structure; Wiley: New York, 1992.

25) (a) Chen, G. J.; Tamborski, C. J. Fluorine Chem. 1990, 46, 137. (b)

Mcloughlin V. C. R.; Thrower, J. Tetrahedron 1969, 25, 5921. (c) Kamigata,

N.; Yoshikawa, M.; Shimizu, T. J. Fluorine Chem. 1998, 87, 91. (d)

Nakamura, T.; Koga, Y. J. Chem. Soc., Perkin Trans. 2 1998, 3, 659. (e)

Dolbier, W. R. Chem. ReV. 1996, 96, 1557. (f) Huang, X.-T.; Long, Z.-Y.;

Chen, Q.-Y. J. Fluorine Chem. 2001, 111, 107. (g) Huang, W.-Y.; Wu,

F.-H. Israel J. Chem. 1999, 39, 167.

(26) (a) Stille, K. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (b) Tamao, K.

J. Organomet. Chem. 2002, 653, 23. (c) Kumada, M. Pure Appl. Chem.

1980, 52, 669.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13483

A R T I C L E S

Facchetti et al.

Scheme 2. Synthesis of Compounds of Family 2^a

a

(

i) Br2; (ii) Zn + HCl; (iii) Cu bronze, C6F13I; (iv) PdCl2dppf; (v) BuLi; (vi) BuLi, TMSCl; (vii) BuLi, CuCl2; (viii) CF3SO3H; (ix) NBS; and (x)

Pd(PPh3)4.

1

affords stannyl derivative 8, which reacts with 2-bromothiophene

to afford 9 in >70% overall yield.

yield by H NMR analysis) was not isolated but lithiated again

and dimerized with CuCl2 to afford bisTMS-quaterthiophene

2

5 in 52% yield. In the last step, the TMS groups of 25 were

The synthetic routes to regioisomeric isoDFH-nTs (2) are

summarized in Scheme 2. The shortest oligomers, isoDFH-2T

and isoDFH-3T, were prepared following the same synthetic

route starting from di- and terthiophene, respectively, which

were regioselectively perbrominated at the 3- and 5-positions

of the external rings to yield 17 and 19. This reaction has never

been attempted with r3T, and it is remarkable that the central

thiophene ring does not undergo electrophilic attack by Br2.

Removal of the terminal bromo substituents of 17 and 19 was

achieved with Zn powder in HCl-EtOH to afford dibromo

derivatives 18 and 20, respectively, in excellent yields. Per-

fluoroalkylation of the latter compounds affords, after column

chromatography, isoDFH-2T and isoDFH-3T, respectively, in

modest yields (∼40%) due to the formation of large quantities

of byproducts. One of these byproducts, triperfluoroalkyl-3T

removed with CF3SO3H, providing isoDFH-4T quantitatively.

The longest oligomers, isoDFH-5T and isoDFH-6T, were

obtained in ∼80% yields by reaction of 27 with bistributyl-

stannyl derivatives 13 and 14, respectively.

Finally, the novel, regiochemically pure â,â′-dialkyl substi-

tuted oligothiophenes isoDH-nTs (n ) 5, 6) were prepared

according to Scheme 3. The key intermediate, bromo derivative

2

8, is unknown and was conveniently prepared via Ni(II)-

catalyzed reaction of the Grignard reagents of 2-bromo-3-

hexylthiophene and 2,5-dibromothiophene in good yield. Bro-

mination of parent compound 29 with either Br2 or NBS affords

a mixture of two bromodithiophenes, 28 and 29, inseparable

by conventional methods. Isomer 29 is formed in surprisingly

high selectivity (∼70%), in agreement with observations of

27

B a¨ uerle et al. for 3-dodecyl-2,2′-dithiophene bromination. The

target compounds isoDH-5T and isoDH-6T were obtained via

Stille coupling of pure 28 with 13 and 14, respectively, and

isolated in ∼80% yields.

2

1, was isolated in moderate yield (27%). The key intermediate

for the synthesis of isoDFH-4T is 3-perfluorohexyldithiophene

23), which is conveniently prepared from parent compound

2. Since the reaction of 23 with n-BuLi affords 5-lithium-3-

(

2

Multinuclear NMR Spectroscopy and Rotational Barriers.

¹H and F NMR spectra were recorded to confirm the molecular

structures and purity of the novel, fluorine-containing DFH-

perfluorohexyl-2,2′-dithiophene (26), direct dimerization of the

latter to give isoDFH-4T is not feasible. Note also that reaction

between bromodithiophene 27 and n-BuLi affords 26, clearly

indicating that prefluoroalkyl functionalization substantially

increases the thiophene ring acidity. Therefore, to achieve

regiochemically correct coupling, compound 23 was lithiated

and quenched with TMSCl. Intermediate 24 (obtained in >95%

19

1

nTs (1) and isoDFH-nTs (2) systems. H NMR spectra of 1

and 2 exhibit, compared to the corresponding fluorine-free DH-

(27) B a¨ uerle, P.; Pfau, F.; Schlupp, H.; Wuerthner, F.; Gaudl, K. U.; Balparda

Caro, M.; Fischer, P. J. Chem. Soc., Perkin Trans. 2 1993, 3, 489.

13484 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Scheme 3. Synthesis of Novel Compounds of Family 4^a

a

(i) PdCl2dppf; (ii) Pd(PPh3)4; and (iii) NBS or Br2.

nT (3) and isoDH-nT (4) analogues, low-field displacements

of the aromatic proton(s) adjacent to the perfluorohexyl chain.

As an example, Figure S1A (Supporting Information) shows

conformational interchange (slow exchange on the ¹⁹F NMR

time scale) between R and S enantiomers. Note that each of

these systems might represent a fast interconversion (on the

NMR time scale) of syn and anti conformers. However, we will

demonstrate that this is not the case (see Discussion). Several

theoretical and experimental studies of oligo- and polythiophene

1

H NMR spectra of DFH-5T and DH-5T. This result is the

first indication that strong σ-electron-withdrawing (-I) fluoro-

carbon substitution decreases thiophene electron density, which

correlates with a shift of the proton resonances toward low

fields.²⁸At room temperature, the F NMR spectra of DFH-

nTs (n ) 2-6) and isoDH-nTs (n ) 3-6) exhibit the usual

30a-d

conformations have been reported.

The theoretical analyses

19

have consistently indicated that syn and anti conformers are

nonplanar. The energies, and thus populations, of these con-

formers are comparable, and their interconversion barriers are

very small (1-2 kcal/mol). A recent experimental study by

Holdcroft et al. also indicates that the barriers to conformational

interchange in oligothiophenes are too low to be measured by

pattern of an aromatic-bonded C6F13 group, with the appropriate

number of ¹F resonances (Figure S1B). In contrast, the ¹⁹

9

F

NMR spectra of isoDFH-2T and isoDFH-3T are more complex.

Particularly interesting is the spectrum of the former compound,

which exhibits patterns in which the R- and â-CF2 signals appear

as AX systems, indicating the presence of a stereogenic center

30e

dynamic NMR spectroscopy, contrasting previous investiga-

(see 20 °C spectrum in Figure 1A). Thus, at room temperature,

the ground-state molecular conformation(s) render the fluorines

of the R- and â-CF2 groups diastereotopic. Since this phenom-

enon has never previously been observed for alkyl-substituted

oligothiophenes (vide infra), it was additionally investigated by

variable-temperature NMR spectroscopy (Figure 1A). On cool-

ing from +20 to - 60 °C, additional CF2 resonances of the γ-

and δ-CF2 appear as AB systems. These strongly coupled

doublets have geminal F-F coupling constant that increase on

moving away from the thiophene core [JF-F (Hz): RR′ (276) <

ââ′ (299) < γγ′ (301) < δδ′ (307)]. A similar increase in JF-F

going from the R- to the terminal CF2 was observed by Hughes

2

9

et al. for fluoroalkyl complexes of (TMEDA)Pd(II). As the

temperature is raised from +20 to +130 °C, pairwise coales-

cence of the diastereotopic resonances is observed, which occurs

at ∼+70 °C for the â-CF2 signals and at ∼+105 °C for R-CF2

signals.

Figure 1. (A) ¹⁹F NMR spectra of isoDFH-2T in toluene-d8 as a function

of temperature. The spectrum at 130 °C was obtained in C2D2Cl4. (B)

Corresponding Eyring plot for the exchange process involving the R-CF2

group.

The origin of isoDFH-2T chirality lies in the noncoplanarity

of the bithiophene core and the relatively large barrier for

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13485

A R T I C L E S

Facchetti et al.

tions on alkyl-substituted tert- and quaterthiophenes where

temperatures is lower than those for isoDFH-2T (100-110 °C),

3

0f,g

barriers in the range of 8-20 kcal/mol were reported.

The

and consequently, the barriers to rotation must be lower. The

former study³also investigated isoDH-2T and concluded that

even at temperatures as low as -130 °C, the NMR spectrum is

consistent with the presence of a conformationally averaged

system.

0e

1

H NMR spectrum of isoDFH-3T is consistent with the

NMR and also shows evidence of a rotameric mixture.

19

F

For the above reasons and the contradictory results reported

on the alkyl-substituted compounds, isoDH-2T was investigated

again in the temperature range of -60 to +130 °C. Our results

are in agreement with those of Holdcroft et al., and the

conformational interconversion barrier could not be obtained

for any of the other isoDH-nTs (n ) 3-6) via NMR. On the

other hand, the gNMR full shape analysis program could be

used to calculate the activation energies of R- and S-isoDFH-

2

T enantiomer interconversion. The geminal F-F coupling

constant of the R-CF2 resonances determined at 20 °C was held

constant for simulation of the high-temperature spectra. The rate

constants were obtained via the best fit between the experimental

q

X-ray Crystallography. X-ray structural analysis was per-

formed on four members of the fluorocarbon-substituted se-

ries: DFH-3T, DFH-4T, isoDFH-5T, and bisTMS-substituted

precursor 25. Single crystals were prepared by slow cooling of

saturated solutions except for 25, where crystals were obtained

by slow sublimation. Important crystallographic data for these

compounds are collected in Table S1. The slipped-stacking

crystal structures of the end-substituted DFH-3T and DFH-4T

systems exhibit a monoclinic unit cell with eight and four

molecules, respectively. Note that all of the unsubstituted

oligothiophene RnTs (n ) 2-6, 8) crystallize in the monoclinic

and calculated spectra. The Eyring eq 1 was used to obtain ∆G ,

q

∆

H , and ∆S . The plot of ln(k/T) vs 1/T shows good linearity,

2

with r ) 0.9956 (Figure 1B). The following activation

parameters were derived: ∆G ) 14.99(5) kcal/mol, ∆H )

2.37(2) kcal/mol, and ∆S ) -0.09(1) kcal/mol K.

q

1

q

ln(k/T) ) ln(k /h) + ∆S /R - ∆H /RT

(1)

B

_T_h_e1⁹F and H NMR spectra of isoDFH-3T at various

1

temperatures are more complex (Figure S2) since this system

can exist in solution as a mixture of diastereoisomeric rotamers

3

1

system as well. The aromatic cores of both structures exhibit

an all-anti, fully planar geometries, with dihedral angles between

the mean plane of the rings <2° (Figure 2A,B). This value

compares well to that reported for r4T but is much smaller

than the 6-9° reported for r3T. The perfluorohexyl substituents

exhibit a zigzag helical conformation characteristic of fluoro-

(e.g., planar A-C, considering only two 180° inter-ring

torsions). Therefore, multiple sets of resonances are possible

for both fluorocarbon chains and aromatic protons. Indeed, the

1

9

F NMR spectrum of isoDFH-3T at 25 °C exhibits multiple

sets of resonances for the perfluorohexyl chains, one of which

dominates over the entire temperature range investigated (-70

to +100 °C). This major set of signals exhibits equivalent

resonances for the R-CF2 groups, indicating that it is associated

with one of the planar symmetric rotamers or more likely with

a mixture of conformers in fast interconversion. On the other

hand, the minority patterns present in the room- and low-

temperature spectra exhibit splitting of the R-CF2 signals,

demonstrating that they are associated with two chiral nonplanar

structures. As the temperature is increased, these peaks broaden

and eventually collapse in the temperature range +(60-90) °C.

3

2

carbon chains and are positioned at ∼140° with respect to

the oligothiophene backbone axes in both cases. The molecular

packing of DFH-4T shares the familiar “herringbone” (HB)

motif found in all members of the RnT series, with an angle of

5

0° between mean planes of adjacent molecules (Figure 2B).

Typical HB angles for oligothiophene RnTs (n ) 4-6, 8) range

between 55 and 70°. The origin of this angle, which is typical

of many aromatic hydrocarbons such as p-oligophenylenes and

acenes, is principally due to intermolecular π-electron repul-

31

F

sion. The minimum interplanar distances between neighboring

molecules in DFH-4T are found to be 3.02 (C5-H7′), 3.52 (C2-

C4′), 3.54 (C4-C7′), and 3.72 Å (S2-C6′), comparable to 3.5-

At +100 °C, the R chains exhibit a single set of resonances.

However, a clear coalescence pattern is not observed, probably

because of the relative low concentration of these species with

respect to the major component(s), and therefore an accurate

determination of the barriers associated with the exchange

processes is not possible. However, the range of coalescence

3

.9 Å in analogous oligothiophenes. The DFH-3T solid-state

packing appears more complex (Figure 2A), with alternating

regions of cofacial π-π stacking (slipped by one thiophene ring)

and HB arrangement (with an angle of ∼48°) of dimeric units

extending along the c axes. The minimal intermolecular

distances found in DFH-3T are 3.80 (S1-C11′), 4.05 (S2-S1′),

and 4.12 Å (S1-C12′) for cofacial dimers, and 3.59 (C9-C15′),

3.69 (C17-C11′), and 3.71 Å (S2-C14′) for HB dimers.

The crystal structures and packing characteristics of â,â′-

disubstituted systems isoDFH-5T and 25 are considerably

different from those of the end-capped compounds (Figure

(

28) It is well-established that the chemical shift of a proton bound to an aromatic

system is related to the effective charge on the carbon atom to which the

proton is bound. Memory, J. D.; Wilson, N. K. NMR of Aromatic

Compounds; John Wiley and Sons: New York, 1982.

(29) Hughes, R. P.; Overby, J. S.; Williamson, A.; Lam, K.-C.; Concolino, T.

E.; Rheingold, A. L. Organometallics 2000, 19, 5190.

(30) (a) Di Cesare, N.; Belletete, M.; Leclerc, M.; Durocher, G. J. Chem. Phys.

A 1999, 103, 803. (b) Bongini, A.; Bottoni, A. J. Phys. Chem. A 1999,

1

03, 6800. (c) Subramanian, H.; Lagowski, J. B. Int. J. Quantum Chem.

998, 66, 229. (d) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R.

1

A. Synth. Met. 1998, 96, 177. (e) Diaz-Quijada, G. A.; Weinberg, N.;

Holdcroft, S.; Pinto, B. M. J. Phys. Chem. A 2002, 106, 1266. (f)

Muguruma, H.; Kobiro, K.; Hotta, S. Chem. Mater. 1998, 10, 1459. (g)

Horne, J. C.; Blanchard, G. J.; LeGoff, E. J. Am. Chem. Soc. 1995, 117,

(31) For a recent review on oligothiophene crystal structures, see: Fichou, D.

J. Mater. Chem. 2000, 10, 571.

(32) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549.

9

551. (h) Bredas, J.-L.; Heeger, A. J. Macromolecules 1990, 23, 1150.

1

3486 J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004

9

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Figure 2. Molecular structure and crystal packing diagram of (A) DFH-3T and (B) DFH-4T. Fluorine atoms (middle) and fluorocarbon chains (bottom)

are removed for clarity.

3A,B). These systems crystallize in a triclinic (Z ) 6) and

between the fluorohexyl chain R-CF2 group and either the sulfur

monoclinic (Z ) 4) unit cell, respectively. The effect of R,ω f

â,â′ regiochemical substitution on the π-core structure is

dramatic with a large torsional angle forced between adjacent

thiophene rings compared to the DFH-nTs series. The magnitude

of this torsional angle is larger between the two thiophene rings

located at the external [31-64° (isoDFH-5T); 45.6° (25)] versus

the internal [1-30° (isoDFH-5T); 0.7° (25)] core positions. To

a lesser degree, this has been observed as well for some â-alkyl-

substituted oligothiophenes³³and is due to steric repulsion

atom or the C -H moiety on the adjacent thiophene ring (vide

infra). Compound 25 has one-half an independent molecule in

â

the asymmetric unit, and the core exhibits both syn (between

outer rings) and anti (between inner rings) configurations. The

molecular packing in the crystal consists of alternating layers

of aromatic TMS and fluorocarbon regions parallel to the bc

plane (Figure 3B). In each layer, the conjugated backbone forms

slipped columnar stacks between the TMS regions along the

b-axes with a minimum intrastack distance of 3.76 Å (S2-C10′).

The minimum intercolumnar distance is only slightly greater

[3.95 Å (S1-S2′′)] but because of the presence of bulky TMS

groups, contacts occur in this direction only at the molecular

termini with most of the core π-orbitals not interacting along

(

33) (a) Tetramethyl-R4T: Barbarella, G.; Zambianchi, M.; Bongini, A.; Antolini,

L. AdV. Mater. 1992, 4, 282. (b) Dibutyl-R6T: Herrema, J. K.; Wildeman,

J.; van Bolhuis, F.; Hadziioannou, G. Synth. Met. 1993, 60, 239. (c) Dihexyl-

R6T: Sato, T.; Fujitsuka, M.; Shiro, M.; Tanaka, K. Synth. Met. 1998, 95,

1

43.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13487

A R T I C L E S

Facchetti et al.

Figure 3. Molecular structure and crystal packing diagram of (A) isoDFH-5T and (B) compound 25. Fluorine atoms (middle) and fluorocarbon chains

bottom) are removed for clarity.

(

the c direction. In the case of isoDFH-5T, two of the three

independent molecules exhibit a syn-anti-anti-anti arrange-

ment (starting from the external ring) and the third, a syn-

anti-anti-syn conformation. The molecular packing (Figure

the aromatic thiophene subunits interact strongly, without any

apparent order, in both a and b directions, with close contacts

at 4.01 (C18-S10′), 3.52 (C10-C43′), 3.58 (C10-C44′), 3.53 (C47-

S13′), and 3.87 Å (C39-S11′).

Finally, note that, regardless of the differences in molecular

structure and packing characteristics, all fluorocarbon-substituted

compounds exhibit similar intermolecular separations between

3A) shows similarities to those of the other systems, with

alternating layers of 5T cores and fluorocarbon chains extending

perpendicular to the c-axes. Despite the poorly conjugated core,

13488 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Table 1. Capillary Melting Points, DSC Thermal Transitions, and Volatility Properties of Solid Compounds of Series 1-5

heating

cooling

TGA

T (°

C)^a

compound

mp (

°

C)

T

DSC

(°

C)

∆H (kcal/mol)

T

DSC

(

°C)

∆H (kcal/mol)

DFH-2T

DFH-3T

DFH-4T

DFH-5T

DFH-6T

isoDFH-4T

isoDFH-5T

isoDFH-6T

DH-2T

97

132

213

252

309

145

160

217

34

97

131

213

168, 250

292, 312

158

168

220

34

11.52

9.29

16.33

0.57, 12.98

9.81, 1.11

14.11

11.02

15.42

86

124

203

244, 152

292, 284

139

159

210

10

11.41

9.25

14.94

0.74, 11.44

8.93, 0.72

12.25

10.62

15.84

9.75

65

91

153

196

232

119

147

187

84

10.04

DH-3T

DH-4T

DH-5T

DH-6T

isoDH-5T

isoDH-6T

83

48, 87

88, 186

258

3.92, 4.53

4.48, 8.15

10.29

15.71

8.62

11.56

4.05

4.67

10.89

78, 31

178, 42

248

309, 300, 295

48, 44, 37

130

28

85

210

250

3.78, 4.18

4.41, 7.78

8.78

14.55

5.98

11.34

3.95

4.57

10.19

130

184

222

270

186

277

30

184

254

310

78

137

34

300, 308, 314

77

147

36

2

3

4

5

6

T

95

97

61

216

257

309

221

262

316

144

184

260

10.78

18.45

10.50

14.97

307

a

Calculated as the temperature where 2% of the material has sublimed at 10^-²Torr.

the oligothiophene and fluorocarbon regions with intercore and

interchain distances comparable to the sum of the respective

sulfur-sulfur (3.4-3.5 Å), sulfur-carbon (3.4-3.5 Å), carbon-

carbon (3.3-3.4 Å), and fluorine-fluorine (3.0-3.1 Å) van der

Waals radius sums.

orientation of the π-conjugated core is a key factor affecting

charge transport, the presence of LC phases might be useful to

further align the molecular components. For relatively short

oligothiophenes, the attachment of alkyl chains to the core is a

prerequisite for the appearance of mesophases. Funahashi et al.

reported that dialkyl terthiophene derivatives exhibit appreciable

charge mobility in their smectic phases, because of the crystal-

Thermal Properties. The thermal properties of compounds

1-5 are reported in Table 1. Within each family, melting points

3

5f

increase with increase in the number of thiophene units. In

addition, R,ω-substituted oligomers DFH-nTs (1) and DH-nTs

like ordering of these phases. Again for terthiophenes, the

appearance of liquid crystalline phases strongly depends on the

35g

(3) melt at considerably higher temperatures than the corre-

substituents at the R,ω-positions. The present DSC measure-

sponding â,â′-substituted isoDFH-nTs (2) and isoDH-nTs (4)

ments were performed under nitrogen at heating and cooling

-

1

isomers, as expected considering that lateral substitution of

rates of 5 °C min , and results are collected in Table 1. DSC

thermograms for end-substituted systems DFH-nTs (1) and DH-

nTs (2) (Figure S3) reveal that some of these compounds

undergo multiple thermal transitions. Upon heating, fluorinated

systems DFH-5T and DFH-6T exhibit the first/second endo-

thermic peaks at 168/250 °C and 292/312 °C, respectively, with

a ∆H of +0.57/12.98 and +9.81/1.11 kcal/mol. Polarized optical

micrographs indicate that when DFH-6T is cooled from the

isotropic liquid phase, an anisotropic phase separates from the

liquid almost without supercooling. The texture is reminiscent

of a smectic liquid crystal (Figure 4B). On the other hand, upon

cooling DFH-5T below the second transition, the liquid turns

into a solid phase that cannot be distinguished by microscopic

means as a liquid crystalline phase or a solid phase. In the alkyl-

substituted family, multiple thermal transitions are observed for

DH-3T (48/87 °C, +3.92/4.53 kcal/mol), DH-4T (88/186 °C,

+4.48/8.15 kcal/mol), and DH-6T (300/308/314 °C, +15.71

kcal/mol). Optical micrographs on cooling clearly indicate the

presence of a LC mesophase, probably smectic, before solidi-

fication for DH-4T (Figure 4C), whereas DH-3T appears to

exhibit a crystal-to-crystal transition before melting. Unfortu-

13,33

rodlike molecules reduces intermolecular interactions.

Fur-

thermore, since 2 and 4 are expected to be less planar, the

decrease in molecular symmetry is associated with a lowering

of the melting point.³⁴Particularly interesting is the effect of

chemical substitution on the cohesive forces operative in these

families of organic materials. Figure 4A plots the melting points

of 1-5 versus n (the number of thiophene rings). In general,

the melting points of fluoroalkyl-substituted oligomers are higher

than those of the corresponding unsubstituted and alkyl-

substituted systems. However, when end-capped systems 1 and

3

are compared to unsubstituted 5, this plot can be divided into

two regions: the first for n > 4, where melting points are

H

F

essentially independent of chemical substitution (H, R , R ),

and the second for n < 4, where DFH-nT melting points are

found to be much higher. This result suggests that in the first

region, the intermolecular cohesive forces are dominated by the

strong π-π interactions of the extended conjugated cores,

whereas the second probably reflects the different specific

gravities of F versus H and different chain-chain interactions

between alkyl and fluoroalkyl substituents.²⁰

The comparative thermal behavior of oligothiophenes 1-5

was additionally investigated by DSC and TGA. Although the

liquid crystalline properties of polythiophenes have been previ-

(

35) (a) Toba, M.; Takeoka, Y.; Rikukawa, M. Synth. Met. 2003, 135-136,

39. (b) Rosu, C.; Manaila-Maximean, D.; Paraskos, A. J. Mod. Phys. Lett.

B 2002, 16, 473. (c) Sharma, S.; Lacey, D.; Wilson, P. Liq. Cryst. 2003,

0, 461. (d) Liu, P.; Nakano, H.; Shirota, Y. Liq. Cryst. 2001, 28, 581. (e)

3

5a,b

ously investigated,

fewer studies have been reported on

Azumi, R.; G o¨ tz, G.; B a¨ uerle, P. Synth. Met. 1999, 101, 544. (f) Funahashi,

M.; Hanna, J. Appl. Phys. Lett. 2000, 76, 2574. (g) Yamada, T.; Azumi,

R.; Tachibana, H.; Sakai, H.; Abe, M.; B a¨ uerle, P.; Matsumoto, M. Chem.

Lett. 2001, 30, 1022. (h) Ponomarenko, S.; Kirchmeyer, S. J. Mater. Chem.

2003, 13, 197.

_o_l_i_g_o_t_h_i_o_p_h_e_n_e_L_C_b_e_h_a_v_i_o_r_.3

5c-g

Since the relative molecular

(34) Brown, R. J. C.; Brown, R. F. C. J. Chem. Educ. 2000, 6, 724.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13489

A R T I C L E S

Facchetti et al.

The comparative volatility properties of series 1-5 (informa-

tion required for control of film growth) were investigated by

-

2

low-pressure TGA at 10 Torr and a temperature ramp of 1.5

C/min. Most of the literature data regarding semiconducting

performance in an FET configuration for unsubstituted nTs (n

5, 6) and alkyl-substituted DH-nTs (n ) 4-6) were obtained

on vacuum-deposited films. However, other than our preliminary

°

)

2

3a

investigation on DH-6T,

no stability or volatility data

regarding the sublimation of these classes of materials are

available. Table 1 also summarizes the sublimation temperature

(TTGA, the temperature where 2% of the material has sublimed)

for 1-5. Comparing these values with the corresponding melting

points, it can be seen that all fluoroalky-substituted DFH-nTs

(1) and isoDFH-nT (2) derivatives sublime below the melting

point by ∼15-80 °C. Interestingly, a similar range is observed

for the unsubstituted RnTs. On the other hand, most of the DH-

nTs (3) and isoDH-nTs (4) alkyl derivatives sublime at much

higher temperatures and in most cases only above the melting

points. Figure S5 shows TGA plots comparing the DFH-nT and

DH-nT series, which clearly demonstrate that quantitative,

smooth, and residue-free sublimation occurs for all fluorinated

derivatives. In contrast, the TGA data of all alkyl-substituted

compounds reveal inflection point(s) (Figure S5B), suggesting

decomposition processes concomitant with sublimation. A

similar trend is observed in the isomeric series isoDFH-nTs and

isoDH-nTs (Figure S6). The unsubstituted RnT (5) derivatives

sublime quantitatively, with the noticeable exception of r6T,

which leaves ∼6% of residue (Figure S3).

Optical Properties. Optical absorption and fluorescence

emission spectra of 1-5 were measured both in solution and

as thin films to assess the effect of both fluoroalkyl- vs alkyl-

and R,ω- vs â,â′-substitutions on oligothiophene absorption and

emission maxima (λabs/λem) and the (optical) HOMO-LUMO

energy gap. Table 2 contains UV-vis/PL data for all compounds

in THF solution. As expected, all of these systems are poorly

solvatochromic, as verified by the weak shift of λabs (<10 nm)

exhibited by the shortest and longest members of series 1-5

Figure 4. (A) Plot of the melting points of 1-5 series [DFH-nTs (0),

DH-nTs ([), isoDFH-nTs (2), isoDH-nTs (b), RnT (×)] versus the number

of thiophene rings (n). The lines are a guide to the eye. Polarized

micrographs on cooling of (A) DFH-6T at 290 °C and (B) DH-4T at 174

(n ) 2, 6) in solvents of different polarity (toluene, CH3CN,

dioxane, CHCl3). The solution absorption spectra of 1-5 are

devoid of any fine structure, whereas splitting of bands due to

vibronic coupling can be clearly seen in the fluorescence spectra

at room temperature (Figures S7, S8). This coupling is due to

planarization of the system upon excitation. The solution ground

state is on average twisted, while the excited state is more planar,

having greater quinoid character, in agreement with computa-

tional studies on oligothiophenes (vide infra). The splitting

periodicity as calculated after Gaussian deconvolution (Figure

°

C.

nately, the DH-6T transitions are above 300 °C and hence not

presently accessible to us. However, it is likely that the high-

temperature mesophase is liquid crystalline, as suggested by

recent results on the corresponding R,ω-didecyl substituted

35h

system. With the exception of isoDH-5T, which exhibits three

transitions upon cooling, all of the remaining R,ω-substituted

systems of series 1 and 3 and the unsubstituted and â,â′-

substituted systems 2, 4, 5 exhibit a single DSC transition both

on heating and cooling (Figure S4). However, the lack of a

second peak does not necessarily exclude the presence of

mesophases before melting. In fact, it has been observed

previously for rigid-rod molecules that the mesophase-to-

isotropic transition is frequently not detectable by either optical

microscopy or DSC analysis, because of the large difference in

the enthalpies associated with the two processes, namely 5-15

kcal/mol for the C-to-M transition and 0.1-0.5 kcal/mol for

the M-to-I transition.³This has been demonstrated in the case

of r6T, where the presence of a mesophase was clearly assigned

by XRD.³

-1

S9) is ∼1400-1500 cm and therefore can be attributed to

one of the ν(CdC) stretching modes of the oligothiophene core.

The large molar absorption coefficients (ꢀ, Table 2) indicate

the dominance in the optical spectra of the allowed conjugated

core π f π* transition. As expected, 1-5 λabs values increase

throughout the series (n ) 2 f 6) for increasing numbers of

2Tf6T

thiophene units. The magnitude of this shift [∆λabs

) λabs

(n ) 6) - λabs (n ) 2)] from the 2T to 6T derivatives is 135,

1

24, and 132 nm for DFH-nTs (1), DH-nTs (3), and RnTs (5),

respectively, with the latter value being close to the 130 and

33 nm measured for the unsubstituted oligothiophenes in CH2-

1

6a

(

36) (a) Johnson, J. F.; Porter, R. S.; Barrall, E. M., II. In Liquid Crystals 2;

Brown, G. H., Ed.; Gordon and Breach: London, 1969. (b) Destri, S.;

Mascherpa, M.; Porzio, W. AdV. Mater. 1993, 5, 43.

6b

13490 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Table 2. Absorption (Maximum of Absorption, λabs nm, and

Fluorescence emission spectra were measured in THF by

-

1

-1

Extinction Coefficient at λabs, ꢀ(M cm )) and Emission Data

-

5

-6

exciting 10 to 10 M solutions of 1-5 at the corresponding

λabs. The shape of the fluorescence excitation spectra tracks those

of absorption. Photoluminescence quantum yields (Φf) were

determined using 9,10-diphenylanthacene as the standard,³⁹and

PL data are collected in Table 2. Intrinsically, Φf is determined

by the relative rate of nonradiative and radiative deactivation.

Φf values are found to increase quasi-monotonically with the

number of thiophene rings. This behavior is similar to that found

in the RnT series in dioxane, where Φf was found to increase

up to six rings and then remain stable or even decrease for longer

(

Maximum of Fluorescence, λem (nm), and Quantum Yield, Φf) of

Compounds 1-5 in Dry THF

a

op

c

₍_e_V₎b

c

compound

λ

max

(ꢀ

)

λ

f

E

g

∆

Φ

f

DFH-2T

DFH-3T

DFH-4T

DFH-5T

308 (17 500)

360 (28 900)

398 (39 000)

421 (40 200)

443 (53 000)

238 (2600)

369

3.63

3.12

2.82

2.64

2.51

4.33

3.32

2.88

2.73

2.60

3.52

3.04

2.80

2.62

2.54

4.29

3.14

2.88

2.70

2.58

3.75

3.14

2.89

2.72

2.61

0.67

0.61

0.41

0.43

0.39

1.18

1.19

0.82

0.67

0.52

0.64

0.52

0.41

0.39

0.41

0.66

0.82

0.56

0.48

0.45

0.66

0.58

0.42

0.40

0.39

0.03

0.26

0.50

0.73

1.00

<0.01

0.13

0.26

0.51

0.80

0.03

0.24

0.47

0.55

0.78

<0.01

0.11

0.38

0.43

0.61

0.01

0.09

0.24

0.39

0.56

417, 437*

458*, 489

493*, 525

515*, 547

308*, 397

429*, 446

465*, 489

498*,(522)

510*, 537

377

424, 443*

463*, 492

491*, 523

515*, 550

314

419, 431*

455*, 480

484*, 515

508*, 543

363

408, 428*

450*, 478

483*, 513

505*, 539

d

DFH-6T

isoDFH-2T

isoDFH-3T

isoDFH-4T

isoDFH-5T

isoDFH-6T

DH-2T

DH-3T

DH-4T

DH-5T

DH-6T

isoDH-2T

isoDH-3T

isoDH-4T

isoDH-5T

isoDH-6T

304 (12 700)

356 (25 500)

393 (38 300)

420 (47 900)

316 (16 800)

374 (22 300)

402 (40 000)

426 (42 900)

440 (52 500)

3

7b

oligomers. With the exception of highly twisted oligomers

isoDFH-2T and isoDH-2T, substitution at either R,ω- or â,â′-

positions is found to increase the solution quantum yield.

Fluorocarbon substitution is found to markedly increase the

solution photoluminescence quantum yields in all cases, with

the only exception being the isoDFH-/isoDH-4T couple, with

the efficiency of DFH-6T approaching unity. This effective

modulation of quantum efficiency by chemical substitution must

be intimately related to changes in the relative positions of the

lowest excited states within the singlet and triplet manifolds

and, hence, to changes in the decay rate via efficient nonradiative

channels, such as intersystem-crossing processes⁴due to

excimer formation (self-quenching) and intramolecular vibra-

d

e

f

269 (5400)

335 (15 000)

377 (28 800)

408 (34 700)

429 (54 100)

304 (12 800)

357 (23 100)

391 (35 000)

418 (46 400)

436 (50 000)

2

3

4

5

6

T

0a

a

Absolute maximum indicated by asterisk, shoulder in parentheses. ^b

∆

40

tional or rotational modes. Finally, when the effect of

c

)

λem - λabs. Measured using 9,10-diphenylanthracene as the standard

d

e

regiochemistry is considered, a lower Φf is observed when the

substituents are displaced from terminal (DFH-/DH-nTs) to

lateral (isoDFH/isoDH-nTs) positions. A similar trend was

observed by Demanze et al. for some members of the dicyano-

(

Φ ) 0.90 in cyclohexane). Measure at ∼60 °C. ꢀ is (10%. Evaluated

f

at the shoulder. The λmax at 248 nm has been attributed to thiophene ring

(

Van Hutten, P. F.; Gill, R. E.; Herrema, J. K.; Hadziioannou, G. J. Phys.

f

Chem. 1995, 51, 3218). At 248 nm.

4

1

_C_l₂₃7a _a_n_d_d_i_o_x_a_n_e_,37b respectively. A much larger displacement

nT series. For conjugated aromatic oligomers and polymers,

an additional efficient source of nonradiative deactivation is low

frequency, interannular torsional modes, which are dominant

is observed in both isoDFH-nT (2) and isoDH-nT (4) families

2

Tf6T

with a ∆λabs

is due mainly to the short wavelength absorption of 238 and

69 nm for isoDFH-2T and isoDH-2T, respectively, compared

of 182 and 160 nm, respectively. This result

4

0c

for twisted interannular configurations.

2

It is interesting to note that when comparing all systems with

the same conjugation core length n, the maximum dispersion

to r2T (304 nm). The former value is close to the absorption

of thiophene (230 nm in THF),³providing additional evidence

that the isoDFH-2T core is essentially deconjugated in solution.

When comparing the absorption of systems with the same core

7c

nT

in λ_e_mis on average less than that for λ_a_b_s, hence [∆λ_e_m(max)

nT

)

(

63 (2T), 12 (3T), 10 (4T), 14 (5T), 7 nm (6T)] < [∆λabs -

max) ) 70 (2T), 56 (3T), 42 (4T), 28 (5T), 23 nm (6T)].

nT

[

∆λabs (x T y) ) |λabs (x) - λabs (y)|; x, y ) 1-4], with the

nT

Furthermore, when comparing the ∆λem /∆λabs of the end-

6

T

exception of DFH-6T and DH-6T for which the ∆λabs (1 T

nT

capped series 1 and 3 (∆λem ) 0-8 nm/∆λabs ) 3-14 nm)

with those of the corresponding â,â′-isomers 2 and 4 (∆λem

2-14 nm/∆λabs ) 9-31 nm), it is evident that both ∆λem /

λabs values of the latter series are indeed larger than those

of the former systems, but ∆λem (2/4) increases in smaller

increments than does the corresponding ∆λabs (2/4). This

3

6

) value shifts bathochomically by ∼3 nm (note that the DFH-

T spectrum was recorded at ∼60 °C), all of the remaining

nT

)

∆

fluoroalkyl-substituted systems 1 and 2 experience, compared

to their alkyl analogues 3 and 4, a hypsochromic shift (5-31

nm), which can be explained by alkyl group inductive and

hyperconjugative effects.³⁸In addition, in R,ω-disubstituted nTs

nT

apparent anomalous difference in ∆λ’s can be explained in terms

of changes in the nuclear geometries of 1-4 upon excitation.

In the ground state, the conjugated thienyl cores are largely

1

and 3, λabs values are bathochromically shifted compared to

those of the corresponding â,â′-isomers 2 and 4, respectively,

probably because â,â′-disubstitution engenders steric interactions

that distort the conjugated π-core from planarity in solution.

The extent of this interaction strongly increases when the length

42

aromatic, with an average interannular bond order of ∼1. Upon

excitation to the Franck-Condon excited state, the nuclear

nT

of the conjugated core is reduced, since ∆λabs (1 T 2) and

(

39) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.

nT

∆

λabs (3 T 4) increase on going from the 6T (23 and 11 nm,

40) (a) Beljonne, D.; Cornil, J.; Friend, R. H.; Janssen, R. A. J.; Bredas, J.-L.

J. Am. Chem. Soc. 1996, 118, 6453. (b) Turro, N. J. Modern Molecular

Photochemistry; University Science Book: Sausalito, CA, 1991. (c)

Berlaman, I. B. Fluorescence Spectra of Aromatic Molecules; Academic

Press: New York, 1971. (d) Pope, M.; Swenberg, C. E. Electronic Processes

in Organic Crystals; Oxford University Press: New York, 1982.

respectively) to the 2T (70 and 53 nm, respectively) derivatives.

(

37) (a) Fichou, D.; Nunzi, J.-M.; Charra, F.; Pfeffer, N. Mol. Cryst. Liq. Cryst.

Sci. Technol., Sect. A 1994, 255, 73. (b) Becker, R. S.; Seixas de Melo, J.;

Macanita, A. L.; Elisei, F. J. Phys. Chem. 1996, 100, 18683. (c) Inoue, S.;

Jigami, T.; Nozoe, H.; Aso, Y.; Ogura, F.; Otsubo, T. Heterocycles 2000,

(41) Demanze, F.; Cornil, J.; Garnier, F.; Horowitz, G.; Valat, P.; Yassar, A.;

Lazzaroni, R.; Bredas, J.-L. J. Phys. Chem. B 1997, 101, 4553.

(42) (a) Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 4457. (b) Bredas, J.-

L.; Themans, B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Phys. ReV.

1984, 29, 6761.

5

2, 159.

(38) Di C e´ sare, N.; Bellet eˆ te, M.; Raymond, F.; Leclerc, M.; Durocher, G. J.

Phys. Chem. A 1997, 101, 776.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13491

A R T I C L E S

Facchetti et al.

geometry relaxes rapidly (<1 ps)⁴³to a more stable configu-

ration, i.e., the equilibrium excited state, with greater quinoid

character and an interannular bond order of ∼2. Therefore, the

first observation indicates that in general the excited states of

1

-5 having the same n differ less electronically than the

corresponding ground states, probably because of the rigidifi-

cation of the excited-state quinoid structure. The second

observation suggests that the quinoid form, which has an

intrinsically higher degree of coplanarity, is apparently more

effective in planarizing the more sterically hindered isoDFH-/

isoDH-nTs isomers than the DFH-/DH-nTs. In other words, the

excited states of 2 and 4 are still less planar than those of 1 and

3

, but much more so than might be anticipated from ground-

state considerations. Similar conclusions were drawn from

comparative optical absorption and PL analyses of regioregular

Figure 5. Plot of Eg^o^pversus 1/n for molecules 1-5. DFH-nTs (0), DH-

nTs ([), isoDFH-nTs (2), isoDH-nTs (b), RnT (×). The points corre-

sponding to isoDFH-3T, isoDFH-2T, and isoDH-2T are not included in

the linear correlation.

(

highly conjugated) and regiorandom (highly twisted) 3-hexylpoly-

4

2b

thiophene.

The change in conformation to a more planar structure and

energy dissipation during the excited-state lifetime are manifest

in Stokes shifts, rigorously defined as the energy difference

lated to infinite length converge to a value of ∼2.1 eV, which

compares well with 2.1-2.3 eV found from extrapolation in

40d

45a,b

between the 0-0 transitions in absorption and emission. Since

-0 transitions are rarely observed in room-temperature solution

other oligothiophene classes

and 2.1-2.5 eV for soluble

4

5c

0

regioregular 3-alkylpolythiophenes.

spectra, it is common procedure to use ∆ ) λem - λabs as an

indication of the magnitude of the Stokes shift. The observed

general trend here is that ∆ decreases with increasing n and

repositioning of substituents from lateral to end positions.

Chemical substitution appears to have only a marginal effect.

The amplitude of this shift generally tracks the lifetime of the

UV-vis/PL data for molecules 1, 3, 5 (n ) 3-6), 2 (n )

4-6), and 4 (n ) 5, 6) as vacuum-deposited thin films are

collected in Table S2. The absorption spectra (Figure S8) exhibit

characteristic peak(s) at high energy (270-280 nm) found in

the spectra of all oligothiophenes and originate from the

4

6

thiophene ring. The position and shape of the high energy π

f π* transition(s) reflect the interplay of molecular structure,

substitution, core length, and solid-state packing. In general

within each series, a bathochromic shift of this absorption is

observed as core length increases. Unlike the solution behavior,

the maxima of alkyl-substituted DH-/isoDH-nTs are shifted to

shorter wavelengths compared to those of fluorocarbon DFH-/

isoDFH-nTs, while â,â′-substituted isoDFH-/isoDH-nTs exhibit

a red shift of the absorption maxima compared to the corre-

sponding end-capped and unsubstituted analogues. As an

example, Figure 6 shows comparative UV-vis/PL spectra of

molecules 1-4 (n ) 5) in solution and as thin films. Note the

similarity between the spectra of R,ω-disubstituted derivatives

with the same core length (n ) 4-6) at long wavelengths, with

a strong absorption at 340-380 nm followed by two weaker

peaks or shoulders at 410-520 nm. The intense peak at high

energies can be attributed to exciton interactions between closed-

packed nearest-neighbor molecules. A consequence of the

herringbone structures is that the unit cell contains at least two

nonequivalent molecular sites. The coupling between the

transition dipole of the molecules at nonequivalent sites leads,

in the case of a rigid infinite lattice, to the well-known Davydov

4

0c,d

excited state,

with longer lifetimes corresponding to in-

creased relative probability of nonradiative decay and correlates

with lower PL quantum efficiency. Indeed, these results are in

good agreement with the present quantitative Φf measurements.

op

Solution optical gaps (Eg ), defined by the 0-0 transition

energies, were estimated from the intercept of the normalized

optical absorption and emission spectra, regarded as mirrors of

the 0-0 transitions.⁴The gaps increase with contraction of

0d

the core, from 2.51 to 2.62 eV for the 6T derivatives (n ) 6) to

op

3

.50-4.33 eV for the shortest 2T (n ) 2) series (Table 2). Eg

op

values remain in a very narrow range (∆Eg e 0.11 eV) for

oligomers 1-5 (n ) 4-6), and increase for shorter oligomers

op

1

-5 (n ) 3) (∆Eg e 0.25 eV) and 1-5 (n ) 2) (∆Eg

e

op

0

.83 eV). Figure 5 shows a plot of Eg versus 1/n, where a

clear correlation can be discerned. Note that the linear fit of

band gaps to 1/n for conjugated heterocyclic oligomers has been

shown to be only approximate, based on high-quality DFT

calculations,⁴⁴but fits extremely well for lengths of 2-6 rings

as studied here. The differences between the extrapolated infinite

4

values from 1/n are typically on the order of 0.2 eV. A linear

increase is observed for all end-capped and unsubstituted DFH-

nT, DH-nT, and RnT systems, whereas the plot of more

sterically hindered isoDFH-nTs and isoDH-nTs deviates upward

for the shortest systems. The projection of the three deviating

points on the line (see Figure 5) can be related to the effective

conjugation length, which is found to be ∼2.5 for isoDFH-3T

and ∼1.5/1.3 thiophene units for isoDH-2T/isoDFH-2T vs a

theoretical value of 3 and 2, respectively. When comparing

systems with the same core length, however, all plots extrapo-

4

0d,46

splitting.

When the dipoles are all parallel, the transition

between the ground state and the lower crystalline excited state

is completely forbidden, thus accounting for the unique intense

peak. In analogy with the results of Garnier et al. on DH-6T,⁴

the weak absorption at longest wavelengths in the DFH-nT and

DH-nT series can be attributed to the 0-0 transition of isolated

6b

(

45) (a) Izumi, T.; Kobashi, S.; Takimiya, K.; Aso, Y.; Otsubo, T. J. Am. Chem

Soc. 2003, 125, 5286. (b) Kirschbaum, T.; Briehn, C. A.; Bauerle, P. J.

Chem. Soc., Perkin Trans. 1 2000, 1211. (c) Lanzi, M.; Costa-Bizzarri, P.;

Della-Casa, C.; Paganin, L.; Fraleoni, A. Polymer 2002, 44, 535

(

43) Kanner, G. S.; Wei, X.; Hess, B. C.; Chen, L. R.; Vardeny, Z. V. Phys.

ReV. Lett. 1992, 68, 538.

(46) (a) Garnier, F. Acc. Chem. Res. 1999, 32, 209. (b) Yassar, A.; Horowitz,

G.; Valat, P.; Wintgens, V.; Hmyene, M.; Deloffre, F.; Srivastava, P.; Lang,

P.; Garnier, F. J. Phys. Chem. 1995, 99, 9155.

(44) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Phys. Chem. A 2002, 106,

1

0596.

13492 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

shoulders. The separation between the better-resolved peaks

-

1

(

1300-1500 cm ) suggests coupling with vibrational modes,

probably thiophene CdC stretches along the molecular back-

bone. On comparing films of identical thickness having

molecules with the same conjugation length, the emission

intensities of fluorocarbon-substituted DFH-nTs and isoDFH-

nTs are up to 3 orders of magnitude greater than those of the

corresponding unsubstituted and alkyl-substituted derivatives.

op

Finally, determination of film Eg parameters is less straight-

forward because of the multiple absorption and emission

op

transitions. However, reasonable estimation of Eg can be

obtained from the onset of the absorption (at 10% of the

op

maximum). Similar to the solution trends, Eg values decrease

as the core conjugation length increases and are larger for the

op

â,â′-substituted systems. In general, film Eg values are smaller

by 0.1-0.4 eV than the corresponding solution values. A similar

trend has been observed for other oligothiophenes.⁴⁷

Electrochemistry. The electrochemical properties of poly-

thiophenes reveal important aspects of chemical and electronic

structures, charge injection and storage mechanisms, substituent

4,48

effects, and physical behavior. In this aspect, oligothiophenes

1

3,48

Figure 6. Optical absorption (UV-vis) and emission (PL) spectra of (A)

THF solution of DFH-5T (thick line) and DH-5T (fine line), (B) THF

solution of isoDFH-5T (thick line) and isoDH-5T (fine line), (C) films

have been significantly less investigated.

Most studies have

been performed on limited numbers of compounds, and more

importantly, results are frequently compared to measurements

performed by other research groups under different conditions

of solvent, electrolyte type, and concentration. Although these

comparisons are useful and have promoted thiophene-based

conductor development, it is difficult to address quantitatively

the effects of chemical and regiochemical substitution and

oligomer dimensions. This is particularly true for the present

systems since the electrochemical effects of σ-only electron-

withdrawing and -donating substituents are expected, particularly

for larger molecular cores, to be relatively modest.

CV for oligomers 1-5 was performed under N2 in 0.1 THF/

TBAPF6 solutions with scanning rates between 60 and 150 mV/

s. Most systems exhibit one or two reversible and/or quasi-

reversible one-electron oxidation and reduction waves within

the range of the solvent and electrolyte window. Figures S10

and S11 show representative voltammograms. When the vol-

tammograms are (quasi)reversible, it is possible to extract formal

(50 nm) of DFH-5T (thick line) and DH-5T (fine line) on glass substrates,

and (D) films (50 nm) of isoDFH-5T (thick line) and isoDH-5T (fine line)

on glass substrates. The PL spectra were obtained by exciting the solution

at the absorption maximum. Intensities are in arbitrary units.

molecules, located in disordered domains and in grain bound-

aries, where molecular misalignment would lead to weak

intermolecular coupling and no splitting of the excited levels.

In fact, the position of this weak absorption rigorously matches

the trend observed in solution, with a decrease of the corre-

sponding energy as the oligothiophene core is elongated. The

second weak absorption at higher energies has been attributed

in the case of DH-6T and r6T to the corresponding CdC

4

6b

vibronic replica (0.19 eV) on the basis of 13 K spectra.

However, from the present room-temperature data, the second

peak lies at an interval of 0.21-0.25 eV, considerably greater

than typical energies of aromatic CdC bond stretches.

The shapes of the â,â′-disubstituted isoDFH-/isoDH-nT film

absorption spectra resemble those of the corresponding end-

capped systems. Particularly informative is the analysis of the

absorption maximum energies. As mentioned previously, the

position of isoDFH-/isoDH-nT λabs is red-shifted compared to

DFH-/DH-nT values, which cannot be explained assuming a

more planar core structure (see the crystal structure discussion

above). In addition, λabs values are blue-shifted compared to

the solution values. These two observations suggest significant

coupling between molecular transition dipoles in the â,â′-

substituted series, generating H-type aggregates. These inter-

actions are weaker than those in the R,ω- families as suggested

by smaller blue shifts of λabs compared to solution values, but

much greater than reported for isomeric mixtures of â,â′-

dihexyl- and â,â′-didecylsexithiophenes, where the film spectra

match those of the isolated molecule (recorded as dilute solutions

or in glass matrixes).⁴

1/2

potentials (E ) as the midpoints between peak potentials for

the forward and reverse scan. Data are summarized in Table 3.

In general, the chemical stability of oxidized and reduced 1-5

species increases with increasing core length and with substit-

uents in the R,ω-positions. This can be judged from the fact

that both E1 and E2 can be measured for almost all of the

longer 1-5 (n ) 5, 6) molecules, whereas among the 2T

derivatives (n ) 2), E1 could only be determined for DFH-

T and DH-2T (oxidation) and DH-2T, isoDH-2T, and r2T

reduction). Furthermore, most of the end-capped DFH-nT and

DH-nT derivatives exhibit two reversible oxidation and reduc-

tion potentials, even for the shorter 3T and 4T cores. In contrast,

the isoDFH-nT-, isoDH-nT-, and RnT-charged species of shorter

oligomers exhibit less stability upon reduction for the two former

systems and upon oxidation for the latter. Interestingly, for

isoDFH-nTs, the solutions around the working electrode show

1/2

2

(

6b

(

47) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.-C.; Van Patten, P. G.;

Film photoluminescence spectra were obtained by excitation

at λabs, and data are presented in Table S2. The spectral shapes

and maxima strongly depend on molecular structure and

substitution, with most plots exhibiting additional peaks and

Bao, Z. Chem. Mater. 2003, 15, 1778.

(48) (a) Domagala, W.; Laokowski, M.; Guillerez, S.; Bidan, G. Electrochim.

Acta 2003, 48, 2379. (b) Jagur-Grodzinski, J. Polym. AdV. Technol. 2002,

1

3, 615-625. (c) Meerholz, K.; Heinze, J. Electrochim. Acta 1996, 41,

1839.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13493

A R T I C L E S

Facchetti et al.

Table 3. Anodic (Ea), Cathodic (Ec), and Half-Wave (E¹^/²) Potentials (V) versus SCE of Compounds 1-5 in Dry THF under Nitrogen^a

oxidation

cathodic

reduction

anodic

half

cathodic

half

1/

2

1/

2

1

/

2

1/2

compound

E

a1

E

a2

E

c1

E

c2

E

1

E

2

E

c1

E

c2

E

a1

E

a2

E

1

E

2

b

DFH-2T

2.14

1.61

1.41

1.22

1.11

2.07

2.01

1.58

1.41

1.20

1.40

1.15

1.02

0.96

0.95

1.99

1.53

1.30

1.16

1.01

2.06

1.57

1.35

1.19

1.06

-1.22

-1.65

-1.58

-1.51

-1.45

-2.60

-2.09

-1.63

-1.56

-1.60

-2.62

-2.23

-2.01

-1.86

-1.83

-3.07

-2.34

-2.20

-1.93

-1.88

-2.60

b

DFH-3T

1.88

1.59

1.45

1.29

1.76

1.50

1.32

1.15

1.82

1.55

1.39

1.22

-1.84

-1.82

-1.72

-1.70

-2.89

-2.23

-1.54

-1.49

-1.37

-1.38

-1.70

-1.69

-1.63

-1.61

-1.59

-1.53

-1.44

-1.42

-1.77

-1.75

-1.67

-1.65

c

DFH-4T

DFH-5T

DFH-6T

b

c

isoDFH-2T

isoDFH-3T

isoDFH-4T

isoDFH-5T

isoDFH-6T

1.94

1.64

1.39

1.48

1.24

1.14

1.30

1.01

0.90

0.86

0.80

1.02

0.74

1.01

0.93

0.88

0.77

0.80

0.81

0.95

0.93

1.87

1.46

1.31

1.52

1.32

1.17

1.35

1.08

0.96

0.91

0.87

1.90

1.55

1.35

-1.54

-1.58

-1.71

-1.49

-2.42

-2.06

-1.89

-1.76

-1.74

-2.97

-2.14

-2.16

-1.79

-1.73

-2.24

-2.00

-1.86

-1.72

-1.66

-1.54

-2.52

-2.14

-1.95

-1.81

-1.78

-3.02

-2.24

-2.18

-1.86

-1.81

-2.42

-2.07

-1.94

-1.78

-1.71

a

DH-2T

a

DH-3T

1.57

1.40

1.18

1.15

1.46

1.27

1.08

1.03

1.51

1.32

1.13

1.09

c

DH-4T

-2.35

-2.15

-2.05

-2.24

-2.05

-1.97

-2.29

-2.10

-2.01

DH-5T

DH-6T

b

isoDH-2T

isoDH-3T

isoDH-4T

isoDH-5T

b

1.26

1.13

1.07

1.01

2.00

1.61

1.22

1.19

0.88

c

1.07

1.00

0.94

-2.45

-2.17

-2.02

-2.38

-2.03

-1.93

-2.41

-2.10

-1.98

1.11

1.06

1.12

1.01

1.00

1.16

1.08

1.12

isoDH-6T

c

2T

3T

4T

5T

6T

1.27

1.40

1.06

1.02

-2.14

-2.02

-1.83

-1.76

-2.54

-2.33

-2.03

-1.91

-2.40

-2.20

-1.93

-1.79

-2.47

-2.07

-1.98

-1.86

1.27

1.17

1.01

0.98

1.22

1.13

a

Referred to Fc /Fc couple in THF (0.50 vs Ag/AgCl; 0.54 vs SCE). Measurements were performed at -10 °C. At -25 °C. ^cAt -20 °C.

+

b

Scheme 4. Proposed Electrochemical Decomposition Pathways for Radical Anions and Radical Cations of isoDFH-4T and isoDH-4T

no apparent change after multiple reductive cycles, despite the

irreversibility of the process, and repetitive CV plots are

essentially superimposable. On the other hand, oxidative cycling

of the isoDH-nT and nT solutions produces a green/blue product

and film around and/or on the working electrode, and in most

cases consecutive forward and reverse scans exhibit increased

current during recording and/or growth of additional waves.

These results suggest that the origin of the chemical instability

upon charging is different for fluoroalkyl and alkyl-unsubstituted

oligothiophenes. Reduction of the core (e.g., isoDFH-4T) results

in preferential negative charge localization over the external

thiophene rings, which may promote fluoride elimination from

the R-chain position (Scheme 4). The resulting soluble structure

will eventually decompose, but not evolve into higher molecular

(e.g., isoDH-4T), the oxidation process generates a radical

cation, which can undergo coupling with neutral molecules to

produce dimers and, subsequently, larger oligomers and poly-

mers via well-known electropolymerization pathways (Scheme

4). Such a process is prevented in DH-nTs by the alkyl

substituents at the R,ω-positions.

5

0

Analysis of the half-wave potentials reveals interesting trends

within and between these systems, in particular:

(

i) Versus unsubstituted oligothiophenes nTs (5) with the

1/2

same core length, the E1 and E2 oxidation and reduction

potentials of the corresponding DFH-/isoDFH-nTs (1/2) are

shifted toward larger values [∆E1 (5 f 1, 2) ) +(0.08-

.52) V, ∆E2 (5 f 1, 2) ) +(0.09-0.70) V], whereas those

of DH-/isoDH-nTs (3/4) are shifted to smaller values [∆E1

5 f 3, 4) ) -(0.01-0.44) V, ∆E2 (5 f 3, 4) ) +(0.12-

.29) V]. In addition, the mean magnitude of these shifts is

greater for the former than for the latter families. These data

1/2

0

1/2

49

weight species. Indeed, base-promoted dehydrofluorination of

,2-dihydroperfluoroalkanoic acids, esters, and amides (which

1/2

(

0

2

can stabilize a negative charge proximate to a -CF2R group)

is known and supports this mechanism. For electron-rich systems

(

50) Can, M.; Sevin, F.; Yildiz, A. Appl. Surf. Sci. 2003, 210, 338. (b) Cha,

S.-K. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 165. (c) Tourillon,

F.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173.

(

49) (a) Liu, J.-T.; Lu, H.-J. J. Fluorine Chem. 2001, 111, 207. (b) Kurykin, M.

A.; Vol’pin, I. M.; German, L. S. J. Fluorine Chem. 1996, 80, 9.

13494 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

not only confirm and quantify the expected accepting and

donating properties of oligothiophene fluoroalkyl and alkyl

substituents but are in agreement with absolute substituent

thiophene core) and maximum lengths (the greatest extent of

the molecule, including the side chains), illustrating the excellent

agreement between computed geometries and experimental

F

H

53

capacities of R versus R chains (vide infra).

crystal structures, particularly for molecular lengths. The

1

/2

(

ii) Within each series, both oxidation and reduction E1

dihedral angles found for the computed gas-phase molecular

structures are generally greater than those found experimentally

in the solid state as might be expected based on solid-state

packing effects. Furthermore, as discussed above, with the

exception of isoDFH-2T and isoDFH-3T, all the compounds

have low rotational barriers, and several local minima are found

and E2¹parameters move to less positive and negative values

as the thiophene core size increases. This trend indicates a

progressive reduction of the (electrochemical) band gap, in

excellent agreement with the aforementioned optical data.

Furthermore, E1¹^/²displacement is much larger for DFH-/

/2

1/2

isoDFH-nTs in oxidation [∆E1 (2T f 6T) ) -(0.35-1.00)

V] than in reduction [∆E1¹(2T f 6T) ) -(0.14-0.17) V],

whereas for DH-/isoDH-nTs, the shift is larger in reduction

52b-d,j

in the gas-phase calculations.

/2

The lengths of the oligothiophene cores are very similar for

various ring substitutions (Figure S12), indicating the typical

rigidity of the conjugated cores parallel to the long molecular

axes, as well as the common all-anti heterocycle conformation

prevalent in all oligomers. Each additional thiophene unit is

computed to increase the core length by 3.91 Å on average (vs

1/2

[

∆E1 (2T f 6T) ) -(0.37-0.72) V] than in oxidation

∆EE1¹(2T f 6T) ) -(0.35-0.48) V]. These trends reveal

/2

[

a markedly different sensitivity of the energy level positions

between fluorocarbon- and alkyl-substituted oligothiophene

families with changes in substituent and conjugation length.

3

.87 Å from the experimental crystal structures),⁵³and the

perfluorohexyl and hexyl chain lengths are computed to be

9.09 and 8.51 Å, respectively, including end group van der

(

iii) The potential difference between subsequent redox events

1/2

[

0

∆E ) |E2 - E1 |] decreases as the core length increases

∆E ) 0.25-0.43 (3Ts), 0.20-0.36 (4Ts), 0.20-0.24 (5Ts),

.15-0.22 V (6Ts)], consistent with reduction of Coulombic

∼

1/2

Waals radii. Crystal structure data provide similar lengths for

the perfluorohexyl and hexyl chains of 9.08 (from DFH-4T)

repulsion between excess charges. A similar trend was observed

54

and 8.40 Å, respectively, probably due to slight conformational

differences. IsoDFH-nT 2 members (n ) 2-5) are calculated

to have slightly bent thiophene cores due to electron-electron

repulsion and steric interactions between the perfluoroalkyl

chains; however, longer isoDFH-6T has sufficient space

between the perfluoroalkyl chains to exhibit a less distorted

geometry, although still with average interplanar dihedral angles

of ∼30°, as shown by the slight deviation in core lengths

between the two series at intermediate oligomer lengths (n)

for unsubstituted oligothiophene RnTs and phenylene p-nPs

series with potentials obtained in CH2Cl2 (oxidation) and DMA

8c

(

reduction).⁴There are two exceptions: the isoDH-5T/isoDH-

6

T couple in oxidation and the entire DFH-nT series in

reduction, which may reflect the interplay of core extension

which should stabilize charge as n increases) and alkyl steric

(

hindrance or substituent effects (which are attenuated when n

increases and therefore less effective for injected electron

stabilization).

(Figure S12). Overall, the perfluoroalkyl-substituted series 1 and

DFT Computation. Theoretical modeling of the electronic

structure of conventional oligothiophenes has previously been

_p_e_r_f_o_r_m_e_d_i_n_p_a_r_a_l_l_e_l_w_i_t_h_e_x_p_e_r_i_m_e_n_t_a_l_w_o_r_k_.13 The parent

2

have larger gas-phase dihedral angles than their alkyl

counterparts (e.g., 12° and 27° vs 7.5° and 0.3° for DFH-6T,

isoDFH-6T, DH-6T, and isoDH-6T, respectively), perhaps due

to greater repulsion between perfluoroalkyl chain fluorine atoms

and the π-orbitals in the central oligothiophene.

oligothiophenes have been studied as polythiophene models and

44,51

as interesting conjugated organic materials in their own right.

The utility of computational modeling has advanced with the

increased accuracy of density functional methods, which can

Similar substituent effects are also observed in computed

dipole moment trends for series 1-5, (Table S3). In most cases,

these largely symmetric molecules have small dipole moments,

computed to be near zero for even thiophene ring numbers and

∼0.6 D for odd thiophene ring numbers, compared to the

experimental dipole moment of thiophene, 0.55 D.⁵⁵The

perfluoroalkyl chains in molecules 1 (n ) 2-6) are essentially

planar (and coplanar with the oligothiophene cores) but have

staggered spiral conformations due to the fluorine-fluorine

repulsions along the chains, and hence, the molecules have

dipole moments ranging from 0.54 to 1.88 D (for DFH-6T and

DFH-3T, respectively). Molecules 2 (n ) 2-6) and 4 (n )

52a

provide accurate estimates of molecular geometries, including

dihedral angles and rotational barriers,⁵

2a-d

dipole moments,⁵²^a^,^e^,^f

as well as electronic structure properties such as electron

affinity,⁵

2h,g

ionization potential, and band gaps.

52h

52i,j

The optimized geometries of compounds 1-5 provide mo-

lecular lengths for comparison with experimental lengths from

crystal structures. Table S3 summarizes molecular core lengths

(maximum ring carbon-ring carbon distance along the oligo-

(

51) (a) Ma, J.; Li, S.; Jiang, Y. Macromolecules 2002, 35, 1109. (b) Springborg,

M.; Schmidt, K.; Meider, H.; De Maria, L. Organic Electronic Materials:

Conjugated Polymers and Low Molecular Weight Organic Solids; Grosso,

G., Ed.; Springer: Berlin, 2001; Vol. 41, p 39. (c) Bredas, J.-L.; Cornil, J.;

Beljonne, D.; Dos Santos, D. A.; Shuai, Z. Acc. Chem. Res. 1999, 32, 267.

2-6) also have larger dipole moments since the â,â′ substituents

(

d) Stafstroem, S.; Bredas, J.-L. THEOCHEM 1989, 57, 393. (e) Ehren-

are predicted to twist the oligothiophene cores slightly. In some

dorfer, C.; Karpfen, A. J. Phys. Chem. 1994, 98, 7492.

(

52) (a) Hehre, W. J. A Guide to Molecular Mechanics and Quantum Chemical

Calculations; Wavefunction, Inc.: Irvine, CA, 2003. (b) Grein, F. J. Mol.

Struct.: Theochem 2003, 624, 23. (c) Mohamed, A. A. Int. J. Quantum

Chem. 2000, 79, 367. (d) Grein, F. J. Phys. Chem. A 2002, 106, 3823. (e)

Prezhdo, O. V. AdV. Mater. 2002, 14, 597. (f) Wang, J.; Johnson, B. G.;

Boyd, R. J.; Eriksson, L. A. J. Phys. Chem. 1996, 100, 6317. (g) Rienstra-

Kiracofe, J. C.; Tschumper, G. S.; Schaefer, H. F.; Nandi, S.; Ellison, G.

B. Chem. ReV. 2002, 102, 231. (h) Zhan, C. G.; Nichols, J. A.; Dixon, D.

A. J. Phys. Chem. A 2003, 107, 4184. (i) Muscat, J.; Wander, A.; Harrison,

N. M. Chem. Phys. Lett. 2001, 342, 397. (j) Hutchison, G. R.; Facchetti,

A.; Ratner, M. A.; Marks, T. J. Unpublished work.

(53) See refs 35a,b and r2T: (a) Visser, G. J.; Heeres; G. J.; Wolters, J.; Vos.

A. Acta Crystallogr., Sect. B 1968, 24, 467. (b) Pelletier, M.; Brisse, F.

Acta Crystallogr., Sect. C 1994, 50, 1942. r4T: (c) Antolini, L.; Horowitz,

G.; Kouki, F.; Garnier, F. AdV. Mater. 1998, 10, 382. (d) Siegrist, T.; Kloc,

C.; Laudise, R, A,; Katz, H. E.; Haddon, R. C. AdV. Mater. 1998, 10, 379.

r6T: (e) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.;

Fave, J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337.

(54) Destri, S.; Ferro, D. R.; Khotina, I. A.; Farina, A. Macromol. Chem. Phys.

1998, 199, 1973.

(55) Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1995.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13495

A R T I C L E S

Facchetti et al.

cases, such as isoDFH-3T, the perfluoroalkyl chains are on the

same side of the molecule, giving a large overall dipole moment

chains appears to influence molecular geometry most, in

particular the core planarity and conjugation, rather than the

overall supramolecular organization. To our knowledge, the only

crystal structure reported for an R,ω-dialkyl-substituted oligo-

thiophene is that of R,ω-dimethylquaterthiophene 31. Compari-

son of DFH-4T and quaterthiophene derivative 25 (and isoDFH-

5T) with 31 and â,â′-dialkylquaterthiophenes 32 clearly indi-

cates that the r4T core geometry is retained for end-capped

systems. Minimal differences in core metrical parameters are

observed. On the other hand, â,â-disubstituted fluorocarbon

derivatives exhibit much greater inter-ring torsional angles

compared to 32, where in the case of 32b and c, the heterocyclic

rings are essentially coplanar. These trends are also observed

in the aforementioned computed gas-phase geometries.

(3.855 D).

Quantitative analysis of the computed electronic energy levels

with respect to experimental data will be discussed in the next

section. However, we can anticipate that oligothiophene sub-

stitution with n-hexyl groups raises both HOMO and LUMO

energies vs the unsubstituted analogues (RnT), while n-

perfluorohexyl groups lower both groups of energy levels (∆E

+

0.11 eV for the DH-6T HOMO and LUMO vs r6T, and

-0.41 eV and -0.38 eV for the DFH-6T HOMO and LUMO,

respectively, vs r6T). The electron-withdrawing nature of the

perfluoroalkyl substitutents is clearly evident in a plot of the

electron density and electrostatic potential. Figure S13 shows

that the oligothiophene cores of DFH-6T and isoDFH-6T have

depleted electron density relative to the alkyl-substituted DH-

6

T and isoDH-6T molecules. However, the shapes of the

HOMOs and LUMOs as well as the topologies of the other

frontier MO vary little across all five series (Figure S14).

Discussion

Solid-State Structure. The combination of single-crystal

XRD data with thermal analysis and solid-state optical spec-

troscopies provides fundamental structural insights into the new

fluoroalkyl-containing DFH-nTs (1) and isoDFH-nTs (2) classes

and key comparisons with fluorine-free oligothiophenes 3-5.

The solid-state structures and packing characteristics of 1 (n

)

3, 4) and 2 (n ) 5) are principally a consequence of the

From the combined XRD and thermal and optical analyses,

the above observations can be extended to the other members

of both series. That the melting points of the longer unsubstituted

RnTs (n g 4) are almost identical to those of the corresponding

DFH-nTs and DH-nTs is clear evidence that intermolecular

forces are dominated by interactions between the essentially

flat cores. When the core length is reduced, the π-π stacking

forces decrease and interactions among the chains prevail, with

the difference between the melting points of DFH-2T (97 °C)

and DH-2T (34 °C) reflecting the characteristics of C12F26 (mp

geometries and assembling characteristics of the two molecular

components, the oligothiophene cores and the fluorocarbon

chains, modulated by their relative arrangements. As in other

polynuclear aromatics, the first component is an intrinsically

flat structure, rich in polarizable π-electron density, which

strongly interacts in the π-stacking direction (perpendicular to

the molecular long axes) to form 2-D crystals. This property is

fundamental to the excellent charge transport characteristics of

RnTs (5) and should facilitate, according to theoretical modeling,

bandlike transport behavior in high-quality single crystals. On

the other hand, fluorocarbon chains are insulating, poorly

polarizable stiff rods with low intrinsic crystallinity and having

the unique characteristic to exhibit, simultaneously, hydrophobic

and lipophobic character. Independent of the chain arrangement

regiochemistry, the first net result of combining these two

elements is formation of lamellar-like packing motifs (Figures

56

)

67 °C) and C12H26 (mp ) -9.5 °C), respectively. Note that

the extrapolated values for n f 0 (considering a linear regression

of melting point versus n) of -17 °C and -116 °C for 1 and 3,

respectively, are much lower than the experimental values

obtained for n ) 0. Such behavior has been also observed for

fluorocarbon-substituted benzenes [F(CF2)nC6H4(CF2)nF] where

melting points are lower than those for the corresponding

perfluoroalkanes F(CF2)2nF and explained by considering that

the small aromatic units act as defects in the fluorocarbon

2

and 3), with clear local phase segregation between oligo-

thiophene cores and fluorocarbon chains. This is true also for

the highly substituted mixed fluorocarbon-TMS structure 25,

where the TMS group “mixes” with the aromatic core, but not

5

9

lattice. Similar conclusions can be drawn here. For the â,â′-

disubstituted series 2 and 4, lower melting points compared to

1 and 3, respectively, indicate overall a smaller degree of

intermolecular interaction. However, the melting points of

fluorocarbon derivatives 2 significantly exceed those of alkyl-

substituted compounds 4 (Table 1). This result at first appears

with the perfluorohexyl chains (Figure 3B). Similar phase

_s_e_g_r_e_g_a_t_i_o_n_h_a_s_b_e_e_n_p_r_o_p_o_s_e_d_f_o_r_D_H_-_n_T_s₍_n₎₄_,5

7a 57b

6

) based

on film XRD analysis, but has never been confirmed with single-

crystal data. On the other hand, the crystal structures of several

â,â′-dialkyl-substituted oligothiophenes frequently exhibit mix-

ing of hydrocarbon and thiophene fragments, which is not

observed in the present fluoroalkyl systems.

5

8

(

58) (a) For 3,3′′′-didodecyl-R4T, 3,3′′′-dihexyl-R4T, and 3,3′′′-dipropyl-R4T,

see: Azumi, R.; Gotz, G.; Debaerdemaeker, T.; Bauerle, P. Chem.-Eur.

J. 2000, 6, 735. (b) For 3′,4′′-didecyl-R4T, see: Wang, S.; Brisse, F.;

Belanger-Gariepy, F.; Donat-Bouillud, A.; Leclerc M. Acta Crystallogr.,

Sect. C 1998, 54, 553. (c) For 3,3′′-dimethyl-R3T, see: Chaloner, P. A.;

Gunatunga, S. R.; Hitchcock, P. B. J. Chem. Soc., Perkin Trans. 2 1997,

1597. (d) For 5,5′′′′′-bis(trimethylsilyl)-4′,3′′′′-dioctyl-R6T and 4′,3′′′′-di-

n-butyl-R6T, see: Herrema, J. K.; Wildeman, J.; van Bolhuis, F.;

Hadziioannou G. Synth. Met. 1993, 60, 239.

When comparing fluoroalkyl series 1 and 2 with alkyl-

substituted series 3 and 4, the positioning of the fluorocarbon

(

56) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bredas, J.-L. AdV. Mater. 2001, 13,

053.

57) (a) Katz, H. E.; Laquindanum, J. G.; Lovinger, A. J. Chem. Mater. 1998,

0, 633. (b) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre,

F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716.

1

(

1

(59) Schulte, A.; Hallmark, V. M.; Twieg, R.; Song, K.; Rabolt, J. F.

Macromolecules 1991, 24, 3901.

13496 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

1

and 2 are blue-shifted vs 3 and 4, demonstrating that the

emitting excited state of the former systems lies 0.2-0.4 eV

-

1

(

1600-3200 cm ) aboVe that of the corresponding alkyl-

substituted systems with the same core length. Although the

extent of the Davydov splitting (WD) in unsubstituted dnT

6

0a

oligothiophenes is still under debate, recent theoretical and

6

0a,b

experimental investigations suggest a value of 0.8-1.2 eV,

although much smaller numbers have also been reported.⁶

Also, assuming that the WD’s of 1-4 are similar to those of

the corresponding RnTs, the energy separation between emitting

states of 1/2 and 3/4 is expected to be lower than the WD values

as the result of the mixing of excitonic transitions with different

bandwidths between upper and lower Davydov excitonic states.

However, the energetic similarities between these parameters

and the combined observations of high emission energies and

high fluorescence efficiencies for 1 and 2 suggest that emission

in the fluorocarbon-substituted nTs takes place from a high-

energy optically allowed excited state (Figure 7). A possible

explanation is that fluorocarbon chains prevent the fast decay

0a

Figure 7. Energy diagram for excitations in films of (A) alkyl- and (B)

fluorocarbon-substituted oligothiophenes. Allowed (solid lines) and forbid-

den (broken lines) optical transitions and nonradiative (wavy lines) decay

processes are shown.

to contradict the molecular structures of fluorocarbon systems

2

(and 25), where a lower degree of core coplanarity is found,

which should correspond to weaker core-core interactions and

therefore more facile melting. However, if molecular packing

alone is considered, the efficient phase segregation promoted

by the fluorocarbon substituents forces the aromatic regions to

interact more strongly than would be expected from molecular

structures alone. Indeed, the minimum core-core distance in

isoDFH-5T (3.52 Å) is similar to those in DFH-4T (3.52 Å)

1

of the Bu to the lower Au state. We are currently investigating

this aspect both theoretically and experimentally. However,

52j

the unprecedented possibility of achieving highly organized

oligothiophene supramolecular assemblies with excellent lumi-

nescence properties opens the possibility for new applications

and deVice fabrication strategies.

Solution Structures and Conformational Analysis. Mo-

lecular conformations in π-conjugated systems result from a

balance between two major opposing effects: steric forces

5

3c

53d

and r4T (3.53 -3.54 Å), but significantly less than that

found in planar isoDH-4T (36b) (3.62 Å).

Additional evidence for comparable intercore interactions is

provided by thin-film optical absorption and emission. The

similarities between the absorption spectra of 1 and 2 with those

of 3 and 4 suggest effective coupling between molecular

transition dipoles in the condensed state. The spectral features

can be interpreted within the framework of Frenkel molecular

excitation theory, which predicts formation of excitonic features

(favoring θ ) 90°, clinical conformations) and π-delocalization

forces (favoring θ ) 0°, syn conformations; or θ ) 180°, anti

conformations). If internal rotation is not greatly hindered,

equilibrium conformation(s) in solution may be significantly

different from those in the solid state. Many factors may

determine the conformational solution phase distribution for

oligothiophenes, including molecular structure (core substitution

and length), solution temperature and concentration, and solvent

polarity.

46e

associated with Davydov splitting. The consequences are that

1

the upper 1 Bu level is accessible by the optical selection rules,

1

whereas the lowest excited level (1 Au) becomes forbidden or

only weakly allowed. Thus, optical transitions between the

ground and the lowest excited state are expected to be very

weak both in absorption and emission. This behavior appears

to be general for conjugated systems forming H-type aggregates

and is observed here for both alkyl-substituted and unsubstituted

oligomers 3-5, in which intense absorption is followed by

ultrafast relaxation of the upper excited states to the lower

excited state, from which almost no fluorescence can be

observed (Figure 7). A well-studied example of this behavior

is r6T, where polycrystalline films deposited at high temper-

ature exhibit photoluminescence quantum yields ∼10 . On the

other hand, we find that all members of fluorocarbon series 1

and 2 exhibit very high solid-state fluorescence yields, which

appear at first to contradict the large intermolecular coupling

suggested by the λabs blue shifts and the shapes of the optical

absorption profiles. Since the film optical absorption spectra of

Useful insights into the “intrinsic” molecular structures of

oligothiophenes 1-5 can be obtained from solution UV-vis/

PL data. Since these measurements were obtained at very low

concentrations (<10 ) with a solvent of low to mid-dielectric

constant (THF, ꢀ ) 7.6), the influence of molecular aggregation

-5

(intermolecular interactions) and differences in molecular dipole

moments (highly polar solvents can preferentially stabilize

conformations with larger dipole moments) should be mini-

61

mized. Furthermore, with the exception of DFH-6T, the

spectra of all compounds were recorded at room temperature.

Therefore, differences in spectral features can be ascribed

principally to the effects of chemical and regiochemical

substitution.

The similarity between the solution optical absorption spectra

of 1-5 (with the exception described below) having the same

core length (Table 2) and the linearity of the plot of the optical

gap vs 1/n (Figure 5) are strong evidence that conjugation among

the core thiophene rings is very similar. Therefore, to a first

approximation, the average conformation in solution, which

results from free rotation about all inter-ring C-C bonds, is

-3

1

and 2 are almost superimposable upon the corresponding

spectra of 3 and 4, respectively, the nature of this electronic

transition and the electronic states involved must be similar.

Furthermore, in addition to different emissive efficiencies, the

emission spectra of 1 and 3 appear substantially different in

line shape compared to those of 2 and 4, respectively (Figure

S8), suggesting that emission takes place from different excitonic

levels. Moreover, (λem)max values of fluorocarbon-substituted nTs

(60) (a) Sun, H.; Zhao, Z.; Spano, F. C.; Beljonne, D.; Cornil, J.; Shuai, Z.;

Bredas, J.-L. AdV. Mater. 2003, 15, 818. (b) Kouki, F.; Spearman, P.; Valat,

P.; Horowitz, G.; Garnier, F. J. Chem. Phys. 2000, 113, 385.

(61) Hernandez, V.; Lopez Navarrete, T. J. J. Phys. Chem. 1994, 101, 1369.

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13497

A R T I C L E S

Facchetti et al.

essentially identical for all oligothiophenes, independent of

substituent effects (identity and position), and core conjugation

length. However, the shorter â,â′-disubstituted systems isoDH-

2

T and isoDFH-nTs (n ) 2, 3), which exhibit shorter λabs and

deviate considerably from the plot of Figure 5, clearly depart

from this picture. These data demonstrate a smaller degree of

overlap between thiophene π-orbitals, and therefore the averaged

core conformation(s) of the latter systems are less planar than

those of the former.

To shed more light on the conformational mobility of 1-5

and how it affects the corresponding substituted polythiophene

structures, optical data were combined with NMR spectroscopy

and DFT computation. With the exception of isoDFH-2T and

isoDFH-3T, the ¹H and F NMR spectra of all systems

including isoDH-2T exhibit a single set of resonances, consistent

with the presence of single species or mixtures of conformers

in rapid interconversion. On the basis of the DFT results for

19

1

-5 and previous experimental and theoretical work on (oligo,

3

0a-e,52a-d,62

poly)thiophenes,

the second scenario reflects the

situation in solution, where many of the envisioned conformers

have comparable energies, and the barriers to their intercon-

version are very small (<1-2 kcal/mol). In contrast, NMR

analysis of isoDFH-2T and isoDFH-3T indicates the presence

of rotamers that interconvert much more slowly. The barrier to

interconversion of isoDFH-2T rotamers is ∼12.4 kcal/mol. This

rotational barrier observed by VT NMR compares favorably

with the rotational energy profile calculated for isoDFH-2T

Figure 8. (A) Computed rotational barriers for isoDFH-2T (open symbols)

and isoDH-2T (filled symbols) and (B) computed energy profile for the

planar syn,syn (A), syn,anti (B), and anti,anti (C) conformations, and

syn,syn (D), syn,anti (E), and anti,anti (F) local minima of isoDFH-3T.

(Figure 8A), which exhibits a single significant energetic

minimum at an inter-ring dihedral angle of 117.2° and a barrier

of 25.1 kcal/mol to reach the 180° planar anti conformation.

Since the computational method does not account for geometric

relaxation during rotation, this value can be taken as an upper

limit, especially since repulsion between the sulfur and per-

fluoroalkyl chains makes the greatest contribution to the

rotational barrier. Furthermore, the modeling clearly shows that

rotations that eclipse the two perfluoroalkyls are unfavorable,

with an estimated barrier of 1592 kcal/mol for the 0° planar

syn conformation. Therefore, these data indicate that, contrary

interconversion barrier is computed to be ∼0.6 kcal/mol. These

modest barriers are in agreement with the averaged conforma-

1

tional structures observed by H NMR (vide supra). Finally, as

far as isoDFH-3T is concerned, we have not computed the entire

2-D torsional potential energy surface, since adequate treatment

of the rotation around both inter-ring bonds would require a

very large computational effort. Instead, Figure 8B shows three

computed syn-syn (D), syn-anti (E), and anti-anti (F) local

minima obtained from relaxation of the three planar (A-C)

diastereomeric rotamers (for the computed structures, see Figure

S15). The latter conformations can be assumed to be upper limits

to energy barriers for interconversions between the various

minima (Figure 8B). These computations provide an estimate

for A h B h C interconversion of 10-20 kcal/mol, although

alternative low-energy pathways cannot be ruled out completely.

Despite the approximations involved, these data suggest two

important conclusions, both supported by the NMR data. First,

many different nonplanar rotamers of isoDFH-3T are populated

in solution. Although the energy differences between these

minima are not large, from composition-equilibrium free-energy

relationships, the corresponding relative populations are sig-

5

1e

to r2T and isoDH-2T (vide infra), syn conformations are

not populated for isoDFH-2T in solution, and the R/S inter-

conversion follows a well-defined pathway.

6

3

nificantly affected. Therefore, energy differences of a few

kilocalories per mole correspond to large differences in con-

former distributions. Second, the interconversion barriers for

isoDFH-3T are estimated to be lower than for isoDFH-2T, but

much larger than those of the other isoDH/isoDFH-nTs.

From these results, important insights into the design and

structure of fluorocarbon-substituted polythiophenes (3-perfluo-

The interconversion barrier (Figure 8A) for the corresponding

R and S rotamers of isoDH-2T is calculated to be only 0.7 kcal/

mol (at 165°, anti conformation), and the barrier to alkyl group

crossing in the 0° planar syn conformation, 42.0 kcal/mol. The

computation indicates that both syn and anti conformers have

similar energies and are therefore present in solution. Their

(

62) In particular, the density functional methods appear to overestimate the

stability of planar π-systems, resulting in an underestimation of dihedral

angles and an overestimate of the rotational barrier. See refs 51e and 52.

(63) Eliel, E. L. Stereochemistry of Carbon Compounds; McGraw-Hill: New

York, 1962.

13498 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Table 4. Electrochemical, Optical, and Computed HOMO-LUMO

Energy Gap (Eg) and Absolute HOMO and LUMO Energies for

Compounds 1-5

energy (eV)

experimental^a

energy gap (eV)

theoretical

HOMO LUMO

CV

b

op

c

th d

compound

HOMO

LUMO

E

g

E

g

E

g

DFH-2T

DFH-3T

DFH-4T

DFH-5T

DFH-6T

isoDFH-2T

isoDFH-3T

-6.90 (-3.27) -6.42 -2.26

3.63 4.16

-6.41

-6.19

-6.03

-5.90

-3.25 -5.88 -2.39 3.16 3.12 3.48

-3.31 -5.58 -2.39 2.88 2.82 3.18

-3.40 -5.36 -2.50 2.63 2.64 2.86

-3.42 -5.20 -2.56 2.48 2.51 2.64

-6.58 -1.80

-5.88 -2.15

4.33 4.79

3.32 3.73

isoDFH-4T -6.36

isoDFH-5T -6.16 (-3.43) -5.33 -2.29

-3.26 -5.58 -2.15 3.10 2.88 3.43

2.73 3.05

isoDFH-6T -6.01

DH-2T

DH-3T

DH-4T

DH-5T

DH-6T

isoDH-2T

isoDH-3T

isoDH-4T

isoDH-5T

isoDH-6T

-3.30 -5.17 -2.29 2.71 2.60 2.88

-2.32 -5.14 -1.01 3.87 3.52 4.14

-2.70 -4.87 -1.52 3.22 3.04 3.35

-2.89 -4.76 -1.80 2.91 2.80 2.97

-3.03 -4.71 -1.96 2.72 2.62 2.75

-3.06 -4.68 -2.07 2.65 2.54 2.61

-6.19

-5.92

-5.80

-5.75

-5.71

Figure 9. Plots of the (A) experimental and (B) computed molecular orbital

energetics of oligothiophene families 1-5 studied vs the number of

heterocycle rings in the oligomer. DFH-nTs and isoDFH-nTs (b), DH-nTs

and isoDH-nTs (2), RnT (×).

(-6.00) -1.82 -5.17 -0.95

(-5.74) -2.60 -4.90 -1.47

4.29 4.22

3.14 3.43

_e_l_e_c_t_r_o_n_a_f_f_i_n_i_t_i_e_s_,65a,b previous work has shown that B3LYP-

derived eigenvalues compare favorably with experimental

-5.91

-5.84

-5.78

-2.66 -4.90 -1.69 3.25 2.88 3.21

-2.98 -4.82 -1.88 2.86 2.70 2.94

-3.03 -4.65 -2.07 2.75 2.58 2.58

5

2g,h

ionization potentials and electron affinities.

The present

2

3

4

5

6

T

(-6.22) -2.42 -5.47 -1.25

(-5.91) -2.77 -5.14 -1.69

(-5.79) -2.90 -4.95 -1.93

3.75 4.22

3.14 3.46

2.89 3.02

systematic agreement between computed HOMO and LUMO

energies and experimental values is shown in Figure S16. The

principal disparity between computed and electrochemical

HOMO and LUMO values derive from the difference in

environments, and previous work has shown that solvation

effects such as polarizability and cavity radius can be used to

-5.85

-5.82

-3.06 -4.87 -2.10 2.79 2.72 2.78

-3.13 -4.79 -2.18 2.69 2.61 2.61

a

The values reported in parentheses are estimated using the optical gap.

b

From electrochemical data: Eg ) E1 (Ox) - E1 ₍_R_e_d₎_.c From

CV

1/2

d

65d

optical data. From DFT computations.

linearly adjust (via a Kamlet-Taft relationship) computed

ionization potentials and electron affinities to compare to

electrochemical data. In particular, the slopes of the linear

roalkyl-pTs) can be drawn. In particular, note that 3-perfluo-

roalkyl-pTs will have significantly greater steric constraints than

the corresponding alkyl-substituted polymers (3-alkyl-pTs). It

is known that the regioregular head-to-tail (H-T) 3-alkyl-pTs

are a class of highly conjugated (semi)conducting polymers.

The corresponding regiorandom or regioregular head-to-head

65c

relations shown in Figure S16 match the behavior predicted from

65c

polarizability effects. However, for the correlations between

electrochemical and computed HOMO values in Figure S16B,

there is somewhat greater scatter, perhaps because of the greater

number of molecules where electrochemical HOMO energies

were estimated from the optical gap. Moreover, the magnitudes

of the solvation effects appear to be larger for molecules with

smaller ionization potentials, although the absolute differences

between computed and experimental HOMO energies across

the molecular range studied here are small (0.38 eV on average,

vs 0.55 eV for the LUMOs). Finally, the energy gaps also exhibit

excellent overall agreement between electrochemical and optical

data and the computed DFT orbital energies, although the

vertical transition energies predicted from theoretical gaps are

(

H-H) systems are less planar and ordered but still exhibit

4

substantial π-delocalization. Our results demonstrate that both

regiorandom and H-H 3-perfluoroalkyl-pTs should not form a

class of highly conjugated polymers. To achieve some degree

of core conjugation in the former systems requires the fluori-

nated substituents be separated by at least 2-3 thiophene rings.

Energy Level Correlations. Important information on the

molecular HOMO and LUMO energy levels can be drawn from

an accurate analysis of electrochemical and optical data

supplemented by theoretical calculations. Table 4 collects

electrochemical, optical, and computed energy gaps for classes

CV

op

44

greater than the Eg or Eg data in Table 4, as expected.

Particularly instructive here are the linear energetic trends

revealed by plotting the experimental and computed orbital

energies versus 1/n (Figure 9). The HOMO and LUMO energy

plots of fluorocarbon-substituted 1/2 and alkyl-substituted 3/4

1

-5. The first important observation is the excellent agreement

CV

op

between solution Eg and Eg data, with differences of ∼0.1-

0.2 eV for almost all compounds. Since these are two indepen-

dently derived measurements, the good agreement supports the

accuracy of the results. Table 4 also summarizes the experi-

mental and computed HOMO and LUMO energies, calculated

(64) The energy level of the normal hydrogen electrode (NHE) is -4.6 eV below

the vacuum level (see Bard, A. J.; Faulkner, L. R. Electrochemical

Methods: Fundamentals and Applications; Wiley: New York, 1984). The

oxidation potential of ferrocene is 0.67 V versus the NHE (see Wang, J.-

F.; Kawabe, Y.; Shaheen, S. E.; Morrell, M. M.; Jabbour, G. E.; Lee, P.

A.; Anderson, J.; Armstrong, N. R.; Kippelen, B.; Mash, E. A.; Peygham-

barian, N. AdV. Mater. 1998, 10, 230). The HOMO/LUMO energies were

determined using the Fc calibration against the vacuum energy level. The

terms HOMO and LUMO are a convenient short hand since the actual

energies correspond to states, not levels.

65) (a) Godby, R. W.; Schluter, M.; Sham, L. J. Phys. ReV. B 1988, 37, 10159.

(b) Stowasser, R.; Hoffmann, R. J. Am. Chem. Soc. 1999, 121, 3414. (c)

Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org.

Chem. 1983, 48, 2877. (d) Jonsson, M.; Houmam, A.; Jocys, G.; Wayner,

D. D. M. J. Chem. Soc., Perkin Trans. 2 1999, 425.

1/2

from the first oxidation and reduction potentials (E1 ), respec-

tively.⁶⁴Because of the good agreement between Eg and Eg ,

HOMO and LUMO energies associated with irreversible oxida-

tive and reductive events are estimated from the corresponding

electrochemically derived LUMO and HOMO energies by

addition and subtraction of the corresponding optical gaps.

While HOMO and LUMO eigenvalues from DFT methods

cannot be formally taken as either ionization potentials or

CV

op

(

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13499

A R T I C L E S

Facchetti et al.

are located below and above those of the unsubstituted oligo-

thiophenes RnTs (5), respectively. This result is in agreement

with established donating and accepting properties of hexyl and

perfluorohexyl substitutents. Furthermore, the slopes (s) of these

plots (Figure S17) are of particular importance, since they

provide quantitative information on substituent effect sensitivity.

The Hammet σ-constants for fluorocarbon chains (e.g., CF3-

CF2 σm ) 0.47, σp ) 0.52) are greater than those of alkyl

substituents (e.g., CH3CH2 σm ) -0.07, σp ) 0.15),²⁰meaning

not only that the EHOMO/LUMO(1/2) < EHOMO/LUMO(5) <

EHOMO/LUMO(3/4) but also that the progressive diminution in

EHOMO/LUMO(1/2) with respect to 5 should be 3-4 times larger

than the corresponding increase in EHOMO/LUMO(1/2). The slopes

for the HOMOs [s ) -2.74 (1/2), -1.22 (5), -0.96 (3/4)] and

LUMOs [s ) 0.40 (1/2), -2.10 (5), -2.93 (3/4)] indicate that

indicate that chemical substitution has, in most of the cases,

little effect on the optical absorption spectra and HOMO-

LUMO gap, which are instead strongly affected by the core

conjugation lengths. PL emission spectroscopy demonstrates that

all oligothiophenes 1-5 are conformationally more rigid in their

excited states, as indicated by coupling of the electronic emission

transitions to vibrational modes. In addition, the emission

quantum efficiencies of DFH/isoDFH-nTs are found to be

significantly greater than those of other oligothiophenes. Optical

data and VT NMR spectroscopies provide insights into the

solution molecular geometries and conformational dynamics.

For the first time, the interconversion energy barrier between

oligothiophene rotamers has been quantified by dynamic NMR

and found to be, in the case of isoDFH-2T, ∼10 kcal/mol, much

larger than that of the corresponding alkyl-substituted isoDH-

2T. This result indicates that fluorocarbon chains on an

oligothiophene core present greater stereoelectronic impediments

to planarization of the polyheterocyclic core.

F

on average, the relative reduction effect of R is ∼1.6, whereas

H

the increment effect of R is ∼0.5, in excellent agreement with

a straightforward substituent effect model.

Another interesting aspect of these energy diagrams concerns

the different sensitivities of the energy levels in 1-5 to the core

length n. For unsubstituted RnTs (5), the reduction of the

HOMO-LUMO gap on going from n ) 2 f 6 is accompanied

by simultaneous, monotonic reduction of ELUMO and increase

of EHOMO. The other oligothiophene families behave differently,

with ELUMO of fluoroalkyl-substituted systems 1 and 2 being

substantially stable across the series and EHOMO being strongly

affected. To a lesser degree, the converse is observed for alkyl

series 3 and 4. These trends, nicely reproduced by DFT

computation, result from the interplay between intrinsic sub-

stituent effects (vide supra), variation in conjugative extension,

and the number of substituents per thiophene unit. As a function

of core length, substituent effects and substituent density fall

with increasing oligomer chain length. Thus, compared to the

unsubstituted systems, greater reduction and increment of

fluoroalkyl- and alkyl-substituted oligothiophene MO energies

are expected for the smaller (2T) vs larger (6T) cores, with the

net result being invariant LUMO energies for 1/2 and invariant

HOMO energies for 3/4 across the spectrum of core lengths

The crystal structures of key fluorocarbon-substituted oligo-

mers are also analyzed, and evidence shows close π-π

intermolecular interactions between the aromatic cores, whereas

the fluorocarbon chains segregate into lamellar structures. From

the combined analysis of the thermal and film optical data, these

considerations can be extended to other members of the

fluoroalkyl families. Surprisingly, despite these strong inter-

molecular interactions, unprecedented high solid-state optical

emission efficiencies are observed in the fluorocarbon-

substituted oligothiophene films.

Finally, electrochemical measurements performed under

stringent, identical experimental conditions allow straightforward

comparison between the redox behaviors of unsubstituted and

fluorocarbon- and alkyl-substituted oligothiophenes. Combining

the electrochemical and optical data, the energy positions of

1

-5 HOMOs and LUMOs are estimated. The shift of DFH/

isoDFH-nT and DH/isoDH-nT HOMO and LUMO energies

with respect RnTs is in quantitative agreement with the expected

effects of fluorocarbon and hydrocarbon substituent-derived

Hammett σ-parameters. Overall, the 1-5 molecular orbital

energy trends within each series are determined by a balance

between substituent nature and density vs the extension of the

aromatic core. The net results from this interplay are frontier

MOs where LUMO and HOMO energies are practically

independent of conjugation length for fluorocarbon- and alkyl-

substituted nTs, respectively. These results are confirmed and

explained by density-functional computational modeling, the

results of which closely parallel experimental trends.

(

Figure 9). Consequently, on going from 6T f 2T derivatives,

the energies of the relevant orbitals (LUMOs and HOMOs for

/2 and 3/4, respectively) vary more than in the unsubstituted

1

oligothiophenes. These results suggest that from the molecular

viewpoint, the energetics involved in injecting electrons (holes)

into LUMO (HOMO) orbitals of 1 and 2 (3 and 4) should be

rather similar, independent of the core length. Therefore, all

device processes and performance metrics intimately related to

the absolute position of these orbitals should be, in principle,

equally manifested by all substituted oligothiophenes.

The above observations and results indicate that the new

oligothiophene series 1-5 are truly complementary in many

aspects of their optoelectronic properties. Indeed, important

consequences of these properties directly impact applications

as semiconducting materials for organic thin-film transistors,

Conclusions

In this contribution, we describe the synthesis and compara-

tive physicochemical properties of five new oligothiophene

classes. The syntheses of end-capped fluorocarbon-substituted

DFH-nTs and of novel, isomerically pure â,â′-disubstituted

isoDFH/isoDH-nT systems are presented in detail. All of the

new compounds have been characterized in depth. Thermal

analysis measurements indicate that all of fluorocarbon-

substituted systems have excellent thermal stability and volatil-

ity. Furthermore, some of these compounds exhibit multiple

thermal transitions before melting, with DFH-6T clearly

exhibiting a smectic LC phase. Optical spectroscopic data

2

as detailed in a closely related contribution.

Acknowledgment. We thank ONR (N00014-02-1-0909) and

the NSF-MRSEC program through the Northwestern Materials

Research Center (DMR-0076097) for support of this research.

This work was also supported by the NASA Institute for

Nanoelectronics and Computing under Award No. NCC 2-3163.

We thank Dr. C. Zuccaccia for helpful discussions of the VT

NMR experiments.

13500 J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004

Building Blocks for Molecular and Polymeric Electronics

A R T I C L E S

Supporting Information Available: Synthetic details and film

deposition methods. ¹⁹F NMR of isoDFH-3T. DSC thermograms

and TGA plots of 2, 4, and 5. FT-IR spectra of 2, 3, and 4.

UV-vis/PL spectra of 1-4 in THF and as thin films. Gaussian

deconvolution of DH-6T and DFH-6T spectra. CV plots of

selected 1-5. Computed HOMO and LUMO orbitals of 1-5

(n ) 6). Computed planar transition states of isoDFH-3T. CIF

and plain text files of DFH-3T, DFH-4T, and isoDFH-5T, 26.

This material is available free of charge via the Internet at

http://pubs.acs.org.

JA048988A

J. AM. CHEM. SOC.

9

VOL. 126, NO. 41, 2004 13501

Article Doi

Building blocks for N-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nTs; n = 2-6). Systematic synthesis, spectroscopy, electrochemistry, and solid-state organization

DOI: 10.1021/ja048988a

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Article abstract of DOI:10.1021/ja048988a

Full text of DOI:10.1021/ja048988a

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