Phenacyl-thiophene and quinone semiconductors designed for solution processability and air-Stability in high mobility n-channel field-effect transistors www.chemeurj.org

FULL PAPER

DOI: 10.1002/chem.200901513

Phenacyl–Thiophene and Quinone Semiconductors Designed for Solution

Processability and Air-Stability in High Mobility n-Channel Field-Effect

Transistors

Joseph A. Letizia, Scott Cronin, Rocio Ponce Ortiz, Antonio Facchetti,*

Mark A. Ratner,* and Tobin J. Marks*^[a]

Abstract: Electron-transporting organic

semiconductors (n-channel) for field-

effect transistors (FETs) that are pro-

quaterthiophene (4), 2,7-bis((5-per-

fluorophenacyl)thiophen-2-yl)-9,10-

values reported to date for a solution-

cast organic semiconductor; one of the

first n-channel polymers, 3, exhibits m_e

ꢀ10^À6cm²V^À1s^À1in spin-cast films

(m_e=0.02 cm²V^À1s^À1for drop-cast 1:3

blend films); and rare air-stable n-

channel material 5 exhibits n-channel

phenanthrenequinone (5), 2,7-bis[(5-

phenacyl)thiophen-2-yl]-9,10-phenan-

threnequinone (6), and 2,7-bis(thio-

phen-2-yl)-9,10-phenanthrenequinone,

(7). Optical and electrochemical data

reveal that phenacyl functionalization

significantly depresses the LUMO en-

ergies, and introduction of the quinone

fragment results in even greater

LUMO stabilization. FET measure-

ments reveal that the films of materials

1, 3, 5, and 6 exhibit n-channel activity.

Notably, oligomer 1 exhibits one of the

highest m_e(up to ꢀ0.3 cm²V^À1s^À1)

ACHTUNGTRENNUNGcessable in common organic solvents or

exhibit air-stable operation are rare.

This investigation addresses both these

challenges through rational molecular

design and computational predictions

of n-channel FET air-stability. A series

of seven phenacyl–thiophene-based

materials are reported incorporating

systematic variations in molecular

structure and reduction potential.

These compounds are as follows: 5,5’’’-

bis(perfluorophenylcarbonyl)-2,2’:5’,-

2’’:5’’,2’’’-quaterthiophene (1), 5,5’’’-bis-

FET

operation

with

m_e=

0.015 cm²V^À1s^À1, while maintaining a

large I_on:off=10⁶for a period greater

than one year in air. The crystal struc-

tures of 1 and 2 reveal close herring-

bone interplanar p-stacking distances

(3.50 and 3.43 ꢀ, respectively), whereas

the structure of the model quinone

compound, 7, exhibits 3.48 ꢀ cofacial

p-stacking in a slipped, donor-acceptor

motif.

ACHTUNGTRENNUNG

Keywords: phenacyl–thiophene

·

2-yl)-4’,4’’-dioctyl-2,2’:5’,2’’:5’’,2’’’-qua-

terthiophene) (3), 5,5’’’-bis(perfluoro-

phenacyl)-4,4’’’-dioctyl-2,2’:5’,2’’:5’’,2’’’-

polymers · quinones · semiconduc-

tors

Introduction

The promise of lower-cost, flexible, large-area electronics

and unique applications has motivated research in the area

of organic electronic materials. Organics offer several key

attractions, including low-temperature solution processing,

readily tunable molecular/polymeric structures, compatibili-

ty with organic substrates, roll-to-roll manufacture and

unique mechanical/device properties. These characteristics

would be invaluable in applications for which low-cost fabri-

cation, mechanical flexibility, large area coverage, and low-

temperature processing are desired, such as, in organic light-

emitting diodes,^[1]sensors,^[2]organic photovoltaics,^[3]and or-

ganic field-effect transistors (OFETs).^[4,5]Organic semicon-

ductors (OSCs) are an essential component of each of these

[a] Dr. J. A. Letizia, S. Cronin, Dr. R. P. Ortiz, Dr. A. Facchetti,

Prof. M. A. Ratner, Prof. T. J. Marks

Department of Chemistry and the Materials Research Center

Northwestern University, 2145 Sheridan Road

Evanston, Illinois, 60208 (USA)

Fax : (+1)847-491-2990

E-mail: a-facchetti@northwestern.edu

ratner@northwestern.edu

t-marks@northwestern.edu

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.200901513 and contains the full

surface microstructure characterization (SEM, AFM, XRD) data.

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

1911

applications, yet fundamental scientific questions and mate-

rials challenges remain concerning long-range charge trans-

port, processability, and ambient stability, especially for elec-

tron transporting (n-channel) OFET materials.^[5]

Enabling semiconductor film solution processability, while

preserving high carrier mobilities, and n-channel FET ambi-

ent stability remain some of the most significant challenges

for organic electronics.^[5]The solubility issue was initially ad-

dressed by simply appending bulky solubilizing substituents

to disrupt tight solid-state packing; however, this frequently

diminishes p–p overlap, leading to lower carrier mobilities.^[6]

In response, several new strategies were developed based on

either geometrically-designed core substituents, which afford

both solubilization and close p–p stacking (e.g., TIPS-substi-

tuted-pentacenes, TIPS: 6,13-bis(triisopropylsilylethynyl))^[7]

or on soluble, thermally labile semiconductor precursors

(e.g., pentacene cycloadducts and ester-functionalized oligo-

thiophenes)^[8]which can yield excellent hole mobilities on

conversion to the semiconductor. Despite these advances,

organic film growth mechanisms on inorganic/organic surfa-

ces and why certain organic materials crystallize with the

proper film morphology/microstructure for FET transport

and others do not, remains mysterious. All that is clear is

that the addition of highly lipophilic substituents to conju-

gated cores promotes the formation of uniform, textured

films, which are essential for device performance.^[9]Several

studies have addressed the importance of gate dielectric sur-

face functionalization, attempting to rationalize the effects

of surface energy, roughness, chemistry,^[10–12]and more re-

cently, viscoelastic properties,^[13]on OSC film growth, mor-

phology, and ultimately carrier mobility patterns. Enhanced

OSC film morphology and microstructure can also be ach-

ieved using a variety of techniques, including substrate rub-

bing/alignment layers, magnetic fields, layer-by-layer film

deposition, vitrification agents, zone casting, zone refining,

and solvent vapor annealing.^[14]Promising results have also

been achieved using external mechanical forces (shear meth-

ods).^[15]Very recently, surface functionalization of the metal-

lic contacts^[16,17]with organic monolayers, combined with

small channel lengths have promoted single-crystal-like mor-

phology for TIPS-pentacene device films, yielding impres-

sive charge transport characteristics.^[17]However, it is still

very difficult to a priori predict from OSC molecular struc-

tures what film morphologies/FET performance parameters

will be achieved, and most of the aforementioned studies

have involved p-channel semiconductors.

Figure 1. Examples of air-stable organic n-type semiconductors.

electron mobilities.^[19]The properties of some of these poly-

mers^[19b]are unique and not fully understood at the moment,

including the fact that substantial carrier mobilities are ex-

hibited by these nearly amorphous solids. Consequently, a

balance between strong electron-withdrawing functionalities,

solubilizing groups, local versus molecular/monomeric

dipole moments, molecular symmetry, and stacking must be

taken into account. 2) Efficient electron injection into the

OSC LUMOs requires the use of low workfunction metal

contacts; however, the corresponding metal surfaces are

then far more reactive than those of high-workfuction noble

metals, resulting in the formation of insulating oxide coat-

ings on the former.^[20]Furthermore, combining a very-low-

workfunction metal with a high-electron-affinity core may

result in electrochemical reactions, leading to chemical reac-

tion/dipole formation at the metal-semiconductor inter-

face.^[21]Therefore, selection of the most effective contact

materials is not obvious for n-channel organic semiconduc-

tors. 3) Finally, mobile electron-carrier stabilization in ambi-

ent is challenging because of charge trapping by device-born

and ambient species.^[22–25]

Initial studies addressing the ambient sensitivity of n-

channel FETs by de Leeuw et al. argued that ambient at-

mosphere H₂O and O₂can trap electron charge carriers, and

it was postulated that OSCs with a reduction potential more

negative than ꢀ-0.66 V vs. SCE are susceptible to H₂O oxi-

dation.^[23]Recent work has revealed that, for several materi-

als, a reduction potential more positive than ꢀÀ0.2Æ0.1 V

vs. SCE is required to stabilize electron charge carriers in

the presence of O₂.^[25–26]Typically, electron trapping by these

gaseous species does not result in chemical degradation of

the semiconductor and is reversed when the devices are sub-

jected to vacuum. Air-stable n-channel FET operation has

been achieved by either inhibiting O₂diffusion to the semi-

conductor film charge-transporting region^[23,27]or by design-

ing a semiconductor architecture that stabilizes mobile elec-

trons sufficiently to render them resistant to O₂trapping

(Figure 1).^[23,25,26]A fluorocarbon substituent O₂barrier

model was proposed by Katz, et. al, for an air-stable N,N’-

fluorocarbon substituted naphthalene diimide (NDI-

Regarding the second of the aforementioned issues, there

are three primary reasons why achieving n-channel OFET

transport is far more challenging: 1) To enable efficient n-

channel transport, planar p-conjugated cores must be func-

tionalized with strong electron withdrawing groups, yielding

flat polar molecules with extremely-large crystal packing af-

finities and very low solubilities. In one study, the polymer

BBL^[18](Figure 1) was solubilized by coordination with

Lewis acids, followed by casting and leaching the acid from

the films. Very recently, we reported soluble and pro

ACHTUNGTRENNcUNG essable

arenedicarboximide-based polymers exhibiting very-high

1912

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

F),^[23,28,29]and has also been

shown to be applicable to per-

fluorinated copper phthalocya-

nine (CuF₁₆Pc)^[27,30–35]and to

fluoroacyl-substituted oligothio-

phenes (e.g., DFCO-4TCO).^[36]

Unfortunately, each of these

materials exhibits device perfor-

mance degradation over the

period of hours to days in air.

Other materials have achieved

n-channel operational stability

in air by having a reduction po-

tential near ꢀÀ0.2 V vs. SCE

or greater, although in some

cases I_on:offis diminished.^[26,37]

These extremely p-deficient air-

stable n-channel molecules are

core-cyanated naphthalene di-

Figure 2. Phenacyl-thiophene and quinone semiconductors and relevant model compounds.

A

none-based subunits, 2,2’-bithiophene-3,3’-oxalaldehyde

(19), 9,10-phenanthrenequinone, or pyrene-4,5,9,10-tetraone

(21), into the core of 1 should lower the reduction potentials

to ꢀÀ0.5 V, À0.4 V, and À0.2 V vs. SCE, respectively. These

reduction potentials span the ꢀÀ0.6 V to ꢀÀ0.2 V vs. SCE

window previously suggested for air-stable n-channel opera-

tion in fluorocarbon-substituted OSCs not subject to ambi-

ent carrier doping when reduction potentials are >ꢀ

À0.1 V. Synthesis and characterization of new compound 7

confirms a cofacial, antiparallel quinone p-stacking motif

with a short 3.48 ꢀ p–p interplanar distance. Synthesis and

characterization of new oligomers 5 and 6 reveals n-channel

FET activity, with 5 having m_e=0.015 cm²V^À1s^À1while main-

taining a large I_on:off=10⁶after greater than one year in air.

This family of materials demonstrates that a balance of

high-crystallinity and processability can be achieved, and

lays the groundwork for incorporating novel, extremely p-

electron deficient monomers into polymeric semiconductors.

dicyanomethylene-substituted

terthienoquinoids

(DCMTs).^[26]Recent work on a solution processed DCMT

derivative has yielded the highest m_eto date for an n-type

air-stable material, 0.16 cm²V^À1s^À1; however, I_on:off=10³is

diminished by substantial doping at V_G=0 V.^[26]

This contribution describes a versatile new class of organ-

ic semiconductors designed to enhance crystallinity, solution

processability, and achieve air-stable n-channel FET opera-

tion. Quaterthiophene is used as the p-core for materials 1–

4 because of its tendency for favorable crystal packing, gen-

erally large carrier mobilities, and amenability to substantial

LUMO energy modulation through substituent effects. Pre-

vious work on fluorinated and non-fluorinated alkyl,^[38–43]

aryl,^[44]and alkyl-carbonyl^[36]substituted thiophenes revealed

that high FET carrier mobility can be achieved while stabi-

lizing LUMO energies to enable n-channel FET operation.

In the present study, phenacyl substituents are appended to

this quaterthiophene p-core to introduce four important

characteristics: 1) Enhanced solubility through symmetry-

breaking induced by the large phenyl-planar p-core dihedral

angle, 2) enhanced crystallinity derived from (perfluoro)

phenyl-(perfluoro) phenyl p-stacking, 3) more positive re-

duction potential resulting from LUMO stabilization by

phenacyl and optional perfluorophenacyl substitution, 4) en-

hanced solution phase rheology and processability by exten-

sion of the phenacyl oligomer to polymeric structures by

means of para substitution of the phenacyl ring.

Materials 1–3 were previously communicated as having

record-setting FET figures of merit for solution-cast oligo-

meric and polymeric semiconductors (Figure 2).^[45]To under-

stand and enhance FET performance further, the effects of

alkyl solubilizing group position on solid-state packing in

polymer 3 is investigated here by relocating the substituents

from the inner to the outer thiophenes in model compound

4. Additional LUMO stabilization is first investigated using

DFT-level computation, revealing that introduction of qui-

Experimental Section

Materials: All reagents were purchased from commercial sources and

used without further purification unless otherwise noted. Anhydrous di-

ethyl ether and THF were distilled from Na/benzophenone, and toluene

was distilled from Na. The Stille reagent 5,5’-bis(tributylstannyl)-2,2’-bi-

thiophene was synthesized according to a published procedure.^[46]Materi-

als

(1), 5,5’’’-bis(phenylcarbonyl)-2,2’:5’,2’’:5’’,2’’’-quaterthiophene (2), poly-

[5,5’’’-(perfluorophenac-2-yl)-4’,4’’-dioctyl-2,2’:5’,2’ꢂ :5’’,2’’’-quaterthio-

phene) (3), (5-bromothien-2-yl)(perfluorophenyl)methanone (13), and

(5-bromothien-2-yl)(phenyl)methanone (14) were synthesized according

5,5’’’-bis(perfluorophenylcarbonyl)-2,2’:5’,2’’:5’’,2’’’-quaterthiophene

AHCTUNGTRENNUNG

to literature procedures.^[45]Conventional Schlenk techniques were used

and reactions were carried out under N₂unless otherwise noted. Micro-

wave-assisted reactions were run in sealed vessels using a CEM Discover

microwave reactor in the temperature-controlled mode. Optical spectra

were recorded by using a Cary Model 1 UV/Visible spectrophotometer.

NMR spectra were recorded by using a Varian Unity Plus 500 spectrome-

ter (¹H, 500 MHz). Electrochemistry was performed by using a C3 Cell

Stand electrochemical station equipped with BAS Epsilon software (Bio-

analytical Systems, Inc., Lafayette, IN).

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1913

T. J. Marks, M. A. Ratner, A. Facchetti et al.

Synthesis of 2-bromo-3-octylthiophene (8): NBS (9.00 g, 50.6 mmol) was

added to a solution of 3-octylthiophene (9.93 g, 50.6 mmol) in 22 mL gla-

cial acetic acid maintained at 158C. After allowing the reaction mixture

to stir for 2 h, it was poured into 200 mL hexane and extracted four times

with 50 mL of water, once with 50 mL NaHSO₄(aq, 5% w/v), once with

50 mL brine, and dried over MgSO₄. Upon filtration and concentration,

13.7 g of colorless oil 8 was obtained (98% yield) and was used without

further purification. Elemental analysis (%) calcd C₁₂H₁₉BrS: C, 52.36;

H, 6.96; found: C, 52.28; H, 7.05; ¹H NMR (CDCl₃): d=7.19 (d, J=

4.0 Hz, 1H), 6.87 (d, J=4.5 Hz, 1H), 2.63 (t, J=5.0 Hz, 2H), 1.60 (t, J=

5.0 Hz, 2H), 1.2–1.3 (b, 10H), 0.89 ppm (t, J=7.5 Hz, 3H); MS (EI): m/z

(%): 274.0 (100) [M⁺].

gen/vacuum three times before 8 mL of anhydrous DMF was added. The

reaction mixture was then heated to 808C for 10 h with stirring. Upon

cooling, the reaction mixture was poured into 100 mL ether and extract-

ed three times with 50 mL of water, once with 50 mL brine, and dried

over MgSO₄. The red solid obtained upon filtration and concentration

was purified by column chromatography on silica gel to give 0.185 g of 4

as a red solid (36% yield). Elemental analysis (%) calcd C₃₀H₆F₁₀O₂S₄:

1

C, 50.28; H, 0.84; found: C, 50.14; H, 0.87; H NMR (CDCl₃): d=7.20 (d,

J=4.5 Hz, 2H), 7.18 (s, 2H), 7.15 (d, J=5.0 Hz, 2H), 2.96 (t, J=8.5 Hz,

4H), 1.64 (m, 4H), 1.2–1.4 (b, 10H), 0.85 ppm (t, J=8.5 Hz, 6H); m.p.

52–558C; MS (EI): m/z (%): 715.8 (100) [M⁺].

Synthesis of 5-(5-(pinacolatoboryl)thien-2-yl)(perfluorophenyl)meth-

Synthesis of 3-octylthiophene-2-carboxaldehyde (9): Reagent 8 (30 g,

0.109 mol) was slowly added to Mg turnings (5.3 g, 0.22 mol) while vigo-

rously stirring in 120 mL dry THF in a round bottom flask with attached

reflux condenser. The reaction mixture was next heated at reflux for 4 h

before it was allowed to cool, filtered through a 0.45 mm syringe filter

into a second dry flask, where DMF (0.28 mol, 20 g) was added, and the

reaction mixture then refluxed overnight. Once cooled, the reaction mix-

ture was poured into 600 mL hexane, extracted six times with 150 mL of

water, and dried over MgSO₄. The oil obtained after filtration and con-

centration was purified by column chromatography on silica gel to give

11.7 g colorless oil (48% yield). Elemental analysis (%) calcd C₁₃H₂₀OS:

C, 69.59; H, 8.98; found: C, 69.52; H, 8.79; ¹H NMR (CDCl₃): d=10.04

(s, 1H), 7.65 (d, J=4.0 Hz, 1H), 7.02 (d, J=4.5 Hz, 1H), 2.97 (t, J=

5.0 Hz, 2H), 1.67 (t, J=5.0 Hz, 2H), 1.2–1.4 (b, 10H), 0.90 ppm (t, J=

7.5 Hz, 3H); MS (EI): m/z (%): 224.2 (100) [M⁺].

AHCTUNGTRENNUNG

A

ACHTUNGTRENNUNG

0.045 mmol), and powdered dry potassium acetate (0.442 g, 4.5 mmol)

before it was evacuated and refilled with nitrogen four times, and 5 mL

toluene added. The reaction mixture was then irradiated with microwaves

for 30 min at a temperature of 1508C. This procedure was repeated five

more times and the combined reaction mixtures poured into 30 mL of

ether, passed through a 3 cm plug of Celite, concentrated in vacuo, and

the resulting oil purified by Kugelrohr distillation (50 mT, 1708C) to give

1.20 g of boronic ester 15 as a colorless solid (33% yield). Elemental

analysis (%) calcd C₁₇H₁₄BF₅O₃S: C, 50.52; H, 3.49; found: C, 50.47; H,

3.40; ¹H NMR (CDCl₃): d=7.98 (d, J=4.0 Hz, 1H), 7.72 (d, J=4.0 Hz,

1H), 1.38 ppm (s, 12H); m.p. 53–558C; MS (EI): m/z (%): 404.08 (100)

[M⁺].

Synthesis of (3-octylthien-2-yl)(perfluorophenyl)methanol (10): A solu-

tion of pentafluorobenzene (4.38 g, 26.0 mmol) in 250 mL dry THF was

cooled to À788C before nBuLi (1.6m in hexanes, 17.9 mL, 28.6 mmol)

was added over 15 min. The reaction mixture was maintained at À788C

for 2 h before 9 (5.83 g, 26.0 mmol) was added and the reaction mixture

allowed to stir for another 6 h at À788C before being quenched with

20 mL 5% HCl (aq) (CAUTION: Allowing the lithium derivative of

pentafluorobenzene to warm to room temperature can result in forma-

tion of a potentially explosive benzyne). The reaction mixture was then

poured into 300 mL hexane and extracted three times with 50 mL of

water, once with 50 mL brine, and dried over MgSO₄. The oil obtained

after filtration and concentration was purified by column chromatogra-

phy on silica gel to give 7.91 g of a colorless oil (77% yield). Elemental

analysis (%) calcd C₁₉H₂₁F₅OS: C, 58.15; H, 5.39; found: C, 58.03; H,

5.36; ¹H NMR (CDCl₃): d=7.23 (d, J=4.5 Hz, 1H), 6.89 (d, J=4.5 Hz,

1H), 6.46 (s, 1H), 2.62 (t, J=5.5 Hz, 2H), 1.2–1.4 (b, 12H), 0.90 ppm (t,

J=7.5 Hz, 3H); MS (EI): m/z (%): 392.1 (100) [M⁺].

Synthesis of 5-(5-(pinacolatoboryl)thien-2-yl)

An 8 mL microwave reaction vessel was charged with (5-bromothien-2-

yl)(phenyl)methanone 14 (0.267 g, 1.00 mmol), bis(pinacolato)diboron

(0.279 g, 1.10 mmol), [Pd(dppf)Cl₂] (25 mg, 0.030 mmol), and powdered

ACHTUNGERTN(NUNG phenyl)methanone (16):

AHCTUNGTRENNUNG

dry potassium acetate (0.196 g, 2.00 mmol) before it was evacuated and

refilled with nitrogen four times and 6 mL toluene added. The reaction

mixture was next irradiated with microwaves for 20 min at a temperature

of 1608C. This procedure was repeated two more times and the combined

reaction mixtures poured into 30 mL of ether, passed through a 3 cm

plug of Celite, concentrated in vacuo, and the resulting dark oil purified

by Kugelrohr distillation (50 mT, 1908C) to give 0.074 g of boronic ester

16 as a light-yellow viscous oil (50% yield). Elemental analysis (%) calcd

C₁₇H₁₉BO₃S: C, 64.98; H, 6.09; found: C, 64.81; H, 5.98; ¹H NMR

(CDCl₃): d=7.89 (d, J=6.5 Hz, 2H), 7.73 (d, J=4.0 Hz, 1H), 7.64 (d, J=

4.0 Hz, 1H), 7.60 (t, J=7.0 Hz, 1H), 7.50 (t, J=8.0 Hz, 2H), 1.38 ppm (s,

12H); m.p. 78–818C; MS (EI): m/z (%): 314.10 (100) [M⁺].

Synthesis of 3,3’,5,5’-tetrabromo-2,2’-bithiophene (17): Bromine (288 g,

1.8 mol) was added slowly over a 1 h period to a stirred solution of 2,2’-

bithiophene (60.0 g, 0.361 mol) in 280 mL chloroform containing 120 mL

glacial acetic acid in a 1 L round bottom flask fitted with a reflux con-

denser (CAUTION: Addition of the first two equivalents of bromine

produces a strongly exothermic reaction). The mixture was then stirred

at reflux for 12 h. Upon cooling to room temperature, a colorless precipi-

tate was isolated by filtration and washed with methanol. The filtrate was

then concentrated and a second crop of tan precipitate was collected and

washed with methanol. The combined solids were next dissolved in meth-

ylene chloride (500 mL), washed four times with 200 mL water, once

with 100 mL brine, and dried over anhydrous MgSO₄. The organic solu-

tion was then filtered, and the solvent removed by evaporation to give

Synthesis of (5-bromo-3-octylthien-2-yl)(perfluorophenyl)methanol (11):

NBS (3.41 g, 19.1 mmol) was added in a single amount to a solution of al-

cohol 10 (7.51 g, 19.1 mmol) in 150 mL glacial acetic acid and allowed to

stir overnight. The reaction mixture was then poured into 300 mL hexane

and extracted four times with 50 mL of water, once with 50 mL NaHSO₄

(aq, 5% w/v), once with 50 mL brine, and dried over MgSO₄. Upon fil-

tration and concentration, 9.03 g of a colorless oil was obtained (98%

yield) and used without further purification. Elemental analysis (%)

1

calcd C₁₉H₁₉BrF₅OS: C, 48.42; H, 4.28; found: C, 48.35; H, 4.31; H NMR

(CDCl₃): d=6.81 (s, 1H), 6.38 (s, 1H), 2.62 (t, J=5.5 Hz, 2H), 1.2–1.4 (b,

12H), 0.90 ppm (t, J=7.5 Hz, 3H); MS (EI): m/z (%): 469.9 (100) [M⁺].

Synthesis of (5-bromo-3-octylthien-2-yl)(perfluorophenyl)methanone

(12): MnO₂(12 g) was suspended in a solution of alcohol 11 (8.99 g,

19.1 mmol) in 100 mL dichloromethane with stirring for 3 days. After re-

moval of solids by filtration and concentration, the oil was purified by

column chromatography on silica gel to give 8.52 g of a colorless oil

(94% yield). Elemental analysis (%) calcd C₁₉H₁₈BrF₅OS: C, 48.42; H,

1

157 g (90% yield) of a colorless powder. H NMR (CDCl₃): d=7.06 ppm

(s, 2H).

Synthesis of 3,3’-dibromo-2,2’-bithiophene (18): Zn powder (31.7 g,

0.485 mol) was added in portions to a vigorously stirred refluxing mixture

of bithiophene 17 (77.9 g, 0.162 mol) in 400 mL of ethanol containing

40 mL water, 100 mL glacial acetic acid, and 8 mL of 3m HCl (aq). After

refluxing for 2 h, the mixture was filtered hot and upon cooling to 08C,

yellow crystals were collected by filtration. The crystals were dissolved in

diethyl ether, the resulting solution washed three times with 200 mL

water, once with 100 mL brine, and dried over anhydrous MgSO₄. The

solution was then filtered and the solvent removed by evaporation to

give 49.4 g (94% yield) of a light-yellow powder. Elemental analysis (%)

1

4.28; found: C, 48.35; H, 4.31; H NMR (CDCl₃): d=7.09 (s, 1H), 2.95 (t,

J=6.0 Hz, 2H), 1.2–1.4 (b, 12H), 0.89 ppm (t, J=7.5 Hz, 3H); MS (EI):

m/z (%): 467.9 (100) [M⁺].

Synthesis of 5,5’’’-bis(perfluorophenacyl)-4,4’’’-dioctyl-2,2’:5’,2’’:5’’,2’’’-qua-

terthiophene (4): A mixture of 12 (0.671 g, 1.43 mmol), 5,5’-bis(tributyl-

stannyl)-2,2’-dithiophene (0.532 g, 0.715 mmol), and tetrakis(triphenyl-

phosphine)palladium(0) (25.0 mg, 0.0215 mmol) was degassed with nitro-

1914

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

calcd C₈H₄Br₂S₂: C, 29.65; H, 1.24; found: C, 29.59; H, 1.14; ¹H NMR

(CDCl₃): d=7.41 (d, J=5.3 Hz, 2H), 7.09 ppm (d, J=5.3 Hz, 2H).

56.85; H, 1.33; found: C, 56.78; H, 1.21; ¹H NMR ([D₆]DMSO): d=8.51

(d, J=8.0 Hz, 2H), 8.40 (s, 2H), 8.27 (d, J=7.5 Hz, 2H), 8.06 (d, J=

4.0 Hz, 2H), 8.02 ppm (d, J=4.5 Hz, 2H); m.p. 351–3548C; MS (EI): m/z

(%): 760.0 (100) [M⁺].

Synthesis of 2,2’-bithiophene-3,3’-dicarboxaldehyde (19): A solution of bi-

thiophene 18 (8.39 g, 25.9 mmol) in 50 mL THF was added dropwise over

1 h to a stirring solution of nBuLi (82.5 mL, 1.6m in hexanes) in 200 mL

THF at À788C. The reaction mixture was allowed to stir for an additional

2 h at À788C before dry DMF (4.17 g, 57.0 mmol) in 50 mL THF was

added dropwise to the reaction mixture over 30 min. The reaction mix-

ture was then allowed to stir for an additional 30 min before it was

quenched with 50 mL of 5% HCl (aq), allowed to warm, washed three

times with 200 mL water, once with 100 mL brine, and dried over anhy-

drous MgSO₄. The red solid obtained upon filtration and concentration

was purified by column chromatography on silica gel to afford 3.19 g of

19 as a red solid that was dried in vacuo and used immediately in the

next reaction. Elemental analysis (%) calcd C₁₀H₆O₂S₂: C, 54.03; H, 2.72;

found: C, 53.90; H, 2.64; ¹H NMR (CDCl₃): d=9.86 (s, 1H), 7.65 (d, J=

5.5 Hz, 2H) , 7.50 ppm (d, J=5.5 Hz, 2H); MS (EI): m/z (%): 222.1

(100) [M⁺].

Synthesis of 2,7-bis((5-phenacyl)thiophen-2-yl)-9,10-phenanthrenequi-

none (6): A 25 mL Schlenk flask with attached condenser was charged

with boronic ester 16 (0.736 g, 2.34 mmol), aryl iodide 22 (0.513 g, de-

gassed, 1.12 mmol), [Pd₂ACTHNUTRGEN(UNG dba)₃] (428 mg, 0.47 mmol), tri-o-tolylphosphine

(142 mg, 0.94 mmol), and was evacuated then back-filled with nitrogen

5x before 15 mL toluene and 3.1 mL Na₂CO₃(aq, 1.5m) were added. The

reaction mixture was heated at reflux overnight and the solids collected

by filtration upon cooling, washed with 10 mL acetone, 10 mL methanol,

and recrystallized 5x from boiling nitrobenzene to give 0.214 g of 6 as a

brown crystalline material (33% yield). Elemental analysis (%) calcd

C₃₆H₂₀O₄S₂: C, 74.46; H, 3.47; found: C, 74.59; H, 3.58; ¹H NMR

([D₆]DMSO): d=8.51 (d, J=8.0 Hz, 2H), 8.40 (s, 2H), 8.27 (d, J=

7.5 Hz, 2H), 8.06 (d, J=4.0 Hz, 2H), 8.02 ppm (d, J=4.5 Hz, 2H); m.p.

312–3158C; MS (EI): m/z (%): 580.2 (100) [M⁺].

Synthesis of 2,7-bis(thiophen-2-yl)-9,10-phenanthrenequinone (7):

A

Synthesis of 2,2’-bithiophene-3,3’-oxalaldehyde (20): The precatalyst 3,4-

25 mL Schlenk flask with attached condenser was charged with 4,4,5,5-

tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane (0.441 g, 2.10 mmol),

dimethyl-5-(2-hydroxyethyl)thiazolium iodide (1.70 g, 6.74 mmol) was

suspended in 180 mL dry DMF before 1,8-diazabicycloACTHNUGRTENUNG[5.4.0]undec-7-ene

aryl iodide 20 (0.460 g, 1.00 mmol), [Pd₂ACHTUNRTGNEUNG(dba)₃] (183 mg, 0.200 mmol), tri-

(DBU, 1.02 g, 6.70 mmol) was added slowly and the solution allowed to

stir for 5 min. Dialdehyde 19 was then added and the reaction mixture al-

lowed to stir for 2 h under nitrogen, before being opened to air and al-

lowed stir overnight. The dark red reaction mixture was then poured into

500 mL of chloroform, extracted 7x with 200 mL of water, once with

200 mL brine, and dried over MgSO₄. A dark colored solid was obtained

upon filtration and concentration, and was purified by vacuum sublima-

tion to give 4.61 g of 20 (94% yield). Elemental analysis (%) calcd

o-tolylphosphine (122 mg, 0.400 mmol), and was evacuated then back-

filled with nitrogen 5ꢃ before 13 mL toluene and 3.1 mL Na₂CO₃(aq,

1.5m) were added. The reaction mixture was next heated at reflux over-

night, and the solids collected by filtration upon cooling, washed with

10 mL acetone, 10 mL methanol, and purified by multiple temperature

gradient vacuum sublimations to afford 0.145 g of 7 as a dark brown crys-

talline solid (39% yield). Elemental analysis (%) calcd C₂₂H₁₂O₂S₂: C,

70.94; H, 3.25; found: C, 70.87; H, 3.17; ¹H NMR ([D₆]DMSO): d=8.35

(d, J=8.0 Hz, 2H), 8.19 (s, 2H), 8.08 (d, J=8.0 Hz, 2H), 7.76 (d, J=

3.5 Hz, 2H), 7.69 (d, J=5.0 Hz, 2H), 7.22 ppm (t, J=4.5 Hz, 2H); m.p.

295–2988C; MS (EI): m/z (%): 372.1 (100) [M⁺].

1

C₁₀H₄O₂S₂: C, 54.53; H, 1.83; found: C, 54.48; H, 1.76; H NMR (CDCl₃):

d=7.62 (d, J=5.0 Hz, 2H), 7.44 ppm (d, J=5.0 Hz, 2H); m.p. 221–

2238C; MS (EI): m/z (%): 220.0 (100) [M⁺].

Synthesis of 5,5’-diiodo-2,2’-bithiophene-3,3’-carboxalaldehyde (21): N-io-

dosuccinimide (4.56 g, 20.2 mmol) was slowly added to a suspension of

quinone 20 (2.23 g, 10.1 mmol) in 20 mL BF₃·Me₂O and allowed to stir

under air in a Nalgene beaker for 4 h before being poured into 100 mL

of water, filtered, washed with 20 mL NaHSO₄(aq, 5% w/v), and recrys-

tallized from xylenes to give 2.68 g dark purple crystalline material (61%

yield). Elemental analysis (%) calcd C₁₀H₂I₂O₂S₂: C, 25.44; H, 0.43;

found: C, 25.36; H, 0.49; ¹H NMR ([D₆]DMSO): d=7.70 ppm (s, 2H);

m.p. 315–3188C; MS (EI): m/z (%): 471.8 (100) [M⁺].

Synthesis of pyrene-4,5,9,10-tetraone (23): Sodium metaperiodate (35.1 g,

164 mmol), water (100 mL), and RuCl₃·3H₂O (0.498 g, 2.40 mmol) were

added to a solution of pyrene (4.05 g, 20.0 mmol) in a mixture of 80 mL

dichloromethane and 80 mL acetonitrile. The reaction mixture was

heated to 408C, allowed to stir overnight, and upon cooling the solids

were collected by filtration. The material was recrystallized from xylenes

to give 0.632 g 23 as a black crystalline material (15% yield). Elemental

analysis (%) calcd C₁₆H₆O₄: C, 73.29; H, 2.31; found: C, 73.26; H, 2.28;

¹H NMR (CDCl₃): d=8.46 (d, J=3.5 Hz, 2H), 8.20 (d, J=2.0 Hz, 2H) ,

7.82 ppm (d, J=8.0 Hz, 2H); MS (EI): m/z (%): 262.0 (100) [M⁺].

Synthesis of 2,7-diiodo-9,10-phenanthrenequinone (22): Iodine (14.6 g,

57.5 mmol) and potassium permanganate (10.3 g, 65.0 mmol) were sus-

pended in a mixture of 100 mL glacial acetic acid and 30 mL acetic anhy-

dride that had been stirring for 5 h. The suspension was stirred vigorously

with a magnetic stirbar and maintained at 58C while H₂SO₄(aq, conc.,

40 mL) was added dropwise (CAUTION: addition of concentrated

H₂SO₄can produce a vigorous exotherm if not performed slowly). The

slurry was then allowed to warm to room temperature before phenan-

threnequinone (10.4 g, 50.0 mmol) was added and the reaction mixture

stirred for 2 days. The solids were next isolated by filtration, washed with

30 mL hexane, 50 mL methanol, 100 mL NaHSO₄(aq, 5% w/v), 50 mL

acetone, and then recrystallized from refluxing xylenes to afford 6.31 g

orange solid (27% yield). Elemental analysis (%) calcd C₁₄H₆I₂O₂: C,

36.55; H, 1.31; found: C, 36.52; H, 1.25; ¹H NMR ([D₆]DMSO): d=8.25

(s, 2H), 8.11 (d, J=7.5 Hz, 2H), 8.10 ppm (d, J=8.0 Hz, 2H); m.p. 302–

3058C; MS (EI): m/z (%): 459.9 (100) [M⁺].

Synthesis of 2,7-diiodopyrene-4,5,9,10-tetraone (24): N-iodosuccinimide

(0.674 g, 3.00 mmol) dissolved in 1 mL H₂SO₄(aq, conc.) was slowly

added to a suspension of tetraone 23 (262 mg, 1.00 mmol) in a mixture of

2 mL trifluoroacetic acid and 5 mL H₂SO₄(aq, conc.). The reaction mix-

ture was allowed to stir for 2 days at 408C before being poured into

20 mL of water, filtered, washed with 20 mL NaHSO₄(aq, 5% w/v), and

recrystallized from nitrobenzene to give 295 mg of 24 as a black crystal-

line material (59% yield). Elemental analysis (%) calcd C₁₆H₄I₂O₄: C,

37.39; H, 0.78; found: C, 37.27; H, 0.69; ¹H NMR ([D₆]DMSO): d=

8.51 ppm (s, 4H); MS (EI): m/z (%): 513.9 (100) [M⁺].

Polymer molecular weight determination: GPC measurements were per-

formed by using a Polymer Laboratories PL-GPC 220 instrument with

1,2,4-trichlorobenzene as the solvent (stabilized with 125 ppm BHT) at

1508C. A set of three PLgel 10 mm mixed columns was used. Samples

were prepared at 1608C. Molecular weights were determined by GPC

using narrow polystyrene standards and are not corrected.

Synthesis of 2,7-bis((5-perfluorophenacyl)thiophen-2-yl)-9,10-phenan-

ACHTUNGTRENNUNG

FET device fabrication and measurement: Prime grade p-doped silicon

wafers (100) having 300 nm thermally grown oxide (Process Specialties

Inc.) were used as device substrates. These were sonicated in methanol,

acetone, propanol, and oxygen plasma cleaned before film deposition.

Trimethylsilyl functionalization of the SiO₂surface was carried out by ex-

posing the cleaned silicon wafers to hexamethyldisilazane (HMDS) vapor

under nitrogen at room temperature for 4 days. Vacuum-deposited films

of oligomers were thermally evaporated onto temperature-controlled

substrates under high-vacuum at a QCM quantified growth rate of 0.1–

CTHUNGTRENNUNG

phine (122 mg, 0.400 mmol), and was evacuated, then back-filled with ni-

trogen five times before 13 mL toluene and 3.1 mL Na₂CO₃(aq, de-

gassed, 1.5m) were added. The reaction mixture was heated at reflux

overnight and the solids collected by filtration upon cooling, washed with

10 mL acetone, 10 mL methanol, and purified by multiple temperature

gradient vacuum sublimations to give 0.190 g of 5 as a dark brown crys-

talline solid (26% yield). Elemental analysis (%) calcd C₃₆H₁₀F₁₀O₄S₂: C,

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1915

T. J. Marks, M. A. Ratner, A. Facchetti et al.

0.2 ꢀ/s. Solution-deposited oligomer and oligomer-polymer blend films

were drop-cast from 0.4 mL toluene or xylenes solutions onto a tempera-

ture-controlled substrate in a solvent-saturated air atmosphere. Films of

polymer 3 were spin-coated and drop-cast from a 500 ppm solution in a

xylenes/diethylamine mixture (9:1 v/v), dried at 1208C in vacuo for 12 h,

and annealing was performed at 1508C and 2508C with negligible en-

hancement in device performance. For FET device fabrication, top-con-

tact gold electrodes (500 ꢀ) were deposited by thermal evaporation

through a shadow mask to define channels with dimensions 100 mm (L)ꢃ

2.00 mm (W). The capacitance of the insulator is 1ꢃ10^À8Fcm^À2for

300 nm SiO₂, and mobility was measured in the saturation regime. TFT

device measurements were carried out at 21–238C in a customized probe

station under high-vacuum (<1ꢃ10^À6Torr) or in air. Coaxial and/or tri-

axial shielding was incorporated into Signaton probes to minimize the

noise level. TFT characterization was performed by using a Keithley

6430 sub-femtoampmeter (drain) and a Keithley 2400 (gate) source

meter, operated by a locally written Labview program and GPIB commu-

nication. Thin films were analyzed by wide-angle X-ray film diffractome-

try (WAXRD) on a Rikagu ATX-G instrument using standard qÀ2q

techniques and CuKa₁radiation. Scanning electron microscopy was per-

formed by using a Hitachi 4800 SEM with samples having a 2 nm Au/Pd

sputtered film.

SEM are then presented. Finally, the charge transport char-

acteristics of the semiconducting films as defined in thin

film transistor configurations are discussed in the context of

the other physical data.

Synthesis: The syntheses of the present phenacyl semicon-

ductors are outlined in Scheme 1. Compounds 1–3 were ob-

tained by Stille coupling of the appropriate phenacyl–thio-

phene bromide with bis-stannylated bithiophene, as previ-

ously reported.^[45]Gel permeation chromatography (GPC)

was used to determine the molecular weight of polymer 3

versus polystyrene, yielding M_W=15,300 D and PDI=2.51.

Model compound 4 was likewise obtained by Stille coupling

of 5-(5-(pinacolatoboryl)thien-2-yl)(perfluorophenyl)metha-

none 15 with 5,5’-bis(tributylstannyl)-2,2’-bithiophene. Phen-

anthrene–quinone-based compounds 5–7 were obtained by

Suzuki coupling of the corresponding boronic ester and 2,7-

diiodophenanthrenequinone (22). Purification of 5 and 7

was achieved by multiple gradient sublimation, and 6 was

purified by multiple recrystallizations from nitrobenzene.

The two other quinine reagents, 5,5’-diiodo-2,2’-bithiophene-

3,3’-oxalaldehyde (21) and 2,7-diiodopyrene-4,5,9,10-tet-

raone (24) were synthesized in five and two steps, respec-

tively, from commercially available starting materials. Cou-

pling of bithiophene quinone 21 with 15 or with 4,4,5,5-tet-

ramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane yields the

desired products (confirmed by EI-MS), however the reac-

tion mixtures decompose below 2508C, precluding product

isolation by sublimation, and are intractable (even using re-

fluxing nitrobenzene or trichlorobenzene). Suzuki coupling

of boronic ester 15 or 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-

1,3,2-dioxaborolane with 24 yielded intractable solids, and

the EI-MS spectra of these materials do not indicate the

presence of the desired products.

Semiconductor building blocks 12–16, 21, 22, and 24 were

all synthesized from commercially available starting materi-

als as shown in Scheme 2. The n-octyl-functionalized aryl

bromide 12, precursor to semiconductor 4, was synthesized

in four steps. First, 3-n-octylthiophene is brominated to give

8, which was then formylated by quenching the Grignard re-

agent with DMF. The lithium salt of pentafluorobenzene

was next reacted with 2-carboxythiophene 9 to afford alco-

hol 10, which was brominated with NBS and oxidized with

MnO₂to afford ketone 12. After purification by means of

column chromatography, 12 was coupled with 5,5’-bis(tribu-

tylstannyl)-2,2’-bithiophene to yield new oligomer 4 in 36%

yield, after purification by column chromatography and Ku-

gelrohr distillation (Scheme 1).

Electrochemistry: Cyclic voltammetry measurements were performed in

an electrolyte solution of 0.1m tetrabutylammonium hexafluorophos-

phate in dry THF. Platinum electrodes were used as both working and

counter electrodes, and Ag wire was used as the pseudo-reference elec-

trode. A ferrocene/ferrocenium redox couple was used as an internal

standard and potentials obtained in reference to the silver electrode were

converted to the saturated calomel electrode (SCE) scale.

Thermal characterization: All materials were dried under vacuum for 3 d

at 1008C before thermal analysis. Thermogravimetric analysis (TGA)

was performed by using a Mettler Toledo TMA/SDTA841e at a ramp

rate of 108Cmin^À1using an aluminum oxide crucible under vacuum (5 T)

for molecular materials or under nitrogen at atmospheric pressure for the

polymer. Differential scanning calorimetry (DSC) was performed by

using a Mettler Toledo DSC823e at a ramp rate of 108Cmin^À1using alu-

minum pans under nitrogen.

Computational methodology: Equilibrium geometry optimizations were

performed in QChem 2.1^[47]at the density functional theory (DFT) level

with a B3LYP functional and the 6-31G* basis set. Single point calcula-

tions using these geometries were then performed at the DFT/B3LYP/6-

31+G* level of theory to obtain molecular orbital energy levels

(QChem) and orbital electron density plots (Spartan ’06). Energy levels

were calibrated to the experimental HOMO/LUMO energies^[39]of sexi-

thiophene.

Single crystal structure determination: Single crystals of 1, 2, and 7 were

grown by gradient sublimation. X-ray single crystal diffraction measure-

ments were performed on a Bruker CCD area detector instrument with

graphite-monochromated MoKa (0.71073 ꢀ) radiation. The data were

collected at 153(2) K, and the structures were solved by direct methods

and expanded using Fourier techniques. Crystallographic details for 7 are

provided in the Supporting Information; those for 1 and 2 are given else-

where.^[45]

Results

Catalytic borylation^[48,49]was employed in the synthesis of

intermediates 15 and 16, precursors to semiconductors 5 and

6, since both the p-deficient quinone core 22 and the phe-

nacyl–thiophene units in 13 and 14 are unstable under stan-

dard metalation conditions. Since borylation and acetal pro-

tection of quinones 21 and 22 was unsuccessful, efforts fo-

cused on catalytic borylations of the phenacyl–thiophene

bromides 13 and 14. Furthermore, because borylation of 13

performed in the standard solvent DMF^[48,50]afforded the

This account begins with a discussion of synthetic strategies

and routes to compounds 1–7. Next, molecular and materials

characteristics such as optical and electrochemical properties

and thermal transitions as determined by UV/Vis/PL spec-

troscopy, cyclic voltammetry, polarized optical microscopy,

and DSC/TGA are outlined. Solid-state microstructures and

surface morphologies in the corresponding thin films as in-

vestigated by film and single-crystal X-ray diffraction and

1916

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

bromo-2,2’-bithiophene

17,

which was selectively debromi-

nated with Zn to yield 3,3’-di-

bromo-2,2’-bithiophene 18. Re-

agent 3,3’-dicarboxy-2,2’-bithio-

phene (19) was obtained by

slowly adding 18 to a dilute so-

lution of nBuLi followed by re-

acting the dilithium derivative

with DMF. The dialdehyde un-

dergoes benzoin condensa-

tion^[51]in the presence of 3,4-di-

methyl-5-(2-hydroxyethyl)thia-

zolium iodide to give the cy-

clized a-hydroxy ketone, which

was then oxidized in air to

afford quinone 20 in 94% yield,

after purification by sublima-

tion. Aryl-diiodide 21 was ob-

tained in 61% yield by BF₃·O-

AHCTUNGTERG(NNUN CH₃)₂-catalyzed iodination of

20 with N-iodo-succinimide. Fi-

nally, commercially available

phenanthrenequinone was iodi-

nated by I₂/KMnO₄to afford 22

in 27% yield after recrystalliza-

tion from xylenes. Diiodinated

pyrenetetraone 24 is obtained

by RuCl₃-catalyzed oxidation of

pyrene with sodium metaperio-

date, followed by iodination

with I₂/KMnO₄.

Semiconductor thermal proper-

ties: Thermogravimetric analy-

sis (TGA) was used to evaluate

the thermal stability of semi-

conductors 1–7 (Figure 3a,

Table 1). A mass loss of 5% is

defined as the threshold for

sublimation (1, 2, 5, 6, 7) or de-

composition (3, 4). The TGA

Scheme 1. Synthesis of semiconductors 1–7.

properties of compounds to be

used in film growth by vacuum

para-hydroxyl substituted pentafluorophenyl derivative, an-

hydrous toluene was used as the reaction medium. Note that

the reactions performed using conventional heating pro-

ceeded in low yields (<5% in this case), however, micro-

wave irradiation resulted in yields of 33% and 50% for 15

and 16, respectively, after purification by Kugelrohr distilla-

tion.

Iodo-functionalized quinone core reagents were used here

since the corresponding aryl-bromide derivatives were un-

reactive under Suzuki coupling conditions. Quinone 21 is

synthesized in five steps from 2,2’-bithiophene. First, 2,2’-bi-

thiophene was tetrabrominated by refluxing with Br₂in a

mixture of chloroform and acetic acid to give 3,5,3’,5’-tetra-

deposition (1, 2, 5, 6, 7) were measured at a pressure of

5 Torr (N₂), and the TGA of materials only to be processed

from solution (3, 4) was performed under N₂at atmospheric

pressure. All of the present molecular materials demonstrate

good thermal stability and volatility, with the onset of subli-

mation at 295 and 3428C for 1 and 2, respectively, and at

375, 4048C, and 3098C for quinone-based 5, 6, and 7, respec-

tively. Polymer 3 and solution-cast molecule 4 decompose at

425 and 3638C, respectively. Differential scanning calorime-

try (DSC) was employed to study the thermal transitions of

1–7 under N₂. DSC plots reveal reversible melting transi-

tions at 294 (1), 306 (2), 352 (5), 316 (6), and 2978C (7) for

the vacuum-deposited materials (Figure 3b and Table 1).

Chem. Eur. J. 2010, 16, 1911 – 1928

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www.chemeurj.org

1917

T. J. Marks, M. A. Ratner, A. Facchetti et al.

PolyACTHNUGRTNEUNGmer 3 exhibits no thermal

transitions in the experimental

window, while material 4 under-

goes a liquid crystalline transi-

tion at 518C and melting at

998C. Thermal data are sum-

marized in Table 1.

Optical properties: Solution

and thin-film UV/Visible ab-

sorption spectra of the new

phenacyl oligomers and poly-

mer 3 are shown in Figure 4,

and the optical spectroscopic

data summarized in Table 2. In

THF solution, molecules 1, 2,

and polymer 3 exhibit absorp-

tion maxima at l=458, 442,

and 460 nm, respectively. The

spectra of 1 and 2 as thin films

reveal hypsochromic shifts of

23 nm and 38 nm, respectively,

while the absorption maximum

Scheme 2. Synthesis of organic semiconductor precursors.

Figure 3. a) Thermogravimetric analysis (TGA) and b) differential scan-

ning calorimetry (DSC) plots of materials 1–7. See Table 1 for data.

Figure 4. Optical absorption spectra of compounds 1–7 in a) THF solu-

tion and b) thin films on glass.

of 3 shifts bathochromically by 16 nm. Compound 4 exhibits

behavior similar to that of polymer 3, with a solution ab-

sorption maximum at l=453 nm and a bathochromic shift

for the film to l=457 nm. Note that the maxima of Qui-

none-based materials 5, 6, and 7 are significantly blue-shift-

ed, with their solution absorption maxima located at l=369,

370, and 318 nm, respectively. As thin films, these materials

exhibit slight peak broadening with absorbtion maxima at

l=371, 365, and 329 nm, respectively.

Table 1. Thermal properties semiconducting of phenacyl oligomers and

polymers

m.p. [8C]

T_DSC[8C]

T_TGA[8C]

Heating

Cooling

1

2

3

4

5

6

7

291–194

302–307

–

294

306

–

51, 99

352

316

297

257

277

–

295

342

425

363

375

404

309

95–99

–

350–353

309–316

294–297

333

281

254

Electrochemical properties: The redox behavior of com-

pounds 1–7 was investigated using cyclic voltammetry

1918

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

Table 2. Summary of optical absorbtion spectroscopic data for materials

1-7.

exhibits a 0.2 V anodic shift in reduction potential compared

to 1, with onset at À1.16 V and with the reduction half-wave

located at À1.23 V. Two irreversible oxidations are also ob-

served for 4 at +1.04 and +1.40 V. The new quinine-con-

taining molecules exhibit significant ꢀ0.6 V cathodic shifts

in their completely reversible reduction behavior. Reduction

onset occurs for 5, 6, and 7, respectively at À0.37, À0.39,

and À0.43 V; the first half-wave potentials are located at

À0.45, À0.46, and À0.54 V; the second half-wave potentials

at À1.02, À1.08, and À1.08 V; and the third half-wave poten-

tials at À1.74, À1.82, and À1.37 V. One irreversible oxida-

tion event is observed for 5 at +1.60 V. It is interesting that

the quaterthiophene-based materials exhibit a 0.3 V catho-

dic shift upon fluorination while the quinone-based materi-

als have onset and reduction half-wave potentials that are

largely independent of ancillary substitution.

soln

film

opt [a]

l_max

l_shoulder

l_max

l_shoulder

E_gap

[eV]

HOMO^[b]LUMO^[c]

[nm]

[eV]

1

2

3

4

5

6

7

458

442

460

453

369

370

318

–

435

404

476

457

371

365

329

495, 373

462

522

469

–

2.40

2.47

2.37

2.41

2.62

2.54

2.56

5.99

5.90

5.72

5.88

6.64

6.56

6.53

À3.59

À3.43

À3.35

À3.47

À4.03

À4.02

À3.97

374, 322

382

340, 354

–

370

[a] Optical band gap calculated from the red edge of the S₀–S₁absorption

band at 1/10 the maximum intensity. [b] HOMO energy estimated by

subtracting the optical gap from LUMO energy. [c] Estimated from the

onset of the first electrochemical reduction using the correction factor

À4.4 V to convert SCE reference to vacuum.^[52]

(Figure 5 and Table 3). Oligomers 1 and 2 exhibit onsets of

reduction at À0.81 and À0.97 V with first reduction half-

wave potentials of À1.03 and À1.31 V, respectively. Two irre-

versible oxidation events are observed for 1 at +1.07 and

+1.22 V, whereas no oxidations are observed for 2. Polymer

3 exhibits three reversible reductions with onset at À1.05 V

and half-wave potentials located at À1.12, À1.24, and

À1.79 V. Two irreversible oxidations are observed for 3 at

+0.96 and +1.13 V. Interestingly, new model compound 4

Thin-film X-ray diffraction and surface microstructure anal-

ysis: Thin-film wide angle X-ray diffraction (WAXRD)

qÀ2q scans of films of compounds 1–7 were performed to

investigate the degree of film crystallinity and molecule/

polyACTHUNRTGNEUNGmer chain orientation with respect to the substrate sur-

face. Scanning electron microscopy (SEM) was employed to

characterize film surface morphology and uniformity. Films

were vacuum deposited or drop-cast on the same substrates

(HMDS treated p⁺⁺-Si-300 nm SiO₂) used to fabricate

TFTs. Thin films of oligomers 1, 2, 5, and 6 exhibit strong

Bragg diffraction features (Figure 6), while those of mole-

cules 4, 7, and polymer 3 do not exhibit detectable reflec-

tions in WAXRD scans. SEM imaging reveals primarily ho-

mogenous films with crystallites and ribbons protruding

from the surface (Figure 7 and Figure 8).

Thin films of oligomers 1 and 2 exhibit strong Bragg re-

flections in qÀ2q scans up to 10^thand 8^thorder, respectively.

The scans indicate single-phase films with d-spacings of

26.6Æ0.1 ꢀ (1) and 26.3Æ0.1 ꢀ (2). As the substrate depo-

sition temperature (T_D) increases, higher intensities and nar-

rower reflections are observed for both films. The qÀ2q dif-

fraction patterns of quinone-based materials 5 and 6 show

similar crystallinity trends with T_D. Oligomer 5 exhibits a

single family of Bragg reflections up to 9^thorder with a d-

spacing of 27.9Æ0.1 ꢀ, indicating a single-phase film. Films

of 6 exhibit reflections up to 7^thorder with a primary d-spac-

ing of 25.90Æ0.08 ꢀ, a second

Figure 5. Cyclic voltammograms of materials 1–7 in THF solution. Data

are collected in Table 3. The dashed line overlaps with the Fc/Fc⁺redox

process.

phase is also present at T_D=

Table 3. Anodic (E_a), cathodic (E_c), and half-wave (E^1/2) potentials (V vs. SCE) from cyclic voltammetry of

semiconductors 1–7.

1508C having a d-spacing of

27.05Æ0.04 ꢀ.

Oxidation^[a]

Cathodic

Reduction

Anodic

E_a2

SEM imaging of vapor-de-

posited oligomer 1 and 2 films

reveals crystallites protruding

from a smooth underlying film

(Figure 7a–f). Crystallite size

increases with T_Dfor both ma-

terials from ꢀ100 nm at 258C

to ribbons> 1 mm long at 908C.

Solution-cast films of 1 exhibit

large crystalline plates having

Cathodic

E_c2

Half-wave

E₂

1/2

E_c1

E_c2

E_c1

E_c3

E_a1

E_a3

E₁

E₃

1

2

3

4

5

6

7

1.07

–

0.96

1.04

1.60

–

1.22

–

1.13

1.40

–

À0.99

À1.27

À1.08

À1.13

À0.40

À0.41

À0.44

À1.12

–

À1.07

À1.35

À1.17

À1.34

À0.50

À0.51

À0.64

À1.18

–

À1.03

À1.31

À1.12

À1.23

À0.45

À0.46

À0.54

À1.15

–

À1.15

À1.88

À1.32

À2.06

À1.24

À1.97

–

À0.86

À0.91

À1.04

À1.69

À1.74

À1.27

À1.18

À1.25

À1.12

À1.79

À1.89

À1.47

À1.02

À1.08

À1.74

À1.82

À1.37

–

[a] Oxidation events were irreversible.

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1919

T. J. Marks, M. A. Ratner, A. Facchetti et al.

Figure 6. X-ray diffraction q-2q plots of semiconductor a) 1, b) 2, c) 5, and d) 6 thin films vapor-deposited (unless otherwise noted) onto Si/SiO₂sub-

strates temperature controlled at the indicated T_D.

flat surfaces and dimensions> 50 mm in SEM images (Fig-

ure 7g) and with birefringence in the optical micrographs

(Figure S1a). Blends of oligomer 1 and polymer 3 also ex-

hibit film surfaces with crystal plates having dimensions of

ꢀ50 mm in SEM images (Figure 7h) and with birefringence

in the optical micrographs (Figure S1b).

SEM imaging of vapor-deposited 5 films reveals surface

morphology similar to that of 1 and 2. Crystallites ꢀ100 nm

long dominate the surface of films grown at T_D=258C,

while longer ribbons> 1 mm protrude from an otherwise

smooth film at higher T_D(Figures 8a–c). Oligomer 6 exhib-

its a surface with fewer crystallites at T_D=258C, which grow

Figure 7. SEM micrographs of semiconductor 1 and 2 thin films vapor-de-

and coalesce into a smooth film at intermediate T_Dbefore

posited onto temperature-controlled substrates at 25 (a, d), 70 (b, e), or

gaps begin to appear in the film at T_D=1508C (Figure 8d–

908C (c, f), as well as solution-cast films of 1 (g, toluene, 500 ppm) and a

f). Films of 7, which do not exhibit detectable X-ray diffrac-

tion, also have a markedly different surface morphology

from the other materials. Films at T_D=258C are amorphous

and become discontinuous as T_Dincreases, with large gaps

forming between islands of 7 (Figures 8g–h).

1:3 blend (h, toluene, 1:1, 500 ppm).

Transistor fabrication and optimization: Thin film transistors

were fabricated with films of 1–7 as the semiconducting

layer. Oligomers 1, 2 and 4–7 were vacuum-deposited (1, 2,

and 4 were also drop-cast), while polymer 3 was spin-cast

and drop-cast. A minimum of 10 devices for each material

was tested under high-vacuum and once fully characterized,

their operational stability in ambient air also characterized.

Oligomers 1, 3, 5, and 6 exhibit exclusively n-type transport

in a top-contact bottom-gate FET geometry (Figures 9, 10,

and Figure S2), while OSC 2 exhibits exclusively p-type be-

havior.

Figure 8. SEM micrographs of semiconductors 5 (a, b, c) and 6 (d, e, f)

thin films vapor-deposited onto temperature-controlled Si/SiO₂substrates

at 25 (a, d,), 120 (b, e), or 1508C (c, f), as well as micrographs of 7 thin

films vapor-deposited onto temperature-controlled Si/SiO₂substrates at

50 (g), 50 (h), or 908C (i).

1920

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

T_D, from m_h=0.012 cm²V^À1s^À1

at 258C (I_on:off=10⁵, V_T=

À33 V), to a maximum m_hof

0.043 cm²V^À1s^À1at T_D=908C

(I_on:off=10⁶, V_T=À19 V). Films

of 2 exhibit ideal saturation be-

havior with only slight contact

resistance (Figure 9d) and com-

pletely air-stable operation.

Solution-cast films of 1 and 2

exhibit impressive FET perf-

ormance

average m_efor drop-cast

films is

0.21(8) cm²V^À1s^À1

(0.27 cm²V^À1s^À1

maximum)

(Figure S2).

The

1

with a I_on:off=4ꢃ10⁵, figures-of-

merit similar to the vapor-de-

posited films. Drop-cast films of

2 exhibit an average m_hof 8(4)ꢃ

10^À4cm²V^À1s^À1with I_on:off

=

Figure 9. FET transfer (a, c) and output (b, d) plots of vacuum-deposited films of 1 (T_D=808C) and 2 (T_D=

908C), respectively.

ꢀ10⁴. Spin-cast and drop-cast

films of polymer 3 exhibit a

mobility of 1.7(6)ꢃ10^À6with an

I_on:offof 2(1)ꢃ10²and V_T=

+52(10) V (Figure S2b). Blends

of 1 and 3 were drop-cast to en-

hance FET performance while

maintaining the favorable solu-

tion rheology of the polymer

(500 ppm concentration at a 1:1

wt. ratio from xylenes), result-

ing in films exhibiting m_e=

0.02(1) cm²V^À1s^À1with I_on:off

3ꢃ10⁴and V_T=+68 V (Fig-

ure S2c).

Films of semiconductors

=

AHCTUNGTRENNUNG

5

and 6 were vacuum-deposited

at T_Ds ranging from 258C

to 1508C. Material 5 exhibits

n-channel FET activity with

performance increasing with

Figure 10. FET transfer (a, c) and output (b, d) plots measured under vacuum (and after 1 year in air for a) of

vacuum deposited films of 5 (T_D=1508C) and 6 (T_D=1508C), respectively.

T_D

from

m_e=5.3(2)ꢃ

10^À3cm²V^À1s^À1(I_on:off=8ꢃ10⁵,

V_T=+33(8) V) at 258C to m_e=

Oligomer and polymer film microstructure was adjusted

to optimize device performance by varying the substrate

temperature (T_D) during vacuum deposition and drop-cast-

ing. Solution-cast films of 1–4 were additionally annealed at

758C, 1508C, and 2508C with negligible effect on FET re-

sponse. However, the performance of vapor-deposited 1

films is enhanced significantly as T_Dincreases (Table 4),

from 258C (m_e=0.18 cm²V^À1s^À1, I_on:off=3ꢃ10⁵, V_T=+68 V),

to a maximum at T_D=808C (m_e=0.45 cm²V^À1s^À1, I_on:off=1ꢃ

10⁸, V_T=+35 V). Under the optimized conditions, 1 exhibits

low contact resistance and excellent saturation behavior in

the output plots (Figure 9b), but does not exhibit FET activ-

ity under air. The FET performance of 2 also increases with

0.017(2) cm²V^À1s^À1(I_on:off=2ꢃ10⁸, V_T=+28 (8) V) at 1508C

(Figure 10 and Table 5). The air stability of 5-based n-chan-

nel FETs was evaluated over a period of 3 months (Fig-

ure 10a),

revealing

stable

performance

of

m_e=

0.015(3) cm²V^À1s^À1(I_on:off=1ꢃ10⁶, V_T=+26(5) V) after the

initial device break-in (the performance of T_D=1508C devi-

ces was unchanged after >1 year in air). The output plot for

5-based devices in Figure 10b reveals negligible contact re-

sistance and excellent saturation behavior. Oligomer 6 also

exhibits n-channel FET activity for T_D>908C (Figure 10c,d

and Table 5).

The performance increases slightly with T_Dto a maximum

m_e=1.2(2)ꢃ10^À5cm²V^À1s^À1(I_on:off=2ꢃ10⁴, V_T=+94(13) V)

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1921

T. J. Marks, M. A. Ratner, A. Facchetti et al.

Table 4. FET performance of vacuum-deposited semiconductor 1 and 2 films as a function of deposition tem-

perature, measured under vacuum. Standard deviations are given in parentheses.

gomer 2 crystallizes in a mono-

clinic cell with a P21/n space

group (Figure 11b,d). The unit

T_D

Semiconductor

[8C]

1

2

cell

dimensions

are

a=

m_e[cm²V^À1s^À1

]

V_T[V]

I_on:off

m_h[cm²V^À1s^À1

]

V_T[V]

I_on:off

6.0174(8), b=7.4293(10), c=

52.685(7) ꢀ, b=93.137(2)8, and

the volume is 2351.8(5) ꢀ³. The

quaterthiophene p-cores of 2

are also packed in a herring-

bone motif at a slightly de-

creased 26.58 angle, with the p–

25

50

70

80

90

0.18(3)

0.17(2)

0.31(3)

0.45(5)

0.17(5)

0.21(8)

68(12)

47(9)

45(7)

35(5)

34(4)

66(4)

3(1)ꢃ10⁵

3(2)ꢃ10⁵

8(2)ꢃ10⁵

1(1)ꢃ10⁸

2(1)ꢃ10⁷

4(3)ꢃ10⁵

1.2(3)ꢃ10^À2

1.4(5)ꢃ10^À2

2.8(8)ꢃ10^À2

3.9(4)ꢃ10^À2

4.3(7)ꢃ10^À2

0.8(4)ꢃ10^À3

À33(9)

À27(4)

À22(5)

À17(3)

À19(5)

À32(6)

2(1)ꢃ10⁵

5(2)ꢃ10⁵

7(2)ꢃ10⁵

2(2)ꢃ10⁶

3(1)ꢃ10⁶

5(3)ꢃ10³

Soln. Cast

p

stacking distance being

Table 5. FET performance of vacuum-deposited 5 and 6 films measured under vacuum. Standard deviations

3.43 ꢀ (C14ÀC16’) and a larger

maximum inter-thiophene tor-

sion of 138. The phenyl groups

are also twisted with respect to

the thiophene p-core at an

angle of 498, but are not closely

p-stacked as in 1. The carbonyl

axis lies 178 out of the plane of

the adjacent thiophene subunit.

are given in parentheses.

T_D

Semiconductor

[8C]

5

6

m_e[cm²V^À1s^À1

]

V_T[V]

I_on:off

m_e[cm²V^À1s^À1

]

V_T[V]

I_on:off

25

50

90

120

150

5.3(2)ꢃ10^À3

9.0(4)ꢃ10^À3

1.2(2)ꢃ10^À2

1.8(4)ꢃ10^À2

1.7(2)ꢃ10^À2

33(8)

34(6)

31(6)

32(5)

28(8)

8(2)ꢃ10⁵

6(1)ꢃ10⁶

1(1)ꢃ10⁷

8(2)ꢃ10⁷

2(1)ꢃ10⁸

–

7.9(3)ꢃ10^À6

8.7(5)ꢃ10^À6

1.2(2)ꢃ10^À5

120(18)

112(22)

94(13)

1(1)ꢃ10³

4(2)ꢃ10³

2(1)ꢃ10⁴

at 1508C. The output plot of 6 films reveals excellent satura-

tion behavior and only slight contact resistance. Owing to

the very-high sublimation temperatures of 5 and 6, the

chemical composition of the evaporation source and the re-

sulting thin films was confirmed after film growth by H¹

NMR and elemental analysis, and was found to be consis-

tent with the pure oligomers.

Crystal structures and solid-state packing: The structures of

oligomers 1 and 2 as well as model compound 7 were deter-

mined by X-ray diffraction of single crystals grown by slow

sublimation. Crystallographic experimental details and de-

rived metrical parameters are summarized in Tables S1–S4

in the Supporting Information. Perfluorophenacyl oligomer

1 crystallizes in a monoclinic cell with a C2/c space group

(Figure 11a,c). Unit cell dimensions are a=53.617(12), b=

7.4030(17), c=6.5735(15) ꢀ, and b=94.259(4)8, giving a

volume of 2602.0(10) ꢀ³. The quaterthiophene cores orient

in a typical herringbone packing motif at an angle of 338,

Figure 11. Crystal structures of semiconductors 1, 2, and 7. Structures 1

and 2 are viewed along the cell b axes (a and b, respectively) and along

the c axes (c and d, respectively). The crystal structure of 7 is viewed

along the a axis (e) and b axis (f).

À

with a p–p stacking distance of 3.50 ꢀ (C14 C15’), and a

maximum inter-thiophene torsion angle of 48. The ancillary

pentafluorophenyl groups are twisted with respect to the

thiophene p-core at an angle of 538 and the carbonyl makes

a 68 dihedral angle with the adjacent thiophene subunit. A

single molecular layer is 26.77 ꢀ thick with the pentafluoro-

phenyl rings not interdigitated, but closely p-stacked

A single molecular layer is 26.51 ꢀ thick with the phenyl

rings not interdigitated. Quinone-based oligomer 7 crystalli-

zes in an orthorhombic cell having the Pbca space group

(Figure 11e,f). Unit cell dimensions are a=12.5516(9), b=

A

12.9708(9), and c=20.2613(14) ꢀ, giving

a volume of

ent than previously observed for planar perfluoroarene-thio-

phene oligomers for which the electron-rich thiophene

moiety overlaps with the electron-poor perfluoroarene, re-

sulting in a face-to-face brick-like stacking and far shorter

intermolecular stacking distances (<3.4 ꢀ).^[44]The present

results reflect the large twist angle between flat quaterthio-

phene core and the pentafluorophenycl groups. Phenacyl oli-

3298.63(14) ꢀ³. The phenanthrenequinone core exhibits a

À

slight 4.78 (C9 C8:C12 C13) torsion angle with the carbon-

yl carbons rotated slightly out of the anthracene plane and

the connected thiophenes tilted at only 8.48 relative the phe-

nanthrenequinone core. The molecules are p-stacked with

the quinones aligned anti-parallel in a cofacial orientation at

À

a close p–p distance of 3.47 ꢀ (S2 C9). These p-stacks align

1922

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Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

along the b axis and make an acute angle of 64.38 relative to

each other (Figure 11e). A donor-acceptor packing motif is

observed in 7, with the molecules slipped 5.7 ꢀ along the

long axis of the molecule, overlapping the electron-deficient

quinone with the electron-rich thiophene fragment.

single crystal X-ray structures, thermal analysis, optical spec-

troscopy, electrochemistry, and DFT-level MO analysis.

The FET device performance of each material was opti-

mized by varying T_Dfor vapor-deposited and the annealing

temperature drop-cast films. Spin-cast films optimized by

varying the solvent and solvent mixtures were also investi-

gated. A wide range of FET device responses and film mi-

crostructures were observed as a consequence of the film

growth optimization studies. Materials 1, 3, 5, and 6 are all

found to exhibit n-channel FET activity under high-vacuum,

and 5 also exhibits air-stable operation over a period greater

than one year. Oligomer 2 exhibits p-channel behavior

under high-vacuum and air, whereas no FET current modu-

lation was observed in this study for films of model com-

pounds 4 and 7.

Quantum chemical modeling: The molecular orbital (MO)

energies and contours were calculated by using the DFT-op-

timized structures of molecules 1, 2, 5, 6, and 7 (Figure 12).

Thin-film microstructure and field-effect transistor device

optimization: Top-contact bottom-gate FETs fabricated with

vapor-deposited films of semiconductors 1, 2, 5, and 6 were

initially measured under vacuum. The performance of 1-

based devices increases with increasing T_D(Figures 9a,b and

Figure 13a), from m_e=0.18(3) cm²V^À1s^À1(V_T=68(12) V,

I

_on:off=3(1)ꢃ10⁵) at T_D=258C to m_e=0.45(5) cm²V^À1s^À1

(V_T=35(5) V, I_on:off=1(1)ꢃ10⁸) at T_D=808C, before dimin-

ishing slightly at T_D=908C (Table 4). This performance en-

hancement corresponds to an increase in crystallite grain

size observed in SEM images from ꢀ100 nm at T_D=258C

to> 1 mm at T_D=908C (Figures 7a–c). WAXRD data for

the films indicate that the overall crystallinity also increases

with T_D, although reflections from a second phase begin to

appear at 908C (Figure 6a). Such a phase-impurity can di-

minish m, as observed in this study, by creating defect sites

Figure 12. Molecular orbital electron density contour plots for oligomers

a) 1, b) 2, c) 5, d) 6, and e) 7 viewed normal to the p-conjugated cores.

The computed p-conjugated quaterthiophene cores of 1 and

2 in the gas phase exhibit minimal intra-tetramer ring tor-

sion, however the phenyl rings are twisted at 53 and 448, re-

spectively, from each quaterthiophene mean plane. The MO

electron density plots reveal that the HOMO and LUMO

are completely delocalized across the quaterthiophene core

and carbonyl groups, with minimal phenyl substituent con-

tribution. The computed HOMO energy for both 1 and 2 is

À6.0 eV, whereas the LUMO energies are À3.6 and

À3.4 eV, respectively. For molecules 5–7, there is <58 com-

puted thiophene-phenanthrenequinone torsion angle, and

the phenyl rings are twisted with respect to the p-core in 5

and 6. The MO contours reveal that the HOMO is delocal-

ized across the phenanthrenequinone and thiophene subu-

nits. Interestingly, the LUMOs in 5–7 are localized on the

phenanthrenequinone subunits (Figures 12c–e). The comput-

ed HOMO energies are À6.6, À6.6, and À6.5 V for 5, 6, and

7, respectively, while the LUMO energies are all À4.0 eV

for 5–7.

Discussion

The correlations between organic semiconductor thin film

morphology and FET performance will first be discussed in

the context of the SEM, WAXRD, and FET data. Detailed

molecular-level analysis will follow to examine the effect of

molecular modifications and to interpret the observed FET

device response. This analysis will utilize data from the

Figure 13. Plot of measured FET m vs. T_Dfor oligomers 1, 2, 5, and 6 (a)

and m vs. number of weeks storage in air for 5 films deposited at the indi-

cated T_D(b). Dashed lines are a guide for the eye.

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1923

T. J. Marks, M. A. Ratner, A. Facchetti et al.

in the film that scatter or trap charge carriers.^[15,41]The d-

spacing of the dominant phase, 26.6Æ0.1 ꢀ, corresponds

well with the height of one molecular layer determined from

the single-crystal structure determination, 26.77 ꢀ, indicat-

ing that the long axis of the molecule is aligned along the

substrate normal.

tions corresponds to a d-spacing of 25.90Æ0.08 ꢀ, arguing

that the molecules are tilted at ꢀ69.08 relative to the sub-

strate surface (calculated using the DFT optimized geometry

plus a 0.3 ꢀ interlayer spacing adjustment). This tilt angle is

similar to the 64.38 mutual angle made by the slipped

dimers in the single-crystal X-ray diffraction derived struc-

ture of 7 (Figure 11). Such a slipped structure can potential-

ly decrease the efficiency of p–p charge transport compared

to completely overlapped/eclipsed oligomers such as in the

crystal structures of 1, 2, and 5. There is a second, minority

phase observed in 6 films grown at T_D=1508C with a

27.05Æ0.04 ꢀ d-spacing, which suggests a more upright mo-

lecular orientation. Although this second phase may exhibit

stronger intermolecular p–p coupling, the dominant, tilted

phase likely limits FET performance. The SEM images of 6

films also reveal a morphology substantially different from

the other oligomers, with a more homogenous surface that

begins to exhibit void spaces of ꢀ1 mm dimensions at T_D=

1508C. Model compound 7 does not exhibit FET activity or

show evidence of thin-film crystallinity under the present

growth conditions.

Materials 1–3 afford uniform films when drop-cast from

common organic solvents, while spin-casting of 3 also yields

highly uniform films. Drop-cast films of oligomers 1 and 2

exhibit outstanding FET response (Figure S2a) with m_e=

0.21(8) cm²V^À1s^À1(V_T=66(4) V, I_on:off=4 (3)ꢃ10⁵) and m_h=

8 (4)ꢃ10^À4cm²V^À1s^À1(V_T=À32(6) V, I_on:off=5 (30)ꢃ10³),

respectively. The FET performance of 1 is one of the most

impressive reported to date for a solution-cast organic semi-

conductor. Indeed, films of both materials are highly crystal-

line and exhibit the same XRD pattern as the vapor-deposit-

ed films (Figure 6). Furthermore, SEM images and optical

microscopy of drop-cast films of 1 reveal very large crystal-

line plates with dimensions up to 0.1 mm (Figure 7 g and

Figure S1a, respectively) which are even larger than what

observed for vapor-deposited 1 films.

Drop-cast and spin-cast films of polymer 3 exhibit a mobi-

lity of 1.7(6)ꢃ10^À6. Although 1 and 2 films exhibit very in-

tense Bragg reflections, 3 films do not, indicating poor film

crystallinity that may impede carrier mobility. To enhance

the film FET performance while maintaining favorable solu-

tion rheology, solutions of oligomer 1 and polymer 3 were

blended. An equal wt.: wt. ratio of molecule : polymer

yields films exhibiting m_e=0.02(1) cm²V^À1s^À1with I_on:off=3ꢃ

10⁴and V_T=+68 V. This unique polymer-oligomer blend so-

lution combines the favorable rheological characteristics of

the polymer solution with the high FET performance of

molecule 1 to yield a more processable solution, affording

excellent semiconductor films for FETs. Molecule 4 was de-

signed to assess whether repositioning the n-alkyl solubiliz-

ing groups from the inner two thiophenes of the quaterthio-

phene core in 3 to the outer thiophenes would result in

higher crystallinity and therefore enhanced FET perfor-

mance. However, films of 4 either vacuum-deposited or

drop-cast do not exhibit Bragg reflections, indicating that

the substituent repositioning does not enhance thin-film

crystallinity.

Non-fluorinated phenacyl molecule 2 exhibits p-channel

FET behavior that also improves with T_D(Figures 9c,d and

Figure 13a) from m_h=0.012(3) cm²V^À1s^À1(V_T=À33(9) V,

I

_on:off=2(1)ꢃ10⁵) at T_D=258C to m_h=0.043(7) cm²V^À1s^À1

(V_T=À19(5) V, I_on:off=3(1) ꢃ10⁶) at T_D=908C (Table 4).

FET performance is unchanged when measured in air. As

with the fluorinated analog 1, this performance enhance-

ment coincides with an increase in average crystallite size

observed in SEM images of 2-derived films from ꢀ100 nm

at T_D=258C to ꢀ1 mm at T_D=908C (Figures 7d–f). The

WAXRD reflection intensity from 2-derived films also in-

creases with increasing T_D, indicating that the overall film

crystallinity is enhanced (Figure 8). The 26.31Æ0.1 ꢀ d-

spacing calculated from the Bragg progression corresponds

well with the height of one molecular layer determined from

the single-crystal structure determination, 26.51 ꢀ, indicat-

ing that the long axis of the molecule is aligned parallel to

the substrate normal.

Quinone-containing oligomer 5 exhibits n-channel FET

behavior when measured both in vacuum and under air

(Figure 10a, b and Figure 13). The FET performance im-

proves with increasing T_Dwhen measured under vacuum

from m_e=5.3(2)ꢃ10^À3cm²V^À1s^À1(V_T=33(8) V, I_on:off=8(2)ꢃ

10⁵) at T_D=258C to m_e=0.018(4) cm²V^À1s^À1(V_T=28(8) V,

I

_on:off=2(1)ꢃ10⁸) at T_D=1508C (Table 5). When measured

in air, V_Texhibits negligible change and m_eexhibits a slight

decrease in magnitude to 0.015(3) cm²V^À1s^À1(V_T=31(12) V,

I

_on:off=5(1)ꢃ10⁶) at T_D=1508C (Figure 13b), which is stable

for greater than one year when stored and measured under

laboratory air. The WAXRD data indicate phase-pure films

having a d-spacing of 27.9Æ0.1 ꢀ, which corresponds closely

with the 28.0 ꢀ height of one molecular layer (determined

from the DFT geometry plus a 0.3 ꢀ interlayer spacing ad-

justment, estimated from that observed in the crystal struc-

ture of 1). This indicates that the long axis of the molecule

is aligned normal to the substrate surface, a motif similar to

that of 1 and 2.

Oligomer 6 also exhibits n-channel FET activity under

vacuum for T_D> 908C (Figure 10c,d and Figure 13a). Per-

formance improves with increasing T_Dfrom m_e=7.9(3)ꢃ

10^À6cm²V^À1s^À1(V_T=120(18) V, I_on:off=1(1)ꢃ10³) at T_D=

908C to m_e=1.2(2)ꢃ10^À5cm²V^À1s^À1(V_T=94(13) V, I_on:off

=

2(1)ꢃ10⁴) at T_D=1508C (Table 5). This diminished m_erela-

tive to the other vapor-deposited oligomers 1, 2, and 5 is

likely attributable to a different thin-film crystal phase that

is revealed by the WAXRD data. Note that no decomposi-

tion is observed upon sublimation of 6. Also, the experimen-

tal and DFT-computed frontier MO energies reveal that 5

and 6 are isoenergetic and isogeometric (Table 2 and

Figure 12), so the difference in FET performance is not in-

trinsically MO-based. The dominant family of Bragg reflec-

1924

www.chemeurj.org

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2010, 16, 1911 – 1928

Phenacyl–Thiophene and Quinone Semiconductors

FULL PAPER

Electronic properties, molecular modeling, and crystal pack-

ing: Although a thin-film microstructure can significantly

impact FET device response, the intrinsic molecular proper-

ties of an organic semiconductor ultimately determine the

performance limits.^[53–55]The primary molecular-level consid-

erations are crystal packing and frontier orbital delocaliza-

tion/energetics/overlap. In this study, optical spectroscopy,

single crystal X-ray structure determination, electrochemis-

try, and DFT-level computational modeling are employed to

probe these properties experimentally and theoretically.

Since charge is transported through the frontier molecular

orbitals of organic semiconductors, their geometries, ener-

gies, and extents of delocalization directly affect carrier sta-

bility and intermolecular hopping rates. Ideally, the HOMO

and LUMO are delocalized over the entire p-core of the

semiconductor for p- and n-channel transport, respective-

ly.^[41,56]Delocalization, such as that observed in the DFT-de-

rived MO contour plots of 1 and 2, enhances the charge

transfer rate by increasing charge stabilization and intermo-

lecular p-overlap while minimizing Marcus reorganization

energies.^[41,45](Figures 12a,b). Interestingly, although the

HOMOs of 5–7 are well delocalized, the LUMOs are local-

ized on the phenanthrenequinone subunits (Figure 12c–e).

This highly localized nature of the LUMO reduces p-over-

lap compared to the ideal delocalization present in 1 and 2

and may favor electron trapping at the quinone site. This lo-

calization may be a contributing factor to the the diminished

m_evalues of compounds 5 and 6 compared to 1, despite the

greater electron affinity.

The MO energies affect charge carrier stability and are a

key parameter in rationalizing the charge carrier signs in

standard OFET devices. In materials 1–4, the phenacyl an-

cillary substituents stabilize the LUMO, diminishing the sus-

ceptibility of the mobile electrons to trapping

(Table 2).^{[13,41,42,57]}In the case of 1 and 3, this stabilization

lowers the LUMOs to À3.6 and À3.4 eV, respectively, suffi-

cient to enable n-channel activity on HMDS-treated SiO₂.

Indeed, these energies are below the threshold postulated

for efficient n-channel activity in the presence of surface hy-

droxyl groups,^[15,58,59]but probably insufficient for n-channel

activity in the presence of air-based trapping species.^[16,37]

Quinone moieties were introduced into the p-core here to

further stabilize the LUMO and enable air-stable n-channel

FET operation. Previous reports on NDI-F^[16,29]and

CuF₁₆Pc^[27,30–32]proposed that LUMO energies below

À3.9 eV are sufficient to enable air-stable n-channel FET op-

eration for perfluoroalkyl-substituted semiconductors. The

present computational modeling reveals that introduction of

either bithiophene-quinone (21), phenanthrenequinone, or

pyrene-4,5,9,10-tetraone (24) fragments can displace the

LUMO energy below this À3.9 eV threshold. Although all

of the proposed quinone-containing cores (21, 22, 24) were

successfully synthesized in this work, only the Suzuki cou-

pling reactions involving 2,7-diiodo-9,10-phenanthrenequi-

none (22) were productive, yielding materials 5–7.

6 does not. The ambient stability of 5 is likely the result of

the more tightly packed perfluorophenyl versus phenyl an-

cillary substituents since the LUMO energies of

5

(À4.03 eV) and 6 (À4.02 eV) as well as their geometries are

similar. This result is in agreement with previous studies sug-

gesting that fluorocarbon substitution of a semiconductor

with a LUMO energy below À3.9 eV creates an O₂-barrier,

preventing O₂from trapping mobile electrons in the FET

channel.^[16]Additionally, semiconductor 5 maintains a high

I_on:offratio of 10⁶since the LUMO energy (À4.03 eV) is

above the À4.2 eV level at which ambient doping has previ-

ously been observed in extremely-high electron-affinity or-

ganic semiconductors.^[26,37]

The crystal packing motifs of 1 and 2 are similar, with the

quaterthiophene cores herringbone-packed at distances of

3.50 and 3.43 ꢀ, respectively (Figure 11). This type of her-

ringbone p-stacking motif has been shown experimentally

and theoretically to afford optimal p-overlap in quaterthio-

phene-based semiconductors.^[44,60]The structure of both ma-

terials reveals two layers of p-stacked molecules per unit

cell, with the out-of-plane phenyl substituents locking the p-

stacked molecules into an orientation normal to the ac

plane. In polymer 3, introduction of the n-octyl solubilizing

group, in combination with the phenyl-thiophene dihedral

angle along the polymer backbone, likely disrupts the her-

ringbone p-stacked layers, resulting in the observed de-

creased thin-film crystallinity and poorer FET performance.

Model compound 4 was synthesized to evaluate how crystal-

linity is affected when the alkyl substituents are moved

toward the edge of the p-core. The crystal lattice is also de-

stabilized with the groups in this location, resulting in 4

being extremely-soluble but producing amorphous films.

The X-ray diffraction-derived structure of quinone model

compound 7 reveals a highly-planar p-core architecture with

molecules cofacially p-stacked (Figure 11e, f). Stacked mol-

ecules are slipped ꢀ5.7 ꢀ along the long axis of the mole-

cule and make a 648 angle from one p-stack with respect to

the next, likely induced by a donor-acceptor interaction of

the electron-rich thiophene fragment with the electron-defi-

cient quinone subunit. The quinones also stack antiparallel

owing to stabilizing dipole–dipole interactions. This slipped

structure, which diminishes intermolecular p–p coupling, is

likely similar to the dominant phase of 6 films, where the

molecules are tilted ꢀ698 relative to the substrate surface.

This structure of 7 provides evidence for the high crystal

packing affinity of quinine-containing materials and also in-

sight into the diminished mobility of 6-derived films.

Conclusions

The present family of phenacyl–thiophene and quinine-

based semiconductors exhibits rich and informative relation-

ships between molecular-level electronic/structural proper-

ties, film processability, thin-film morphology, and p-/n-chan-

nel FET device characteristics. The design strategy to con-

currently enhance solubility and crystal packing yields high-

As predicted, the fluorinated oligomer 5 exhibits air-

stable n-channel operation, while the non-fluorinated analog

Chem. Eur. J. 2010, 16, 1911 – 1928

ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

1925

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with m_eup to ꢀ0.3 cm²V^À1s^À1for solution-cast films of 1

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Acknowledgements

We thank the Northwestern MRSEC (NSF-MRSEC DMR-0520513) and

AFOSR (FA9550-08-1-0331 for support of this research. R.P.O. acknowl-

edges the MICINN of Spain for a personal postdoctoral grant.

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