Alkylenesulfanyl-bridged bithienyl cores for simultaneous tuning of electronic, filming, and thermal properties of oligothiophenes

Article abstract of DOI:10.1021/ol801261r

(Chemical Equation Presented) (Graph Presented) DPY and DPE alkylenesulfanyl-bridged bithienyls were prepared by a highly effective ring-closing reaction via arylalkylsulfonium intermediate and used as inner cores in oligothiophenes. HOMO-LUMO energy levels, conformational flexibility, and intrinsic asymmetry of the cores are reflected in the electronic, film-forming, and thermal properties of the corresponding oligomers.

Full text of DOI:10.1021/ol801261r

ORGANIC
LETTERS
2008
Vol. 10, No. 17
3665-3668
Alkylenesulfanyl-Bridged Bithienyl Cores
for Simultaneous Tuning of Electronic,
Filming, and Thermal Properties of
Oligothiophenes
Maria Luisa Navacchia,† Manuela Melucci,*,† Laura Favaretto,† Alberto Zanelli,†
Massimo Gazzano,† Alessandro Bongini,‡ and Giovanna Barbarella†
Consiglio Nazionale delle Ricerche (CNR), Istituto per la Sintesi Organica e la
FotoreattiVita` (ISOF), Via Gobetti, 101, I-40129, Bologna, Italy, and Dipartimento di
Chimica ‘G. Ciamician’, UniVersita` di Bologna, Via Selmi 2, I-40126 Bologna, Italy
mmelucci@isof.cnr.it
Received June 4, 2008
ABSTRACT
DPY and DPE alkylenesulfanyl-bridged bithienyls were prepared by a highly effective ring-closing reaction via arylalkylsulfonium intermediate
and used as inner cores in oligothiophenes. HOMO-LUMO energy levels, conformational flexibility, and intrinsic asymmetry of the cores are
reflected in the electronic, film-forming, and thermal properties of the corresponding oligomers.
Thiophene-based compounds have found remarkable ap-
plications as electroactive and light-emitting materials in a
variety of (opto)-electronic devices, as optical transducers
in biosensors and as fluorescent markers for biopolymers.1
The search for design strategies to tune the electronic
properties, mantaining both solubility and desired self-
assembly features for low-cost processing, is actively being
pursued.1a Thienyl ring chemical modifications,2 shape
engineering,3 and supramolecular organization through spe-
(1) (a) Chem. ReV. 2007, 104, 4. Theme issue on organic electronics
and optoelectronics. (b) Barbarella, G.; Melucci, M.; Sotgiu, G. AdV. Mater.
2005, 17, 1581–1593.
† Istituto per la Sintesi Organica e la Fotoreattivita` (ISOF).
‡ Universita` di Bologna.
10.1021/ol801261r CCC: $40.75
Published on Web 07/30/2008
2008 American Chemical Society

cific noncovalent interactions4 are among the most investi-
gated approaches. Introduction of planar electron-rich or
electron-deficient fused heterocycles into the oligothiophene
backbone influences the frontier orbitals energies and the
self-assembly capability.5 Moreover, the inner planar core
combined with alkyl ends promotes liquid crystallinity (LC).6
Less attention has been devoted to the effects of the insertion
of alkyl-bridged heterocycles.7 Methylene- or alkoxyalkyl-
bridged bithienyl cores induce conformational changes that
strongly affect the extent of π-π delocalization and thus
the spectral features.7e,f
deprotection/bromination was obtained in one pot by treating
4 with PBr3
yield). Ring closure of 5 occurred spontaneously on standing
at room temperature in fluorobenzene (70%
at room temperature(67% yield, Scheme 1).
Scheme 1a
Here, we introduce nonsymmetric and conformationally
flexible ꢀ,ꢀ′-alkylenesulfanyl-bridged bithienyls (DPE and
DPY) as inner cores for alkyl-ended oligothiophenes. An
unprecedented and direct synthetic approach to DPE and
DPY, based on an intramolecular C-S bond formation, is
presented. Subsequent chemical manipulation through Pd-
catalyzed cross-couplings furnished new asymmetric olig-
othiophenes. We demonstrate that in this way a fine tuning
of electronic, self-organization, and thermal behavior of the
final oligomer can be simultaneously achieved.
a[Pd(0)] ) [Pd(PPh3)4]; THP ) tetrahydro-4H-pyranyl.
The synthesis of 5,6-dihydrodithieno[3,2-b:2′,3′-d]thiepine
(DPE) was efficiently achieved by one-pot [Pd(PPh3)4]-
catalyzed Stille coupling reaction between the stannyl
derivative 18 and bromoderivative 2 (toluene, 112 °C, 40 h,
80% yield, Scheme 1). Reaction of 1 with 2-bromo-3-
(bromomethyl)thiophene led to a complex mixture of uni-
dentified products. Therefore, the synthesis of 5H-dithieno[3,2-
b:2′,3′-d]thiopyrane DPY relied on the key intermediate
bithiophene 4, which was prepared via Stille coupling
between compounds 1 and 3 (45 h, 65% yield). The
From a mechanistic point of view, the formation of DPE
and DPY can be explained in terms of intramolecular SN2
reaction with formation of a sulfonium salt, followed by
bromide-assisted demethylation (Scheme 2).9 This hypothesis
Scheme 2
(2) (a) Murphy, A. R.; Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066–
1096. (b) Electronic Materials: The Oligomer Approach; Mu¨llen, K.,
Wegner, G., Eds.; Wiley-VCH: New York, 1998. (c) Roncali, J. Chem.
ReV. 1997, 97, 173–205.
(3) (a) Nicolas, Y.; Blanchard, P.; Levillain, E.; Allain, M.; Mercier,
N.; Roncali, J. Org. Lett. 2004, 6, 273–276. (b) Bilge, A.; Zen, A.; Forster,
F.; Li, H.; Galbrecht, F.; Nehls, B. S.; Farrell, T.; Neherb, D.; Scherf, U. J.
Mater. Chem. 2006, 16, 3177–3182. (c) Yu, K.; Chernichenko, Sumerin,
V. V.; Shpanchenko, R. V.; Balenkova, E. S.; Nenajdenko, V. G. Angew.
Chem., Int. Ed. 2006, 45, 7367–7370.
was confirmed by the isolation of bromo-derivatives 5 and
6, which spontaneously convert to DPY and DPE.
(4) (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491–1546. (b) Kato, T.; Mizoshita, N.;
Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68.
DPY and DPE were then used to build the asymmetric
oligothiophenes 10 and 11 through a procedure based on
Stille coupling. To this purpose, bistannanes 7 and 8 were
coupled to 2-bromo-5-n-butyl-thiophene 9 under standard
conditions (Scheme 3). Noteworthy, despite the relatively
poor reaction yield, our approach is more convenient than
the ring-by-ring growth required to synthesize asymmetric
nonidentical alkyl-ended oligothiophenes, since no tedious
purifications to remove “symmetrical” side products are
required.10 To evaluate the role of conformational flexibility
and molecular asymmetry in oligomers 10 and 11, the
symmetric and planar derivative 1411 was prepared by using
dithienothiophene, DTT12 as inner core. For the synthesis
of this oligomer, dibromination of DTT followed by Stille
(5) Kim, H.-S.; Kim, Y.-H.; Kim, T.-H.; Noh, Y.-Y.; Pyo, S.; Yi, M. H.;
Kim, D.-Y.; Kwon, S.-K. Chem. Mater. 2007, 19, 3561–3567. (b) Tian,
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14996–14997.
(6) Melucci, M.; Favaretto, L.; Bettini, C.; Gazzano, M.; Camaioni, N.;
Maccagnani, P.; Ostoja, P.; Monari, M.; Barbarella, G. Chem. Eur. J. 2007,
13, 10046–10054.
(7) For fluorene-thiophene co-oligomers, see: (a) Surin, M.; Sonar, P.;
Grimsdale, A. C.; Mu¨llen, K.; De Feyter, S.; Habuchi, S.; Sarzi, S.; Van
der Auweraer, M.; De Schryver, F. C.; Cavallini, M.; Moulin, J.-F.;
Biscarini, F.; Femioni, C.; Lazzaroni, R.; Lecle`re, P. J. Mat. Chem. 2007,
17, 728–735. (b) Tian, H.; Wang, J.; Shi, J.; Yan, D.; Wang, L.; Geng, Y.;
Wang, F. J. Mater. Chem. 2005, 15, 3026–3033For all-thienyl-based
systems, see: (c) Jousselme, B.; Blanchard, P.; Levillain, E.; Delaunay, J.;
Allain, M.; Richomme, P.; Rondeau, D.; Gallego-Planas, N.; Roncali, R.
J. Am. Chem. Soc. 2003, 125, 1363–1370. (d) Pasini, M.; Destri, S.; Botta,
C.; Porzio, W. Tetrahedron 1999, 55, 14985–14994. (e) Benincori, T.;
Bongiovanni, G.; Botta, C.; Cerullo, G.; Lanzani, G.; Mura, A.; Rossi, L.;
Sannicolo′, F.; Tubino, R. J. Phys. ReV. B 1998, 58, 9082–9086. (f) Zerbi,
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Phys. 1993, 98, 4531–4541.
(9) (a) Shevchenko, N. E.; Nenajdenko, V. G.; Balenkova, E. S. Synthesis
2003, 1191–1200. (b) Koval′, I. V. Russ. J. Org. Chem. 2007, 43, 319–
346.
(8) Barbarella, G.; Zambianchi, M. Tetrahedron 1994, 50, 11249–11256.
3666
Org. Lett., Vol. 10, No. 17, 2008

Scheme 3
coupling with 2-tributylstannyl-5-n-buthyl- thiophene 13 was
characterized by highly rigid and fully planar conformation
and strong angular tension. On the contrary, high conforma-
tion flexibility was found for DPE, characterized by two
isomeric forms with two thienyl rings tilted by 24° and 38°,
respectively. The calculated energy barrier to interconversion
is 3.7 Kcal/mol, with the first isomer being more stable by
0.8 Kcal/mol. DPY resulted to be a more rigid system than
DPE, with a preferred conformation characterized by a nearly
coplanar arrangement of the two thienyl rings forming a
dihedral angle of about 17°.
found to be the most efficient pathway (Scheme 3).
The optical properties of isolated cores and oligomers,
calculated and experimental HOMO-LUMO energy values
are listed in Table 1. A marked red shift of the maximum
Table 1. Summary of Electronic Properties of Isolated Cores
and Corresponding Oligomers
λmax
λPL
Eox
Ered EHOMO ELUMO
(eV) (eV)
UV and PL spectra of oligomers 10, 11, and 14 (Figures
SI_8-SI_16, Supporting Information) show a trend parallel
to that observed for the isolated cores, indicating that the
properties of the inner core are reflected in the electronic
properties of the final oligomer. As shown in Table 1, a red
shift of both absorption and emission wavelengths was
observed for 10 and 11 with respect to 14. Noteworthy, a
marked red shift was observed also with respect to conven-
tional alkyl-ended quaterthiophenes (about 50 nm).4a,17
In agreement with the trend of λmax, the electrochemical
HOMO-LUMO energy gap (Eg)14 of 10 and 11 (Figure
b
item (nm)a
(nm)a
Eg
(V)c (V)c
DPY 356 406
DPE 327 398
DTT 291 343
3.06
3.26
3.88
-5.29d -1.38d
-5.44d -1.23d
-5.60d -1.09d
10
11
14
440 500, 527, 580 2.46 0.74 -1.86 -5.48e -2.88e
417 486, 511, 570 2.58 0.82 -2.0 -5.52e -2.7e
397 445, 468, 520 2.75 0.81 -2.2 -5.55e -2.5e
a In CH2Cl2, for PL λexcitation ) 325 nm. b Eg ) 1240/λonset c Versus
.
SCE; E°ferrocene ) 0.41V versus SCE. d DFT calculated values. e Estimated
from the peak potentials of the CVs as EHOMO ) -4.50 + e(Esce + Epa1);
ELUMO ) -4.50 + e(Esce + Epc1).18
absorption wavelength (λmax) was found for DPY and DPE
(356 and 327 nm, respectively) compared to the fully
conjugated DTT (291 nm). Although somewhat unexpected,
this result was confirmed by DFT B3LYP/6-31G* calcula-
tions on frontier energy orbitals13 (energy gaps 3.91, 4.21,
and 4.51 eV, respectively).14 It is worth noting that a similar
trend was observed for simple15 and condensed7f,16 five-,
six-, and seven-membered cyclic systems and ascribed to
the strong angular tension of the five-membered ring. In
agreement with this, our calculations showed that DTT is
Figure 1.
(a) CVs and (b) LSVs at 0.2 V s-1 of 10, 11, and 14
(10) (a) Casado, J. M.; Ruiz Delgado, C.; Rey Mercha´n, M. C.;
Herna´ndez, V.; Lo´pez Navarrete, J. T.; Pappenfus, T. M.; Williams, N.;
Stegner, W. J.; Johnson, J. C.; Edlund, A. B.; Janzen, D. E.; Mann, R. K.;
Orduna, J.; Villacampa, B. Chem. Eur. J. 2006, 12, 5458–5470. (b)
Funahashi, M.; Hanna, J. I. AdV. Mater. 2005, 17, 594–598.
(11) Single-crystal X-ray analysis on a dihexyl-ended analogue showed
an almost planar conformation. See ref 6.
(∼1 mmol L-1) in CH3CN/CH2Cl2 (1:1) with 0.1 M (C4H9)4NClO4.
1) was lower than that of 14 (2.60 and 2.8 vs 3.0 eV).19
The CVs of 11 and 14 display oxidation waves with the
maximum at 0.82 V (E° ) 0.78 V, quasi-reversible) and
0.81 V (E° ) 0.78 V, reversible), respectively. On the
(12) San Miguel, L.; Porter, W. W., III; Matzger, A. J. Org. Lett. 2007,
6, 1005–1008. (b) Frey, J.; Proemmel, S.; Armitage, M. A.; Holmes, A. B.
Org. Synth. 2006, 83, 209–216.
(13) Cartesian coordinates of all the optimized structures are reported
as Supporting Information, and energy and shape of the orbitals are shown
in the graphical abstract.
(17) Melucci, M.; Zambianchi, M.; Zanelli, A.; Camaioni, N.; Gazzano,
(14) Calculated as ELUMO - EHOMO from values in Table 1.
(15) Scott, A. I. Interpretation of the UltraViolet Spectra of Natural
Products; Pergamon Press: Oxford, 1964; pp 48-51.
M.; Bongini, A.; Barbarella, G. Chem. Phys. Chem. 2007, 8, 2621–2626.
(18) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Funda-
mentals and Applications, 2nd ed.; John Willey & Sons: Hoboken, 2001;
(16) Coppo, P.; Turner, M. J. Mater. Chem. 2005, 15, 1123–1133
.
pp 54-63. (b) Trasatti, S. Pure Appl. Chem. 1986, 58, 955–966.
Org. Lett., Vol. 10, No. 17, 2008
3667

contrary, the CV of 10 displays an irreversible oxidation
wave with the maximum at 0.74 V (E1/2 ) 0.69 V). In
the region of the negative potentials, the linear sweep
voltammetries (LSVs, Figure 1b) show the reduction wave
of 10 with the maximum at -1.86 V (E1/2 ) -1.80 V)
but allowed only an indicative assignment of reduction
potentials for 11 and 14 because the processes occur below
-1.9 V.20 Taking into account that the peak potentials
shift only about 0.05 V changing the scan rate from 0.2
to 0.02 V s-1, the data unambiguously show that the
reduction wave of 10 occurs at a potential less negative
than 11 (about 0.14 V), which in turn is reduced at a
potential less negative than 14 (about 0.2 V). Remarkably,
the lowering of the LUMO level obtained by DPY respect
to DTT insertion is comparable to that observed upon
introduction of perfluoroalkyl chains in quaterthiophene,
a functionalization leading to high n-type FET charge
mobility.21
aggregation of 10, molecular asymmetry affects the aggrega-
tion kinetics.
In agreement with the observed thin film morphologies,
the melting points of 10 and 11 were markedly lower than
that of 14 (61, 38, and 130 °C, respectively, determined by
the Koffler method). Differential scanning calorimetry (DSC)
and hot-stage POM revealed LC behavior for compound 14,
with a Schlieren textures converting to nematic droplets
above 150 °C (Figure SI_4, Supporting Information). No
multiple transitions were found for 10 and 11 in the DSC
traces independently of scan rates. However, on cooling of
the melt, hot-stage POM analysis on 10 revealed a shim-
mering birefringent texture at about 40 °C that persisted at
room temperature upon cooling (Figure SI_5, Supporting
Information). Interestingly, a similar behavior was observed
for hexynyl-hexyl end-substituted asymmetric quater-
thiophene.10b The X-ray diffraction pattern of melt-quenched
films of 10 (Figure 2b) suggests that the molecules are
arranged in parallel layers spaced by 2.12 nm. The molecular
length in the extended all-trans configuration being about
2.5 nm, the molecules must be somewhat tilted with respect
to the substrate (as in smectic C phase). Such orientation is
favorable to charge transport encouraging investigations in
melt-processed films of 10 as the active elements in FET
devices.22
Polarized optical microscopy (POM) on cast films of 14
exhibited the deposition of micrometer sized birefringent
fibrils (see Supporting Information). In the case of 10, the
growth of birefringent rods was observed only after 48 h
from the deposition (Figure 2a). On the contrary, compound
In conclusion, we have presented an unprecedented ring
bridging strategy for the tuning of HOMO-LUMO energies
in oligothiophenes. Alkylenesulfanyl bridging could be
efficiently achieved by a simple one-pot cyclization reaction,
starting from 3-alkylbromo,3′-methylen-sulfanyl bithienyl
units. Introduction of alkylsulfanyl-bridged cores into olig-
othiophenes led to an increase in electron affinity comparable
to that obtained employing electron-withdrawing substituents.
Furthermore, a simultaneous tuning of filming and thermal
properties could also be achieved. Such results may aid in
the design of innovative self-assembling, multifunctional
oligothiophenes, as modifications of the alkylsulfanyl bridge
(length, insertion of stereocenters) and of the aromatic
backbone (size, end-substitution, shape) can be envisaged.
Figure 2. Cast film of 10 from CH2Cl2: (a) optical micrograph with
crossed polars showing birefringent rods, (b) X-ray profile after
melt-quenching and proposed molecular organization.
11 showed an amorphous morphology independent of the
solvent and casting conditions employed. Clearly, the self-
organization capability of asymmetric 10 and 11 in solution-
cast films is lower than that of 14 but higher than that of
conventional alkyl-ended oligothiophenes.2a This suggests
that the π-core of 10, 11, and 14 promotes aggregation and
that the more planar the core, the higher is the π-stacking
tendency. However, as indicated by the slow rate of
Acknowledgment. This work was partially supported by
project FIRB RBNE03S7XZ_005 (SYNERGY). Thanks are
due to Cristian Bettini (Mediteknology SRL) for helpful
assistance.
Supporting Information Available: Experimental pro-
cedures and full characterization for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(19) Electrochemical Eg was 0.2-0.3 eV wider than the optical one
because it is estimated from peak potential instead of redox potentials.
(20) Under these conditions, the reduction processes appeared irrevers-
ible.
OL801261R
(21) (a) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.;
Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13480–13501.
(22) Maunoury, J. C.; Howse, J. R.; Turner, M. L. AdV. Mater. 2007,
19, 805–809.
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Org. Lett., Vol. 10, No. 17, 2008