Synthesis and Characterization of Radial Oligothiophenes: A New Class of Thiophene-Based Conjugated Homologues

Article abstract of DOI:10.1021/ol026292l

(Matrix Presented) A series of thiophene-based homologues with an aromatic core surrounded by terthiophene "arms" with acetylene linkages has been synthesized by using Sonogashira coupling methods. The homologues were investigated spectroscopically in sol

Full text of DOI:10.1021/ol026292l

ORGANIC
LETTERS
2002
Vol. 4, No. 18
3043-3046
Synthesis and Characterization of Radial
Oligothiophenes: A New Class of
Thiophene-Based Conjugated
Homologues
Ted M. Pappenfus and Kent R. Mann*
Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
mann@chem.umn.edu
Received June 3, 2002
ABSTRACT
A series of thiophene-based homologues with an aromatic core surrounded by terthiophene “arms” with acetylene linkages has been synthesized
by using Sonogashira coupling methods. The homologues were investigated spectroscopically in solution and in the solid state. They display
extended π-conjugation through the aromatic core that affects the strong emission and redox properties.
Conjugated organic oligomers and macromolecules continue
to receive considerable attention for electronic devices such
as field-effect transistors and light-emitting diodes.1 For
example, we recently reported a terthiophene-based quino-
dimethane as an n-channel conductor in a thin-film transis-
tor.2 As part of an ongoing research project to create materials
with favorable structural and optoelectronic properties, we
now report the synthesis and electronic properties of a new
class of radial oligothiophenes with oligophene “arms”
around a central aromatic core attached with alkynyl linkages.
have thoroughly demonstrated the utility of triple bonds in
a series of oligo(R-thiopheneethynylenes) and studied their
use in molecular scale electronic devices.5 Although similar
“starlike” molecules and polymers have been prepared with
thioether linkages6 and direct thiophene-aromatic core
bonds,7-10 these systems are not anticipated to support
extended π-conjugation through the core. The ethynyl
(3) (a) Diercks, R.; Armstrong, J. C.; Boese, R.; Vollhardt, K. P. C.
Angew. Chem., Int. Ed. Engl. 1986, 36, 268-269. (b) Boese, R.; Green, J.
R.; Mittendorf, J.; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1643-1645.
(4) Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489-
2496.
(5) (a) Pearson, D. L.; Jones, L., III; Schumm, J. S.; Tour, J. M. Synth.
Met. 1997, 84, 303-306. (b) Pearson, D. L.; Tour, J. M. J. Org. Chem.
1997, 62, 1376-1387. (c) Wu, R.; Schumm, J. S.; Pearson, D. L.; Tour, J.
M. J. Org. Chem. 1996, 61, 6906-6921. (d) Tour, J. M. Chem. ReV. 1996,
96, 537-553.
Vollhardt et al. first reported the synthesis of hexaethynyl-
benzene and its derivatives under high-temperature Sono-
gashira conditions.3 Also, tetraethynylthiophene has been
reported by Whitesides and Neenan.4 Tour and co-workers
(1) (a) Torsi, L.; Dodabalapur, A.; Rothberg, L. J.; Fung, A. W. P.; Katz,
H. E. Science 1996, 272, 1462-1464. (b) Horowitz, G.; Delannoy, P.;
Bouchriha, H.; Deloffre, F.; Fave, J. L.; Garnier, F.; Hajlaoui, R.; Heyman,
M.; Kouki, F.; Valat, P.; Wintgens, V.; Yassar, A. AdV. Mater. 1994, 6,
752-755.
(2) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.;
Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184-
4185.
(6) Inoue, S.; Nishiguchi, S.; Murakami, S.; Aso, Y.; Otsubo, T.; Vill,
V.; Mori, A.; Ujiie, S. J. Chem. Res. (S) 1999, 596-597.
(7) Geng, Y.; Fechtenkotter, A.; Mullen, K. J. Mater. Chem. 2001, 11,
1634-1641.
(8) Wu, I.-Y.; Lin, J. T.; Tao, Y. T.; Balasubramaniam, E. AdV. Mater.
2000, 12, 668-669.
(9) Kotha, S.; Chakraborty, K.; Brahmachary, E. Synlett 1999, 10, 1621-
1623.
10.1021/ol026292l CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/02/2002

Scheme 1a
a Reagents and conditions: (a) NBS, AcOH:CHCl3, 0 °C; (b) (trimethylsilyl)acetylene, Pd(PPh3)2Cl2, CuI, (iPr)2NH, THF, rt; (c) K2CO3,
CH2Cl2:MeOH, rt; (d) Pd(PPh3)2Cl2, CuI, (iPr)2NH, THF:toluene, reflux.
linkages between the oligothiophene arm and the benzene
core enhance conjugation by allowing the arms and core to
achieve coplanarity.
Radial oligothiophenes 5-8 were synthesized according
to the general route in Scheme 1. Bromination of the phenyl-
capped oligomer 1 with N-bromosuccinimide (NBS), fol-
Table 1. Physical Properties of Homologues 4-8
a Dichloromethane solutions. Reported values of ꢀ are not corrected for overlap with other absorption bands. b Thin films. Reported values are uncorrected
ATR values. c Dichloromethane solutions measured at room temperature with λex ) 435.8 nm. d Quantum yields measured relative to [Ru(bpy)3](PF6)2 in
acetonitrile, Φ ) 0.062.14 e Thin films on quartz slides. f Potential vs Ag/AgCl in a 0.1 M TBAPF6/CH2Cl2 solution. All E° ′ values, unless otherwise noted.
g Irreversible process Epa value provided. h Measurement precluded due to limited solubility.
3044
Org. Lett., Vol. 4, No. 18, 2002

lowed by a Sonogashira5,11 coupling with (trimethylsilyl)-
acetylene afforded the silylated oligomer 3. Protodesilylation
with potassium carbonate gave the acetylide arm 4 in
quantitative yield. To force complete substitution of the
appropriate aromatic halide, 9, 20, and 36 molar equiv of 4
were added to produce 5, 6, and 7, respectively, using
Sonogashira coupling.
The electronic spectra of the arm and radial homologues
were recorded in dichloromethane (Table 1); spectra of the
benzene-core series are shown in Figure 1. The absorption
Figure 2. Electronic absorption spectrum (-) and emission
spectrum (‚‚‚) for thin films of 7.
series for the solid and solution spectra. The series shows
surprisingly good linearity. Typically, a plot of ∆E vs 1/n
results in a linear relationship for π-conjugated oligomers
and a saturation limit for ∆E vs n is reached for a given
oligomer length.15 Clearly, this series of homologues is
unique as no hint of a saturation limit for ∆E is evident as
the number of thiophene rings is increased. To our knowl-
edge, such a relationship is unprecedented in thiophene-based
conjugated oligomers.
Figure 1. Electronic absorption spectra recorded in dichlo-
romethane for 4 (-), 5 (---), 6 (‚‚‚), and 7 ()).
maximum (λmax) for the lowest π-π* transition increases
with the number of arms around the benzene core. The trend
is consistent with a previously reported series of phenyl-
ethynyl-substituted benzene systems12 and is evidence of
extended π-conjugation through the benzene core. Homo-
logue 8 with the thiophene core exhibits a maximum
absorption at 402 nm and is substantially red-shifted relative
to the arm (374 nm), again suggesting the formation of a
delocalized π-system through the core.
Solid-state spectra of the homologues were also recorded.
Figure 2 shows the electronic absorption spectrum of a thin
film of 7 cast from a dichloromethane solution onto a cubic
zirconia ATR crystal. The spectrum is red-shifted compared
with the solution phase and exhibits low energy fine structure.
Both qualities are characteristic of solid-state spectra of
conjugated homologues and polymers.13 The large red-shift
has generally been attributed to enhanced planarity of the
molecule in the solid state. However, the origin of the fine
structure is still debatable and will not be discussed here.
Figure 3 shows plots of the lowest energy transition vs
the number of thiophene rings for the arm and benzene-core
Figure 3. Energy (∆E) of the lowest electronic transition for
solution (b) and solid (O) for 4-7 vs the number of thiophene
rings in the conjugated homologue.
The emissive properties of the homologues were also
studied in solution and in the solid state (Table 1, Figures 2
and 4). As in the case for the absorption spectra, the emission
spectra of the homologues display a distinct red-shift as the
number of thiophene rings increases. The radial homologues
exhibit good quantum efficiencies (g20%) in dichloro-
methane solution at room temperature (relative to the [Ru-
(10) (a) Cherioux, F.; Guyard, L. AdV. Func. Mater. 2001, 11, 305-
309. (b) Cherioux, F.; Maillotte, H.; Audebert, P.; Zyss, J. Chem. Commun.
1999, 2083-2084. (c) Cherioux, F.; Guyard, L.; Audebert, P. Chem.
Commun. 1998, 2225-2226.
(11) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467. (b) Takahashi, S.; Hagihara, N.; Kuroyama, Y.; Sonogashira, K.
Synthesis 1980, 627.
(14) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583-
5590.
(12) Kondo, K.; Yasuda, S.; Tohoru, S.; Miya, M. J. Chem. Soc., Chem.
Commun. 1995, 55-56.
(13) For example, see the following and references therein: Koren, A.
B.; Curtis, M. D.; Kampf, J. W. Chem. Mater. 2000, 12, 1519-1522.
(15) (a) Jestin, I.; Frere, P.; Mercier, N.; Levillain, E.; Stievenard, D.;
Roncali, J. J. Am. Chem. Soc. 1998, 120, 8150-8158. (b) Guay, J.; Kasai,
P.; Diaz, A.; Wu, R.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4, 1097-
1105.
Org. Lett., Vol. 4, No. 18, 2002
3045

Figure 5. Cyclic voltammogram of 6 (ν ) 100 mV/s) in 0.1 M
TBAPF6/CH2Cl2 solution.
Figure 4. Emission spectra of 4 (-), 5 (---), 6 (‚‚‚), 7 ()), and 8
(- ‚ -) in dichloromethane (relative intensity displayed).
processes are observed for 6 at E° ′ ) 0.87, 1.06, and 1.51
V. The large current “spike” for the return of the third process
is attributed to precipitation of the highly charged species
generated in the third anodic process on the electrode surface.
A reversible reduction (E° ′ ) -1.49 V) followed by an
irreversible reduction (Epc ) -1.81 V) is also observed for
6. The observation of reversible oxidations and reductions
in thiophene-based homologues are rare and offer attractive
prospects for device applications. These applications and
further investigations of the redox behavior of these mol-
ecules will be reported soon.
(bpy)3](PF6)2 (Φ ) 0.062) standard14). Emission spectra of
solid films are red-shifted relative to the solution spectra,
indicating the excited states of the thin films have greater
conjugation lengths than in solution.
The redox properties of the homologues were investigated
with cyclic voltammetry and Osteryoung square-wave vol-
tammetry. The TMS-protected arm 3 displays two reversible
one-electron oxidations with E° ′ ) 1.00 and 1.42 V. These
two processes correspond to the sequential formation of a
radical cation and dication. The protonated arm 4 exhibits
irreversible oxidation processes at Epa ) 1.06 and 1.56 V
and no stable oxidized species on the CV time scale are
observed.
The radial homologues 5, 6, and 8 display stable oxidized
species on the CV time scale. The redox behavior of 7 could
not be determined due to limited solubility. The potential of
the first oxidation for each homologue is listed in Table 1.
Each homologue exhibits a lower first oxidation potential
than the arm indicative of a more delocalized oxidized
species. A representative cyclic voltammogram is provided
for homologue 6 in Figure 5. Three distinct oxidation
Acknowledgment. This research was supported by the
National Science Foundation under Grant CHE-9307837. We
thank Daron Janzen for assistance with the emission data
acquisition.
Supporting Information Available: Experimental pro-
cedures and absorption and emission spectra of the homo-
logues. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL026292L
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Org. Lett., Vol. 4, No. 18, 2002