Synthesis and evaluation of 2,5-linked alternating pyridine-thiophene oligomers

Article abstract of DOI:10.1021/ol1007907

The first iterative access to alternating 2,5-linked pyridine-thiophene (Py-Th) oligomers is presented. These oligomers exhibit strong absorption and emission, even in the solid state (picture of the longest oligomer, above). Protonation leads to large red shifts in emission and, unlike most known thiophene-containing oligomers, they are readily reduced but not oxidized. These species represent a promising new class of materials for further study and potential application.

Full text of DOI:10.1021/ol1007907

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2598-2601
Synthesis and Evaluation of 2,5-Linked
Alternating Pyridine-Thiophene
Oligomers
Silvia V. Rocha and Nathaniel S. Finney*
Institute of Organic Chemistry, UniVersity of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
finney@oci.uzh.ch
Received April 7, 2010
ABSTRACT
The first iterative access to alternating 2,5-linked pyridine-thiophene (Py-Th) oligomers is presented. These oligomers exhibit strong absorption
and emission, even in the solid state (picture of the longest oligomer, above). Protonation leads to large red shifts in emission and, unlike
most known thiophene-containing oligomers, they are readily reduced but not oxidized. These species represent a promising new class of
materials for further study and potential application.
Polythiophenes and oligothiophenes are among the most
intensively studied classes of organic electronic materials,
having found application in, e.g., light-emitting diodes
(LEDs) and solar cells.1,2 This is in large part the result of
their facile synthesis and the ease with which their optical,
electronic, and material properties can be tuned by structural
modification. Over time, two notable strategic trends have
emerged: an increasing emphasis on the synthesis and study
of thiophene oligomers of defined structure and the incor-
poration of electron-deficient aromatic components into the
electron-rich oligothiophenes.
more laborious (and thus costly) relative to polymer syn-
thesis, these objectives can be realized via the controlled
synthesis of oligomers. The second trend, the donor-acceptor
(D-A) approach to electronic materials,3 grew out of a desire
to reduce the band gap of poly- and oligothiophenes, which
is influenced by the donor HOMO and the acceptor LUMO
energies. Oligomeric alkylthiophenes are in widespread use,
and there are now numerous examples of thiophene-
containing D-A oligomers.4
Our interests began with the observation that, despite being
the archetypal electron-deficient heterocycle, pyridine has
been conspicuous in its absence in the development of D-A
oligomers.5–8 We report here the first controlled, iterative
The first trend is driven by the need for materials with
uniform purity, regiochemistry, and properties. While it is
(1) For recent monographs, see: (a) Gyo¨rgy, I. Conducting Poly-
mers;Springer: Berlin, 2008. (b) Petty, M. C. Molecular Electronics; Wiley:
(3) For representative reviews, see: (a) Roncali, J. Chem. ReV. 1997,
97, 173–206. (b) Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga,
E. E.; Meijer, E. W. Mat. Sci. Eng. 2001, 32, 1–40. (c) Meijer, H. Angew.
Chem., Int. Ed. 2005, 44, 2482–2506.
Chichester, 2008
.
(2) For representative reviews, see:(a) Mishra, A.; Ma, C.-Q.; Ba¨uerle,
P. Chem. ReV. 2009, 109, 1141–1276. (b) Osaka, I.; McCullough, R. D.
Acc. Chem. Res. 2008, 41, 1202–1214. (c) Perepichka, I. F.; Perepichka,
D.; Meng, H.; Wudl, F. AdV. Mater. 2005, 17, 2281–2305. (d) Groenendaal,
L.; Jonas, F.; Freitag, D.; Pielartzik, H.; R. Reynolds, J. R. AdV. Mater.
(4) For representative examples, see: (a) Karsten, B. P.; Janssen, R. A. J.
Org. Lett. 2008, 10, 3513–3516. (b) Facchetti, A.; Yoon, M.-H.; Stern, C. L.;
Katz, H. E.; Marks, T. J. Angew. Chem., Int. Ed. 2003, 42, 3900–3903. (c)
van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chem.sEur.
J. 1998, 4, 1235–1243. See also refs 1–3.
2000, 12, 481–494
.
10.1021/ol1007907  2010 American Chemical Society
Published on Web 05/04/2010

synthesis of 2,5-linked pyridine-thiophene oligomers and
show that they have remarkable optical and electrochemical
properties which make them a promising new class of
electronic materials. Of particular note are long absorption
and emission wavelengths, especially in protonated form,
solid-state luminescence, and facile electrochemical reduc-
tion.
Scheme 1. Synthesis of Two-Ring Precursors
Beginning with 3-butylthiophene, two-ring precursors were
synthesized:9 25a was prepared by regioselective lithiation,10
transmetalation with zinc chloride, and Negishi coupling with
2-tert-butoxy-5-bromopyripine (Scheme 1)11,12 in the pres-
ence of an N-heterocyclic carbene ligand;13 deprotection and
sulfonylation provided the corresponding triflate 25aTf.14
Four-, six- and eight-ring oligomers (25b-d) were prepared
by efficient homologation with 25aTf via iterative lithiation/
Negishi coupling (Scheme 2).
Scheme 2. Iterative Oligomer Synthesis
The oligomers show a steady increase in λmax of absorption
and emission with chain length (Table 1, Figure 1), ac-
companied by a rise in quantum yield. The longest, 25d, is
strongly absorbent in the visible region and is more emissive
than comparable D-A oligothiophenes of similar length:5
its quantum yield and extinction coefficient approach those
of benchmark fluorophores such as fluorescein or rhodamine
6G.18
An additional attribute of pyridine-thiophene oligomers
is that each pyridine provides a well-defined site for
(5) Since the inception of this work, there have been reports describing
pyridine-thiophene and pyrimidine-thiophene oligomers. See: (a) Fukumoto,
H.; Kumagai, A.; Fujiwara, Y.; Koinuma, H.; Yamamoto, T. Heterocycles
2006, 68, 1349-1357. (b) Kumagai, A.; Fukumoto, H.; Yamamoto, T. J.
Phys. Chem. B 2007, 111, 8020–8026. (c) Yue, W.; Tian, H.; Hu, N.; Geng,
Y.; Wang, F. Cryst. Growth Des. 2008, 8, 2352–2358. (d) Chevallier, F.;
Charlot, M.; Katan, C.; Mongin, F.; Blanchard-Desce, M. Chem. Commun.
2009, 692–694. (e) Ortiz, R. P.; Casado, J.; Hernandez, V.; Navarrete,
J. T. L.; Letizia, J. A.; Ratner, M. A.; Facchetti, A.; Marks, T. J. Chem.sEur.
J. 2009, 15, 5023–5039.
(6) For conducting and/or electroluminescent polymers containing
pyridine groups, see : (a) Zhou, Z.; Maruyama, T.; Kanbara, T.; Ikeda, T.;
Ichimura, K.; Yamamoto, T.; Tokuda, K. Chem. Commun. 1991, 1210–
1212. (b) Yamamoto, T.; Maruyama, J. T.; Zhou, Z.; Ito, T.; Fukuda, T.;
Yoneda, Y.; Begum, F.; Ikeda, T.; Sasaki, S.; Takezoe, H.; Fukuda, A.;
Kubotall, K. J. Am. Chem. Soc. 1994, 116, 4832–4845. (c) Monkman, A. P.;
Halim, M.; Samuel, I. D. W.; Horsburgh, L. E. J. Chem. Phys. 1998, 109,
10372–10378. (d) Meghdadi, F.; Leising, G.; Wang, Y. Z.; Gebler, D. D.;
Swager, T. M.; Epstein, A. I. Synth. Met. 1999, 102, 1085–1086. (e) Feller,
F.; Monkman, A. P. Synth. Met. 2001, 116, 149–152. (f) Eichen, Y.;
Nakhmanovich, G.; Gorelik, V.; Epshtein, O.; Poplawski, J. M.; Ehrenfre-
und, E. J. Am. Chem. Soc. 1998, 120, 10463–10470. (g) Wang, C.;
Kilitziraki, M.; McBride, J. A. H.; Bryce, M. R.; Horsburgh, L. E.; Sheridan,
A. K.; Monkman, A. P.; Samuel, I. D. AdV. Mater. 2000, 12, 217–222.
(7) For polymers containing both pyridine and thiophene rings, see: (a)
Yamamoto, T.; Zhou, Z.; Kanbara, T.; Kizu, K.; Tsukasa Maruyama, T.;
Yoshiyuki Nakamura, Y.; Lee, B.-L.; Ooba, N.; Tomaru, S.; Kurihara, T.;
Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem. Soc. 1996, 118, 10389–
10399. (b) Irvin, D. J.; DuBois, C. J., Jr.; Reynolds, J. R. Synth. Met. 2001,
119, 321–322. (c) DuBois, C. J., Jr.; Larmat, F.; Irvin, D. J.; Reynolds,
J. R. Synth. Met. 2001, 119, 321. (d) DuBois, C. J.; Reynolds, J. R. AdV.
Mater. 2002, 14, 1844–1846. (e) Jenkins, I. H.; Salzner, U.; Pickup, P. G.
Chem. Mater. 1996, 8, 2444–2450.
postsynthetic modulation of optical properties. Indeed, we
find that protonation of the oligomers with trifluoroacetic
acid (TFA) leads to significant red shifts in emission (Table
1, Figure 2).15,16 This bodes well for further manipulating
optical and electronic properties by alkylation or Lewis acid
coordination.17
(11) Negishi, E.; Zeng, X.; Tan, Z.; Qian, M.; Hu, Q.; Huang, Z. In
Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; De Meijere, A.,
Diederich, F., Eds.; Weinheim: Wiley-VCH, 2004; pp 815-889.
(12) Wang, P.-S.; Liang, C.-K.; Leung, M. Tetrahedron 2005, 61, 2931–
2939.
(8) We have found 2,6-biarylpyridines to be useful fluorophores in the
context of chemosensor development. See: (a) Mello, J. V.; Finney, N. S.
Angew. Chem., Int. Ed. 2001, 40, 1536–1538. (b) Fang, A. G.; Mello, J. V.;
Finney, N. S. Org. Lett. 2003, 5, 967–970. (c) Fang, A. G.; Mello, J. V.;
Finney, N. S. Tetrahedron 2004, 60, 11075–11087. (d) Mello, J. V.; Finney,
N. S. J. Am. Chem. Soc. 2005, 127, 10124–10125.
(13) Jafarpour, L.; Stevens, E. D.; Nolan, S. P. Organomet. Chem. 2000,
606, 49–54.
(14) For aryl methyl ether cleavage with Py·HCl, see: Loren, J. C.;
Gantzel, P.; Linden, A.; Siegel, J. S. Org. Biomol. Chem. 2005, 3, 3105–
3116.
(15) We have previously seen similar protonation effects in 2,6-
biarylpyridines. See ref 8.
(9) See the Supporting Informationfor complete experimental and
spectroscopic details.
(10) For preparation of 3-butylthiophene, see: McCullough, R. D.; Lowe,
R. D.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58, 904–912.
For regioselective lithiation, see: Smith, K.; Barratt, M. L. J. Org. Chem.
2007, 72, 1031–1034.
(16) The red-shift in emission is accompanied by a ca. 3-fold decrease
in emission intensity. The emission response to added TFA is instantaneous.
However, 1H NMR studies show conversion of protonated oligomers to
the corresponding pyridols (via loss of the tert-butyl group) over the course
of several hours.
Org. Lett., Vol. 12, No. 11, 2010
2599

conjugates.2 They are waxy solids and are readily soluble
in most organic solvents, allowing them to be spin-coated
onto inorganic substrates such as glass. We observe red-shifts
in emission for 25c (λmax ) 500 nm) and 25d (λmax ) 535
nm) in thin films on glass (Figure 4),9 consistent with
aggregation or increased planarization in the solid state. The
absence of self-quenching makes these and related oligomers
attractive candidates for incorporation into light-emitting
devices.
Table 1. Optical Properties of 25a-da
ε/105
M-1 cm-1
λmax(abs)/
nm -/+ TFAc
λmax(em)/
b
nm -/+ TFAc, d
φc, d
25a
25b
25c
25d
290/295
363/390
394/407
410/422
360/441
438/500
483/528
501/560
0.05
0.60
0.69
0.74
4.1
7.0
a Values for longest wavelength λmax b In CHCl3. c In CH3CN. d Quantum
.
yields for neutral species relative to PPO (φ ) 0.94 in cyclohexane) for
25a, DPA (φ ) 1, in cyclohexane) for 25b-d. PPO ) 2,5-diphenyloxazole,
9,10-diphenylanthracene.
Figure 2. Absorbance and emission of 25a-d with added TFA.
Figure 1. Absorbance and emission of 25a-d.
Plots of absorption and emission energy vs 1/n are linear
(n ) number of PyTh units, Figure 3)9,19 for both neutral
and protonated forms of the oligomers. This indicates that
the maximum conjugation length has not yet been reached,
and longer oligomers are expected to have even longer
wavelength absorption and emission maxima. (For example,
the 16-ring compound is extrapolated to emit at 560/590 nm
in neutral/protonated form.)
Beyond strong solution-phase emission, 25b-d are visibly
luminescent in the solid state, which distinguishes them from
typical oligothiophenes and the few known pyridine-thiophene
Figure 3. Plot of 1/n vs absorption or emission energy for 25a-d
(n ) no. of Py-Th units).
(17) For a recent example in which Lewis acid coordination to a nitrogen
atom significantly alters the band gap of a D-A oligomer, see: Welch, G. C.;
Coffin, R.; Peet, J.; Bazan, G. C. J. Am. Chem. Soc. 2009, 131, 10802–
Finally, cyclic voltammetry reveals that while these
oligomers cannot be oxidized (in THF), they are electron
acceptors. The two-ring compound, 25a, shows a single
reduction at Ec ) -3.4 V vs Fc/Fc+ (-2.9 V vs Ag/AgCl).9
The number of electrons accepted by the oligomers increases
with chain length, and 25d can accept up to five electrons
10803
.
(18) Fluorescein in EtOH: ε ) 95000 M-1 cm-1; φ ) 1.0. Rhodamine
6G in EtOH: ε ) 116000 M-1 cm-1; φ ) 0.95. Data obtained from the
PhotochemCAD database. See : Du, H.; Fuh, R. A.; Li, J.; Corkan, A.;
Lindsey, J. S. Photochem. Photobiol. 1998, 68, 141–142
.
(19) Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379
.
2600
Org. Lett., Vol. 12, No. 11, 2010

over the range -2.1 to -3.3 V (Figure 5). Compounds 25b
and 25c show intermediate behavior.
tert-butyl radical from highly reduced intermediates, and this
issue will need to be addressed by the preparation of
oligomers with nonlabile terminating groups. However, this
electrochemical behavior still differentiates these oligomers
from the majority of reported D-A oligomers, including
those containing pyridine and thiophene rings, which are
readily oxidized but not reduced.5a-c,20
In conclusion, we have reported the concise iterative
synthesis of a new class of D-A oligomers based on
thiophene and pyridine units. The longer oligomers show
strong absorption and emission, which can be tuned by
protonation of the pyridine units. In addition, they remain
emissive in the solid state, and undergo ready electrochemical
reductions. These oligomers thus have a range of potential
applications, such as use as electron-transporting or elec-
troluminescent materials. Ongoing work includes further
solid-state characterization of 25d and its analogues, synthesis
of longer oligomers, and evaluation of electroluminescence,
as well as computational modeling.
Figure 4
.
Solid-state emission of 25c,d.
Acknowledgment. We thank Ylenia Maniglio (EMPA;
Swiss Federal Laboratories for Materials and Research) for
solid-state emission measurements, Dr. Matthias Nagel
(EMPA) for assistance with CV measurements, and Drs.
Roland Hany and Frank Nu¨esch (EMPA) and Dr. Richard
Muller (Sandia National Laboratories) for helpful discussions.
We thank the University of Zurich and the SNF for financial
support.
Supporting Information Available: Experimental pro-
cedures; characterization of all new compounds; cyclic
voltammetry details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 5
.
Reductive portion of cyclic voltammograms for 25b-d.
(1 mM in THF, 0.1 M Bu4NClO4, scan rate 50 mV s-1, glassy
carbon working electrode, Pt wire counter electrode, Ag/AgCl
reference electrode, ferrocene (Fc) internal reference.)
OL1007907
(20) Oligothiophenes capped on both ends by fluoroalkyl, fluorocarbonyl,
or fluorobenzene groups are representative exceptions. For examples, see:
Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem.
Soc. 2005, 127, 1348–1349. (b) Yoon, M.-H.; DiBenedetto, S. A.; Russell,
M. T.; Facchetti, A.; Marks, T. J. Chem. Mater. 2007, 19, 4864–4881. For
a pyrimidine-thiophene conjugate that can undergo two-electron reduction,
see ref 5e.
While the CV measurements were conducted in freshly
distilled, anhydrous THF, the reductions of 25c and 25d are
not fully reversible. Potential complicating factors include
the possibility of protonation by residual H2O and loss of
Org. Lett., Vol. 12, No. 11, 2010
2601