optical (NLO) materials,10 organic field effect transistors
(OFETs),5 and photoluminescence.3a
Scheme 1. Synthesis of Tetraethynyl OPV Building Block 10
In this contribution, a new class of skewed, H-shaped
π-conjugated co-oligomers based on linear OPE and OPV
fragments were synthesized and investigated. These oligo-
mers, termed “H-mers” due to their unique H-shaped
π-topology as illustrated in the graphical abstract, possess
distinct π-electron delocalization patterns with three linear
conjugation paths a, b and c, where path c (highlighted bold)
represents the longest linear conjugation route. Paths d, e,
and f, indicated by dashed lines, are cross-conjugated, along
which weak electronic communications may be induced as
well.11 The design criteria of the H-mer structure were aimed
to create a series of new 2D π-fluorophore cores whose
electronic and photonic behavior can be flexibly manipulated
or finely tuned by means of chemical functionalization with
various electroactive and chromophoric groups.
Synthetic routes to an OPE/OPV H-mer 11a and its donor/
acceptor (D/A) functionalized derivatives 11b-d have been
successfully developed. In Scheme 1, the synthesis of a
tetraethynylated OPV building block 10 essential for the
construction of H-mers is described. The synthesis began
with bromination of 2-aminobenzoic acid (1), leading to
2-amino-5-bromobenzoic acid (2). Compound 2 underwent
diazotization followed by treatment with KI to afford
5-bromo-2-iodobenzoic acid (3), which was then subjected
to a Fischer esterification with MeOH in the presence of
H2SO4, yielding methyl ester 4. Compound 4 was reduced
into benzyl alchohol 5 by diisobutylaluminum hydride
(DIBAL). Oxidation of 5 with pyridinium dichromate (PDC)
resulted in 5-bromo-2-iodobenzaldehyde (6) in a high yield.
Compound 6 underwent a Wittig-Horner reaction with the
phosphonate ylide that was generated by treating 7 with NaH,
affording dibromodiiodo-OPV percursor 8. Sonogashira
coupling of 8 with trimethylsilylacetylene (TMSA) under the
catalysis of Pd/Cu gave compound 9, which was further
desilylated with K2CO3 to afford the desired tetraethynyl
OPV 10 in an almost quantitative yield.
The facile preparation of 10 allowed the construction of
symmetrically functionalized H-mers to be readily performed
via a Sonogashira cross-coupling protocol. As shown in
Scheme 2, a basic H-mer scaffold 11a, a donor (D)-
functionalized H-mer 11b, and an acceptor (A)-functionalized
H-mer 11c could be obtained in decent yields through the
cross-coupling reactions.
The synthesis of a D-A-substituted, electron “push-pull”
H-mer 11d, however, required a different strategy in which
an unsymmetrically silyl-protected precursor 14a was neces-
sary. Compound 14a was prepared via a sequence of
Sonogashira coupling and Wittig-Horner reactions as out-
lined in Scheme 3. Selective removal of the TMS groups in
14a with K2CO3 afforded bis-terminal alkyne 14b, which
was subjected to Sonogashira coupling with methyl 2-iodo-
benzoate to yield compound 15a. Desilylation of 15a with
TBAF followed by Sonogashira coupling with 4-iodo-N,N-
dimethylaniline employing PdCl2(PPh3)2/CuI as catalyst and
piperidine as base furnished D-A substituted H-mer 11d in
a satisfactory yield.
The electron-donating and -accepting groups appended to
the phenyl termini of the H-mer backbone were anticipated
to significantly modulate π-electronic characteristics.12 The
substitution effects on structures and electron density of the
H-mers 11a-d13 were evaluated by ab initio calculations (HF/
3-21G) using Spartan’06.14 The computational results are
quite revealing in terms of the sensitivity of FMOs to the
electronic nature of the substituents; D and A functionalities
are found to result in different energetics and distributions
of FMOs (see Figure 1a and the Supporting Information).
The calculated HOMO-LUMO gaps of four H-mers are 11a
(5) Sun, X.; Liu, Y.; Chen, S.; Qiu, W.; Yu, G.; Ma, Y.; Qi, T.; Zhang,
H.; Xu, X.; Zhu, D. AdV. Funct. Mater. 2006, 16, 917.
(6) (a) Bilge, A.; Zen, A.; Forster, M.; Li, H.; Galbrecht, F.; Nehls,
B. S.; Farrell, T.; Neher, D.; Scherf, U. J. Mater. Chem. 2006, 16, 3177.
(b) Zen, A.; Bilge, A.; Galbrecht, F.; Alle, R.; Meerholz, K.; Grenzer, J.;
Neher, D.; Scherf, U.; Farrell, T. J. Am. Chem. Soc. 2006, 128, 3914.
(7) (a) Hauck, M.; Scho¨nhaber, J.; Zucchero, A. J.; Hardcastle, K. I.;
Mu¨ller, T. J. J.; Bunz, U. H. F. J. Org. Chem. 2007, 72, 6714. (b) McGrier,
P. L.; Solntsev, K. M.; Scho¨nhaber, J.; Brombosz, S. M.; Tolbert, L. M.;
Bunz, U. H. F. Chem. Commun. 2007, n/a, 2127. (c) Zucchero, A. J.; Wilson,
J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 11872. (d) Gerhardt,
W. W.; Zucchero, A. J.; Wilson, J. N.; South, C. R.; Bunz, U. H. F.; Weck,
M. Chem. Commun. 2006, n/a, 2141. (e) Wilson, J. N.; Bunz, U. H. F.
J. Am. Chem. Soc. 2005, 127, 4124. (f) Zhou, N.; Wang, L.; Thompson,
D. W.; Zhao, Y. Tetrahedron Lett. 2007, 48, 3563.
(8) (a) Miao, Q.; Chi, X.; Xiao, S.; Zeis, R.; Lefenfeld, M.; Siegrist, T.;
Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 1340. (b)
Tolosa, J.; D´ıez-Barra, E.; Sa´nchez-Verdu´, P.; Rodr´ıguez-Lo´pez, J. Tetra-
hedron Lett. 2006, 47, 4647. (c) Wang, H.-Y.; Wan, J.-H.; Jiang, H.-J.;
Wen, G.-A.; Feng, J.-C.; Zhang, Z.-J.; Peng, B.; Huang, W.; Wei, W. J.
Polym. Sci.: Part A: Polym. Chem. 2007, 45, 1066.
(9) Grunder, S.; Huber, R.; Horhoiu, V.; Gonza´lez, M. T.; Scho¨nen-
berger, C.; Calame, M.; Mayor, M. J. Org. Chem. 2007, 72, 8337.
(10) (a) Yi, Y.; Zhu, L.; Shuai, Z. Macromol. Theory Simul. 2008, 17,
12. (b) Slepkov, A. D.; Hegmann, F. A.; Tykwinski, R. R.; Kamada, K.;
Ohta, K.; Marsden, J. A.; Spitler, E. L.; Miller, J. J.; Haley, M. M. Opt.
Lett. 2006, 31, 3315.
(12) Meier, H. Angew. Chem., Int. Ed. 2005, 44, 2482.
(13) The NMe2 and COOMe groups of 11b-d were replaced with NH2
and COOH groups in the calculations to reduce computational expense.
Details are given in the Supporting Information.
(11) (a) Gholami, M.; Tykwinski, R. R. Chem. ReV. 2006, 106, 4997.
(b) Zhao, Y.; Tykwinski, R. R. J. Am. Chem. Soc. 1999, 121, 458.
(14) Spartan’06 software, Wavefunction Inc., Irvine, CA.
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