Efficien t Isom er -P u r e Syn th esis of a
Ben zo[b]th iop h en e An a logu e of
P en ta cen e†
,‡
§
|
Brigitte Wex, Bilal R. Kaafarani, and
Douglas C. Neckers*,§
Center for Photochemical Sciences, Bowling Green State
University, Bowling Green, Ohio 43403, and School of
Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, Georgia 30332
F IGURE 1. General structure of polyacene (1) and polybenzo-
b]thiacene (2).
[
Received December 3, 2003
Abstr a ct: Isomer-pure thieno[3,2-f:4,5-f ′]bis[1]benzothio-
phene (2, n ) 2) has been synthesized in an efficient four
step approach without column chromatography.
F IGURE 2. Structures of [5]heterohelicenes 3 and 4.
isomeric thiophene analogues of pentacene. In future
work, we will demonstrate that this unit provides a
synthon for target polyacenes.
Thiophene and other fused sulfur containing com-
pounds make up to 10-30% of the aromatic fraction of
1
crude oil. The unique reactivity and electronic properties
Benzo[1,2-b:5,4-b′]dithiophene (2, n ) 1) has been
7
of thiophene derivatives have led to many applications,
synthesized. Both benzodithiophene isomers were ob-
2
particularly as oligomers and polymers. Polyacenes (1)
tained using a series of organolithium reactions to
generate dithienylmethane, followed by a formylation and
acid-catalyzed cyclization to generate the central arene
moiety. The electronic properties of only the syn-anti
have recently gained attention as charge-transport
materials in both theoretical and experimental studies.3
Current synthetic procedures, however, are limited.
Heptacene (1, n ) 7) is the largest reported, having been
8
isomer, benzo[1,2-b:4,5-b′]dithiophene, and derivatives
4
9
last synthesized in 1952. Despite interesting electronic
thereof have been extensively studied. An improvement
properties, polyacenes have major drawbacks. For all
of the overall yield of this synthetic scheme resulted when
hydrobromic acid (HBr) was replaced with polyphos-
phoric acid (PPA) as the acid catalyst in the cyclization
reaction.10 The main drawbacks of PPA remain the high
practical purposes, hexacene (1, n ) 6) is the largest
5
stable polyacene, since oxidative stability and solubility
in common organic solvents decrease with increasing
length of the polyacene.
1
1
viscosity, the poor solubility, and the subsequent neu-
tralization step.
Our approach to the problem involves the development
of analogues of polyacenes having a benzo[b]thiophene
repeat unit (Figure 1). In this paper, we demonstrate an
Previously prepared [5]heterohelicenes 312 and 4 (Fig-
ure 2) exhibit nonlinear optical properties and optical
6
13
efficient synthetic approach for 2 (n ) 2), one of the
activity. These compounds were prepared by oxidative
1
4
photolysis of the 1,2-dithienylethenes.
†
15
Contribution No. 521 from the Center for Photochemical Sciences.
This paper is dedicated to Professor Dr. J . W. Neckers on the
Laquindanum et al. reported substituted anthra-
dithiophenes (ADT). These compounds showed signifi-
‡
occasion of his 102nd birthday.
§
Bowling Green State University.
Georgia Institute of Technology.
|
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(
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0.1021/jo035769j CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/14/2004
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