Effect of Ring Fusion on the Electronic Absorption and Emission Properties of Oligothiophenes

Article abstract of DOI:10.1021/jo035241e

A series of fused-ring oligothiophenes were synthesized by a combination of Stille and oxidative coupling reactions. Compounds with the same number of double bonds, but varying in extent of planarization, display a similar longest wavelength absorption maximum in solution. However, the introduction of sulfur linkages into these oligothiophenes leads to a blue shift of the maximum emission wavelength and a correspondingly smaller Stokes shift.

Full text of DOI:10.1021/jo035241e

Effect of Rin g F u sion on th e Electr on ic
Absor p tion a n d Em ission P r op er ties of
Oligoth iop h en es
Xinnan Zhang and Adam J . Matzger*
Department of Chemistry and the Macromolecular Science
and Engineering Program, The University of Michigan,
Ann Arbor, Michigan 48109-1055
matzger@umich.edu
F IGURE 1. Chemical structures of thieno[3,2-b]thiophene (1),
dithieno[3,2-b:2′,3′-d]thiophene (2), oligothiophenes (n ) 3,
R-terthiophene, 3; n ) 4, R-quaterthiophene, 4; n ) 6,
R-sexithiophene), linear acenes (n ) 5, pentacene), and
thienoacenes.
Received August 25, 2003
Abstr a ct: A series of fused-ring oligothiophenes were
synthesized by a combination of Stille and oxidative coupling
reactions. Compounds with the same number of double
bonds, but varying in extent of planarization, display a
similar longest wavelength absorption maximum in solution.
However, the introduction of sulfur linkages into these
oligothiophenes leads to a blue shift of the maximum
emission wavelength and a correspondingly smaller Stokes
shift.
Oligothiophenes have received much attention as
organic materials due to their excellent electronic and
1
optical properties. R-Sexithiophene (Figure 1), the R-linked
hexamer of thiophene, and its derivatives have been
successfully employed as active components in organic
field-effect transistors2
-4
and light-emitting devices.5
Compared to pentacene, an important small molecule
semiconductor that is rigidly planar, oligothiophenes can
F IGURE 2. Chemical structures of fused-ring oligothiophenes
used to explore the influence of planarization on the electronic
absorption and emission spectra of thiophene materials.
6
easily twist from planarity, thus disrupting conjugation
7
and potentially affecting band gap in the solid state. An
intriguing approach for the design of conjugated small
molecules is to combine the stability of the thiophene ring
units.10 In addition, the R-linked dimer of dithieno[3,2-
b:2′,3′-d]thiophene (2) and its alkylated derivatives have
with the planarity of linear acenes to produce thienoacenes
proven to be effective as the active layer in organic thin
8
(
Figure 1). As of yet, only isolated examples of conju-
11,12
film transistors.
5
Higher thienoacenes with n ) 4 and
gated oligomers containing fused-ring thiophenes have
been reported, and therefore, clear trends in the effect
of this structural perturbation on the electronic proper-
ties of oligothiophenes have not yet emerged. The syn-
thesis and characterization of oligomers of 3,6-dimeth-
have also been reported, and in the case of the former
a crystal structure has been obtained which indicates
13
planarity of these fused-ring systems. However, perhaps
because of the lack of convenient synthetic routes, devices
14
utilizing these compounds have not been studied.
9
ylthieno[3,2-b]thiophene has been accomplished. However,
To systematically explore the effect of increasing
degree of ring fusion on the inherent electronic properties
of R-oligothiophenes, we have synthesized a series of
oligomers in which two or more of the thiophene rings
are replaced by thieno[3,2-b]thiophene (1) and dithieno-
the steric repulsion between the â-methyl groups pre-
vents coplanarity of adjacent thieno[3,2-b]thiophene
*
Corresponding author.
1) M u¨ llen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim; New York, 1998.
2) Horowitz, G.; Fichou, D.; Peng, X. Z.; Xu, Z. G.; Garnier, F. Solid
State Commun. 1989, 72, 381-384.
3) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002,
4, 99-117.
4) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270-
71.
5) Uchiyama, K.; Akimichi, H.; Hotta, S.; Noge, H.; Sakaki, H.
Synth. Met. 1994, 63, 57-59.
6) Holmes, D.; Kumaraswamy, S.; Matzger, A. J .; Vollhardt, K. P.
C. Chem. Eur. J . 1999, 5, 3399-3412.
7) DiC e´ sare, N.; Belletete, M.; Donat-Bouillud, A.; Leclerc, M.;
Durocher, G. Macromolecules 1998, 31, 6289-6296.
(
[3,2-b:2′,3′-d]thiophene (2) units (Figure 2). These com-
(
pounds, in combination with the fully planar pen-
14,15
tathienoacene
(9), provide the first well-defined series
(
1
(
(10) For a more recent example of a â-substituted oligomer contain-
ing a dithieno[3,2-b:2′,3′-d]thiophene unit and its conversion to the S,S-
dioxide, see: Sotgiu, G.; Zambianchi, M.; Barbarella, G.; Aruffo, F.;
Cipriani, F.; Ventola, A. J . Org. Chem. 2003, 68, 1512-1520.
(11) Li, X. C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti,
S. C.; Feeder, N.; Clegg, W.; Teat, S. J .; Friend, R. H. J . Am. Chem.
Soc. 1998, 120, 2206-2207.
2
(
(
(
(12) Morrison, J . J .; Murray, M. M.; Li, X. C.; Holmes, A. B.;
Morratti, S. C.; Friend, R. H.; Sirringhaus, H. Synth. Met. 1999, 102,
987-988.
(
8) A related approach to a mixed thiophene-pyrrole fused-ring
system, N-functionalized dithieno[3,2-b:2′,3′-d]pyrroles, has recently
been demonstrated: Ogawa, K.; Rasmussen, S. C. J . Org. Chem. 2003,
(13) Mazaki, Y.; Kobayashi, K. J . Chem. Soc., Perkin Trans. 2 1992,
761-764.
6
8, 2921-2928.
9) Nakayama, J .; Dong, H. B.; Sawada, K.; Ishii, A.; Kumakura, S.
Tetrahedron 1996, 52, 471-488.
(
(14) Mazaki, Y.; Kobayashi, K. Tetrahedron Lett. 1989, 30, 3315-
3318.
1
0.1021/jo035241e CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/15/2003
J . Org. Chem. 2003, 68, 9813-9815
9813

TABLE 1. Lon gest Wa velen gth Electr on ic Absor p tion
a n d Em ission Ma xim a of a Ser ies of Oligoth iop h en es in
CH2Cl2 Solu tion
F IGURE 3. Chemical structures of methylene-bridged bithio-
phene 10 and a double-bridged terthiophene 11.
were purified by one or more sublimations until they were
greater than 99% pure by gas chromatography. R-Qua-
terthiophene (4)2
0,21
was used as a reference oligomer for
the spectroscopic studies described below.
The optical data of the oligothiophenes22 and appropri-
ate reference compounds are summarized in Table 1.
Oligomers 3, 7, 8, and 9 all have six double bonds but
differ in the number and position of sulfur linkages. The
nonfused oligomer 323 displays a λmax in the absorption
spectrum at 354 nm. It is generally recognized that
increasing the planarity of aromatic systems leads to
more extensive conjugation and a corresponding decrease
in the HOMO-LOMO gap. Such a red shift in wavelength
absorption of a planarized methylene-bridged bithiophene
2
4
25
1
0, and a double-bridged terthiophene 11 (Figure 3)
has been reported.Theλmax ofmethylene-bridged bithiophene
0 is red-shifted (311 nm) compared to R-bithiophene (302
nm). Terthiophene 11 displays a bathochromic shift
∆λmax ) 22 nm) when compared to the unconstrained
1
(
model compound 3.
The above arguments suggest that oligomers 7 and 8,
which have more sulfur linkages and increased planarity,
will display spectra bathochromically shifted relative to
in which to probe the effect of ring fusion on the electronic
spectra of oligothiophenes. These results provide a basis
for the design of new materials with specified electronic
properties.
3
. In fact, these oligomers display longest wavelength
absorption maximum values very close to 3: 352 and 350
nm, respectively (Figure 4). Furthermore, the completely
fused-ring compound 9 is also remarkably unperturbed
relative to the unconstrained reference compound (3) with
an absorption at 357 nm (Figure 4). This trend is also
Synthesis of the planarized oligomers was achieved by
a modular approach using a combination of cross-coupling
strategies and oxidative couplings (see the Supporting
Information). The key building blocks, thieno[3,2-b]-
thiophene16 (1) and dithieno[3,2-b:2′,3′-d]thiophene (2),
were synthesized according to the literature procedures
starting from 3-bromothiophene. The oligomers 5-7 were
synthesized by Stille coupling of appropriate stannyl and
17
11
evident in the R-linked dimer of 2, which has an equal
number of double bonds as R-quaterthiophene (4) and
displays an absorption maximum at the same wavelength
(390 nm). The insensitivity of the position of the absorp-
bromo compounds catalyzed by Pd(PPh
3 4
) and CuI.
tion maximum to the extent of ring fusion likely results
from a cancellation of effects. Planarization is acting to
narrow the HOMO-LUMO gap while the increased
sulfur substitution is widening it. Because the LUMO of
R-terthiophene has the appropriate symmetry for inter-
acting with the sulfur atoms introduced at the positions
in 9, this effect is presumed to result from a destabiliza-
tion of this orbital. A related effect is also evident in
thiophenol, which has a π-π* absorption band at 238
Compound 5 has been reported previously; however, no
1
8
spectroscopic data was measured. One of the greatest
challenges was the purification of the oligomers because
homocoupling of the stannyl and bromo compounds
occurred as side reactions in all syntheses. R-Bithieno-
[3,2-b]thiophene (8) was prepared by an oxidative coup-
ling of the lithiated 1. The choice of metal proved critical
in obtaining the product free of impurities. Employing
CuCl
inated19 1 in approximately 30% yield as evidenced by
GC-MS. Substitution of Fe(acac) as oxidative coupling
reagent avoids halogenation products.19 Compounds 5-8
2
as coupling reagent led to the formation of chlor-
3
(
20) Kamal, M. R.; Al-Taweel, S. A.; El-Abadelah, M. M.; Ajaj, K.
M. Phosphorus Sulfur Silicon Relat. Elem. 1997, 126, 65-74.
21) Horowitz, G.; Deloffre, F.; Garnier, F.; Hajlaoui, R.; Hmyene,
(
M.; Yassar, A. Synth. Met. 1993, 54, 435-445.
(
15) Sato, N.; Mazaki, Y.; Kobayashi, K.; Kobayashi, T. J . Chem.
Soc., Perkin Trans. 2 1992, 765-770.
16) Fuller, L. S.; Iddon, B.; Smith, K. A. J . Chem. Soc., Perkin
(22) The solubility of all oligomers in common organic solvents was
2 2
low. Generally, in benzene and CH Cl , oligomer 7 is the most soluble,
followed by 8, 6, 5 and finally the nonfused oligomer R-quaterthiophene
(4) is the least soluble.
(
Trans. 1 1997, 3465-3470.
(
(
17) J ong, F. D.; J anssen, M. J . J . Org. Chem. 1971, 36, 1645-1648.
18) Blazy, F.; Bonastre, J .; Pfisterg. G Bull. Soc. Chim. Fr. 1968,
(23) Garcia, P.; Pernaut, J . M.; Hapiot, P.; Wintgens, V.; Valat, P.;
Garnier, F.; Delabouglise, D. J . Phys. Chem. 1993, 97, 513-516.
(24) Amer, A.; Burkhardt, A.; Nkansah, A.; Shabana, R.; Galal, A.;
Mark, H. B.; Zimmer, H. Phosphorus Sulfur Silicon Relat. Elem. 1989,
42, 63-71.
4
247-4250.
19) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.;
Servet, B.; Ries, S.; Alnot, P. J . Am. Chem. Soc. 1993, 115, 8716-
721.
(
8
(25) Roncali, J .; Thobiegautier, C. Adv. Mater. 1994, 6, 846-848.
9
814 J . Org. Chem., Vol. 68, No. 25, 2003

of double bonds, dramatic changes are observed in the
emission spectra even for molecules differing only in the
extent of ring fusion. Although compounds 3, 7, 8, and 9
have similar absorptions, the emission maxima span
nearly 50 nm (Figure 4). Oligomer 3, which has three
thiophene rings, displays the longest wavelength emis-
sion maximum of the series: 433 nm. The introduction
of one sulfur linkage leads to ca. 20 nm blue shift in
emission. Fused-ring oligomers 7 and 8 show emission
maxima at 414 and 412 nm respectively. Compound 9,
which has one more sulfur linkage, displays the shortest
emission λmax of the series (388 nm). This trend indicates
that for compounds with an equal number of double
bonds, increasing the number of sulfur linkages leads to
a substantial blue shift of the emission maximum. This
dramatic difference in Stokes shifts can be explained by
noting that a higher degree of ring fusion increases the
rigidity in the oligomers, thus reducing the degrees of
freedom and the conformational reorganization in the
excited state. A closer geometric match between the
ground and excited-state results in a smaller Stokes shift.
Although isomers 7 and 8 show similar emission
maxima at 414 and 412 nm, a significant difference is
observed between isomers 5 and 6 (439 nm, 453 nm). This
effect is consistent with a greater degree of conjugation
for the bithiophene-containing oligomer, 6, in the excited
state. Introducing another double bond and removing all
ring fusion yields 4 and causes an additional bathochro-
mic shift in the emission spectrum (482 nm).
F IGURE 4. Comparison of UV-vis absorption and emission
spectra of oligothiophenes with an equal number of double
bonds in CH
2
Cl
2
solution.
In summary, a series of oligothiophenes were synthe-
sized that differ in the extent of ring fusion. The optical
data indicate that with the same number of double bonds
in a molecule, the introduction of sulfur linkages into
oligothiophenes leads to a minor shift in the electronic
absorption spectra and a dramatic blue shift of the
emission maximum. Increasing the number of double
bonds leads to a red shift of both absorption and emission.
Understanding these trends is critical for the design of
new emissive materials in which fused thiophene rings
nm,26 a value substantially shorter than that of benzene
(252 nm). An additional trend evident in the spectra of
the oligomers with six double bonds, that is not apparent
in the position of the longest wavelength absorption
maximum, is that increasing rigidity along the series 3
to 7, 8 to 9 results in a narrowing of the absorption
envelope and a general blue shift of the profile. Com-
pound 3 has a long wavelength absorption tail associated
with transitions occurring from a planar conformation.
The general broadness of the band suggests important
contributions from absorption of nonplanar conformers.
In contrast, 9 is rigidly planar and therefore has an
absorption profile that is more narrow with a clearly
observable vibronic structure. Partially planarized oli-
gomers 7 and 8 display intermediate broadening.
In oligomers 5 and 6, the number of double bonds
increases to seven and a correspondingly longer wave-
length absorption is observed (371 and 370 nm, respec-
tively). R-Quaterthiophene (4), which has eight double
bonds, has a λmax in the absorption spectrum at 390 nm.
Significantly, these data show that both fused and
nonfused ring oligothiophenes with an equal number of
double bonds have similar longest wavelength absorption
maxima. As expected, an increase in the number of
double bonds leads to a red shift of the absorption
spectrum.
27
replace the simple R-linked thiophenes. Efforts are
underway to determine the effect of ring fusion on the
crystal packing of these oligomers and to produce longer
chain fused-ring oligothiophenes with diminished HOMO-
LUMO gaps for device applications.
Ack n ow led gm en t. This work was supported by
start-up funds from the University of Michigan, the
donors of the Petroleum Research Fund, administered
by the American Chemical Society, and a 3M Nonten-
ured Faculty Award. We thank J ames P. J ohnson for
providing the fluorescence spectrum and a sample of 9.
Su p p or tin g In for m a tion Ava ila ble: Synthetic procedure
and spectral data for compounds 5-8. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O035241E
Although the longest wavelength absorption maximum
in this oligomer series depends primarily on the number
(27) The thieno[3,2-b]thiophene-based copolymer poly(9,9′-dioctylflu-
orene-alt-thieno[3,2-b]thiophene) was recently reported to exhibit
better electroluminescence performance than the corresponding
bithiophene-based copolymers. Lim, E.; J ung, B.; Shim, H. Macromol-
ecules 2003, 36, 4288-4293.
(
26) Tumanov, A. A.; Sennikov, P. G.; Egorochkin, A. N.; Kosolapova,
I. G.; Skobeleva, S. E.; Chumakov, L. V.; Bodrikov, I. V. Zh. Obshch.
Khim. 1983, 53, 766-771.
J . Org. Chem, Vol. 68, No. 25, 2003 9815