Electronic tuning of thiazolyl-capped π-conjugated compounds via a coordination/cyclization protocol with B(C6F5)3

Article abstract of DOI:10.1021/ol102282x

Electronic tuning of thiazolyl-capped π-conjugated systems via a coordination/cyclization protocol with B(C₆F₅)₃ effectively enhances an electron-accepting character giving rise to lower reduction potentials and increases

Full text of DOI:10.1021/ol102282x

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5470-5473
Electronic Tuning of Thiazolyl-Capped
π-Conjugated Compounds via a
Coordination/Cyclization Protocol with
B(C6F5)3
Andre Job,† Atsushi Wakamiya,*,‡,§ Gerald Kehr,† Gerhard Erker,*,† and
Shigehiro Yamaguchi*,‡
Department of Chemistry, Graduate School of Science, Nagoya UniVersity,
Furo, Chikusa, Nagoya 464-8602, Japan, and Organisch-Chemisches Institut,
UniVersita¨t Mu¨nster, Corrensstrasse 40, D-48149 Mu¨nster, Germany
wakamiya@scl.kyoto-u.ac.jp; erker@uni-muenster.de; yamaguchi@chem.nagoya-u.ac.jp
Received September 28, 2010
ABSTRACT
Electronic tuning of thiazolyl-capped π-conjugated systems via a coordination/cyclization protocol with B(C6F5)3 effectively enhances an electron-
accepting character giving rise to lower reduction potentials and increases thermal stability.
π-Conjugated materials are key components for organic
electronics. In their molecular designs, the crucial issues are
the modification of the electronic structure at will as well as
the control of the solid-state structure to gain appropriate
intermolecular interactions.1 In particular, the former issue
is of importance to attain the required color emission, to
facilitate the charge injection from the electrodes, or to
achieve ambipolar carrier transport. Various methodologies
for electronic tuning have been proposed. For example,
Marks and co-workers demonstrated that the introduction of
perfluorophenyl groups to the termini of an oligothiophene
skeleton switched its semiconducting properties from p-type
to n-type.2 Consequently, they succeeded in the synthesis
of efficient n-type organic semiconductors for organic thin-
film transistors. Recently, Bazan and co-workers also re-
ported the electronic tuning of N-heteroaryl-containing
π-conjugated skeletons based on the complexation with
Lewis acids, such as B(C6F5)3.3 They found that this
modification is effective in band gap control of the ben-
zothiadiazole-containing conjugated oligomers.
In the molecular design of new π-electron materials,
another important point is to employ an inherently superior
skeleton. In this regard, thiazolyl-containing π-conjugated
frameworks have several advantages as candidates for n-type
† Universita¨t Mu¨nster.
‡ Nagoya University.
§ Present address: Institute for Chemical Research, Kyoto University,
Uji, Kyoto 611-0011, Japan.
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J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (b)
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10.1021/ol102282x  2010 American Chemical Society
Published on Web 11/03/2010

semiconductors.4-8 They exhibit relatively high electron-
accepting properties, in contrast to the electron-donating
thiophene analogue, facile functionalization for further
modification,4 and formation of close π stacking in the solid
state.5-7 For the modification of this skeleton, we now
applied a reaction recently reported by the authors,9 as shown
in Scheme 1. The treatment of the imidazole ring with the
Scheme 2
Scheme 1
intramolecular nucleophilic aromatic substitution at the ortho
C-F moiety of one of the C6F5 groups led to the cyclized
products 3a and 3b in 73% and 48% yield, respectively. The
obtained compounds 3 have good solubility in common
organic solvents, such as chloroform and THF.
highly Lewis acidic borane B(C6F5)3 forms a B-N adduct,
which is further treated with a strong base, such as meth-
yllithium, to form a cyclized skeleton. We envisioned that
the synergistic effects of the resulting intramolecular B-N
coordination10,11 and the extension of the π conjugation with
the C6F4 ring would effectively reduce the LUMO level and
enhance the electron-accepting properties. We here report
this “coordination/cyclization” protocol as an effective
pathway to modify thiazolyl-capped π-conjugated skeletons.
The synthetic outline is shown in Scheme 2. As the core
moiety for the thiazolyl-capped π-conjugated skeletons, we
chose two kinds of representative skeletons, fluorene and
bithiophene. Using dithiazolylfluorene 1a and dithiazolyl-
bithiophene 1b as a precursor, the treatment of these
compounds with B(C6F5)3 in toluene produced the
thiazolyl-B(C6F5)3 complexes 2a and 2b in good yield of
78% and 83%, respectively. Subsequently, the obtained
compounds were reacted with LDA in toluene/THF (10/1)
at room temperature followed by heating at 60 °C. After
lithiation at the 2-position of the thiazolyl ring, the subsequent
The thermal stability of the products was investigated by
thermogravimetric analysis. While complexes 2a and 2b have
a low decomposition temperature with a 5% weight loss (Td5)
at 225 and 233 °C, respectively, both cyclized products 3a
and 3b show high thermal stability with Td5 at 387 and 376
°C, respectively. These results indicate that the (C6F5)2BC6F4
cyclization significantly increases the thermal stability. We
were able to sublimate 3b around 350 °C at 0.1 mmHg to
give single crystals suitable for X-ray structure analysis
(Figure 1).
(4) (a) Jenkins, I. H.; Pickup, P. G. Macromolecules 1993, 26, 4450.
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2003, 36, 7986
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S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 14996. (b) Mamada, M.;
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19, 5404. Yamashita, Y. Chem. Lett. 2009, 38, 870
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Erker, G.; Mu¨ck-Lichtenfeld, C.; Grimme, S. Eur. J. Inorg. Chem. 2002,
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Figure 1. Crystal structure of 3b: (a) ORTEP drawing with 50%
probability for thermal ellipsoids and (b) packing structure (per-
fluorophenyl groups on the boron atom are omitted for clarity).
Ed. 2006, 45, 3170
.
(11) (a) Cheng, C.-C.; Yu, W.-S.; Chou, P.-T.; Peng, S.-M.; Lee
G.-H.; Wu, P.-C.; Song, Y.-H.; Chi, Y. Chem. Commun. 2003, 2628. (b)
Klappa, J. J.; Geers, S. A.; Schmidtke, S. J.; MacManus-Spencer, L. A.;
McNeill, K. Dalton Trans. 2004, 883. (c) Liu, Q.-D.; Mudadu, M. S.;
Thummel, R.; Tao, Y.; Wang, S. AdV. Funct. Mater. 2005, 15, 143. (d)
Yoshino, J.; Kano, N.; Kawashima, T. Chem. Commun. 2007, 559. (e) Zhao,
For gaining a large degree of electronic coupling and thus
a high carrier mobility, the π-stacking motif with a large
Q.; Zhang, H.; Wakamiya, A.; Yamaguchi, S. Synthesis 2009, 127
.
Org. Lett., Vol. 12, No. 23, 2010
5471

overlap of π-orbitals with the neighboring molecules is
crucial.12 In the structure of 3b, while the thiophene and
thiazole ring is slightly twisted with a dihedral angle of 37°,
the inner bithiophene unit and the terminal boron-bridged
perfluorophenylthiazoyl units have planar conformation with
dihedral angles of 0.4° and 7.3°, respectively. Overall, the
dihedral angle between the central thiophene plane and the
terminal C6F4 ring plane is 38.9° (Figure 1a). In the packing
structure, despite the bulky (C6F5)2B moiety, 3b forms a
π-stacked one-dimensional array (Figure 1b). The closest
intramolecular distance between the thiazolyl moiety and the
terminal C6F4 moiety is 3.53 Å (C6···C11).
We investigated the photophysical properties of two series
of compounds, 1a-3a and 1b-3b. Figure 2 shows the
UV-vis absorption and fluorescence spectra of 1a-3a in
THF as a representative example. The data for all compounds
are summarized in Table 1. Both series show a similar trend
to each other. In the absorption spectra, while the complex-
ation with B(C6F5)3 only results in subtle red shifts in the
absorption maximum wavelength (λmax), the cyclization
results in significant red shifts in the λmax, together with the
significant increase of the molar absorption coefficients
compared to those of 1 and 2. In the fluorescence spectra,
while the (C6F5)3B adducts 2 also show slight red shifts in
the emission maxima compared to 1, the cyclized products
3a and 3b show blue and yellow emissions with significantly
red-shifted emission maxima by 63 and 72 nm compared to
those of 2a and 2b, respectively. While the fluorene
derivative 3a has a smaller quantum yield than the non-
modified 1a, the bithiophene derivative 3b maintains a
comparable quantum yield with that of 1b.
Table 1. Photophysical Data for 1a-3a and 1b-3b in THF
absorption
fluorescence
τs kr
[ns] [108 s-1
a
b
c
λabs
ε
λem
[nm]
ΦF
knr
compd [nm] [M-1 cm-1
]
]
[108 s-1
]
1a
2a
3a
1b
2b
3b
346
352
399
381
388
432
48200
46900
59700
32500
33200
41400
375 0.82 1.2
413 0.56 1.1
476 0.40 1.0
442 0.18 0.48
476 0.13 0.52
548 0.15 0.89
6.8
5.1
4.0
3.8
2.5
1.6
1.5
4.0
6.0
17.1
16.7
9.5
a Only the longest absorption maxima are shown. b Emission maxima
upon excitation at the absorption maximum wavelengths. c Absolute
quantum yield determined by a calibrated integrating sphere system (within
errors of (3%).
tion). While the parent compound 1a shows almost no change
both in absorption and fluorescence spectra, (C6F5)3B-
coordinated and (C6F5)2BC6F4-cyclized derivatives 2a and
3a show red shifts in the fluorescence spectra with increasing
polarity of the solvent. According to the Lippert-Mataga
plot,13 the dipole moment changes from the ground state to
the excited state are 19.6 and 20.3 D for 2a and 3a,
respectively. These results demonstrated that both the
(C6F5)3B coordination and (C6F5)2BC6F4 cyclization enhance
the intramolecular charge transfer character in the excited
state to a comparable extent.
To elucidate the electronic effects of the present modifica-
tions on the photophysical properties, DFT calculations were
conducted on model compounds 1a′-3a′, which are 9,9-
dimethylfluorene derivatives of 1a-3a (Figure 3). According
to the TD-DFT calculations, the observed longest absorption
bands in 1a-3a are assignable to the π-π* transitions. In
the parent compound 1a′, the Kohn-Sham (KS) HOMO and
LUMO are delocalized over the whole π skeleton with the
potentials of -5.62 and -1.78 eV, respectively. The (C6F5)3B
coodination in 2a′ decreases not only the LUMO level by
-1.10 eV but also the HOMO level to a similar extent. As
a result, the transition energies for 1a′ and 2a′ are comparable
to each other, despite the significant differences in the
HOMO and LUMO energy levels. In contrast, in 3a′, while
the (C6F5)2BC6F4 cyclization further decreases the LUMO
level, the HOMO level is increased due to the extension of
the π conjugation with the additional C6F4 rings. Conse-
quently, compound 3a shows the significantly red-shifted
absorption band. This coordination/cyclization protocol also
makes the π orbital and π* orbital more localized on the
inner fluorene π-conjugated moiety and the terminal B-N-
coordinated thiazole moieties, respectively. This increases a
To gain deeper insight into the excited state dynamics,
we measured the time-resolved fluorescence spectra. We
determined the fluorescence lifetimes for all compounds and
calculated the radiative (kr) and nonradiative (knr) decay rate
constants from the singlet excited state, based on the
equations of kr ) ΦF/τs and knr ) (1 - ΦF)/τs. Comparison
of these data demonstrated that the (C6F5)3B coordination/
cyclization protocol tends to slightly decrease the kr values,
whereas 1a-3a and 1b-3b show a different trend in the knr
values from each other.
We also examined the solvent effects in the fluorescence
spectra for the fluorene series 1a-3a (Supporting Informa-
(12) (a) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bre´das, J.-L. AdV. Mater.
2001, 13, 1053. (b) Kim, E.-G.; Coropceanu, V.; Gruhn, N. E.; Sa´nchez-
Carrera, R. S.; Snoeberger, R.; Matzger, A. J.; Be´das, J.-L. J. Am. Chem.
Soc. 2007, 129, 13072.
(13) Although the Lippert-Mataga equation originally assumes the
presence of a molecular dipole, it has been demonstrated to be applicable
to several quadrupole extended π-conjugated molecules. See: (a) Strehmel,
B.; Sarker, A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.; Neckers, D. C.
J. Am. Chem. Soc. 1999, 121, 1226. (b) Krebs, F. C.; Spanggaard, H. J.
Org. Chem. 2002, 67, 7185. (c) Zhao, C.-H.; Sakuda, E.; Wakamiya, A.;
Yamaguchi, S. Chem.-Eur. J. 2009, 15, 10603.
Figure 2. Photophysical properties of 1a-3a: (a) absorption and
(b) fluorescence spectra in THF.
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Org. Lett., Vol. 12, No. 23, 2010

Figure 4. Cyclic voltammograms of (a) 1a-3a and (b) 1b-3b in
THF, measured with [n-Bu4N][PF6] (0.1 M) as a supporting
electrolyte at a scan rate of 100 mV s-1
.
Figure 3. Plot of the Kohn-Sham HOMO and LUMO energy
structure is also effective in stabilizing the produced anionic
species.
levels for 1a′-3a′ and pictorial presentations of HOMOs and
LUMOs of 1a′ and 3a′. The π-π* transition energies and oscillator
strengths are calculated by TD DFT at the B3LYP/6-31G(d) level.
In summary, we have demonstrated that the (C6F5)3B
coordination/cyclization protocol is effective for tuning the
electronic structure of thiazoyl-capped π-conjugated com-
pounds. The electron affinity can be increased by the
(C6F5)2BC6F4 cyclization. This modification also endows
charge transfer character in their photophysical properties.
It is also worth noting that (C6F5)2BC6F4 cyclization is
effective for increasing the thermal stability. Further study
of the development of new n-type semiconducting materials
using this protocol is now in progress.
quadrupolar acceptor-donor-acceptor character, resulting
in the more significant intramolecular charge transfer char-
acter in the excited state in 2a and 3a.
How the present modifications affect the electrochemical
properties is also an important issue in this chemistry. We
carried out the measurements of cyclic voltammetry in THF to
determine the reduction potentials (Figure 4). While the parent
compounds 1a and 1b only show irreversible reduction waves
with the peak potentials (Epc) at -2.86 and -2.44 V (vs a
ferrocene/ferrocenium couple), respectively, the (C6F5)2BC6F4-
cyclized 3a and 3b show two-step reduction waves at more
positive potentials Epc ) -1.81 V (E1/2 ) -1.74 V) and -1.96
V for 3a and -1.70 V (E1/2 ) -1.61 V) and -1.89 V for
3b, with increased reversibility for the first reduction waves.
These results well demonstrate the effect of the (C6F5)2BC6F4
cyclization for lowering the LUMO level. On the basis of the
observed first reduction potentials, the LUMO levels of 3a and
3b are estimated to be 3.1 and 3.2 eV, respectively.14 These
values are sufficiently low to facilitate electron injection from
the cathode in electron-transporting devices.1a,b For the (C6F5)3B-
coordinated 2a and 2b, we observed only irreversible
reduction waves around -2.3 and -2.1 V, respectively,
indicating their instability under the measurement condition.
These results demonstrate that the (C6F5)2BC6F4-cyclized
Acknowledgment. This work was supported by the IRTG
program of Mu¨nster University-Nagoya University and
Grants-in-Aid (No. 19685004 and 19675001) from the
Ministry of Education, Culture, Sports, Science, and Tech-
nology, Japan.
Supporting Information Available: Experimental details,
thermogravimetric analysis, photophysical data, results of
theoretical calculations, and crystallographic data in CIF
format. These materials are available free of charge via the
Internet at http://pubs.acs.org.
OL102282X
(14) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Ba¨ssler,
H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551. The LUMO levels were
estimated based on the fact that the reduction potential (E1/2) of ferrocene
corresponds to 4.80 eV.
Org. Lett., Vol. 12, No. 23, 2010
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