Fluorescent cyclopentadithiophene derivatives having phenyl-substituted silyl moieties

Article abstract of DOI:10.1016/j.tetlet.2011.05.128

4,4-Dimethylcyclopenta[2,1-b:3,4-b′]dithiophene derivatives bearing trimethyl-, dimethylphenyl-, diphenylmethyl-, or triphenyl-silyl moieties were synthesized. The introduction of the silyl moieties onto cyclopenta[2,1-b:3,4- b′]dithiophene induced fluorescent emission as well as the bathochromic shift of wavelength at the maximum absorption and fluorescence. It was found that the larger number of phenyl group on silyl moiety resulted in the higher fluorescence quantum yield.

Full text of DOI:10.1016/j.tetlet.2011.05.128

Tetrahedron Letters 52 (2011) 4039–4041
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fluorescent cyclopentadithiophene derivatives having phenyl-substituted
silyl moieties
⇑
Hitoshi Hanamura, Ryoko Haneishi, Nobukatsu Nemoto
Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, Tamura-machi, Koriyama, Fukushima 963-8642, Japan
a r t i c l e i n f o
a b s t r a c t
4,4-Dimethylcyclopenta[2,1-b:3,4-b0]dithiophene derivatives bearing trimethyl-, dimethylphenyl-,
diphenylmethyl-, or triphenyl-silyl moieties were synthesized. The introduction of the silyl moieties onto
cyclopenta[2,1-b:3,4-b0]dithiophene induced fluorescent emission as well as the bathochromic shift of
wavelength at the maximum absorption and fluorescence. It was found that the larger number of phenyl
group on silyl moiety resulted in the higher fluorescence quantum yield.
Article history:
Received 28 April 2011
Revised 20 May 2011
Accepted 27 May 2011
Available online 2 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Cyclopentadithiophene
Fluorescence
Substituent effect
Silyl group
Phenyl group
Conjugated polymers1 obtained by polymerization of fused ring
derivatives have emerged as attractive materials for flexible, low
cost and low power electro-optic devices. Polyfluorenes are repre-
sentatives of conjugated polymers which are suitable for applica-
tions in electronics and optoelectronics, such as organic light
emitting diodes (OLEDs);2 while thiophene-based polymers and
oligomers, including fused ring bithiophene derivatives3 such as dit-
hienothiophene4 derivatives, exhibit efficient charge transport in
organic field-effect transistors (OFETs).5 On the other hand, cyclo-
penta[2,1-b:3,4-b0]dithiophene (CPDT)6 is composed of a fused ring
bithiophene derivative and regarded as analogous structure of fluo-
rene where the benzene rings are replaced by thiophene rings. CPDT
derivatives have been rigid precursors of polymeric semiconducting
materials for the development of organic photovoltaic devices;7
however, there have been few reports on the fluorescent properties
of CPDT derivatives or conjugated polymers based on CPDT.8
In the meantime, the incorporation of silyl substituent onto aro-
matic species has resulted in the high fluorescence quantum yield,9
as the photochemistry of organosilicon compounds has been re-
ported in the recent review.10 In addition, Karatsu et al.11 reported
the fluorescent properties of silyl substituted anthracene deriva-
tives and revealed that the introduction of bulky phenyl group onto
the silyl moieties induces the high fluorescence quantum yield as
well as the pure-blue electroluminescence.
ties investigated by absorption and emission spectroscopy as well
as quantum calculation using density functional theory (DFT)
method. It will be revealed that the introduction of plural phenyl
groups onto the silyl moieties remarkably increases the fluores-
cence quantum yield.
Scheme 1 describes the synthesis of CPDT derivatives having silyl
substituents (CPDT1–CPDT4) from 2,6-dibromo-4,4-dimethylcy-
clopenta[2,1-b:3,4-b0]dithiophene (1), which was prepared by the
modified method of literature (see Scheme S1 in the Supplementary
data).6a,b
Namely, an addition of chlorosilane reagent having different
substituents after the lithiation reaction of 1 using n-butyllithium
in the presence of N,N,N0,N0-tetramethylethylenediamine (TMEDA)
in diethyl ether afforded CPDT1–CPDT4 with moderate yields (see
Experimental details described in the Supplementary data).
From these points of view, we describe here the synthesis of
CPDT derivatives having silyl substituents and their optical proper-
Scheme 1. Synthesis of 4,4-dimethylcyclopenta[2,1-b:3,4-b0]dithiophene deriva-
tives having silyl substituents.
⇑
Corresponding author. Tel./fax: +81 24 956 8812.
E-mail address: nemoto@chem.ce.nihon-u.ac.jp (N. Nemoto).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.128

4040
H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
Table 1
Next the optical properties of CPDT1–CPDT4 were investigated.
Optical properties of CPDT derivatives
Figure 1 shows the absorption spectra of CPDT1–CPDT4 and 4,4-
dimethylcyclopenta[2,1-b:3,4-b0]dithiophene (DMCPDT).
Table 1 summarizes the optical properties of CPDT1–CPDT4 and
DMCPDT.
a
Compound
kabs/nm (
e
/L molꢀ1 cmꢀ1
)
kem/nm
UF
DMCPDT
CPDT1
CPDT2
CPDT3
CPDT4
319 (15000) 329 (11800)
334 (22000) 345 (17500)
337 (26600) 349 (20900)
341 (29900) 352 (23900)
344 (31500) 355 (26100)
378
382
385
387
389
0.01
0.03
0.08
0.23
0.70
In the absorption spectra of CPDT1–CPDT4, bathochromic and
hyperchromic effects were observed by the introduction of silyl
groups onto CPDT skeletons, presumably because of
r–p –
and r⁄
p⁄ conjugations between the silyl groups and aromatic moieties.9,10
The bathochromic effect has been known to be induced by lowering
the energy gap between the highest occupied molecular orbital
(HOMO)and lowest unoccupiedmolecularorbital(LUMO) states be-
a
Fluorescence quantum yields (UFs) were determined by using pyrene (UF
:
0.19)9e as a standard in CHCl3.
cause of both destabilization of HOMO state through
r-p conjuga-
tion and stabilization of LUMO state through r⁄ p⁄ conjugation.
–
The hyperchromic effects would be caused by the enhancement of
transition moment based on the increase in the dipole moments of
the HOMO and LUMO states owing to the
HOMO and the r⁄ p⁄ conjugation in the LUMO. In addition, the
bathochromic shift of the maximum absorption and the increase
of the molar extinction coefficient ( ) were observed by the intro-
r–p conjugation in the
–
e
duction of phenyl group onto the silyl moieties. Namely, the intro-
duction of phenyl group onto silyl moieties was also revealed to
induce the bathochromic and hyperchromic effects.
We calculated the HOMO and LUMO energy levels of CPDT1–
CPDT4 and DMCPDT with density functional theory (DFT) method
at the B3LYP/6-31G(d) level of theory12 for the confirmation of the
effects of the introduction of silyl moieties on the absorption spec-
tra. Figure 2 depicts the energy diagrams for the HOMO and LUMO
energy levels as well as the energy gap between LUMO and HOMO.
The difference in the LUMO energy level between DMCPDT
(ꢀ1.01 eV) and CPDT1 (ꢀ1.16 eV) and that in the HOMO energy le-
vel between DMCPDT (ꢀ5.18 eV) and CPDT1 (ꢀ5.11 eV) means
Figure 2. Energy diagrams of CPDT derivatives. Calculated using DFT method at the
B3LYP/6-31G(d) level.
that stabilization of LUMO state through r⁄ p⁄ conjugation and
destabilization of HOMO state through r–p conjugation are in-
–
duced by the introduction of trimethylsilyl moieties. Thus, the
introduction of trimethylsilyl moieties caused the decrease in the
energy gap between LUMO and HOMO, resulting in the bathochro-
mic shift of the maximum absorption. The replacement of methyl
group by phenyl one on silyl moieties was found to decrease both
HOMO and LUMO energy levels as shown in Figure 2. The degree of
decrease of LUMO energy level was rather higher than that of
HOMO energy level with increase in the number of phenyl group
on silyl moieties, inducing the rather bathochromic shift of the
maximum absorption with increase in the number of phenyl group
on silyl moieties.
Figure 3 depicts the fluorescence spectra of CPDT1–CPDT4 and
DMCPDT in CHCl3 at ambient temperature.
The wavelength at maximum emission (kem) and fluorescence
quantum yield (UF) of CPDT1–CPDT4 and DMCPDT are also sum-
marized in Table 1. In the fluorescence spectra of CPDT1–CPDT4,
bathochromic effects were observed by the introduction of silyl
groups onto CPDT skeletons. The UFs of unsubsitituted CPDT
(DMCPDT) and CPDT1 were 0.01 and 0.03, respectively, indicating
that the introduction of trimethylsilyl moieties onto CPDT skeleton
rather increased UF. Namely, the fluorescent ability could be
Figure 1. Absorption spectra of CPDT derivatives in CHCl3 at ambient temperature
(Conc.: 2.0 ꢁ 10ꢀ6 mol/L).
Figure 3. Fluorescence spectra of CPDT derivatives in CHCl3 at ambient temper-
ature (kex: 317 nm, conc.: 2.0 ꢁ 10ꢀ6 mol/L).

H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
4041
attached by the introduction of silyl moieties onto CPDT skeleton,
Supplementary data
which exhibits very low fluorescent ability.
In addition, the UF of CPDT2 was 0.08 and 2.7 times as large as
that of CPDT1, indicating that the introduction of a phenyl group
onto silyl moieties drastically increase UF. It should be still remark-
able that the increase in the number of phenyl group on silyl moi-
eties enhances the fluorescent efficiency, as it was observed that
the UFs of CPDT3 and CPDT4 were 0.23 and 0.70, respectively. Fur-
ther detailed analysis of photochemical processes including the
determination of the rate constants as radiative rate constant,
intersystem crossing rate constant, and non-radiative rate constant
would be necessary for the clarification of the reason for the in-
crease in UF; however, the plausible reasons for the increase in
UF are as follows.10 In the case of the present CPDT1–CPDT4, the
larger number of phenyl groups on the silyl moieties resulted in
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.05.128.
References and notes
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In summary, we achieved four cyclopenta[2,1-b:3,4-b0]dithi-
ophene derivatives having silyl moieties. The introduction of the
silyl moieties onto cyclopenta[2,1-b:3,4-b0]dithiophene induced
the bathochromic shift of wavelength at the maximum absorption
and fluorescence because of both destabilization of HOMO state
through
–
r–p conjugation and stabilization of LUMO state through
r⁄
p⁄ conjugation. It should be remarkable that the increase in the
10. Karatsu, T. J. Photochem. Photobiol. C: Photochem. Rev. 2008, 9, 111.
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efficiency, and the highest UF in the present series of CPDT deriv-
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Acknowledgments
The authors would like to appreciate Ms. Satoko Tokiwa and Ms.
Nami Sugashima, Nihon University College of Engineering World-
wide Research Center for Advanced Engineering and Technology
(NEWCAT) for performing NMR measurements and Dr. Yoshio Sai-
to, Associate Professor of Nihon University, for performing HR-MS
measurements.