Excited-state property of 1-(4-cyanophenyl)-2-(4-methoxyphenyl)-1,1,2,2- tetramethyldisilane

Article abstract of DOI:10.1246/cl.2007.1168

Diphenyldisilane derivative having electron-donating and -withdrawing groups in each phenyl group, 1-(4-cyanophenyl)-2-(4-methoxyphenyl)-1,1,2,2- tetramethyldisilane was synthesized, and its excited-state property was studied. Intramolecular charge-transf

Full text of DOI:10.1246/cl.2007.1168

1168
Chemistry Letters Vol.36, No.9 (2007)
Excited-state Property of 1-(4-Cyanophenyl)-2-(4-methoxyphenyl)-1,1,2,2-tetramethyldisilane
Hiroshi Hiratsuka,ꢀ1 Hiroaki Horiuchi,1 Yuki Takanoha,1 Hiroshi Matsumoto,1 Takeo Yoshihara,1
Tetsuo Okutsu,1 Keisuke Negishi,2 Soichiro Kyushin,1 and Hideyuki Matsumoto1
1Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu 376-8515
2Gunma University Advanced Technology Center, Kiryu 376-8515
(Received June 6, 2007; CL-070613; E-mail: hiratuka@chem-bio.gunma-u.ac.jp)
Diphenyldisilane derivative having electron-donating and
-withdrawing groups in each phenyl group, 1-(4-cyanophenyl)-
2-(4-methoxyphenyl)-1,1,2,2-tetramethyldisilane was synthe-
sized, and its excited-state property was studied. Intramolecular
charge-transfer character through the disilanyl group was con-
firmed; the dipole moment of the excited-state has been
estimated to be 28 debye.
in acetonitrile at ca. 270 and 240 nm with molar absorption
coefficient of 10900 and 23800 Mꢁ1 cmꢁ1, respectively. The
absorption spectrum observed in cyclohexane is very similar to
that in acetonitrile, indicating that there is no significant solvent
effect on the absorption process. This absorption spectrum can
not be reproduced by the sum of the spectra of anisole, benzoni-
trile, and hexamethyldisilane, suggesting that there is some inter-
action between the donor and acceptor moieties. The spectrum
can be explained by the calculated results shown by the stick
spectrum in Figure 1d, which were obtained by TD-DFT ap-
proach using B3LYP/6-31G(d,p) for the optimized structure ob-
tained by DFT using B3LYP/6-31G(d,p). The intense first and
It is well known that pentamethylphenyldisilane and its
derivatives exhibit dual fluorescence: the fluorescence from the
locally excited state in phenyl ring (LE fluorescence) and the in-
tramolecular charge-transfer excited state (ICT fluorescence).1–6
Its charge-transfer nature was intensively discussed by Shizuka
et al.,1–3 and Sakurai et al.,4–6 and in the case of (4-cyanophenyl-
ethynyl)pentamethyldisilane it was concluded that the charge
was transferred from the disilanyl group to 4-cyanophenyl
group.7 Dipole moments in the excited state of pentamethyl-
phenyldisilane derivatives were estimated to be ca. 5–12 debye.3
Diphenyldisilane derivatives having electron-donating or
-withdrawing groups in each phenyl group have been studied
as one of functional materials with the nonlinear optical property
because of their intramolecular charge-transfer (ICT) character
of the excited state. Mignani et al. synthesized this kind of di-
phenyldisilane derivatives to study their nonlinear optical prop-
erty and examined electronic absorption spectra and frequency-
independent second-order polarizability.8 Hutten and Hadziioan-
nou also studied electronic absorption spectra and hyperpolariz-
ability of diphenyldisilanes having the dimethylamino donor and
perfluoroalkylsulfonyl acceptor in comparison with the results
obtained by MO calculations.9 However, they did not study
the excited-state properties such as fluorescence property and
excited-state dipole moment.
third bands are assigned to the transition from
to ¼
ground
I
0:695ꢀHOMO!LUMO þ ꢂ ꢂ ꢂ (ꢀ ¼ 305 nm, oscillator strength f ¼
III
0:22) and to ¼ 0:641ꢀ(HOMOꢁ1)!LUMO þ ꢂ ꢂ ꢂ (ꢀ ¼ 254 nm,
f ¼ 0:29), respectively. As can be understood from the pictures
of HOMOꢁ1, HOMO, and LUMO shown in Figure 2, these two
transitions have the ICT character from the donor 4-methoxy-
phenyl to acceptor 4-cyanophenyl moiety through the disilanyl
group. Upon the excitation of CMDSi at 270 nm, fluorescence
spectra were observed with a maximum at around 470 nm (a),
450 nm (b), 435 nm (c), and 410 nm (d), respectively, as shown
in Figure 1. The Stokes shift between the first absorption
maximum (ꢁabs) and fluorescence maximum (ꢁfl) increases
with increase of solvent polarity as follows; 12600 cmꢁ1 in
cyclohexane (d), 14000 cmꢁ1 in chloroform (c), 14800 cmꢁ1 in
n-butyronitrile (b), and 15800 cmꢁ1 in acetonitrile (a): the latter
is remarkably large and comparable with that observed for
4-(N,N-dimethylamino)benzonitrile (DMAB).11 This solvent
In this communication, we report the excited-state property
of 1-(4-cyanophenyl)-2-(4-methoxyphenyl)-1,1,2,2-tetramethyl-
disilane (abbreviated to be CMDSi), which has the electron-do-
nating methoxy and -withdrawing cyano groups in each phenyl
group as shown in Chart 1.
CMDSi was synthesized from 4-chloroanisole and 4-bromo-
benzonitrile and was identified by the spectroscopic methods and
elementary analysis.10
Figure 1 shows the absorption (full line), fluorescence
(broken line), and excitation spectra (dotted line) of CMDSi in
acetonitrile (a), n-butyronitrile (b), chloroform (c), and cyclo-
hexane (d), respectively. There are two absorption maxima
Figure 1. Absorption (full line), fluorescence (broken line), and
excitation spectra (dotted line) of CMDSi in acetonitrile (a), n-
butyronitrile (b), chloroform (c), and cyclohexane (d). Calculat-
ed transitions for CMDSi are shown as sticks in (d).
Chart 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.9 (2007)
1169
This work was partly supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture Sports,
Science and Technology (MEXT) of Japanese government.
References and Notes
1
H. Shizuka, H. Obuchi, M. Ishikawa, M. Kumada, J. Chem.
Soc., Chem. Commun. 1981, 405; H. Shizuka, Y. Sato, M.
Ishikawa, M. Kumada, J. Chem. Soc., Chem. Commun. 1982,
439; H. Shizuka, K. Okazaki, M. Tanaka, M. Ishikawa, M.
Sumitani, K. Yoshihara, Chem. Phys. Lett. 1985, 113, 89; H.
Hiratsuka, Y. Mori, M. Ishikawa, K. Okazaki, H. Shizuka, J.
Chem. Soc., Faraday Trans. 2 1985, 81, 1665.
Figure 2. Molecular orbitals of CMDSi mainly related to the
first and third transitions.
dependence of the Stokes shift supports that these fluorescence
bands are of the ICT character from the 4-methoxyphenyl to
4-cyanophenyl moiety.
2
M. Yamamoto, T. Kudo, M. Ishikawa, S. Tobita, H. Shizuka,
J. Phys. Chem. A 1999, 103, 3144.
~
~
The Stokes shift (ꢁabs ꢁ ꢁfl) is related with the difference in
dipole moments between the ground (ꢂg) and emitting states
(ꢂe) by the Lippert–Mataga equation as follows;12
3
4
H. Shizuka, H. Hiratsuka, Res. Chem. Intermed. 1992, 18, 131.
H. Sakurai, H. Sugiyama, M. Kira, J. Phys. Chem. 1990, 94,
1837; M. Kira, T. Miyazawa, N. Mikami, H. Sakurai, Organo-
metallics 1991, 10, 3793; H. Sakurai, J. Abe, K. Sakamoto,
J. Photochem. Photobiol., A 1992, 65, 111.
M. Kira, T. Miyazawa, H. Sugiyama, M. Yamaguchi, H.
Sakurai, J. Am. Chem. Soc. 1993, 115, 3116.
Y. Tajima, H. Ishikawa, T. Miyazawa, M. Kira, N. Mikami, J.
Am. Chem. Soc. 1997, 119, 7400; H. Ishikawa, Y. Shimanuki,
M. Sugiyama, Y. Tajima, M. Kira, N. Mikami, J. Am. Chem.
Soc. 2002, 124, 6220.
2
3
~
~
hcðꢁabs ꢁ ꢁflÞ ¼ 2ðꢂe ꢁ ꢂgÞ =ð4ꢃ"0a Þ
ð1Þ
ꢃ ð f ꢁ fnÞ þ const
"
5
6
where h, c, "0, and a are Planck’s constant, velocity of light,
permittivity of vacuum, and Onsager radius of the sample mole-
cule, respectively. f ¼ ð" ꢁ 1Þ=ð2" þ 1Þ and fn ¼ ðn2 ꢁ 1Þ=
"
ð2n2 þ 1Þ, where " and n are dielectric constant and refractive
~
~
index of the solvent, respectively. By plotting hcðꢁabs ꢁ ꢁflÞ
7
8
K. A. Horn, R. B. Grossman, J. R. G. Thorne, A. A. Whitenack,
J. Am. Chem. Soc. 1989, 111, 4809.
against (f ꢁ fn), a rather good linear relationship was obtained
"
as shown in Figure 3. The slope and the intercept (const of eq 1)
of this line were estimated to be 1:69 ꢃ 10ꢁ19 and 2:41 ꢃ
10ꢁ19 J, respectively. Assuming ꢂg ¼ 6:4 debye ¼ 1:5 ꢃ
10ꢁ29 C m and a3 ¼ 5:3 ꢃ 10ꢁ28 m3, as obtained by MO calcula-
tion on CMDSi, ꢂe was estimated to be 28 debye. This value is
greater than that determined for the ICT state of DMAB (23
debye).11 It is noted that almost no LE emission was observed
in the fluorescence spectrum, suggesting that the intramolecular
charge-transfer process is very fast in CMDSi.
G. Mignani, A. Kramer, G. Puccetti, I. Ledoux, G. Soula,
J. Zyss, R. Meyrueix, Organometallics 1990, 9, 2640; G.
Mignani, A. Kramer, G. Puccetti, I. Ledoux, J. Zyss, G. Soula,
Organometallics 1991, 10, 3656; G. Mignani, M. Barzoukas,
J. Zyss, G. Soula, F. Balegroune, D. Grandjean, D. Josse,
Organometallics 1991, 10, 3660.
9
P. F. van Hutten, G. Hadziioannou, R. Bursi, D. Feil, J. Phys.
Chem. 1996, 100, 85.
10 The Grignard reagent of 4-chloroanisole was allowed to react
at 0 ꢅC with 1,2-dichloro-1,1,2,2-tetramethyldisilane to give
1-chloro-2-(4-methoxyphenyl)-1,1,2,2-tetramethyldisilane. 4-
Bromobenzonitrile was treated with tert-butyllithium in tetra-
hydrofuran at ꢁ100 ꢅC to produce 4-lithiobenzonitrile, and in-
to this solution 1-chloro-2-methoxyphenyl-1,1,2,2-tetrameth-
yldisilane was added to prepare CMDSi (51% overall yield).
Data for CMDSi: 1H NMR (CDCl3) ꢅ 0.28 (s, 6H), 0.32 (s,
6H), 3.79 (s, 3H), 6.85 (d, 2H, J ¼ 8:5 Hz), 7.22 (d, 2H, J ¼
8:5 Hz), 7.40 (d, 2H, J ¼ 8:1 Hz), 7.53 (d, 2H, J ¼ 8:1 Hz);
13C NMR (CDCl3) ꢅ ꢁ 4:3, ꢁ4:0, 55.0, 111.8, 113.6, 119.2,
128.4, 130.8, 134.2, 135.1, 146.8, 160.2; 29Si NMR (CDCl3):
ꢅ ꢁ 22:1, ꢁ20:6; MS: m=z (%) 325 (Mþ, 50), 310 (10), 165
(100), 160 (13). Anal. Calcd for C18H23NOSi2: C, 66.14; H,
6.99; N, 4.30%. Found: C, 66.41; H, 7.12; N, 4.30%.
The fluorescence lifetime (ꢄf) in cyclohexane was deter-
mined to be 3:4 ꢄ 0:3 ns,13 being a little longer than that of (4-
cyanophenyl)pentamethyldisilane in pentane.5 The lifetime in
acetonitrile was determined to be ca. 100 ps. This solvent effect
on the fluorescence lifetime has the same tendency as that
observed for the ‘‘b emission band’’ observed for DMAB (ꢄf is
1.84 ns in methylcyclohexane and shorter than 10 ps in acetoni-
trile).14 It was also confirmed that CMDSi is much more stable
for the 253.7-nm light photolysis in acetonitrile under Ar than
the unsubsituted 1,1,2,2-tetramethyl-1,2-diphenyldisilane.
11 E. Lippert, W. Lueder, H. Boos, in Advances in Molecular
Spectroscopy, Pergamon Press, London, 1962, p. 443.
12 E. Lippert, Z. Naturforsch. A 1955, 10, 541; E. Lippert, Z.
Electrochem. 1957, 61, 962; N. Mataga, Y. Kaifu, M. Koizumi,
Bull. Chem. Soc. Jpn. 1955, 28, 690.
13 Fluorescence lifetime was determined by means of the time-
correlated single photon-counting technique using an Edin-
burgh Analytical Instruments FL900CDT spectrometer system
in the nanosecond time-region or a femtosecond laser system
(Spectra Physics mode-locked Ti:sapphire laser; Tsunami)
combined with a detector MCP-PMT (Hamamatsu R3809U-
51) system in the picosecond time-region.
~
~
Figure 3. Plot of hcðꢁabs ꢁ ꢁflÞ versus (f ꢁ fn) based on the
"
Lippert–Mataga equation; cyclohexane (1), chloroform (2), n-
butyronitrile (3), and acetonitrile (4). The correlation coefficient
was 0.966.
14 D. Huppert, S. D. Rand, P. M. Rentzepis, P. F. Barbara, W. S.
Struve, Z. R. Grabowski, J. Chem. Phys. 1981, 75, 5714.
Published on the web (Advance View) August 18, 2007; doi:10.1246/cl.2007.1168