Time-resolved fluorescence of α,ω-di(1-naphthyl)oligosilanes and 1-naphthyloligosilanes: Intramolecular excimer formation and charge-transfer interactions

Article abstract of DOI:10.1007/s11164-012-0653-3

The intramolecular photochemical processes excimer formation and charge-transfer (CT) complex formation were investigated by comparing the behavior of α,ω-di(1-naphthyl)permethyloligosilanes ((1-naphthyl)-(SiMe₂) _n -(1-naphthyl); NS n N, n = 1, 3, and 6) and 1-(1-naphthyl)permethyloligosilanes ((1-naphthyl)-(SiMe₂) _n -Me; NS n, n = 1, 3, and 6) by use of stationary and time-resolved fluorescence (TR-FL) measurements. Formation of excimer and CT complexes is highly dependent on the silicon chain length and polarity of the medium. Graphical Abstract: Intramolecular excimer formation between the two naphthyl groups and charge transfer interactions between the naphthyl and silane moieties were investigated by use of a time-correlated single-photon counting method.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-012-0653-3

Res Chem Intermed
DOI 10.1007/s11164-012-0653-3
Time-resolved fluorescence of a,x-di(1-
naphthyl)oligosilanes and 1-naphthyloligosilanes:
intramolecular excimer formation and
charge-transfer interactions
•
Takashi Karatsu Takashi Nakamura
•
•
Masato Terasawa Shiki Yagai Akihide Kitamura
•
•
•
Yoshinobu Nishimura Iwao Yamazaki
Received: 24 October 2011 / Accepted: 17 November 2011
Ó Springer Science+Business Media B.V. 2012
Abstract The intramolecular photochemical processes excimer formation and charge-
transfer (CT) complex formation were investigated by comparing the behavior of a,x-di
(1-naphthyl)permethyloligosilanes ((1-naphthyl)-(SiMe2)n-(1-naphthyl); NSnN, n = 1, 3,
and 6) and 1-(1-naphthyl)permethyloligosilanes ((1-naphthyl)-(SiMe2)n-Me; NSn, n = 1, 3,
and 6) by use of stationary and time-resolved fluorescence (TR-FL) measurements. For-
mation of excimer and CT complexes is highly dependent on the silicon chain length and
polarity of the medium.
Keywords Organosilane ꢀ Oligosilane ꢀ Excimer ꢀ Charge transfer ꢀ
Time-resolved fluorescence
Introduction
Organosilanes are compounds consisting of alkyl and/or aryl substituted silicon
atoms. Silicon atoms have unique characteristics, for example different bond lengths
and bond angles compared with their carbon analogues. Their photochemical
behavior has therefore been the subject of investigation. Professor Mizuno et al.
Dedicated to Professor Kazuhiko Mizuno on his retirement from Osaka Prefecture University.
T. Karatsu (&) ꢀ M. Terasawa ꢀ S. Yagai ꢀ A. Kitamura
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
e-mail: karatsu@faculty.chiba-u.jp
T. Nakamura ꢀ Y. Nishimura ꢀ I. Yamazaki
Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Present Address:
Y. Nishimura
Department of Chemistry, University of Tsukuba, Tsukuba, Japan
123

T. Karatsu et al.
reported the intramolecular excimer fluorescence (FL) and photochemical cyclo-
addition reaction of styrene and stilbene derivatives as two photochemically active
parts linked by dimethylsilyl and dimethylsiloxy chains [1, 2]. Organosilanes also
have interesting properties, for example chain-length-dependent UV absorption
caused by r-conjugation in which the silane chain acts as one of the chromophores.
Photophysical processes of phenyldisilanes have been extensively investigated to
elucidate the interactions between the aryl group and the disilane unit. The proposed
origin of this interaction is a r–p interaction between the p-orbital of the aryl group
and the Si–Si r-orbital, or a 3d-p-orbital interaction [3–6]. Detailed investigation of
the anti conformation of 1,2-diphenyltetramethyldisilane indicated that interactions
in the HOMO and LUMO are assigned to a rSiSi–paryl and a p*SiSi–p*aryl
interactions, respectively [7].
Our interests include the synthesis of r–p conjugated systems containing selected
aryl groups (naphthyl and/or anthryl) and an oligosilane linker unit, the elucidation of
photophysical processes such as excimer/exciplex formation, intramolecular charge
transfer (CT), and electronic energy-transfer processes of these compounds, and
comparison of this behavior with that of the carbon analogues. Excimer formation has
been reported for a,x-dipyrenylpermethyloligosilanes [8, 9]. We have reported
preliminary results for oligosilanes such as dimethyl-1,1-di(1-naphthyl)silane
(NS1N), tetramethyl-1,2-di(1-naphthyl)disilane (NS2N), hexamethyl-1,3-di(1-naph-
thyl)trisilane (NS3N), octamethyl-1,4-di(1-naphthyl)tetrasilane (NS4N), dodecameth-
yl-1,6-di(1-naphthyl)hexasilane (NS6N), 1,1-di(9-anthryl)dimethylsilane (AS1A),
1,2-di(9-anthryl)tetramethyldisilane (AS2A), 1,3-di(9-anthryl)hexamethyltrisilane (AS3A),
1,4-di(9-anthryl)octamethyltetrasilane (AS4A), and 1,6-di(9-anthryl)dodecamethyl-
hexasilane (AS6A) [10] and investigated a series of anthryloligosilanes by use of a
time-resolved (TR) single-photon counting method [11]. A p–p interaction between
the two aromatic groups and a r–p interaction between the aromatic group and the
silicon chain were observed for these compounds. In these cases, intramolecular
excimer emission was observed in cyclohexane when n C 2. The strongest excimer
emission at n = 2 is in contrast with the Hirayama-rule (n = 3) proposed for carbon
analogues [12]. For formation of the excimer of dianthryldisilane, the FL behavior of
the positionally isomeric di(9-, 1-, or 2-anthryl)tetramethyldisilanes was investigated
by use of a TR single-photon counting method [13]. The steric effect of the anthryl
group varied with the angle of the anthracene plane and the disilane unit, and,
therefore, affected the interaction between the two chromophores and ultimately the
formation process and stability of the excimers. 1,3-Di(9-anthryl)propane derivatives
were reported to efficiently produce an intramolecular [4 ? 4] cycloadduct and,
therefore, emitted no excimer FL [14, 15]. AS2A, however, emitted strong excimer
FL, indicating that intramolecular cycloaddition is a minor process. AS1A has been
reported to produce a unique intramolecular [2 ? 4] cycloadduct, [16–18] whereas
longer chain analogues (n = 2 and 3) gave [4 ? 4] cycloadducts between the 9,10 and
90,100, and 1,4 and 90,100 positions of the anthracenes, respectively [18].
We have reported that the CT properties of the naphthyl oligosilanes are more
prominent than those of the anthryl oligosilanes. CT-FL was observed for the
NSnN when n C 4 and ASnA (n C 6) in acetonitrile. We have also reported that the
123

Time-resolved fluorescence of a,x-di(1-naphthyl)oligosilanes
intramolecular energy-transfer process for 1-(9-anthryl)dimethyl-1-(1-naphthyl)
silane (AS1N), 1-(9-anthryl)tetramethyl-2-(1-naphthyl)disilane (AS2N), 1-(9-anthryl)
hexamethyl-3-(1-naphthyl)trisilane (AS3N), 1-(9-anthryl)octamethyl-4-(1-naphthyl)
tetrasilane (AS4N), and 1-(9-anthryl)dodecamethyl-6-(1-naphthyl)hexasilane
(AS6N), occurs very efficiently compared with for the carbon analogues [19–21],
and that FL, which indicates energy transfer from the naphthyl to the anthryl
moieties, was observed first, before CT-FL of the charge transfer between the
anthryl and the hexasilyl moieties was seen [22].
In this study, we investigated the photophysical properties and, specifically,
the relaxation processes for the electronic excitation energy, for the series of
compounds NSnN (n = 1, 3, and 6), trimethyl-1-(1-naphthyl)silane (NS1), hep-
tamethyl-1-(1-naphthyl)trisilane (NS3), and tridecamethyl-1-(1-naphthyl)hexasilane
(NS6) (Fig. 1), by use of a picosecond time-correlated single-photon counting
method. The contribution of the two interactions, p–p and r–p, were investigated by
changing the chain length and solvent polarity.
Experimental
Materials
Di(1-naphthyl)oligosilanes (NSnN, n = 1, 3, and 6) and (1-naphthyl)oligosilanes
(NSn, n = 1, 3, and 6) were prepared as reported elsewhere [3, 10, 23].
Tetrahydrofuran (THF) and diethyl ether, dehydrated and stabilizer-free, (Kanto
Chemical) were used as received. Dichlorodimethylsilane was purchased from Shin-
Etsu Chemical, and 1-chloropentamethyltrisilane, 1-chlorotridecamethylhexasilane,
1,3-dichlorohexamethyltrisilane, and 1,6-dichlorododecamethylhexasilane were
synthesized as reported elsewhere [24].
In a typical run, a solution of monochloro or dichlorooligosilane in THF was
slowly added to a solution of 1-lithionaphthalene, prepared from 1-bromonaphtha-
lene, and a solution of n-butyllithium (1.6 M in hexane) in THF–diethyl ether (8:5)
at -50 °C. The solution temperature was gradually increased and stirring was then
maintained overnight at room temperature. After workup, the product was separated
by flash chromatography (hexane–benzene 1:1) and purified by recrystallization
from hexane.
Me
Si
Me
Si
Me
Si
Me
Si
Me
n
n
n
n
Me
Me
Me
Me
n = 1, 2, 3, 6
n = 1, 3, 6
n = 1, 2, 3, 6
n = 1, 2, 3, 6
NSnN
NSn
ASnA
ASnN
Fig. 1 Structures of NSnN, NSn, ASnA, and ASnN
123

T. Karatsu et al.
Spectroscopy
The absorption and FL spectra were measured in spectroscopic grade cyclohexane
or acetonitrile (Dotite) by use of a Jasco V-570 spectrophotometer and an FP-6600
spectrofluorimeter, respectively. For the FL measurements, sample solutions were
passed through and purged with dry argon for at least 15 min before measurement.
The FL decay curves and TR-FL spectra were measured at Hokkaido University
with a picosecond pulse laser and a single-photon timing apparatus [19, 20, 22]. The
laser system was a mode-locked Ti: sapphire laser (Coherent, Mira 900) pumped by
an argon ion laser (Coherent, Innova 300) combined with a pulse picker (Coherent,
model 9200, repetition rate 2.9 MHz). The third harmonics (284 nm) generated by
an ultrafast harmonic system (Inrad, model 5-050) were used as the excitation light
source. The decay curve was obtained by monitoring the emission with a
microchannel-plate photomultiplier (Hamamatsu R2809U-01). The pulse width of
the instrumental response function was 30 ps (FWHM). The picosecond TRFL
spectra were obtained from the decay curves monitored at different wavelengths
ranging from 300 to 600 nm with a 0.625 nm step. The TR-spectra were constructed
from a series of decay curves by plotting the FL intensities at the given delay times
as a function of emission wavelength.
Results and discussion
Absorption spectra of NSn and NSnN (n = 1, 3, and 6) were measured in
cyclohexane as shown in Fig. 2a, b, respectively. The absorption maxima shifted to
longer wavelengths as the silicon-linker lengths became longer. The band widths
also became larger and were clearly observed around 260 and 290 nm. This shift is
explained by the r–p conjugation between the r-orbital of the Si–Si bond and the
p-orbital of the naphthalene ring. In addition, increases in the molar extinction
coefficient (e) were observed at approximately 245 nm with an increase from n = 1
to 3, and approximately 260 nm with an increase of n = 3–6. The larger e values are
because of the appearance of the absorption of the silicon-linker unit. The
absorption of the linker unit shifted to longer wavelengths with increasing chain
length because of r-conjugation [10]. The relative absorbance at 260 and 290 nm
(Abs260/Abs290) of NS6 was twice as large as that of NS6N because the portion of
the naphthyl moiety aligned with the linker unit in the latter compound was doubled.
The shape of the absorption spectra remained essentially the same in cyclohexane
and acetonitrile. The absorption maxima of NS1N were, however, shifted 2 nm to
longer wavelength than those of NS1, indicating that there is an interaction between
the two naphthyl groups in NS1N.
Figure 3 shows the stationary FL spectra of NSn and NSnN for n = 1 (a and d)
n = 3, (b and e), and n = 6, (c and f) in argon-purged cyclohexane (a, b, and c) and
acetonitrile (d, e, and f). The spectra were measured after excitation at 280 nm. The
effects of silyl chain length, number of naphthyl groups, and solvent on the FL
spectra were investigated.
123

Time-resolved fluorescence of a,x-di(1-naphthyl)oligosilanes
Fig. 2 Absorption spectra of
(a) 2.0
NSn (a) and NSnN (b) (n = 1,
3, and 6) in cyclohexane
NS1
NS3
NS6
1.5
1.0
0.5
0
200
240
280
320
Wavelength / nm
2.0
(b)
NS1N
NS3N
NS6N
1.5
1.0
0.5
0
200
240
280
320
Wavelength / nm
The FL spectra of NS1 and NS1N had vibrational fine structure only, and the
spectral shape did not change significantly in the two different solvents. These
spectra were assigned to local emission (LE) from the naphthyl moiety. The
spectrum of NS1N had wider skirts at the low energy side of the emission band
compared with that of NS1. In cyclohexane, the FL maximum (kmax) of NS3 was
located at approximately 350 nm, and a broad shoulder was observed at
approximately 400 nm. In contrast, kmax of NS3N appeared at approximately
400 nm and this FL was assigned to intramolecular excimer emission. In
acetonitrile, the FL of NS3 appeared at 330 and 480 nm, which were assigned to
LE and CT-FL, respectively. The FL of NS3N was at approximately 450 nm, and
this band was much broader than that of NS3. It could be divided into two Gaussian
bands with kmax at 405 and 480 nm, which were assigned to the excimer and CT
emission, respectively. This excimer emission appeared at the same wavelength as
that of NS3N in cyclohexane. The FL spectra of NS6 and NS6N had large band
widths and Stokes shifts. These spectra were almost identical in each solvent, but
kmax in acetonitrile appeared at a much longer wavelength than that in cyclohexane.
The shapes of the spectra were independent of the number of naphthyl groups in the
123

T. Karatsu et al.
1.6
1.2
0.8
0.4
(a)
(d)
(b)
(e)
(c)
(f)
NS1
NS1N
NS3
NS3N
NS6
NS6N
0.0
1.2
NS3
NS3N
NS6
NS6N
NS1
0.8
0.4
0.0
NS1N
300
400
500
600 300
400
500
600 300
400
500
600
Wavelength / nm
Wavelength / nm
Wavelength / nm
Fig. 3 Stationary fluorescence spectra of NSn and NSnN for n = 1, a and d, n = 3, b and e, and n = 6,
c and f, in argon-purged cyclohexane (a, b, and c), and acetonitrile (d, e, and f). The FL band of NS3N at
approximately 450 nm in e was divided into two Gaussian bands with kmax at 405 (dotted line) and
480 nm (dash–dotted line); these were assigned to the excimer and CT emission, respectively
molecule. Therefore, the emissions was assigned to the CT emission associated with
the silane linker.
TR-FL spectra and their time profiles were measured by use of a picosecond
single-photon counting instrument. Figure 4 shows the TR-FL spectra of NS1 (left)
and NS1N (right), measured in cyclohexane, with their time profiles observed at 330
and 360 nm. The FL spectral shape of NS1 was time-independent and its intensity
decayed with a single exponential profile and a lifetime of 14.2 ns (Table 1). In
contrast, the FL of NS1N decayed with a double exponential profile at 360 nm with
103
NS1
NS1
NS1N
5.42 –
6.45 ns
1.81 –
2.32 ns
1.03 –
102
101
Em 330 nm
Em 360 nm
IRF
100
103
1.29 ns
277 –
NS1N
102
101
330 ps
52.8 –
Em 330 nm
Em 360 nm
IRF
66.1 ps
0 –
5.28 ps
100
300 400 500 600 300 400 500 600
Wavelength / nm Wavelength / nm
0
10 20 30 40
Time / ns
Fig. 4 TR-FL spectra of NS1 and NS1N in argon-purged cyclohexane (left), and time-correlations of
NS1 (right top) and NS1N (right bottom) at 330 and 360 nm. The excitation was performed at 280 nm.
IRF is instrumental response function
123

Time-resolved fluorescence of a,x-di(1-naphthyl)oligosilanes
Table 1 Fluorescence lifetimes of local, excimer, and CT emissions in deaerated solution
Compound In cyclohexane
sf/ns (weight)
In acetonitrile
sf/ns (weight)
NS1
NS1N
NS3
NS3N
NS6
NS6N
a
14.2 (1.00)a
2.50 (0.561), 11.2 (0.439)a
0.0423 (0.211), 0.394 (0.559), 0.692 (0.230)b 0.0235 (0.369), 0.124 (0.534), 1.55 (0.097)c
0.261 (0.892), 7.52 (0.108)b
0.0689 (0.651), 0.772 (0.349)b
0.0499 (0.615), 0.682 (0.385)b
12.3 (1.00)a
3.15 (0.631), 10.9 (0.369)a
0.0298 (0.361), 0.193 (0.616), 1.12 (0.024)c
0.101 (0.698), 2.01 (0.302)c
0.114 (0.847), 2.11 (0.321)c
kobs was 360 nm
b
c
kobs was 450 nm
kobs was 510 nm
lifetimes of 2.5 ns (56.1 %) and 11.2 ns (43.9 %). The spectra of both compounds
in acetonitrile were similar to those in cyclohexane, and the corresponding lifetimes
were 12.3 ns for NS1 and 3.15 ns (63.1 %) and 10.9 ns (36.9 %) for NS1N. This
behavior is explained by the existence of a conformer that enables better
delocalization of the excitation energy between the two naphthyl moieties across
the silicon atom in NS1N.
TR-FL spectra of NS3 and NS3N in cyclohexane can be seen in Fig. 5. The
initial bands of the TR-FL spectra of NS3 and NS3N in cyclohexane were broad and
had CT character. The FL kmax did not shift very much for NS3, and the FL decayed
quickly in 42.3 ps (21.1 %), 394 ps (55.9 %), and 692 ps (23.0 %) at 450 nm. For
NS3N, however, a double exponential decay profile with lifetimes of 261 ps
(89.2 %), and 7.52 ns (10.8 %) was observed. The latter long lifetime component
was observed for NS3N only, and not for NS3, and was assigned to the excimer FL.
The initial spectra of the TR-FL for NS3 and NS3N were essentially similar in
acetonitrile (Fig. 6). After excitation for 1 ns, the FL kmax shifted to a longer
103
NS3N
NS3
NS3
102
101
IRF
5.42 –
6.45 ns
Em 450 nm
1.81 –
2.32 ns
1.03 –
100
103
1.29 ns
NS3N
IRF
Em 450 nm
277 –
330 ps
52.8 –
66.1 ps
0 –
102
101
5.28 ps
100
300 400 500
Wavelength / nm Wavelength / nm
300 400 500 600
600
0
10 20 30 40
Time / ns
Fig. 5 TR-FL spectra of NS3 and NS3N in argon-purged cyclohexane (left), and time-correlations of
NS3 (right top) and NS3N (right bottom) at 450 nm. The excitation was performed at 280 nm
123

T. Karatsu et al.
103
102
101
Em.410
NS3
NS3
NS3N
Em.510
IRF
5.42 –
6.45 ns
1.81 –
2.32 ns
1.03 –
1.29 ns
277 –
330 ps
52.8 –
66.1 ps
0 –
100
103
Em.410
Em.510
IRF
NS3N
102
101
100
5.28 ps
300 400 500 600 300 400 500
0.0 0.5 1.0 1.5 2.0
Time / ns
Wavelength / nm
Wavelength / nm
Fig. 6 TR-FL spectra of NS3 and NS3N in argon-purged acetonitrile (left), and time correlations of NS3
(right top) and NS3N (right bottom) at 410 and 510 nm. The excitation was performed at 280 nm
wavelength for NS3, but a broadened peak was observed for NS3N. Time profiles
observed at 410 and 510 nm decayed with triple components with similar lifetimes
but different contributions. NS3 at 510 nm decayed with 23.5 ps (36.9 %), 124 ps
(53.4 %), and 1.55 ns (9.7 %) lifetimes, and the FL of NS3N decayed with 29.8 ps
(36.1 %), 193 ps (61.6 %), and 1.12 ns (2.4 %) lifetimes, which are faster than
those of NS3. This result may be because of the presence of the two naphthyl
moieties, where CT is possible in the molecule.
The TR-FL of NS6 and NS6N are shown in Fig. 7. Similar to the stationary state
FL measurement, the TR spectral kmax of NS6N shifted to a longer wavelength than
that of NS6. The kmax of NS6 and NS6N were almost identical, however, the
spectral band width was larger for NS6N than NS6. The FL decayed with a double
exponential profile at 450 nm for NS6 and 510 nm for NS6N in both cyclohexane
and acetonitrile. In cyclohexane, NS6 decayed over 68.9 ps (65.1 %) and 772 ps
(34.9 %) whereas NS6N decayed over 49.9 ps (61.5 %) and 682 ps (38.5 %).
In cyclohexane
NS6
In acetonitrile
NS6
NS6N
NS6N
5.42 –
6.45 ns
1.81 –
2.32 ns
1.03 –
1.29 ns
277 –
330 ps
52.8 –
66.1 ps
0 –
5.28 ps
300
600
300 400 500
600
300 400 500
300 400 500 600
500
400
600
Wavelength / nm Wavelength / nm
Wavelength / nm Wavelength / nm
Fig. 7 TR-FL spectra of NS6 and NS6N in argon-purged cyclohexane (left) and acetonitrile (right). The
excitation was performed at 280 nm
123

Time-resolved fluorescence of a,x-di(1-naphthyl)oligosilanes
In acetonitrile, lifetimes of the short and long components were two and three times
longer, respectively, than those measured in cyclohexane: 101 ps (69.8 %) and
2.01 ns (30.2 %) for NS6, and 114 ps (67.9 %) and 2.11 ns (32.1 %) for NS6N in
acetonitrile. The shift of kmax to the longer wavelengths and the spectral broadening
were larger in acetonitrile than in cyclohexane, which may be because of solvent
reorientation during the CT process [25]. The CT state is more stabilized in a polar
medium.
The direction of the CT process in aryloligosilanes has not been resolved [3–6].
A proposed orthogonal intramolecular CT (OICT) r(Si–Si)–p* mechanism suggests
that CT occurs from the silane chain to the aryl group [5, 6], whereas a 2pp* ? 3dp
mechanism [3, 4, 26] advocates CT in the opposite direction. Our experimental
results could not verify either proposed mechanism. However, we have performed
DFT calculations in an attempt to gain greater insight into this issue [27]. The fully
optimized ground state structure of NS6 and its frontier orbitals are depicted in
Fig. 8. Information about the transition calculated time-dependent density
LUMO+2
254.4 nm
f =0.6580
-0.00273
-0.00537
279.0 nm
f =0.0050
280.4 nm
306.9 nm
f =0.3463 f =0.0222
LUMO+1
LUMO
-0.03846
39%+28%
HOMO
69%
90%
90%
HOMO-1
-0.20302
-0.21548
HOMO-2
-0.23898 au
Fig. 8 Molecular orbitals and their energy levels, and the transition configuration calculated using a TD-
DFT B3LYP/6-31G method. The ground state structure was fully optimized using the same basis set
123

T. Karatsu et al.
functional theory (TD-DFT) method is also shown. The HOMO is delocalized over
all of the molecule including the naphthyl moiety and the silane chains. The LUMO,
on the other hand, is located on the naphthyl moiety only. This result seems to
suggest that at least a HOMO ? LUMO transition accompanies the silane to aryl
charge transfer. In this case, the CT depends on silane chain length and supports the
argument for a shift of the absorption band to longer wavelengths as chain length
increases. Longer chain lengths have increasing electron-donating ability, corre-
spondingly increasing the energy level of the HOMO, and therefore greater
electron-donating CT ability also [28]. It is important to note, however, that this
result does not consider any conformational changes in the excited state.
Conclusions
The photophysical characteristics of 1-naphthyloligosilanes that lead to formation of
excimers or CT complexes are strongly dependent on silicon chain length and the
polarity of the medium.
When n = 1, the CT ability of silane is weak, and the excited state is only located
on the one naphthyl moiety in NS1. For NS1N, however, the excitation energy is
delocalized between the two naphthyl moieties.
When n = 3, both p–p* interaction and r–p* CT interaction are affected during
FL. In a nonpolar solvent, for example cyclohexane, the CT complex is less stable,
and, therefore, NS3N forms an excimer. In a polar solvent, however, the CT
complex is more stabilized, and therefore CT emission is observed at a longer
wavelength (lower energy) than that of the excimer emission.
When n = 6, no significant spectral differences between NS6 and NS6N were
observed in cyclohexane or acetonitrile, although the emission kmax was located at a
longer wavelength in acetonitrile. These results indicate that only r–p* interaction
was observed through FL because of the higher CT character of the longer silane
chain. Therefore, formation of a CT complex was observed even in cyclohexane.
Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (no. 20550056),
Scientific Research on Priority Areas (417, no. 17029013), and the G-COE program from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government and the COE
start-up program from Chiba University.
References
1. H. Maeda, H. Yagi, K. Mizuno, Chem. Lett. 33, 388 (2004)
2. H. Maeda, K. Nishimura, K. Mizuno, M. Yamaji, J. Oshima, S. Tobita, J. Org. Chem. 70, 9693
(2005)
3. H. Shizuka, Y. Sato, Y. Ueki, M. Ishikawa, M. Kumada, J. Chem. Soc. Faraday Trans. 1 80, 341
(1984)
4. H. Shizuka, H. Obuchi, M. Ishikawa, M. Kumada, J. Chem. Soc. Faraday Trans. 1 80, 383 (1984)
5. M. Kira, T. Miyazawa, H. Sugiyama, M. Yamaguchi, H. Sakurai, J. Am. Chem. Soc. 115, 3116
(1993)
6. C.G. Pitt, R.N. Carey, E.C. Toren Jr, J. Am. Chem. Soc. 94, 3806 (1972)
7. H. Tsuji, Y. Shibano, T. Takahashi, M. Kumada, K. Tamao, Bull. Chem. Soc. Jpn. 78, 1334 (2005)
123

Time-resolved fluorescence of a,x-di(1-naphthyl)oligosilanes
8. D. Declercq, P. Delbeke, F.C. de Schryver, L.V. Meervelt, R.D. Miller, J. Am. Chem. Soc. 115, 5702
(1993)
9. D. Declercq, F.C. de Schryver, R.D. Miller, Chem. Phys. Lett. 186, 467 (1991)
10. T. Karatsu, T. Shibata, A. Nishigaki, K. Fukui, A. Kitamura, Chem. Lett. 30, 994 (2001)
11. T. Karatsu, T. Shibata, A. Nishigaki, A. Kitamura, Y. Hatanaka, Y. Nishimura, S. Sato, I. Yamazaki,
J. Phys. Chem. B 107, 12184 (2003)
12. F. Hirayama, J. Chem. Phys. 42, 3163 (1965)
13. K. Takahashi, Y. Nishimura, T. Arai, S. Yagai, A. Kitamura, T. Karatsu, J. Photochem. Photobiol. A
218, 204 (2011)
14. H. Bouras-Laurent, J.-P. Desvergne, in Photochromism, ed. by H. Du¨rr, H. Bouras-Laurent (Elsevier,
Amsterdam 1990), pp. 561–630, and cited therein
15. H.-D. Becker, in Advances in photochemistry, vol. 15, ed. by D.H. Volman, G.S. Hammond,
K. Gollnick (Wiley, New York, 1990), pp. 139–227
16. H. Sakurai, K. Sakamoto, A. Nakadaira, M. Kira, Chem. Lett. 14, 497 (1985)
17. M. Daney, C. Vanucci, J.-P. Desvergne, A. Castellan, H. Bouas-Laurent, Tetrahedron Lett. 26, 1505
(1985)
18. K. Takahashi, Y. Takanashi, S. Yagai, A. Kitamura, T. Karatsu, J. Photopolym. Sci. Technol. 23, 789
(2010)
19. Y. Nishimura, A. Yasuda, S. Speiser, I. Yamazaki, Chem. Phys. Lett. 323, 117 (2000)
20. M. Hasegawa, S. Enomoto, T. Hoshi, K. Igarashi, T. Yamazaki, Y. Nishimura, S. Speiser,
I. Yamazaki, J. Phys. Chem. B 106, 4925 (2002)
21. S. Speiser, Chem. Rev. 96, 1953 (1996)
22. T. Karatsu, M. Terasawa, S. Yagai, A. Kitamura, T. Nakamura, Y. Nishimura, I. Yamazaki,
J. Organomet. Chem. 689, 1029 (2004)
23. M. Kumada, M. Yamaguchi, Y. Yamamoto, J. Nakajima, K. Shiina, J. Org. Chem. 21, 1264 (1956)
24. H. Gilman, S. Inoue, J. Org. Chem. 29, 3418 (1964)
25. T. Hayashi, N. Mataga, Y. Sakata, S. Misumi, M. Morita, J. Tanaka, J. Am. Chem. Soc. 98, 5910
(1976)
26. A.G. Evans, B. Jerome, N.H. Rees, J. Chem. Soc. Perkin Trans 2, 447 (1973)
27. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zak-
rzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels,
K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci,
C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma,
D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefa-
nov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill,
B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople,
Gaussian 98, Revision A.11.4 (Gaussian Inc., Pittsburgh, 1998)
28. M.K. Raymond, J. Michl, Int. J. Quantum Chem. 72, 361 (1999)
123