π-π and σ-π interactions in α,ω-Di-(9-anthryl) and Di-(1-naphthyl) oligosilanes studied by time-resolved fluorescence in solution

Article abstract of DOI:10.1021/jp0355760

The photophysical properties of two series of oligosilanes, (1-naphthyl)-(SiMe₂)_n-(1-naphthyl) (1-5) and (9-anthryl)-(SiMe₂)_n-(9-anthryl) (6-10) with n = 1-4 and 6, were investigated. In these compounds, two types of interactions, a π-π interaction between two aromatic groups and a σ-π interaction between aromatics and a silicon chain unit, were observed. Intramolecular excimer emission was observed in cyclohexane when n ≥ 2. The strongest excimer emission of 2 and 7 is different from the Hirayama rule (n = 3) proposed for carbon analogues and also shows that intramolecular cycloaddition is minor. The time-resolved fluorescence spectra of an anthryl series revealed that the time constant of excimer formation varied depending on the chain length (82-152 ps). In the cases of 4, 5, and 10, charge-transfer (CT) emission was observed in acetonitrile or THF. The time constant of the CT-state formation for 10 was relatively slow (45 ps), which may indicate a conformational change. Monomer emission from the locally excited state was observed for 1 and 6 in both cyclohexane and acetonitrile.

Full text of DOI:10.1021/jp0355760

12184
J. Phys. Chem. B 2003, 107, 12184-12191
π-π and σ-π Interactions in r,ω-Di-(9-anthryl) and Di-(1-naphthyl) Oligosilanes Studied
by Time-Resolved Fluorescence in Solution
Takashi Karatsu,*,† Toshifumi Shibata,† Atsuko Nishigaki,† Akihide Kitamura,*,†
Yusuke Hatanaka,‡ Yoshinobu Nishimura,‡ Shin-ichiro Sato,‡ and Iwao Yamazaki*,‡
Department of Materials Technology, Faculty of Engineering, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan, and Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan
ReceiVed: June 4, 2003; In Final Form: September 4, 2003
The photophysical properties of two series of oligosilanes, (1-naphthyl)-(SiMe2)n-(1-naphthyl) (1-5) and
(9-anthryl)-(SiMe2)n-(9-anthryl) (6-10) with n ) 1-4 and 6, were investigated. In these compounds, two
types of interactions, a π-π interaction between two aromatic groups and a σ-π interaction between aromatics
and a silicon chain unit, were observed. Intramolecular excimer emission was observed in cyclohexane when
n g 2. The strongest excimer emission of 2 and 7 is different from the Hirayama rule (n ) 3) proposed for
carbon analogues and also shows that intramolecular cycloaddition is minor. The time-resolved fluorescence
spectra of an anthryl series revealed that the time constant of excimer formation varied depending on the
chain length (82-152 ps). In the cases of 4, 5, and 10, charge-transfer (CT) emission was observed in
acetonitrile or THF. The time constant of the CT-state formation for 10 was relatively slow (45 ps), which
may indicate a conformational change. Monomer emission from the locally excited state was observed for 1
and 6 in both cyclohexane and acetonitrile.
Introduction
R,ω diaryl oligosilanes have attracted much attention because
they have interesting photochemical characteristics compared
to those of the carbon chain analogues.1-4 Di-(9-anthryl)-
dimethylsilane (5) is reported to produce unique [4 + 2]
cycloadducts,1 which are very similar to the photoadduct
produced by the bis-9-anthrylethylenes.5 The R,ω bis-3-pyrenyl
oligosilanes are also reported to produce excimer emission by
photoexcitation.3 In that report, two pyrenyl groups are proposed
to interact even in the ground-state-based chemical shift of the
1H NMR spectra, and a strong excimer emission was observed
in the case of the disilane unit as a linker. It is to be noted that
a charge-transfer (CT) state of diaryloligosilanes is formed in
addition to excimer formation. The direction of CT between
the aromatic group and disilane unit is a subject of contro-
versy.6-10
Figure 1. Di-(1-naphthyl) and di-(9-anthryl) oligosilanes evaluated
in this study.
In this report, we examined the photophysical properties of
the R,ω di-(1-naphthyl) and di-(9-anthryl) oligosilanes [Np-
(SiMe2)n-Np (1-5), Ant-(SiMe2)n-Ant (6-10), n ) 1-4 and
6] (Figure 1) in which π -π interactions between the two aryl
groups and σ-π interactions between the aryl group and the
silicon chain unit are expected to occur. It is very important to
compare the ability of the π -π interaction of these compounds
with those of a carbon chain analogue. Although a compound
with a carbon chain length of n ) 3 is known to have the highest
ability to produce an excimer (known as the Hirayama rule,
Figure 2),11 1,3-di-(9-anthryl)propane does not produce efficient
excimer emission because the excimer is the key intermediate
to give the [4 + 4] cycloadduct.12-14 The bond length of the
Si-Si or S-C bond is longer than the C-C bond; therefore,
Figure 2. Hirayama rule (top) and the sandwich dimer of di-(9-
anthryl)-propane (bottom left). Calculated structure of 6 (bottom right).
* To whom correspondence should be addressed. E-mail: karatsu@
faculty.chiba-u.jp. Phone: +81-43-290-3366. Fax: +81-43-290-3039.
† Chiba University.
the conformations before and after making the excimer should
be very different. In addition, changing the silane chain lengths
‡ Hokkaido University.
10.1021/jp0355760 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/11/2003

Fluorescence Spectra of R,ω Diaryloligosilanes
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12185
makes it easier to change the oxidative and reductive energy
levels versus the aromatic energy level. This may allow one to
understand the mechanism of CT-state formation.
Experimental Section
Compounds 1-10 were prepared from 1-naphthyllithium and
9-anthryllithium (obtained from aryl bromide and 1.6 M
n-butyllithium in hexane) and 1,n-dichloropermethyloligosilanes
(Cl-(SiMe2)n-Cl). The 1,2-dichlorotetramethyldisilane (n ) 2)
was purchased from Tokyo Kasei, and the other dichlorotri-,
dichlorotetra-, and dichlorohexasilanes (n ) 3, 4, 6) were
synthesized from dodecamethylcyclohexasilane and pentachlo-
rophosphine as previously reported.15 1-Pentamethyl-disilyl-
naphthalene (11) and 9-pentamethyldisilylanthracene (12) were
synthesized from the lithio-aromatics and chloropentamethyl-
disilane. The chloride was prepared by the reaction of am-
monium chloride with a hexamethyldisilane and sulfuric acid
mixture as previously reported.16 All of the compounds were
1
characterized using a JEOL SX-400 H NMR spectrometer.
The absorption spectra and steady-state fluorescence spectra
were measured using Hitachi U-3000, Hitachi F-4010, and
JASCO FP6600 spectrophotometers.
The molecular orbital calculation was performed by MOPAC
98.17
The fluorescence decay curves and time-resolved fluorescence
spectra were measured with a picosecond pulse laser and a
single-photon timing apparatus at Hokkaido University.18 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 (280 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 photomul-
tiplier (Hamamatsu R2809U-01). The pulse width of the
instrumental response function was 30 ps (fwhm). The pico-
second time-resolved fluorescence spectra were obtained from
the decay curves monitored at different wavelengths ranging
from 300 to 600 nm with a 0.625-nm step. The time-resolved
spectrum was constructed from a series of decay curves by
plotting the fluorescence (FL) intensities at the given delay times
as a function of the emission wavelength. The time constant of
the rise-up excimer component was obtained from the plots of
excimer emission intensities measured from the time-resolved
FL spectra versus time after the laser pulses.
Figure 3. 1H NMR chemical shift of di-(1-naphthyl) and di-(9-anthryl)
oligosilanes.
naphthalene (11), respectively. For the anthryl derivatives, H
at the 1, 4, 3, 2, and 10 positions of 9-(pentamethyldisilyl)-
anthracene (12) were observed at 7.42, 7.44, 8.00, 8.34, 8.43
ppm, respectively, and at 7.13, 7.35, 7.95, 8.24, and 8.40 ppm,
respectively, for 7. The signals of 1, 4-6, 9, and 10 appeared
at similar positions to the signals of disilanylaromatics 11 and
12, respectively. This indicates that the contribution of a
sandwich-type conformer in a ground-state conformational
equilibrium is very small.
The ground-state potential energy surface of rotamerism was
calculated by the MOPAC98/PM3 method. The calculation of
phenylpentamethyldisilane has been reported using a more
reliable method (CASSCF6-31G*);7c however, the molecules
here are too large to use such a basis set. The Si-C and Si-Si
bonds are longer; therefore, the activation energy for rotation
should be smaller than the C-C bond energy. Figure 4 shows
the calculated potential energy surfaces of rotation around the
Si-Si bonds of 2, 7, 11, and 12 and their carbon chain
analogues. The optimized structure has a Si-Si bond perpen-
dicular to the aromatic plane, which is favorable for σ-π
interactions between π orbitals of aromatics and σ orbitals of
Results and Discussion
Ground-State Conformation Studied by 1H NMR and MO
Calculations. The ground-state conformations for 1-10 were
1
considered by the chemical shift of the H NMR spectra. For
all of the 1-naphthyl and 9-anthryl compounds, eight and five
aromatic proton signals and one to three singlet methyl signals
from the dimethylsilyl units were observed. Figure 3 shows the
change in the chemical shift of the aromatic signals with silicon
chain length. The aromatic signals appeared as a singlet or
complicated coupling signals,indicating that a rapid rotation
along the Si-C or Si-Si bonds made the methyl groups and
two aromatic environments equivalent. In both series of naphthyl
or anthryl silanes, chemical shifts for the compounds with chain
lengths of two and three appeared in a higher magnetic field
region than those of the other compounds. For example, H at
positions 2-8 on a naphthalene ring were observed at 7.80,
7.20, 7.80, 7.57, 7.39, 7.39, 7.80 ppm for 2 and at 7.84, 7.46,
7.84, 7.63, 7.46, 7.46, 7.98 ppm for 1-(pentamethyldisilyl)-

12186 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Karatsu et al.
Figure 6. Fluorescence spectra of 1-5 in deaerated c-hexane and
acetonitrile.
Figure 4. Calculated potential energy surfaces for the rotation of the
Si-Si bond (PM3) of (a) 11 (circles) and 12 (squares), (b) 2, and (c)
6 and their carbon chain analogues.
Figure 7. Fluorescence spectra of 6-10 in deaerated c-hexane and
acetonitrile.
mol-1, respectively (Figure 4a). They are much smaller than
the value of 17 kJ mol-1 calculated for 1-naphthylCMe2-CMe3.
In addition, conformations were considered for 2 and 7 by
PM3. The potential energy surfaces of rotation around the
Si-Si bond were compared with those of carbon analogues as
shown in Figure 4b and c. For 2, two isomeric conformers exist
depending on the orientation of the two 1-naphthyl groups. One
is that two naphthyl groups stack on top of each other by only
one benzene ring when two Si-C bonds take an eclipsed
conformation to each other. This structure has a C2 axis
perpendicular between the Si-Si bonds. The eclipsed conformer
of another series has a structure such that two naphthyl groups
can completely stack face-to-face; therefore, it has mirror plane
symmetry (C2V). The former and latter series are expressed as
anti and syn, respectively, in Figure 4b. The rotational barrier
for the C-C bond with the eclipsed conformation of two
Figure 5. Absorption spectra of 1-5 and 6-10 in c-hexane.
the S-Si bond. The activation energies for the rotation of the
Si-Si bond for 11 and 12 were calculated to be 1.3 and 2.1 kJ

Fluorescence Spectra of R,ω Diaryloligosilanes
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12187
Figure 8. Time-resolved fluorescence spectra of 6-9 in cyclohexane and THF.
TABLE 1: Calculated Heat of Formation (∆∆H/kJ mol-1
)
of 8 is explained by the small population of the conformers
that leads to the excimer in equilibrium in the ground state and
the long distance between intramolecular chromophores due to
the long Si-Si bond length of the excimer.
for the Conformers of 8 by the PM3 Method
t
g+
g-
t
tt ) 0
g+
g+ ) 0.4
gg ) 18.1
g+g-
g- ) 0.4
g+
g-
g+g- ) 8.8
gg ) 18.1
Absorption and Steady-State Fluorescence Measurement.
Figure 5a and b indicates the absorption spectra of 1-5 and
6-10 in cyclohexane and acetonitrile, respectively. The absorp-
tion spectra of 1-5 and 6-10 are shifted about 15 nm to longer
wavelengths than that of naphthalene or anthracene. They are
very similar to each other, indicating neither a strong interaction
between the two aromatic chromophores nor a dependence of
the σ conjugation on the chain length. For 5 and 10, the
absorption band assigned to the hexasilyl group appeared at
g-
1-naphthyl groups is twice as high as that for the Si-Si bond.
This barrier becomes much higher for 1,2-di(9-anthryl)ethane
as shown in Figure 4c. The optimized structures of 7, 8, and 12
described above are similar to the recently reported structures
determined by X-ray crystallography.19
The heat of formation energies for the stable conformers
consisting of Si-Si-Si bonds of 8 were calculated as shown
in Table 1. The tt conformer is used as the starting conformer.
One of the Si-Si bonds is then twisted clockwise or counter-
clockwise to make two gauche conformers, g+ and g-. Similarly,
the gg and g+ g- conformers were made and optimized by the
PM3 method. There are almost no differences in energy between
the anti, g+, and g- conformers; however, there is a significant
difference between gg and g+g-. A weak excimer fluorescence
1
around 260 nm, and this is evident for 5. From the H NMR
analysis, compounds with n ) 2 to 3 show a weak interaction
between the two overlapping aromatic rings, as indicated by
the higher field shifts of the aromatic protons as mentioned
above.
Figures 6 and 7 show the fluorescence spectra of 1-5 and
5-10, respectively, in cyclohexane and acetonitrile. Compounds

12188 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Karatsu et al.
TABLE 2: Time Constant of Decay and Rise-up Components of the Fluorescence of 6-9a
in c-hexane
in THF
compds
420 nm
520 nm
420 nm
520 nm
6
7
8
9
4.90
5.60
4.96
5.63
82.1 ps + 248 ps + 7.76
-74.4 ps + 260 ps + 7.30
99.8 ps + 5.07 + 20.6
-85.0 ps + 5.32 + 19.1
126 ps + 1.55
-92.6 ps + 1.28
122 ps + 9.47
-98.8 ps + 7.71
152 ps + 2.82
-152 ps + 2.81
141 ps + 17.8
-116 ps + 8.30
a Values without units are presented in ns, and negative values at 520 nm are rise-up components.
Figure 9. Decay profiles of the fluorescence spectra of 6-9 observed at 440 and 520 nm in cyclohexane and THF.
1 and 5 produced only monomer-like fluorescence in both
solvents. However, a silicon chain length of more than 2 showed
excimer emission in cyclohexane. For 2-5, the intensity of the
excimer emission decreased with increasing chain length in
cyclohexane. For 6-10, the fluorescence behavior is somewhat
different from that of 1-5. Excimer emission with a large Stokes
shift was observed for 7 (Figure 5a). This shift (λmax ) 520
nm) is much larger than those observed for 8 and 9, indicating
the closely stacked conformation of 7 in its excimer. Steric
hindrance between the anthryl groups and dimethylsilane chain
unit may interrupt the conformation to produce close stacking
for 8 and 9. The λmax values at 620, 570, and 460 nm for the
excimer emission were reported for anthracenophanes with 1,
2, and 3 phenyl rings in anthracene stacked face-to-face,
respectively.20 In addition, there is the Hirayama rule that states
that an excimer is most stable with a three-carbon chain unit;
1,3-di(9-anthryl)propane undergoes efficient [4 + 4] cycload-
dition showing no excimer emission.13,14 In our case, 7 has
unique characteristics in which the Si-Si bond is longer than
the C-C bond; the two anthracene rings produce sandwich-
like stacking with a large distance between the 9-9′ positions
and a larger distance between the 10-10′ positions. The nearest
distance between the 9-9′ positions is calculated to be 3.55 Å
by the PM3 method for the eclipsed conformation of the ground
state (7) as shown in Figure 2 (bottom right). Therefore, it has
a strong excimer emission without undergoing efficient photo-
cyclization. This may be attributed to two reasons: (1) a fairly
long Si-Si bond length provides an appropriate distance
between the two anthracene rings for excimer formation and
(2) the conformation of the central Si-Si bond is restricted to
gauche by the steric hindrance of the methyl substitutions on
the Si atoms.
Solvent Effect and CT Emission. The λmax of the emission
of 1-3 and 5-9 in acetonitrile did not show a significant
difference from that in cyclohexane. However, the λmax of the
emission of 4, 5, and 10 shifted to longer wavelengths in
acetonitrile than in cyclohexane. These shifts were quite different
from those observed for the excimer emissions of 2 and 7. The
emission that showed a large shift can be assigned to the CT
emission because excimer emission does not show such a large
shift.
There have been many studies on the CT mechanism of aryl
disilanes.6-10 In our experiment, the CT emission band was
observed for the naphthylsilanes with n > 3 but was not
observed in the case of anthrylsilanes for 1 < n < 4 in a polar
solvent. These results are consistent with the mechanism
proposed by the σ f π* OICT mechanism,8,9 where CT
occurred from the silane to the aryl unit. The electron-donating
ability of the silane unit increases with increasing chain length,
and the electron-accepting ability of the anthryl group is lower
than that of the naphthyl group. Another mechanism of 2pπ*
f 3dπ6,7 is proposed as a alternative mechanism for the
phenyldisilane derivatives, and this mechanism does not seem
to contradict the experimental result. However, note that the
π*-state energy level of the naphthyl moiety is higher than that
of the anthryl moiety and that the energy of the 3dπ state
decreases with increasing silane chain length. Also noteworthy
is that the energy of the 3dπ state of the silane is expected to
be higher than that of the 2pπ* state of naphthalene. It follows
that as a favorable mechanism charge transfer occurs from the

Fluorescence Spectra of R,ω Diaryloligosilanes
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12189
Figure 10. Extraction of the rise-up components from the fluorescence spectra of 7-9. The observed points of the rise-up and simulation curves
of 50 (-), 100 (‚‚‚), 150 (- - -), 200 (- - -), and 400 ps (- ‚ - ‚), respectively. The bold lines were fitted using a nonlinear least-squares fitting
method.
silane unit to the naphthyl or anthryl group. The OICT
mechanism of σ f π*8,9 may also be appropriate, at least in
these cases.
Time-Resolved Fluorescence Measurement. The time-
resolved fluorescence spectra and their time profiles are shown
in Figures 8 and 9, respectively. The spectra are normalized at
the maximum wavelength. The spectra of 6 did not change with
time and decayed with the lifetimes of 4.90 and 5.60 ns (440
nm) in c-hexane and THF, respectively. For compounds 7 and
8, the FL maxima are shifted to longer wavelengths in the later
time region. The maxima are shifted to 520 and 550 nm for 7
and 8, respectively. For compounds 7-9, the rise-up components
were extracted from the spectra. The excimer component was
estimated by the method in which the monomer emission
observed just after laser excitation was subtracted from each
spectrum (Figure 8). The obtained intensities (I) of the excimer
emission at λmax were plotted versus time (Figure 10). The
Figures show the curves fitted using a nonlinear least-squares
method and the simulation curves using various time constants
(50, 100, 150, 200, and 400 ps). For compound 7 in c-hexane,
Figure 11. Time and solvent dependence of the shift of the excimer
the fluorescence around 440 nm decays in three components:
a fast-decaying component with a 100-ps lifetime, an intermedi-
ate decay component with 248-ps lifetime, and a long compo-
nent with a 7.76-ns lifetime. However, the fluorescence at 520
nm is composed of a rising component with a 100-ps rise time
and decay components of 5.07 and 20.6 ns. The two long-lived
components may imply the existence of two kinds of excimers.
These behaviors are very similar in THF. The spectra of 8 in
c-hexane did not significantly change but showed only a small
increase in the intensities around 550 nm. The fluorescence at
440 nm decays with lifetimes of 126 ps and 1.55 ns, and the
emission at 520 nm rises with a 122-ps rise time and decays
with a 9.47-ns lifetime. The spectral behavior of 9 was similar
to that of 8. These fast decay of the monomer FL and the rise
of the excimer FL are consistent with excimer formation. The
time constants showed almost no solvent dependence.
emission λmax for 7-9.
after excitation. This result is consistent with the result from
the 1H NMR analysis for the ground-state conformation
mentioned above. The FL quantum yields (Φf) for 6-10 were
0.40, 0.07, 0.05, 0.35, and 0.40, respectively, and these values
may partially reflect the inverse population of the sandwich-
like conformers in the ground state.
Figure 11 shows that the excimer FL λmax shift occurred on
a slower time scale after laser excitation. These shifts were
between 10 and 25 nm and occurred on the 20-ns time scale
depending on the chain lengths and solvents. The absolute shifts
are greater in THF than in c-hexane, indicating more stabilizing
excimer formation in THF than in c-hexane. The time scale of
this phenomenon is much slower than that observed in the
picosecond time frame mentioned above.
The time-resolved fluorescence indicates that the conformers
produce excimer emission from a weakly coupled dimer, even
in the ground state, because excimer emission rapidly rises just
Figure 12 shows the time-resolved fluorescence spectra of
10 in c-hexane and acetonitrile. 10 is the only compound in
which CT emission was observed in acetonitrile by a steady-

12190 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Karatsu et al.
Figure 12. Time-resolved spectra of 10 in (a) c-hexane and (b) acetonitrile.
stabilization of the excimer structure. This is more effective in
a polar solvent than in a nonpolar solvent as observed here.
This process might be slow because it accompanies the
reorientations of a large number of solvent molecules and a
readjustment of the face-to-face excimer. This process is slow
like the motion of two magnets after they have attracted each
other.
Conclusions
In conclusion, for the 2-5 and 7-10 cases, intramolecular
excimer emissions were observed in cyclohexane, and the
strongest excimer fluorescence was observed for 2 and 7 (chain
length n ) 2). CT emission was observed for 4, 5, and 10,
indicating charge transfer from the silane unit to the naphthyl
or anthryl group and suggesting a σ-π interaction in acetonitrile.
Only excimer emission was observed for 2, 3, and 7-9 in
acetonitrile as similarly seen in cyclohexane. Only monomer-
type emission was observed for 1 and 6 in both cyclohexane
and acetonitrile. The time-resolved fluorescence measurements
showed that the excimer fluorescence rise-up times (about 90,
120, 140 ps for 7-9, respectively) were weakly time-dependent
and independent of the solvent viscosity and polarity. These
results reveals that the average conformation is more stretched
for the longer-chain compounds; however, there is a significant
population of conformers that already have a weak interaction
in the ground state. In acetonitrile, the CT fluorescence rise time
was 45 ps, indicating a rapid conformational transformation from
the Franck-Condon state to the CT state.
Figure 13. Proposed potential energy surface of excimer formation.
state fluorescence measurement in the di-(9-anthryl)-oligosilane
series. The spectra did not show any change between just after
the laser excitation and 35 ns, indicating a very small contribu-
tion of the excimer formation in c-hexane. However, in
acetonitrile, the time-resolved spectra showed a time depen-
dence, and the fluorescence decayed in three components: 45
ps and 1.72 and 5.6 ns at λobs ) 440 nm, with an observed
47-ps rise-up and a 5.0-ns decay at λobs ) 570 nm. The
picosecond time scale change must be attributed to the CT
process, and this process may require a conformational change
as proposed by the OICT mechanism.9,10
Figure 13 shows a schematic representation of the potential
energy surfaces of the ground state and the excited-state excimer
formation. The excimer-formation process might involve two
processes having different time orders. The first process is a
fast process of around 100 ps corresponding to excimer
formation from the Franck-Condon state; however, this process
may occur for the conformer that has a sandwich-like conforma-
tion in the ground state. The second process might be a
stabilizing process including a solvent reorientation similar to
the reported TICT of bianthryl.21 The formation of the excimer
induces reorientations of the solvent molecules and then more
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research on Priority Areas (417, no.
14050022) and Scientific Research (no. 14550786) from the
Ministry of Education, Culture, Sports, Science, and Technology
(MEXT) of the Japanese Government to T.K.
References and Notes
(1) (a) Sakurai, H.; Sakamoto, K.; Nakadaira, A.; Kira, M. Chem. Lett.
1985, 497. (b) Daney, M.; Vanucci, C.; Desvergne, J.-P.; Castellan, A.;
Bouas-Laurent, H. Tetrahedron Lett. 1985, 26, 1505.

Fluorescence Spectra of R,ω Diaryloligosilanes
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12191
(2) Pitt, C. G.; Carey, R. N.; Toren, E. C., Jr. J. Am. Chem. Soc. 1972,
94, 3806.
(3) (a) Declercq, D.; Delbeke, P.; de Schryver, F. C.; Meervelt, L. V.;
Miller, R. D. J. Am. Chem. Soc. 1993, 115, 5702. (b) Declercq, D.; de
Schryver, F. C. Miller, R. D. Chem. Phys. Lett. 1991, 186, 467. (c) Declercq,
D.; Hermans, E.; de Schryver, F. C. Miller, R. D. Proc. - Indian Acad.
Sci., Chem. Sci. 1993, 105, 451.
(4) (a) Karatsu, T.; Shibata, T.; Nishigaki, A.; Fukui, K.; Kitamura,
A. Chem. Lett. 2001, 994. (b) Fang, M.-C.; Watanabe, A.; Matsuda, M.
Macromolecules 1996, 29, 6807.
(5) Becker, H.-D. AdVances in Photochemistry; Volman, D. H.,
Hammond, G. S., Gollnick, K., Eds.; Wiley & Sons: New York, 1990;
Vol. 15, pp 139-227.
(6) Pitt, C. G.; Carey, R. N.; Toren, E. C., Jr. J. Am. Chem. Soc. 1972,
94, 3806.
(7) (a) Shizuka, H.; Sato, Y.; Ueki, Y.; Ishikawa, M.; Kumada, M. J.
Chem. Soc., Faraday Trans. 1 1984, 80, 341. (b) Shizuka, H.; Okazaki,
K.; Tanaka, M.; Ishikawa, M.; Sumitani, M.; Yoshihara, K. Chem Phys.
Lett. 1985, 113, 89. (c) Yamamoto, M.; Kudo, T.; Ishikawa, M.; Tobita,
S.; Shizuka, H. J. Phys. Chem. A 1999, 103, 3144.
(8) Horn, K. A.; Grossman, R. B.; Throne, J. R. G.; Whitenack, A. A.
J. Am. Chem. Soc. 1989, 111, 4809.
(10) (a) Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1974, 96, 791. (b)
Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1975, 97, 4879.
(11) Hirayama, F. J. Chem. Phys. 1965, 42, 3163.
(12) Bouras-Laurent, H.; Desvergne, J.-P. Photochromism; Du¨rr, H.,
Bouras-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; Chapter 14, pp 561-
630 and references therein.
(13) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P. Pure Appl. Chem.
1980, 52, 2633.
(14) Bergmark, W. R.; Jones, G., II; Reinhardt, Th. E.; Halpern, A. M.
J. Am. Chem. Soc. 1978, 100, 6665.
(15) Gilman, H.; Inoue, S. J. Org. Chem. 1964, 29, 3418.
(16) Kumada, M.; Yamaguchi, M.; Yamamoto, Y.; Nakajima, J.; Shiina,
K. J. Org. Chem. 1956, 21, 1264.
(17) Stewart, J. J. P. MOPAC 98; Fujitsu Limited: Tokyo, Japan,
1998.
(18) (a) Nishimura, Y.; Yasuda, A.; Speiser, S.; Yamazaki, I. Chem Phys.
Lett. 2000, 323, 117. (b) Hasegawa, M.; Enomoto, S.; Hoshi, T.; Igarashi,
K.; Yamazaki, T.; Nishimura, Y.; Speiser, S.; Yamazaki, I. J. Phys. Chem.
B 2002, 106, 4925.
(19) Yang, D.-D. H.; Yang, N. C.; Steele, I. M.; Li, H.; Ma, Y.-Z.;
Fleming, G. R. J. Am. Chem. Soc. 2003, 125, 5107.
(20) Hayashi, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Morita, M.;
Tanaka, J. J. Am. Chem. Soc. 1976, 98, 5910.
(9) (a) Kira, M.; Miyazawa, T.; Sugiyama, H.; Yamaguchi, M.; Sakurai,
H. J. Am. Chem. Soc. 1993, 115, 3116. (b) Sakurai, H.; Sugiyama, H.; Kira,
M. J. Phys. Chem. 1990, 94, 1837. (c) Tajima, Y.; Ishikawa, H.; Miyazawa,
T.; Kira, M.; Mikami, N. J. Am. Chem. Soc. 1997, 119, 7400.
(21) Kang, T. J.; Kahlow, M. A.; Giser, D.; Swallen, S.; Nagarajan, V.;
Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 6800.