Homolytic cleavage of C-Si bond of p-trimethylsilylmethylacetophenone upon stepwise two-photon excitation using two-color two-laser flash photolysis

Article abstract of DOI:10.1016/j.cplett.2005.03.127

C-Si bond cleavage of p-trimethylsilylmethylacetophenone(1) occurred in a higher triplet excited state (T_n), giving mainly p-acetylbenzyl radical with the transient absorption in the region of 295-360 nm, with a quantum yield of 0.046 ± 0.008 using the two-color two-laser photolysis techniques. In contrast, the C-Si bond cleavage of p- trimethylsilylmethylbenzophenone(2) was absent in the T_n state whose energy is larger than the C-Si bond cleavage energy. The results can explain existence of a bond cleavage crossing between potential surfaces of the T _n state and a dissociative state of the C-Si bond for 1, but not for 2.

Full text of DOI:10.1016/j.cplett.2005.03.127

Chemical Physics Letters 407 (2005) 402–406
www.elsevier.com/locate/cplett
Homolytic cleavage of C–Si bond of
p-trimethylsilylmethylacetophenone upon stepwise
two-photon excitation using two-color two-laser flash photolysis
a
Xichen Cai , Masanori Sakamoto , Michihiro Hara , Susumu Inomata ,
a
a
b
b
Minoru Yamaji , Sachiko Tojo , Kiyohiko Kawai , Masayuki Endo ,
a
a
a
a
Mamoru Fujitsuka , Tetsuro Majima
a,
*
a
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
b
Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan
Received 26 August 2004; in final form 25 March 2005
Available online 14 April 2005
Abstract
C–Si bond cleavage of p-trimethylsilylmethylacetophenone(1) occurred in a higher triplet excited state (Tn), giving mainly p-ace-
tylbenzyl radical with the transient absorption in the region of 295–360 nm, with a quantum yield of 0.046 0.008 using the two-
color two-laser photolysis techniques. In contrast, the C–Si bond cleavage of p-trimethylsilylmethylbenzophenone(2) was absent in
the Tn state whose energy is larger than the C–Si bond cleavage energy. The results can explain existence of a bond cleavage crossing
between potential surfaces of the Tn state and a dissociative state of the C–Si bond for 1, but not for 2.
ꢀ 2005 Elsevier B.V. All rights reserved.
1. Introduction
triplet excited state (T1) by using the two-color two-laser
irradiation method [8]. The stepwise photocleavage of
two C–O bonds in 4-benzoylphenoxymethyl-substitued
naphthalenes through the Tn state with three-step excita-
tion by three-color three-laser irradiation method was
also shown [9]. It seems that the reactivity of the Tn state
is different from that of the T1 state. It is well known
that the C–Si bond is photochemically stable [10]. To
our best knowledge, there are no reports on the C–Si
bond cleavage under the direct light irradiation [10]. It
is suggested that the C–Si bond will not dissociate in
the S1 and T1 states due to the larger dissociation energy
of the C–Si bond than the pertinent state energies. In
contrast, in higher excited states, the C–Si bond may
cleave due to much higher excitation energies than the
dissociation energy of the C–Si bond.
The higher excited states show the higher reactivities
compared with the lowest excited state from the view-
point of excitation energy [1]. Some reactions, which
are still inert in the lowest excited state, could occur in
higher excited states. Therefore, investigation of the
properties of molecules in the higher excited singlet
(Sn) and triplet (Tn) states is an attractive subject [2–5].
The formation of carbazyl radical from carbazole in
the Sn and Tn states has been reported based on the tran-
sient absorption measurements using the double excita-
tion method [6,7]. We have recently reported that the
C–O bond in 1- and 2-[(4-benzoylphenoxy)methyl]naph-
thalenes cleaves in the Tn state, but not in the lowest
Herein, we report, for the first time, occurrence of the
C–Si bond cleavage of p-trimethylsilylmethylacetophe-
none (1) in the Tn state, but absence of decomposition
*
Corresponding author. Fax: +81 6 6879 8499.
E-mail address: majima@sanken.osaka-u.ac.jp (T. Majima).
0009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.03.127

X. Cai et al. / Chemical Physics Letters 407 (2005) 402–406
403
O
O
0.03
a
B
A
0.02
0.10
0.05
0.00
b
H C
3
0.15
0.01
0.00
CH
Si(CH )
2
3 3
CH Si(CH )
2 3 3
300 320 340 360
c,d
λ (nm)
1
2
0.0
0.5
1.0
0.10
0.05
0.00
Scheme 1. Structures of acetophenone and benzophenone derivatives
(1 and 2) used in the present study.
t (µs)
of p-trimethylsilylmethylbenzophenone (2) in the Tn
state. The structures of 1 and 2 are shown in Scheme
1. The occurrence of the C–Si bond fission depends
not only on the Tn state energy but also on the crossing
between the potential surfaces of the Tn and a dissocia-
tive state for the C–Si bond.
300
350
400
λ (nm)
450
500
550
Fig. 1. Transient absorption spectra obtained at 200 ns after the first
266-nm laser flash (dotted line) and at 100 ns after the second 355-nm
laser flash (solid line) during the two-color two-laser flash photolysis of
1. The delay time of the second laser after the first laser was 100 ns.
The broken spectrum shows the bleaching of 1(T1) without formation
of any peak. The insets show the spectral change in the region of 295–
360 nm, obtained by subtraction of the broken spectrum from the solid
one (A), and time profiles of the transient absorption (B) at 380 nm
using one 266-nm (6 mJ pulseÀ1) laser (a), or 266- and 355-nm
(60 mJ pulseÀ1) two-laser (b), and at 317 nm using one 266-nm laser
(c), or 266- and 355-nm two-laser (d) during the one- or two-laser flash
photolysis. (c) and (d) are overlapped.
2. Experimental
2.1. Materials
p-Trimethylsilylmethylacetophenone and p-trimethy-
lsilylmethylbenzophenone were synthesized according
to the literature [11]. Acetonitrile (spectral grade) was
purchased from Nacalai Tesque Inc. Sample solutions
were freshly prepared and deoxygenated by bubbling
with Ar. All experiments were carried out at room
temperature.
those of triplet acetophenone [12,13]. Therefore, the
transient absorption spectrum with a peak at 380 nm
is assigned to the T1 state of 1 (1(T1)) formed with a for-
mation quantum yield (UT) of 1.0 [13]. The extinction
coefficient of 1(T1) at 380 nm (e380) was calculated to
be 8000 500 MÀ1 cmÀ1 based on e330 = 7160 MÀ1
cmÀ1 of acetophenone(T1) as the reference [13]. Upon
266 or 355 nm laser photolysis of acetonitrile solution
of 2, a transient absorption spectrum with a peak at
530 nm was also assigned to 2(T1) (Fig. 2) since it was
similar to the absorption profiles of triplet benzophe-
none [12,13]. From the observation that no residual
transient absorption was left after depletion of the
absorption spectra of 1(T1) and 2(T1), it is inferred that
the C–Si bond does not dissociate in 1(T1) and 2(T1).
When 1(T1) was excited by the second 355-nm laser
pulsing, an immediate bleaching of the absorption of
1(T1) was observed, while almost no change was around
317 nm (Fig. 1, solid line). No recovery in DOD was ob-
served after the bleaching for 1(T1) (Fig. 1, inset B,b).
These results suggest that an absorption band appeared
in the region of 295–360 nm as shown in Fig. 1, inset A
is probably assigned to the p-acetylbenzyl radical,
although the transient absorption spectrum of p-acetylb-
enzyl radical has not been reported. The transient
absorption spectra of various p-substituted benzyl radi-
cals (p-X-benzyl radical: X = F, Cl, Br, CH3, OCH3,
CN, and NO2) have been observed to have a peak in
the region of 295–360 nm [14,15]. p-Acetylbenzyl radical
2.2. Two-color two-laser flash photolysis
Two nanosecond Nd:YAG laser systems (Contin-
uum, Surelite II-10; 5-ns fwhm, 10 Hz, and Brilliant,
Quantel; 5-ns fwhm, 10 Hz) were employed for laser
flash photolyses at 266, 355, and 532 nm. The probe
light was obtained from a 450 W Xe-lamp (Osram
XBO-450). The probe beam was arranged to the en-
trance side of the laser beam. The probe beam was then
focused on a monochromator (Nikon G250). The out-
put of the monochromator was monitored using a pho-
tomultiplier tube (PMT) (photomultiplier tube, PMT;
Hamamatsu Photonics R928). The signal from the
PMT was recorded on a transient digitizer (TDS 580D
four channel digital phosphor oscilloscope, 1 GHz,
Tektronix). A Hamamatsu Photonics multi-channel
analyzer (C5967) system was used for measurement of
the transient absorption spectra.
3. Results and discussion
Upon 266-nm laser photolysis of acetonitrile solution
of 1, a transient absorption spectrum with a peak at
380 nm was obtained (Fig. 1). The transient absorption
with a lifetime of several microseconds was efficiently
quenched by oxygen. These properties are similar to

404
X. Cai et al. / Chemical Physics Letters 407 (2005) 402–406
0.020
0.4
0.3
0.2
0.1
0.015
0.010
0.005
0.000
10
20
30
40
I (mJ/pulse)
Fig. 3. Plots of DDOD380 vs., 355-nm laser power, I.
350
400
450
(nm)
500
550
λ
Fig. 2. Transient absorption spectra obtained at 200 ns after the first
355-nm laser flash (dotted line) and at 100 ns after the second 532-nm
laser flash (solid line) during the two-color two-laser flash photolysis of
2. The delay time of the second laser after the first laser was 100 ns.
ber of the second laser photons absorbed by 1(T1) using
an actinometry of T–T absorption of benzophenone at
525 nm in benzene (eT = 6500 MÀ1 cmÀ1 at 525 nm)
[12,13].
is one of p-X-benzyl radicals and, therefore, it is as-
sumed to have the similar absorption to those of various
p-X-benzyl radicals. Because there is no report of e value
of p-acetylbenzyl radical, we assume that e317 of p-ace-
tylbenzyl radical is similar to that of benzyl radical
(e317 = 12000 1500 MÀ1 cmÀ1) [14,15]. The transient
absorption spectrum of p-acetylbenzyl radical
(DDOD317 in Fig. 1, inset A) was overlapped with that
of 1(T1) in the region of 295–360 nm, and obtained by
subtraction of the broken spectrum (with the bleaching
of 1(T1) but no formation of any peak) from the solid
one. Therefore, the formation of p-acetylbenzyl radical
can be calculated from DDOD317. The concentration of
On the other hand, bleaching in the absorption spec-
trum of 2(T1) was not observed when 2(T1) was excited
by the second 532-nm (Fig. 2) or 355-nm laser pulsing.
These observations indicate that 2(Tn) undergoes a non-
radiative deactivation to the 2(T1) within the second
laser-pulse duration.
The triplet energies of 1(T1) and 2(T1) (E1(T1)
=
292 kJ molÀ1 and E2(T1) = 286 kJ molÀ1) determined
from the origins of phosphorescence spectra of 1 and 2
obtained in an mixture of methanol and ethanol (1:1
v/v) at 77 K are larger than the C–Si bond dissociative
energies (D(C–Si)) estimated to be 262 kJ molÀ1 for 1
and 234 kJ molÀ1 for 2, respectively, by using the
semi-empirical PM3 calculations. For the bond cleav-
age, distribution of the excitation energy to the cleaving
bond in the molecule is essential [10,16–18]. Hence, the
absence of C–Si bond cleavage suggests that the excita-
tion energy is not sufficiently distributed to the C–Si
bond in 1(T1) and 2(T1), or, in other words, that a large
energy barrier (E) may exist between the potential sur-
faces of the T1 state and the C–Si bond dissociation.
From the obtained results, it is inferred that the
occurrence of the C–Si bond cleavage in the Tn state is
controlled by not only the energy but also the electronic
delocalization of the molecule. The occurrence of the
C–Si bond cleavage in 1(Tn) and the absence of that in
2(Tn) can be explained by electronic delocalization in
the Tn state including the C–Si bond or the existence
of a thermally activated crossing between potential sur-
faces of the Tn state and a p, r* or a r, r* state leading
to the C–Si bond dissociation according to a selection
rule for bond cleavage (avoided crossing) [19]. A sche-
matic energy diagram of photoexcited 1 upon stepwise
laser photolysis is shown in Scheme 2.
1(T1)
disappeared
([1(T1)] = (DDOD380 = 0.022)/
(e380 = 8000 500 MÀ1 cmÀ1) = 3 · 10À6 M) was similar
to that of the p-acetylbenzyl radical formed ([p-acetylb-
enzyl
radical] = (DDOD317 = 0.031)/(e317 = 12000
1500 MÀ1 cmÀ1) = 3 · 10À6 M) within the second-laser
duration, where DDOD380 and DDOD317 were the de-
crease and increase of the transient absorption at 380
and 317 nm, assigned to 1(T1) and p-acetylbenzyl radi-
cal, respectively. Therefore, it is suggested that the dis-
appeared 1(T1) changes mainly to p-acetylbenzyl
radical and trimethylsilyl radical through the C–Si bond
cleavage within the second-laser duration. The forma-
tion of trimethylsilyl radical was not observed with the
transient absorption measurements because of no
absorption in the region of 300–600 nm [14].
The DDOD380 value increased with increasing the sec-
ond 355-nm laser. When DDOD380 was plotted against
the second 355-nm laser power, I (Fig. 3), a straight line
was obtained. From the slope of the line, the quantum
yield (U) of the C–Si bond cleavage from 1(Tn) was esti-
mated to be 0.046 0.008 from DDOD380 and the num-

X. Cai et al. / Chemical Physics Letters 407 (2005) 402–406
405
the Tn state are effected by the phenyl and methyl
substituents.
Tn
kdis
4. Conclusions
hν2
S1
T1
The C–Si bond cleavage from 1(Tn) with
U = 0.046 0.008 has been observed by using the two-
color two-laser flash photolysis techniques for the first
time. The disappeared 1(T1) changes mainly to p-ace-
tylbenzyl and trimethylsilyl radicals through the C–Si
bond cleavage. The occurrence of the C–Si bond cleav-
age depends on both excitation energy and crossing be-
tween potential surfaces of the Tn and the C–Si bond
dissociation of 1. On the other hand, C–Si bond cleav-
age does not occur from 2(Tn). These results indicate
that the photochemical properties of the Tn state are
quite different from those of the T1 state, and strongly
depend on the molecules. The present study shows pos-
sibilities that new reactions may be found in the Sn and
Tn states of various molecules by using the two-color
two-laser flash photolysis technique.
∆E
C • + • SiMe3
hν1
S0
ISC
D(C-Si) = 262 kJ mol-1
0
Reaction coordinate
Scheme 2. An energy diagram of photoexcited 1 by two-color two-
laser photolysis techniques. hm1: the first 266-nm laser excitation; hm2:
the second 355-nm laser excitation; ISC: intersystem crossing; kdis: rate
constant of the C–Si bond dissociation; DE: energy barrier between the
potential surfaces; CÅ: p-acetylbenzyl radical; ÅSiMe3: trimethylsilyl
radical.
When the S1 state of 1 is produced upon the first 266-
s
À1) intersystem
nm laser irradiation, a very fast (ꢀ1011
crossing (ISC) from the S1 to the T1 state proceeds [1].
No C–Si bond cleavage occurred from the S1 and T1
states although E1(S1) and E1(T1) are higher than
D(C–Si), suggesting that DE is too large for a bond cleav-
age crossing to proceed from the potential surfaces of
the S1 or T1 states to that of the C–Si bond dissociation.
On the other hand, the C–Si bond dissociated from
1(Tn) generated upon excitation of 1(T1) by the second
355-nm laser photolysis. It seems that E1(Tn) is much
higher than D(C–Si), and that DE is small for the crossing
between potential surfaces of the Tn state and the C–Si
bond dissociation of 1. Although U of the C–Si bond
cleavage from 1(Tn) was 0.046 0.008 and the non-reac-
tive process is the major route for relaxation of 1(Tn),
the disappeared 1(T1) changes mainly to p-acetylbenzyl
and trimethylsilyl radicals through the C–Si bond cleav-
age. Because 1(T1) can be selectively excited to give
1(Tn), one can control the occurrence of the C–Si bond
cleavage from 1(Tn) to give p-acetylbenzyl and trimeth-
ylsilyl radicals using the two-color two-laser flash pho-
tolysis technique. The occurrence of the chemical
reactions can be initiated from the Tn state, even when
such chemical reactions can not proceed in the T1 state.
The similar mechanism can be considered for 2(T1) and
2(Tn) where the C–Si bond cleavage is absent. DE is still
high for the crossing between potential surfaces of the
Tn state and the C–Si bond dissociation of 2. It is sug-
gested that the Tn state energy is still localized on the
benzophenone moiety in 2(Tn). In other words, the exci-
tation energy is not sufficiently dispersed from the ben-
zophenone moiety to the C–Si bond even in 2(Tn). The
difference between 1 and 2 is only phenyl and methyl at-
tached to the carbonyl group. The different experimental
results for 1(Tn) and 2(Tn) indicate that the properties of
Acknowledgments
This work has been partly supported by a Grant-in-
Aid for Scientific Research on Priority Area (417), 21st
Century COE Research, and others from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT) of Japanese Government. The authors also
thank to JSPS fellowship for X.C.
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