Time-resolved EPR and laser photolysis investigations of photoinduced ω-bond dissociation in an aromatic carbonyl compound having triplet π,π* character

Article abstract of DOI:10.1016/j.cplett.2005.09.123

Photochemical properties of photoinduced ω-bond dissociation in p-phenylbenzoylbenzyl phenyl sulfide (PPS) having the lowest triplet state (T₁) of π,π* character in solution were investigated by time-resolved EPR and laser flash photolysis techniques. PPS was found to undergo photoinduced ω-bond cleavage in the excited lowest singlet state (S₁(n,π*)) with a quantum yield (Φ_rad) of 0.15 for the radical formation, which was independent of excitation wavelengths. Based on the facts of the observation of the absorption spectrum of triplet PPS upon triplet sensitization of xanthone, and absence of CIDEP signal, ω-cleavage was shown to be absent in the T₁(π,π*) state of PPS. Considering the electronic character of the excited and dissociative states of PPS, a schematic energy diagram for the ω-bond dissociation of PPS was shown.

Full text of DOI:10.1016/j.cplett.2005.09.123

Chemical Physics Letters 417 (2006) 211–216
www.elsevier.com/locate/cplett
Time-resolved EPR and laser photolysis investigations
of photoinduced x-bond dissociation in an aromatic carbonyl
compound having triplet p,p* character
a,
a
b
b
*
Minoru Yamaji , Susumu Inomata , Satoru Nakajima , Kimio Akiyama ,
b
a
c
Shozo Tero-Kubota , Seiji Tobita , Bronislaw Marciniak
a
Department of Chemistry, Gunma University, Tenjin-cyo, 1-5-1, Kiryu 376-8515, Japan
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
b
c
Faculty of Chemistry, Adam Mickiewicz University, Poznan 60-780, Poland
Received 16 September 2005; in final form 26 September 2005
Available online 24 October 2005
Abstract
Photochemical properties of photoinduced x-bond dissociation in p-phenylbenzoylbenzyl phenyl sulfide (PPS) having the lowest trip-
let state (T ) of p,p* character in solution were investigated by time-resolved EPR and laser flash photolysis techniques. PPS was found
to undergo photoinduced x-bond cleavage in the excited lowest singlet state (S (n,p*)) with a quantum yield (Urad) of 0.15 for the radical
formation, which was independent of excitation wavelengths. Based on the facts of the observation of the absorption spectrum of triplet
PPS upon triplet sensitization of xanthone, and absence of CIDEP signal, x-cleavage was shown to be absent in the T (p,p*) state of
1
1
1
PPS. Considering the electronic character of the excited and dissociative states of PPS, a schematic energy diagram for the x-bond dis-
sociation of PPS was shown.
ꢀ
2005 Elsevier B.V. All rights reserved.
1
. Introduction
deed the bond enthalpies for the breaking C–Cl, C–S and
C–Br bonds in the benzophenone derivatives were smaller
than the corresponding triplet energies [20–22] while
p-hydroxymethylbenzophenone having an enthalpy for
the C–O bonding larger than the triplet energy was inert
to photodecomposition [20]. Interestingly, with p-bromom-
Photoinduced bond dissociation of aromatic carbonyl
compounds has been widely documented, such as Norrish
Type I and II reactions, where carbon–carbon bond fission
occurs at the a- and b-positions of the carbonyl, respectively
[
1–19]. Recently, we have been studying a new-type photo-
ethylbenzophenone having the C–Br bond, the S (n,p*)
1
induced homolytic bond cleavage which occurs neither at
a- nor b-position, but at the x-position of benzophenone
derivatives by using time-resolved EPR and laser photolysis
techniques [20–22]. x-Bond cleavage of benzophenone
derivatives having C–S, C–Cl and C–Br bonds was charac-
state was also shown to be reactive for x-cleavage [21].
With p-benzoylbenzyl phenyl sulfide, the quantum yield
for x-cleavage was shown to depend on the excitation wave-
length [22]. The reactivity of x-bond cleavage seems to be
closely related to the spin multiplicity and electronic charac-
ter of the excited states, bond enthalpies and leaving groups.
In this context, x-cleavage reactivity of aromatic ke-
terized to occur mainly in the T (n,p*) state. It seems that
1
for occurrence of x-bond dissociation, the enthalpy of the
cleaving bond must be smaller than the triplet energy. In-
tones having the T state of p,p* character should be our
1
next interest. When a phenyl group is introduced in benzo-
phenone to become phenylbenzophenone, it is known that
*
Corresponding author. Fax: +81 277 301212.
E-mail address: yamaji@chem.gunma-u.ac.jp (M. Yamaji).
the electronic character of the T state changes from n,p*
1
0
009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.09.123

212
M. Yamaji et al. / Chemical Physics Letters 417 (2006) 211–216
to p,p* [1]. In the present work, reaction profiles of photo-
induced x-bond dissociation in a phenylbenzophenone
derivative having a phenylthiyl moiety as a leaving group,
i.e., p-phenylbenzoylbenzyl phenyl sulfide (PPS) are inves-
tigated by means of laser photolysis and time-resolved
EPR (TR-EPR) techniques. Based on the electronic char-
acter of the excited and dissociative states of PPS, a sche-
matic energy diagram for the x-bond dissociation of PPS
is illustrated for understanding the photochemical proper-
ties of x-bond dissociation in solution.
analyzed by using the least-squares best-fitting method.
The transient absorption spectra were taken with an
USP-554 system from Unisoku with which one can take
a transient absorption spectrum with single laser pulse.
The TR-EPR measurements were performed with a Var-
ian E-109E X-band EPR spectrometer without field modu-
lation as reported previously [24]. Transient EPR signals
generated by the pulsed laser irradiation were detected by
the diode of the EPR spectrometer and transferred to a
boxcar integrator (EG&G Model 4121B) for the TR-EPR
spectra. The frequency of microwave and the strength of
magnetic field were measured by a microwave counter
O
(
(
Echo Electronics EMC-14) and an NMR field meter
Echo Electronics EFM-2000AX), respectively. The third
CH2
S
harmonics (355 nm) of the Nd:YAG laser (Quanta-Ray
INDI-40-20, Spectra Physics, 20 Hz) was used as the light
source. Sample solutions for CIDEP measurements were
constantly deoxygenated by Ar gas bubbling and flowed
into a quartz cell in the EPR resonator. The TR-EPR mea-
surements were carried out in butyronitrile solution
PPS
ꢀ
3
ꢀ3
2
. Experimental
p-Phenylbenzoyl toluene (PBT) was synthesized by usual
(5.0 · 10 mol dm ) at 295 K.
3. Results and discussion
Friedel-Crafts acylation of toluene with p-phenylbenzoyl
chloride. PBT was refluxed in CCl4 in the presence of
SO Cl and dibenzoyl peroxide, providing 4-(p-phen-
Fig. 1 shows absorption and phosphorescence spectra of
PPS in ACN at 295 K and in a glass matrix of a mixture of
methanol and ethanol (1:1 v/v) at 77 K, respectively. The
absorption bands at 290 nm having a molar absorption
2
2
ylbenzoyl)benzyl chloride (PBC). p-Phenylbenzoylbenzyl
phenyl sulfide (PPS) was synthesized by the reaction of
PBC with thiophenol in the presence of K CO in acetone.
4
3
ꢀ1
ꢀ1
2
3
coefficient of the order of 10 dm mol cm can be char-
acterized due to the S (p,p*) S0 transition while that at
PPS was purified by repeated recrystallizations from hex-
ane. Xanthone (XT) was recrystallized from ethanol for
purification.
Acetonitirile (ACN), methanol, ethanol, butyronitrile
were distilled for purification and used as solvents. ACN
and butyronitrile were used for absorption measurements
and TR-EPR measurements at 295 K, respectively, while
a mixture of methanol and ethanol (1:1 v/v) was for phos-
phorescence measurement at 77 K.
2
350 nm can be assigned to the S (n,p*) band. It was con-
1
firmed that the phosphorescence excitation spectrum of
PPS agreed well with the corresponding absorption spec-
trum. The energy level of the lowest triplet (T ) state of
1
ꢀ1
PPS was determined to be 60.4 kcal mol from the phos-
phorescence origin. The feature of the phosphorescence
spectrum of PPS is similar to that of p-phenylbenzophe-
none whose triplet character is known to be of p,p* [1].
Absorption and emission spectra were recorded on a U-
best 50 spectrophotometer (JASCO) and a Hitachi F-4010
fluorescence spectrophotometer, respectively.
Therefore, the electronic character of the T state of PPS
is assigned to be of p,p*.
1
All the samples for transient absorption measurements
were degassed in a quartz cell with a 1 cm path length by
several freeze–pump–thaw cycles on a high vacuum line.
The concentration of PPS for direct laser photolysis was
adjusted to achieve the optical density at the excitation
wavelength (266, 308 or 355 nm) being ca. 0.7 in ACN.
Transient absorption measurements were carried out at
1
3
2
1
0
x 20
2
95 K. A XeCl excimer laser (308 nm, Lambda Physik,
Lextra 50) and third (355 nm) and fourth harmonics
3
+
(
266 nm) of a Nd :YAG laser (JK Lasers HY-500; pulse
abs
phos
500
width 8 ns) were used as light sources for flash photolysis.
The number of the repetition of laser pulsing in a sample
was less than four pulses to avoid excess exposure. The de-
tails of the detection system for the time profiles of the
transient absorption have been reported elsewhere [23].
The transient data obtained by laser flash photolysis was
0
300
400
600
Wavelength / nm
Fig. 1. An absorption spectrum of PPS in ACN at 295 K and a
phosphorescence spectrum of PPS in a mixture of methanol and ethanol
(1:1 v/v) at 77 K.

M. Yamaji et al. / Chemical Physics Letters 417 (2006) 211–216
213
[
28]). By using Eqs. (1) and (2), the Urad values were deter-
mined to be 0.14 ± 0.02, 0.15 ± 0.02 and 0.16 ± 0.02 upon
66, 308 and 355 nm laser photolyses, respectively. The ob-
0
0
.2
.1
0
2
tained results show that the Urad value does not depend on
the excitation wavelength.
In order to investigate x-cleavage reactivity in the trip-
let state of PPS, triplet sensitization by using of xanthone
(
(
(
x 10
XT) was carried out. Since the triplet energy of XT
ꢀ
1
74.0 kcal mol
[28]) is larger than that of PPS
ꢀ1
60.4 kcal mol ), efficient triplet energy transfer is possi-
ble from triplet XT to PPS. The triplet–triplet absorption
peak of XT is located at 625 nm in ACN [22,29]. When
4
00
500
600
Wavelength / nm
3
55 nm laser photolysis was carried out in a XT
ꢀ
3
ꢀ4
ꢀ3
(
0.01 mol dm )-PPS (5.0 · 10 mol dm ) system, the
Fig. 2. Transient absorption spectra observed at 500 ns in degassed ACN
solid) and at 4.8 ls in aerated ACN (dotted) of PPS after 266 nm laser
(
intensity at 625 nm due to triplet XT decreased with a de-
6
ꢀ1
pulsing at 295 K.
cay rate (kobsd) of 8.3 · 10 s (see inset in Fig. 3). The
transient absorption obtained at 700 ns, when the triplet
XT was completely decayed, is shown in Fig. 3. Appar-
ently, the absorption spectrum is due to triplet PPS. After
depletion of the triplet absorption spectrum, any transient
Fig. 2 shows transient absorption spectra observed after
66 nm laser pulsing in degassed and aerated ACN solu-
2
tions of PPS at 295 K. The transient absorption spectrum
at 500 ns obtained in the degassed solution is similar to that
of triplet p-phenylbenzophenone, and its intensity was
accelerated to decrease in the presence of dissolved oxygen.
Therefore, the absorption spectrum at 500 ns is ascribed to
triplet PPS. The transient absorption spectrum obtained at
0
.5
0
4
.8 ls after quenching of triplet PPS by dissolved oxygen is
similar to that of the phenyl thiyl radical (PTR;
0
1
0
.05
3
ꢀ1
ꢀ1
Time /µs
e = 2000 ± 100 dm mol cm
at 450 nm [25]). Theses
observations indicate that x-bond dissociation of PPS oc-
curs to produce PTR upon direct excitation, competing
with formation of triplet PPS via intersystem crossing.
The counter radical of PTR, p-phenylbenzoylbenzyl radical
(
PBBR) whose absorption spectrum is expected to appear
0
4
00
500
Wavelength / nm
600
around 320 nm, was not clearly seen in the wavelength re-
gion 300–700 nm. Upon 355 and 308 nm laser photolysis of
PPS in ACN, it was confirmed that the absorption spec-
trum of PTR appeared after complete quenching of triplet
PPS by dissolved oxygen.
Fig. 3. A transient absorption spectrum obtained at 700 ns upon 355 nm
laser photolysis of a XT (0.01 mol dm )-PPS (5.0 · 10 mol dm
system in ACN. Inset; a temporal absorbance change at 625 nm for
triplet XT.
ꢀ
3
ꢀ4
ꢀ3
)
The quantum yield (Urad) of the radical formation upon
laser pulsing of PPS was determined with the use of Eq. (1).
ꢀ
1
ꢀ1
;
U
rad ¼DA450 4
e
I
ð1Þ
50 abs
1
1
5
0
5
0
where DA450, e450 and Iabs are, respectively, the absorption
change at 450 nm after quenching of triplet PPS by dis-
solved oxygen, the molar absorption coefficient of PTR
at 450 nm (2000 dm mol cm [25]) and the number of
the photon flux of a laser pulse at the excitation wave-
length. The quantity of Iabs was determined by using the
absorption of triplet benzophenone (BP) in ACN as an
actinometer [26]
3
ꢀ1
ꢀ1
BP
BP BP
DAT ¼e U
I
abs
;
ð2Þ
T
ISC
BP
T
BP
T
BP
0
5
10
where DA , e and UISC are, respectively, the initial
[
PPS] / 10–4 mol dm–3
absorbance at 520 nm for the formation of triplet benzo-
phenone obtained immediately after laser pulsing, the mo-
lar absorption coefficient of triplet BP at 520 nm in ACN
Fig. 4. The rate (kobsd) for the decay of triplet XT plotted against [PPS]
upon 355 nm laser photolysis in XT (0.01 mol dm )-PPS systems in ACN
at 295 K.
ꢀ3
3
ꢀ1
ꢀ1
(
6500 dm mol cm [27]) and triplet yield of BP (1.0

214
M. Yamaji et al. / Chemical Physics Letters 417 (2006) 211–216
0
absorption was not seen in the wavelength region studied,
50–700 nm. The decay rates, kobsd of triplet XT are plot-
ted as a function of the concentration of PPS, [PPS] in
Fig. 4. Since the plots give a straight line, the kobsd can
be formulated by:
program contained in MOPAC 97, being D H(PPS) =
f
ꢀ
1
ꢀ1
3
77.5 kcal mol , D H(PBBR) = 76.1 kcal mol
(PTR) = 56.1 kcal mol . The D(C–S) value was calculated
to be 54.7 kcal mol for PPS by using Eq. (4).
and D H
f
f
ꢀ
1
ꢀ
1
D
f
HðPPSÞ¼D
f
HðPBBRÞþD
f
HðPTRÞꢀDðC–SÞ
ð4Þ
k
obsd ¼k
0
þk
q
½PPSꢁ;
ð3Þ
The obtained value of D(C–S) is close to that
where k and k , respectively, represent the decay rate of
0
q
ꢀ1
(52.7 kcal mol ) of BBPS [22].
triplet XT in the absence of PPS and the rate constant
for quenching of triplet XT by PPS. From the intercept
and slope of the line, the values of k and k were deter-
Based on the obtained results, a schematic energy dia-
gram of photoexcited PPS is drawn in Scheme 1 including
the C–S bond cleavage processes. The dissociation profile
in the excited states of PPS can be interpreted to be a ther-
mally activated crossing with dissociative potential surfaces
leading to free radicals. The plausible dissociative poten-
tials are of p,r* and p,p*, and the energy level of the for-
mer would be located lower than that of the latter at a
reaction coordinate. According to a crossing rule, excited
states of p,p* would correlate with a r,r* state which leads
to a pair of r radicals whereas those of n,p* would interact
0
q
6
ꢀ1
9
3
ꢀ1 ꢀ1
mined to be 3.5 · 10 s
The determined value is close to that (8.6 ·
) for XT sensitization of p-benzoylbenzyl
and 8.3 · 10 dm mol
s .
k
q
9
3
ꢀ1 ꢀ1
1
0 dm mol
s
phenyl sulfide (BBPS) having an x-dissociative T (n,p*)
1
state. We have reported that transient absorption due to
triplet BBPS did not appear upon XT sensitization of
BBPS [22]. From these observations and considerations,
it is inferred that x-bond dissociation is absent in the
T (p,p*) state of PPS, and concluded that upon direct pho-
1
toexcitation of PPS, x-cleavage proceeds in the S (n,p*)
1
state of PPS competing with the intersystem crossing to
3(
σ, σ*)
the T state since the U
value for PPS is independent
rad
1
of excitation wavelength. It should be noted that the effi-
1(
π, σ*)
ciency (a ) for x-cleavage in the triplet state of benzophe-
dis
none derivatives strongly depends on the electronic
character, (n,p*) or (p,p*).
S2(π,π*)
S1(n,π*)
3
3
T1(π, π*)
The above assignment of the reactive state was sup-
ported by the TR-EPR result. We tried to detect CIDEP
signals from photodecomposed PPS. However, no CIDEP
signal was observed upon 355 nm photolysis of PPS,
whereas the transient absorption measurements clarified
free radical formation. The TR-EPR result strikingly con-
trasts with that obtained in the photolysis of BBPS. The
strong net-emissive CIDEP spectrum due to the triplet
mechanism was observed upon photolysis of BBPS [22].
These results indicate that the spin multiplicity in the
photocleavage process of PPS is different from that of
BBPS. When the C–S bond dissociates in the singlet state
of PPS, a singlet p,r radical pair of PBBR and PTR may
be initially produced in a solvent cage according to the spin
conservation rule. In singlet radical pairs, geminate recom-
bination is allowed to proceed to reform the parent mole-
cule in the solvent cage. The rate of radical escaping
from the solvent cage is considered to be smaller than that
∆
Edis
PBBR + PTR
D(C-S)
S0 (PPS)
Reaction Coordinate
Scheme 1.
3(
σ,σ*)PPS
3(
π,σ*)BBPS
BBPS
1
0
ꢀ1
T
(
ꢂ10
at 295 K, leading to a low escaping yield from solvent cages
Urad = 0.15). It was impossible to determine the net quan-
tum yield (U ) of the C–S bond dissociation in the S state
s
[30,31]) of geminate recombination in solution
1(n, π*)
PPS
T1
(π, π*)
∆Edis
(
dis
1
Radicals
by nanosecond laser spectroscopy since the geminate
recombination between PBBR and PTR would be an event
during the laser pulse duration (ꢂ8 ns) in the present sys-
D(C-S)
Ground state of BBPS or PPS
C-S Bond distance
tem. The value of U is estimated to be larger than that
dis
of Urad (0.15).
The enthalpy of the C–S bond in PPS, D(C–S) was ob-
tained on the basis of the heat of formation (D H) for PPS,
PBBR and PTR computed by using a semi-empirical PM3
f
Fig. 5. The energy diagram for the T1 state and the correlating
dissociative potential surface of PPS compared with that of BBPS.

M. Yamaji et al. / Chemical Physics Letters 417 (2006) 211–216
215
Table 1
Quantum yields (U) for radical formation, efficiencies (adis) for x-cleavage in the triplet state, triplet energies (E
T
), bond enthalpies (D(C–S)) and electronic
a
character of the T
1
states
ꢀ1
ꢀ1
Compound
U
266
U
308
U
355
a
dis
E
T
(kcal mol
)
D(C–S) (kcal mol
)
1
T character
BBPS
PPS
a
0.65
0.14
0.64
0.15
1.0
0.16
1.0
0
68.4
60.4
52.7
54.7
n,p*
p,p*
Data for BBPS are cited from [22].
with a p,r* state [32]. With respect to PPS, the S (n,p*)
to the x-bond according to a crossing rule (avoided cross-
1
state interacts with a singlet p,r* potential for the C–S
anti-bonding. From the fact that the transient absorption
of triplet PPS was obtained upon direct excitation, it is in-
ing) [32].
4. Conclusion
ferred that the x-cleaving process in the S state of PPS
1
competes with intersystem crossing from the S (n,p*) to
By means of TR-EPR and laser flash photolysis tech-
niques, x-bond cleavage of PPS is characterized, and pho-
tochemical properties obtained are summarized in Table 1
along with those of BBPS. x-Bond dissociation in PPS oc-
1
the T (p,p*) state, which is an allowed transition according
1
to the El-Sayed rule [33]. Therefore, the intersystem cross-
1
1
ꢀ1
ing rate of PPS may be as large as that (1 · 10
s
[34]) of
benzophenone. This estimation is rationalized by the above
consideration that the rate of the C–S bond fission compet-
curs only in the S (n,p*) state independent of excitation
1
wavelengths with a radical yield, Urad of 0.15. Due to a
3
itive with intersystem crossing in the S state may be in the
large energy barrier between the T (p,p*) and the (r,r*)
1
1
1
1
ꢀ1
magnitude of 10
s
.
potential surfaces, x-cleavage is absent in the T (p,p*) state
1
In the present work, although it is obvious that the x-
of PPS. It is considered that energy barriers for distribut-
ing the excited energy from the aromatic carbonyl moiety
to the anti-bonding orbital of the leaving group may con-
trol the reactivity of x-bond dissociation independent of
electronic character of the excited states.
ꢀ
1
bond enthalpy (54.7 kcal mol ) in PPS is smaller than
ꢀ
1
the triplet energy (60.4 kcal mol ) of PPS, it is revealed
that x-dissociation does not proceed in the T (p,p*). This
1
may be due to a large energy barrier (DE ) for bond dis-
dis
3
sociating between the T (p,p*) and a dissociative (r,r*)
1
potential surface. For the case of BBPS having an x-disso-
ciative T (n,p*) having a unity efficiency of x-fission (a ),
Acknowledgments
1
dis
the DEdis value was found to be close to zero [23]. The exis-
tence of an energy barrier can be understood by consider-
ing the difference in energy levels of triplet states and
dissociative potential surfaces for PPS and BBPS (see
Fig. 5). By introducing a phenyl group into BBPS, the elec-
tronic character of triplet PPS becomes of p,p* with a
This work was supported by a Scientific Research
Grant-in-Aid from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
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ꢃ
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[
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