Photoinduced ω-bond dissociation of m-halomethylbenzophenones studied by laser photolysis techniques and DFT calculations. Substituted position effects

Article abstract of DOI:10.1039/b702598j

Photochemical profiles of ω-cleavage of carbon-X (X = Br and Cl) bonds in m-bromo- and m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo ω-bond cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds. From these observations, it was inferred that the ω-bond of these m-halomethylbenzophenones (m-HMBP) cleaves in the lowest excited singlet state (S₁(n,π*)) upon direct excitation. Upon triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption for an Ac-m-BMBP system, and an efficiency of the C-Br bond cleavage in the lowest triplet state (T₁(n,π*)) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an Ac-m-CMBP system. Absence of C-Cl bond cleavage in the triplet state of m-CMBP indicated the reactive state of m-CMBP for ω-cleavage is only the S ₁(n,π*) state. Based on the efficiencies and DFT calculations for excited state energies, photoinduced ω-bond dissociation of m- and p-HMBPs was characterized. the Owner Societies.

Full text of DOI:10.1039/b702598j

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Photoinduced x-bond dissociation of m-halomethylbenzophenones studied
by laser photolysis techniques and DFT calculations. Substituted position
effectsw
Minoru Yamaji,*a Michiyo Ogasawara,a Kazuhiro Kikuchi,a Satoru Nakajima,b
d
Shozo Tero-Kubota,b Bronislaw Marciniakc and Koichi Nozakiz
Received 19th February 2007, Accepted 13th April 2007
First published as an Advance Article on the web 11th May 2007
DOI: 10.1039/b702598j
Photochemical profiles of o-cleavage of carbon–X (X = Br and Cl) bonds in m-bromo- and
m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis
techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo o-bond
cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were
determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds.
From these observations, it was inferred that the o-bond of these m-halomethylbenzophenones
(m-HMBP) cleaves in the lowest excited singlet state (S1(n,p*)) upon direct excitation. Upon
triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption
for an Ac–m-BMBP system, and an efficiency of the C–Br bond cleavage in the lowest triplet state
(T1(n,p*)) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an
Ac–m-CMBP system. Absence of C–Cl bond cleavage in the triplet state of m-CMBP indicated
the reactive state of m-CMBP for o-cleavage is only the S1(n,p*) state. Based on the efficiencies
and DFT calculations for excited state energies, photoinduced o-bond dissociation of m- and
p-HMBPs was characterized.
photolysis techniques.36–40 o-Cleavage of C–S, C–Cl and
C–Br bonds has been characterized to occur mainly in the
1. Introduction
Photoinduced bond cleavage is one of the well-known reac-
lowest triplet (T1) state. It seems that for occurrence of o-bond
dissociation, the enthalpy of the cleaving bond must be smaller
chemical reactions of carbonyl compounds, Norrish type I and
than the triplet energy. Indeed the bond enthalpies for the
tions in photochemistry and photobiology. As with photo-
II reactions where carbon–carbon bond fission occurs at the a-
and b-positions of the carbonyl, respectively, have been widely
cleavable C–Cl, C–S and C–Br bonds in the BP and NPK
derivatives were smaller than the corresponding triplet ener-
gies36,37,39,40 while BP derivatives having an enthalpy for the
documented by means of product analysis and time-resolved
transient measurements as well as carbon–heteroatom disso-
C–O or C–Si bonding larger than the triplet energies was inert
to photodecomposition.35a,39 Interestingly, in some BP and
NPK derivatives, it is shown that the excited singlet states are
reactive for o-cleavage,37 or that the quantum yields depend
on the excitation wavelength.39,40 It has been revealed that the
reactivity of o-bond cleavage is closely related to the spin
multiplicity and electronic character of the excited states, bond
enthalpies and leaving groups. We have studied o-cleavage at
the para-position of benzophenones. Since benzophenone is
one of the most fundamental compounds in photochemical
and photobiological investigations, its photophysical pro-
cesses are well understood through pico- and nano-second
time-resolved measurements.41 The lowest excited triplet state
(T1) of benzophenone is produced within 10 ps in solution due
to a fast intersystem crossing upon photoexcitation. It does
not seem that the substitution with alkyl groups at the para-
and meta-positions affects the photophysics for triplet forma-
tion. It is reported that photolysis of 2-methylbenzophenone
efficiently provides the corresponding triplet, which readily
undergoes intramolecular H-atom abstraction.42,43 However,
the effect of the substituted position to o-cleavage of benzo-
phenones has not been examined.
ciation occurring at the a- and b-positions of aromatic carbo-
nyls.1–28 Less attention, however, has been paid to
photoinduced rupture of remote carbon–heteroatom bonds
in aromatic carbonyl compounds, that is at positions other
than the a- or b-position of the carbonyl.29–35 Recently, we
also have been studying photolytic bond cleavage which
occurs neither at a- nor b-position, but namely, at the
o-position of benzophenone (BP) and naphthyl phenyl ketone
(NPK) derivatives by using time-resolved EPR and laser
a Department of Chemistry, Gunma University, Kiryu 376-8515,
Japan. E-mail: yamaji@chem-bio.gunma-u.ac.jp
b Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Sendai 980-8577, Japan
c Faculty of Chemistry, Adam Mickiewicz University, 60-780 Poznan,
Poland
d Department of Chemistry, Graduate School of Science, Osaka
University, Toyonaka, Osaka 560-0043, Japan
w Electronic supplementary information (ESI) available: Transient
absorption spectra obtained upon 266 nm laser photolysis of o-methyl
and o-chloromethylbenzophenones, and DFT calculation results. See
DOI: 10.1039/b702598j
z Present address: Graduate School of Science and Engineering,
University of Toyama, Gofuku, Toyama 930-8555, Japan.
ꢀc
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In the present study, photoinduced o-bond dissociation in
meta-substituted halomethylbenzophenones (m-HMBP) was
examined by laser photolysis techniques. It is revealed that
the substituted position on HMBP affects the reactivities of
o-cleavage in the lowest excited singlet and triplet states. The
differences in these reactivities are rationalized with the aid of
DFT calculations.
and Cl atoms and lanl2dzdp for the Br atom in which the
inner-shell electrons being replaced by Los Alamos relativistic
effective core potential.47 In calculation of the potential energy
curves of o-cleavage, the geometrical optimizations for the
lowest triplet (T1) states were performed at an unrestricted
DFT level with a triplet spin multiplicity using the Gaussian 03
program package,48 while the geometry of the lowest singlet
(S1) states was calculated using the analytical TDDFT gradi-
ent with the TURBOMOLE 5.7 program package.49 Atomic
spin densities, SD on a C(a) were computed using Mulliken
population analysis of the atomic orbitals of T1 states. All
quantum chemical calculations were carried out on a lab-
developed PC cluster system consisting of 32 Pentium IV
CPUs (3.0–3.4 GHz).
2. Experimental
m-Methylbenzophenone (m-MBP) was purchased commer-
cially. m-Chloromethylbenzophenone (m-CMBP) was synthe-
sized by a reaction of m-MBP with sulfuryl chloride in CCl4 in
the presence of benzoyl peroxide. m-Bromomethylbenzophe-
none (m-BMBP) was synthesized by a reaction of m-MBP with
N-bromosuccinimide in benzene in the presence of benzoyl
peroxide. m-Halomethylbenzophenones (m-HMBP; m-BMBP
and m-CMBP) were purified by repeated recrystallizations
from hexane. Acetone (Ac), acetonitrile (ACN), methanol,
ethanol and butyronitrile were distilled for purification.
Diethyl ether (spectroscopy grade, Kanto) and isopentane
(spectroscopy grade, Fluka) were used as supplied. ACN
and butyronitrile were used as the solvents at 295 K while
EPA (diethyl ether-isopentane-ethanol, 5 : 5 : 2 v/v/v) and a
mixture of methanol and ethanol (1 : 1 v/v) were used as
matrices at 77 K. Absorption and emission spectra were
recorded on a U-best 50 spectrophotometer (JASCO) and a
Hitachi F-4010 fluorescence spectrophotometer, respectively.
Time-resolved EPR measurements were carried out using an
X-band EPR spectrometer (Varian E-109E) without magnetic
field modulation as reported previously.44 A XeCl excimer
laser (Lambda Physik COMPex 102, 308 nm, 20 Hz) was used
as a pulsed light source. Sample solutions for the CIDEP
measurements were constantly deoxygenated by argon gas
bubbling and flowed into a quartz cell in the EPR resonator.
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 concen-
tration of m-HMBP for 266 nm laser photolysis was adjusted
to achieve the optical density at 266 nm being ca. 0.7 in ACN.
Transient absorption measurements were carried out at 295 K.
Fourth harmonics (266 nm) of a Nd3+:YAG laser (JK Lasers
HY-500; pulse width 8 ns) and a XeCl excimer laser (308 nm;
Lambda Physik, Lextra 50; pulse width 20 ns) at 308 nm were
used for flash photolysis. The number of the repetition of laser
pulsing in a sample was less than four pulses to avoid excess
exposure. The details of the detection system for the time
profiles of the transient absorption have been reported else-
where.45 The transient data obtained by laser flash photolysis
was analyzed by using the least-squares best-fitting method.
The transient absorption spectra were taken with a USP-554
system from Unisoku with which one can take a transient
absorption spectrum with a one-shot laser pulse.
3. Results
3.1 Absorption and emission measurements
Fig. 1 shows absorption spectra of m-MBP, m-BMBP and
m-CMBP in ACN at 295 K and their phosphorescence spectra
in EPA glass at 77 K. They are similar to each other. It was
confirmed that the phosphorescence excitation spectra agreed
well with the corresponding absorption spectra. From the
phosphorescence origins, the energy levels of the lowest triplet
(T1) states of m-MBP, m-CMBP and m-BMBP were deter-
mined to be 68.9 kcal molꢁ1. From the similarity in the
phosphorescence spectra of m-CMBP and m-BMBP with that
of m-MBP, it is inferred that the electronic character of the
lowest excited triplet state of m-CMBP and m-BMBP is
of n,p*.
3.2 Direct excitation
Fig. 2 shows transient absorption spectra obtained after 266
nm laser pulsing in the degassed ACN solutions of m-MBP, m-
CMBP and m-BMBP at 295 K, respectively. The transient
absorption spectra at 100 ns for m-MBP and m-CMBP are due
to the triplet states of m-MBP and m-CMBP since they are
similar to that of benzophenone. The transient absorption
spectrum having the absorption maximum at 335 nm observed
at 100 ns for m-BMBP, however, cannot be due to triplet m-
BMBP, but should be assigned to that of the m-benzoylbenzyl
Electronic-state energy and geometry calculations were
performed using density functional theory (DFT) and time-
dependent DFT (TDDFT) with a Becke style three-parameter
hybrid exchange functional and Lee Yang Parr correlation
functional (B3LYP).46 Polarized valence triple-z basis sets
were employed for the calculations: 6-311G(d) for C, H, O,
Fig. 1 Absorption spectra in ACN at 295 K and phosphorescence
spectra in a mixture of methanol and ethanol (1 : 1 v/v) at 77 K for (a)
m-MBP, (b) m-CMBP and (c) m-BMBP .
ꢀc
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The CIDEP measurement was carried out for m-HMBP in
butyronitrile at room temperature, but CIDEP signal was
absent at 500 ns after 308 nm laser photolysis of m-CMBP
and m-BMBP.
3.3 Triplet sensitization
Triplet sensitization of Ac (1.0 mol dmꢁ3) was carried out in
ACN solutions of m-MBP, m-BMBP and m-CMBP by using a
XeCl excimer laser (308 nm). Fig. 3 shows transient absorption
spectra obtained upon acetone sensitization of m-MBP, m-
CMBP and m-BMBP using 308 nm laser pulse. The transient
absorption spectra for the Ac–m-MBP and m-CMBP systems
are due to triplet m-MBP and m-CMBP, respectively. The
intensity of the absorbance at 525 nm for triplet m-MBP and
at 320 nm for triplet m-CMBP increases according to the first-
order kinetics (2.9 ꢃ 106 sꢁ1 for m-MBP and 2.6 ꢃ 106 sꢁ1 for
m-CMBP) as shown in insets in Fig. 3a and b, respectively.
From these observations, it can be recognized that triplet
energy transfer proceeds from triplet Ac to m-MBP or
m-CMBP to produce the corresponding triplet states. After
depletion of triplet m-CMBP, no residual transient absorption
was seen. This indicates that triplet m-CMBP does not under-
go o-cleavage. The transient absorption spectrum for the
Ac–m-BMBP system obtained at the time (2.4 ms) of the
Fig. 2 (a) A transient absorption spectrum observed at 100 ns after
266 nm laser pulsing in the ACN solution of m-MBP. (b) Transient
absorption spectra observed at 100 ns (broken line) and 1 ms (solid
line) after 266 nm laser pulsing in the ACN solution of m-CMBP.
Inset: a temporal absorbance change at 335 nm in the presence of the
dissolved oxygen in the ACN solution. (c) A transient absorption
spectrum observed at 100 ns after 266 nm laser pulsing in the ACN
solution of m-BMBP.
radical (m-BBR) since it resembles the absorption spectrum of
the p-benzoylbenzyl radical (p-BBR) having a molar absorp-
tion coefficient of 7600 ꢂ 400 dm3 molꢁ1 cmꢁ1 at 320 nm.36,37
As with m-CMBP, when triplet m-CMBP was quenched by the
dissolved oxygen (see inset in Fig. 2b), the absorption spec-
trum obtained at 1 ms after 266 nm laser pulsing was similar to
that of m-BBR. These observations that m-BBR was formed
upon photolysis of m-CMBP and m-BMBP indicate that
m-HMBP undergoes o-bond dissociation to form m-BBR in
ACN at room temperature.
The quantum yield (Frad) of the m-BBR formation upon
266 nm laser photolysis of m-HMBP was determined with the
use of eqn (1) based on the absorption change (DA335) at
335 nm due to m-BBR obtained at 100 ns for m-BMBP and at
1 ms for m-CMBP in the presence of the dissolved oxygen.
ꢁ1
Frad ¼ DA335Deꢁ1
I
ð1Þ
335 abs
where De335 and Iabs are, respectively, the molar absorption
coefficient change of m-BBR at 335 nm and the number of the
photon flux of a laser pulse absorbed by m-HMBP. The De335
value was assumed to be the same as that (7500 dm3 molꢁ1
cmꢁ1) of the p-BBR at 320 nm.36,37 The quantity of Iabs was
determined by using the absorption of triplet benzophenone
(BP) in ACN as an actinometer.50
BPꢁ1
Iabs ¼ DABTPeBTPꢁ1
F
ð2Þ
ISC
Fig. 3 (a) A transient absorption spectrum observed at 850 ns after
308 nm laser pulsing in an Ac(0.7 mol dmꢁ3)–m-MBP(8 ꢃ 10ꢁ4 mol
dmꢁ3) system. Inset: a temporal absorbance change at 525 nm. (b) A
transient absorption spectrum observed at 650 ns after 308 nm laser
pulsing in an Ac(0.7 mol dmꢁ3)/m-CMBP(8 ꢃ 10ꢁ4 mol dmꢁ3) system.
Inset: a temporal absorbance change at 320 nm. (c) A transient
absorption spectrum observed at 2.4 ms after 308 nm laser pulsing in
an Ac(0.7 mol dmꢁ3)–m-BMBP(8 ꢃ 10ꢁ4 mol dmꢁ3) system. Inset: a
temporal absorbance change at 335 nm.
where DABTP, eBTP and FIBSPC are the initial absorbance at 520 nm
for the formation of triplet benzophenone obtained immedi-
ately after laser pulsing, the molar absorption coefficient of
triplet BP at 520 nm in ACN (6500 dm3 molꢁ1 cmꢁ1 51
and
)
triplet yield of benzophenone (1.0), respectively.42 By using
eqn (1) and (2), the Frad value was determined to be 0.085 ꢂ
0.04 for m-CMBP and 0.25 ꢂ 0.02 for m-BMBP.
ꢀc
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tization were determined by eqn (4) and (5), respectively.
FTET ¼ DAs5e2n5se5ꢁ215 aꢁb1s
I
ð4Þ
ð5Þ
sens
rad
ꢁ1
I
F
¼ DAs3e3n5sDeꢁ1
335 abs
sens
335
where DAs5e2n5sand DA
denote the maximum absorption
changes due to the formation of triplet m-MBP at 525 nm
and m-BBR at 335 nm , respectively (see insets in Fig. 3). The
molar absorption coefficient, e525, of triplet m-MBP at 525 nm
was determined to be 6700 dm3 molꢁ1 cmꢁ1 by comparison
with that of benzophenone (6500 dm3 molꢁ1 cmꢁ1 at
520 nm)51 and the value 7500 dm3 molꢁ1 cmꢁ1 was used as
the molar absorption coefficient change, De335 of BBR at 335
nm. The value of Iabs at 308 nm, the photon flux absorbed by
Ac, was determined by eqn (2). The obtained values of FTET
sens
rad
and F
for the Ac–m-MBP and m-BMBP systems are
plotted as a function of [Q] in Fig. 4b. Both the values non-
Fig. 4 (a) The rate (kobsd) for the formation of triplet m-MBP, triplet
m-CMBP and m-BBR obtained upon 308 nm laser photolysis of the
Ac (0.7 mol dmꢁ3)–m-MBP (J), m-CMBP (&) and m-BMBP (K)
systems, respectively, plotted as a function of [Q]. (b) The quantum
yields (FTET and Fsraends) for triplet m-MBP (J) and m-BBR (K)
formation upon 308 nm laser photolysis of Ac–m-MBP and m-BMBP
systems, respectively, plotted as a function of [Q]. The solid curves
were calculated by eqn (6) and (7) for FTET and Fsraends, respectively.
linearly increase with increasing [Q]. On the other hand, FTET
sens
rad
and F
eqn (6) and (7).
are related with the kinetic parameters, k0 and kq by
FTET ¼ kq½QꢄaTETFAc ðk0 þ kq½QꢄÞꢁ1 Q for m-MBP ð6Þ
ISC
sens
rad
F
¼ kq½QꢄaT aTETFAc ðk0 þ kq½QꢄÞꢁ1
rad
ISC
ð7Þ
Q for m-MBP
Ac
ISC
where aTET, aTrad and F
are the efficiency for triplet energy
maximum absorbance at 335 nm (see inset Fig. 3c) was similar
to that of m-BBR. In other words, m-BBR is generated via the
triplet state of m-BMBP upon sensitization. The rise rate of
the m-BBR formation was 1.7 ꢃ 106 sꢁ1. The observed rates
(kobsd) for the triplet formation of m-MBP and m-CMBP, and
the m-BBR formation from triplet m-BMBP are plotted as a
function of the concentration, [Q] of the quencher, Q in Fig. 4.
Since the plots give straight lines, the kobsd can be formu-
lated by
transfer from triplet Ac to m-MBP or m-BMBP, the efficiency
for the radical formation in the triplet state of m-BMBP, and
the triplet yield of Ac (1.0), respectively.52 By best-fitting eqn
(6) and (7) to the experimental values of FTET and FrTad
respectively, with the use of the k0 and kq values obtained
above, the aTET for the Ac–m-MBP system was obtained to be
0.65 ꢂ 0.03 while a product value of aTradaTET for the Ac–
m-BMBP system was determined to be 0.26 ꢂ 0.02. By
adopting the aTET value (0.65) of the Ac–m-MBP system to
the Ac–m-BMBP system, the aTrad value for the Ac–m-BMBP
system is determined to be 0.40 ꢂ 0.04. The determined arTad
values are listed in Table 1 along with those of p-HMBP.
,
kobsd ¼ k0 þ kq½Qꢄ
ð3Þ
where k0 and kq, respectively, represent the decay rate of
triplet acetone in the absence of the quencher and the rate
constant for quenching triplet Ac by the quencher. From the
intercept and slope of the line, the values of k0 and kq were,
respectively, determined to be 8.0 ꢃ 105 sꢁ1 and 2.9 ꢃ 109 dm3
molꢁ1 sꢁ1 for m-MBP, 2.5 ꢃ 109 dm3 molꢁ1 sꢁ1 for m-CMBP
and 1.3 ꢃ 109 dm3 molꢁ1 sꢁ1 for m-BMBP.
3.4 DFT calculations
Fig. 5a and b show potential energy curves in m-BMBP and
m-CMBP, respectively, calculated as a function of the distance
between the benzylic carbon and the halide atom, X, d(C–X).
These curves were obtained from constrained geometry opti-
mizations for the S1 and T1 states with the following assump-
tions. For the T1 states, the angle of X–C(a)–C(4) was kept
The quantum yields (FTET and Fsraends) for the formation of
triplet m-MBP and m-BBR from m-BMBP upon triplet sensi-
Table 1 Photophysical and photochemical parameters for m-HMBP obtained in the present worka
d
e
f
X
ETb/kcal molꢁ1 kqc/109 dm3 molꢁ1 sꢁ1 arSad
aTrad
BDE(C–X) /kcal molꢁ1 SDg
DEScalh/kcal molꢁ1 DETcali/kcal molꢁ1
Br
68.9
(n.d.)j
68.9
1.3
(2.9)
2.5
0.25
0.40
58.0
(0.41) (0.53) (53.0)
ꢁ0.009
6.5
(1.2)k
13.0
4.6
(2.6)k
10.7
(0.054)k
Cl
0.085
(B0)
0 70.6
(0.51) (65.6)
ꢁ0.006
(68.4)
(3.7)
(0.027)k (10.9)k
(6.3)k
a
Data in parentheses are for p-HMBP obtained previously.37 b Triplet energies determined from the 0–0 origins of the phosphorescence spectra at
c
d
77 K. Quenching rate constants of triplet Ac by m-HMBP. Efficiencies of the radical formation in the S1 state. Efficiencies of the radical
e
f
formation in the T1 state. Bond dissociation energies of the C–X bond estimated by MP3 calculations. Spin densities on the CH2 atom at the
g
h
geometry optimized for T1. Calculated activation energies for C–X bond dissociation in the S1 state. Calculated activation energies for C–X
i
j
bond dissociation in the T1 state. Not determined. Obtained in the present work.
k
ꢀc
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tion of m-HMBP proves that the T1 state of m-BMBP is
dissociative while that of m-CMBP is inert to decomposition.
From the observations for m-CMBP, it is concluded that the
C–Cl bond of m-CMBP cleaves only in the excited singlet
state, and that the dissociation process competes with inter-
system crossing to the triplet manifold. With m-BMBP, for-
mation of the triplet state was not observed in transient
absorption upon direct excitation at room temperature, and
CIDEP signal was absent upon 308 nm laser photolysis of m-
BMBP. The absence of CIDEP signal suggests that the m-BBR
formation upon photolysis of m-BMBP is not contributed
from the bond-dissociative triplet manifold. It should be noted
that photolysis of p-CMBP shows net-emissive CIDEP due to
p-BBR because the C–Cl bond cleaves in the triplet state.37
The large spin–orbit coupling interaction of the halogen atom
eliminates the electron spin polarization due to the radical pair
mechanism, even if the spin polarization is created during the
interaction between the pair radicals in the solvent cage.25a
Based on these photolytic results of m-BMBP, it seems that
deactivation processes of the S1 state of m-BMBP are gov-
erned by the efficient C–Br bond dissociation, which may
hamper intersystem crossing from the S1 to the triplet state
of m-BMBP at room temperature.
Fig. 5 Potential energy curves of the S1 (triangles) and T1 (circles) of
(a) BMBP and (b) CMBP calculated at the B3LYP/6-311G(d) level as
a function of a distance of C–X bond, d(C–X). Filled and unfilled
symbols denote m-HMBP and p-HMBP, respectively.
larger than 901 during the geometry optimization to avoid the
complex formation between X radical and m-BBR. The geo-
metry optimization for the S1 state without any constraints
revealed that the phenyl ring was perpendicular to the halo-
methylphenyl group whereas the dihedral angle between two
phenyl rings was calculated to be about 501 for both m-HMBP
in the ground state and m-BBR. Since it is presumably hard
for such a large rotation of the phenyl ring to occur in solution
within the very short lifetime of the S1 state (a few pico-
seconds),1 the dihedral angle between two phenyl rings was
kept equal to or greater than 501 for the geometry optimiza-
tion. From the potential energy curves, the activation energies
4.2 Efficiencies of x-cleavage
We have determined the quantum yields, Frad of the free
radical formation upon direct excitation of m-HMBP. As
mentioned above, the m-BBR formation from photoexcited
m-HMBP originates from the C–X bond dissociation in the S1
state. The actual efficiencies for the C–X bond rupture in the
S1 states are, however, unable to be determined with our
nanosecond laser system since some of the singlet radical pairs
produced in solvent cages may undergo the fast geminate
recombination within the laser pulse duration (ca. 20 ns).
Values of these efficiencies should be larger than those of the
Frad obtained. In contrast, values of efficiencies, aScal for free
radical formation due to the C–X bond fission in the S1 state
can be regarded as being equal to those of the obtained
quantum yields, Frad since the C–X bond cleaves only in the
S1 state upon direct photoexcitation of m-HMBP. Here, we
assume the same efficiency of the geminate recombination of
the in-cage singlet biradical pair formed in the S1 states
between m- and p-BMBPs. The aScal value (0.25) of m-BMBP
is smaller than that of p-BMBP (0.41).37 The S1 state of
m-CMBP is reactive for o-cleavage (aScal = 0.085) while that
of p-CMPB is inert (acSal B0).37 Based on these aScal values, effects
of substituted positions in HMBPs to o-cleavage are recognized.
When the C–Br bond dissociates in the triplet state of
for the o-cleavage in the S1 and T1 states (DEScal and DEcTal
,
respectively) were estimated as the difference in potential
energy between the excited state and the transition state (TS)
which gives the maximum when the C–X bond is elongated.
The DEScal and DETcal values are listed in Table 1.
The bond dissociation energy of the C–X bond in m-HMBP,
BDE(C–X) was obtained on the basis of the heat of formation
(DfH) for m-BMBP and m-CMBP, m-BBR and halogen atoms
(X = Br and Cl) computed using a semi-empirical PM3
program contained in MOPAC 97. These were DfH (m-
BMBP) = 15.5 kcal molꢁ1, DfH (m-CMBP) = 5.2 kcal molꢁ1
DfH (m-BBR) = 46.8 kcal molꢁ1, DfH (Brꢅ)=26.7 kcal molꢁ1
,
,
and DfH (Clꢅ)=29.0 kcal molꢁ1. The BDE(C–X) value is
calculated to be 58.0 kcal molꢁ1 for m-BMBP and 70.6 kcal
molꢁ1 for m-CMBP by using eqn (8).
Df Hðm-HMBPÞ ¼ Df Hðm-BBRÞ
þ Df HðXꢅÞ-BDEðCꢁXÞ
ð8Þ
3
m-BMBP, a triplet radical pair, (m-BBR + Brꢅ)cage of m-BBR
and Br radical may be initially produced in a solvent cage
according to the spin conservation rule. Because of the large
spin–orbit coupling interaction due to the heavy Br atom, the
singlet–triplet (S–T) mixing may be efficient to the triplet in-
cage biradical pair to some extent. The triplet radical pair
escapes from the solvent cage competing with the S–T mixing
that results in geminate recombination. Thus, the aTrad value
(0.40) should be smaller than that of the intrinsic efficiency of
the C–Br bond rupture in the triplet manifold of m-BMBP.
4. Discussion
4.1 Spin multiplicities of dissociative states
Transient absorption measurements upon laser flash photo-
lysis of m-CMBP and m-BMBP in the present work show that
they undergo o-cleavage to generate m-BBR. Triplet sensitiza-
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The residual efficiency (1 ꢁ aTrad = 0.60) should be due to
intersystem crossing from the T1 to the ground state and the
geminate recombination induced by the heavy Br atom. When
we compare the aTrad values between m- and p-BMBPs here-
after, we assume the same ratio of the rate for the escape of the
triplet in-cage radical pair from the solvent cage to the rate for
the geminate recombination of the singlet in-cage radical pair
converted from the triplet radical pair by the S–T mixing due
to the heavy atom effect.
via intersystem crossing, but undergoes the C–Br bond scission
to produce m-BBR and Br radical with an efficiency (aScal) of
0.25, which is smaller than that of p-BMBP (0.41).37 Triplet
sensitization has shown that the T1 state also undergoes
o-cleavage of the C–Br bond with a smaller efficiency (arTad
=
0.40) than that of triplet p-BMBP (0.53).37 It is revealed that
the S1 and T1 states of m-BMBP are reactive toward o-bond
dissociation as well as those of p-BMBP.
4.4 Understanding of substituted position effects based on
DFT calculations
4.3 Energy diagrams for excited states of m-HMBP
Based on the experimental results obtained, energy diagrams
for the deactivation processes including the C–X bond clea-
vage of photoexcited m-CMBP and m-BMBP are depicted in
Scheme 1. The dissociation mechanism in the excited states of
aromatic molecules can be interpreted to be a thermally
activated crossing of the reactive excited state with dissociative
potential surfaces leading to free radicals.23 The electronic
configurations of the plausible potentials for the C–X bond
rupture in m-HMBP are of p,s* and s,s*. It was originally
suggested that radical cleavages of excited states proceed by
avoided crossings between states of the same overall symme-
try.53,54 In the present case, the S1 and T1 states of n,p* would
correlate with the corresponding singlet and triplet p,s*
potentials. With the diagram for m-CMBP in Scheme 1, the
S1(n,p*) undergoes C–Cl bond cleavage along the correlated
singlet p,s* potential to produce m-BBR and Clꢅ. This process
is competitive with intersystem crossing to the T1 state where
the C–Cl bond does not dissociate. The lifetime of the S1 state
of benzophenone is known to be as short as 10 ps,41 the rate of
o-cleavage from the S1 state of m-CMBP is of the magnitude
DFT calculations for HMBP demonstrate the presence of
activation energies, DEScal and DEcTal for o-bond dissociation.
Since energy barriers between excited states and dissociative
potential surfaces are essential for photoinduced bond rup-
ture, we can elucidate the relationship between the activation
energies and efficiencies of o-cleavage. At first, the DEScal value
(6.5 kcal molꢁ1) of m-BMBP is much larger than that of
p-BMBP (1.2 kcal molꢁ1). The smaller o-cleavage efficiency
(0.25) in the S1 state of m-BMBP than that of p-BMBP (0.41)
can be explained by this difference in the DEScal value. The
activation energy may also facilitate the reaction in the T1
state of BMBPs. The larger DETcal value (4.6 kcal molꢁ1) for
m-BMBP than that for p-BMBP (2.6 kcal molꢁ1) corresponds
to the smaller aTrad value (0.40) for m-BMBP than that for
p-BMBP (0.53). The other factor to affect the aTrad values of
BMBPs may be spin densities, SD, on the CH2 group at the
T1 geometry, which would correlate with the spin density of a
s* orbital for the C–X bond. The calculated SD values are
listed in Table 1. Squared values of SD are efficient in
comparison. The fact that the squared SD value for triplet
m-BMBP is smaller than that for triplet p-BMBP indeed
rationalizes the smaller o-cleavability. In contrast, comparison
with SD values would be less useful for triplet CMBPs. The
BDE value (70.6 kcal molꢁ1) for m-CMBP is obviously larger
than the triplet energy (68.9 kcal molꢁ1). Since o-cleavage is
absent in triplet m-CMBP, there would be a large activation
energy for the C–Cl bond dissociation shown as DE in Scheme
1. In fact, the calculated DETcal value (10.7 kcal molꢁ1) for
m-CMBP is larger than that for triplet p-CMBP (6.3 kcal
molꢁ1) where o-cleavage efficiently proceeds. Finally, the
of 109–1010 ꢁ1. The absence of bond cleavage in the T1 state
s
(aTrad = 0) may be due to the BDE (70.6 kcal molꢁ1) for the
C–Cl bond being slightly larger than the triplet energy (68.9
kcal molꢁ1), which may yield a large activation barrier, DE to
a dissociative triplet p,s* potential. The triplet state of
m-CMBP is fated to deactivate only by intersystem crossing to
the ground state. It is noteworthy that the reactivities in the S1
and T1 states of m-CMBP toward o-cleavage of the C–Cl bond
differ from those of p-CMBP (aScal B 0 and arTad = 0.51).37 On
the other hand, the S1 state of m-BMBP does not deactivate
Scheme 1 Schematic energy diagrams for the deactivation and bond dissociation processes of excited m-CMBP (left) and m-BMBP (right).
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difference in DEScal of CMBPs cannot be understood from the
DFT calculation data. The DEScal value of m-CMBP (13.0 kcal
molꢁ1) is relatively larger than that of p-CMBP (10.9 kcal
molꢁ1). o-Bond dissociation is, however, recognized to take
place in the S1 state of m-CMBP whereas it is absent from
triplet p-CMBP. The electronic character and state energy of
the S1 states of both m- and p-CMBPs are similar. Concerning
C–Cl bond cleavage in the S1 state, the BDE(C–Cl) values
differ by 5 kcal molꢁ1 between m- and p-CMBPs. From this
consideration, it is inferred that the degree of correlation
between the S1 state and the corresponding dissociative
1(p,s*) potential surface is affected by the BDE value. There
would be no interaction between the S1 state and 1(p,s*)
potentials for p-CMBP (unavoided crossing) while a weak
avoided crossing may cause between the potential curves for
m-CMBP due to the relatively large BDE(C–Cl).
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5. Conclusions
Photochemical properties of o-bond cleavage in m-HMBP
have been studied by laser photolysis techniques and DFT
calculations. Based on the efficiencies of free radical forma-
tion, it is shown that reactivity of o-cleavage clearly depends
on the substituted positions of HMBPs. The reaction processes
and efficiencies of o-cleavage in excited m-HMBP have been
rationalized with the aid of DFT calculations. With m-CMBP,
o-cleavage does not occur in the T1 state, but it does in the S1
state. The dissociation process in the S1 state of m-CMBP
competes with intersystem crossing to the T1 state. The
absence of dissociation in the T1 state of m-CMBP is explained
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BDE(C–Cl) being larger than the triplet energy. These proper-
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cleavage in p-CMBP. In contrast to CMBP, both the S1 and T1
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system crossing from the S1 to T1 states. These properties are
similar to those of p-BMBP. Substituted position effects
between m-BMBP and p-BMBP are shown as a reduction in
the efficiencies, acSal and arTad. It seems that the aScal values of
BMBPs are affected by an activation barrier, DEScal whereas
spin densities on a s* orbital of the C–Br bond are crucial to
the reactivity of o-cleavage in the T1 states of BMBPs.
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Acknowledgements
This work was partially supported by a Scientific Research
Grant-in-Aid from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. The authors thank Prof.
Seiji Tobita at Gunma University for his critical reading of the
manuscript.
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