C-O-bond cleavage of esters with a naphthyl group in the higher triplet excited state during two-color two-laser flash photolysis

Article abstract of DOI:10.1002/chem.200600822

A C-O-bond cleavage of esters having a naphthyl group, NpCO-OR and RCO-ONp (Np = α- and βnaphthyl (^αNp and ^βNp, respectively), R = Ph and Me), was found during the two-color two-laser flash photolysis in acetonitrile. The C-O-bond cleavage occurred when NpCO-OR and RCO-ONp were excited to the singlet excited states (S₁). On the other hand, no reaction occurred from the lowest triplet excited states (T ₁). When NpCO-OR(T₁) and RCO-ONp(T₁) were excited to the higher triplet excited states (T_n) using the second laser during the two-color two-laser flash photolysis, the C-O-bond cleavage occurred. The C-O-bond cleavage quantum yield (Φ) was estimated from the plots of the T₁state esters disappeared within a laser flash versus the second laser intensities. The C-O-bond cleavage in ^βNpCO- OPh(T_n) occurred more efficiently than in ^αNpCO- OPh(T_n) and that in PhCO-O^βNp(T_n) occurred more efficiently than in PhCO-O^αNp(T_n). The Φ value for ester with Ph and β-Np groups was larger than that for ester with Ph and α-Np groups. The Φ value for MeCO-O^αNp(T _n) was similar to those for PhCO-ONp(T_n), while that for MeCO-OβNp(T_n) was much smaller than those for PhCO-ONp(T _n) and MeCO-O^αNp(T_n). On the other hand, no C-O-bond cleavage was observed in NpCO-OMe(T_n). The Φ value depended on the characters of the groups (Np, Ph, and Me) on the ester. Whether R is Ph or Me with or without π electron, respectively, is important for the C-O-bond cleavage. In other words, electronic delocalization of the T _n state including Np and ester groups is necessary for the occurrence of the C-O-bond cleavage in NpCO-OR(T_n) and RCO-ONp(T_n).

Full text of DOI:10.1002/chem.200600822

FULL PAPER
DOI: 10.1002/chem.200600822
ꢀ
C O-Bond Cleavage of Esters with a Naphthyl Group in the Higher Triplet
Excited State during Two-Color Two-Laser Flash Photolysis
[a]
Xichen Cai,[a] MasanoriSakamoto, Minoru Yamaji,[b] Mamoru Fujitsuka,[a] and
Tetsuro Majima*[a]
ꢀ
Abstract: A C O-bond cleavage of
was estimated from the plots of the T1-
state esters disappeared within a laser
flash versus the second laser intensities.
for MeCO-ObNp(Tn) was much smaller
than those for PhCO-ONp(Tn) and
MeCO-OaNp(Tn). On the other hand,
esters having a naphthyl group, NpCO-
OR and RCO-ONp (Np=a- and b-
naphthyl (aNp and bNp, respectively),
R=Ph and Me), was found during the
two-color two-laser flash photolysis in
The C O-bond cleavage in bNpCO-
no C O-bond cleavage was observed
ꢀ
ꢀ
OPh(Tn) occurred more efficiently
than in aNpCO-OPh(Tn) and that in
PhCO-ObNp(Tn) occurred more effi-
ciently than in PhCO-OaNp(Tn). The F
value for ester with Ph and b-Np
groups was larger than that for ester
with Ph and a-Np groups. The F value
for MeCO-OaNp(Tn) was similar to
those for PhCO-ONp(Tn), while that
in NpCO-OMe(Tn). The F value de-
pended on the characters of the groups
(Np, Ph, and Me) on the ester. Wheth-
er R is Ph or Me with or without p
electron, respectively, is important for
ꢀ
acetonitrile. The C O-bond cleavage
occurred when NpCO-OR and RCO-
ONp were excited to the singlet excit-
ed states (S1). On the other hand, no
reaction occurred from the lowest trip-
let excited states (T1). When NpCO-
OR(T1) and RCO-ONp(T1) were excit-
ed to the higher triplet excited states
(Tn) using the second laser during the
two-color two-laser flash photolysis,
ꢀ
the C O-bond cleavage. In other
words, electronic delocalization of the
Tn state including Np and ester groups
is necessary for the occurrence of the
ꢀ
C O-bond cleavage in NpCO-OR(Tn)
and RCO-ONp(Tn).
Keywords: esters · flash photolysis ·
higher triplet excited state · laser
chemistry · photochemistry
ꢀ
the C O-bond cleavage occurred. The
C O-bond cleavage quantum yield (F)
ꢀ
Introduction
matic groups such phenyl, naphthyl, pyrenyl, etc., X=halo-
gen, carbonates, phosphites, sulfides, etc.) in the S1 or T1 ex-
cited state has been widely studied.[4] The S1 and T1 states
are not the only reactive excited states for the bond cleav-
age.[5] Bond cleavages of molecules in the higher excited
states occur to give the corresponding radicals. For example,
formation of carbazyl radical from carbazole in the highly
excited singlet (Sn) and triplet (Tn) states has been reported
using the double excitation method.[6,7] Recently, we have
also proven the formation of naphthylmethyl radicals from
Free radicals as reactive intermediates have been extensive-
ly investigated. The generation and detection of radicals are
very important in physics, chemistry, biology, applied sci-
ence, and technology. Free radicals generated from homolyt-
ic bond cleavages can be used practically as radical initiators
of polymerization.[1,2] Photochemical generation of radicals
usually occurs via the precursors in the singlet or triplet
lowest excited states (S1 and T1, respectively).[3,4] For exam-
ꢀ
ꢀ
ꢀ
ple, homolytic CH2 X-bond cleavage of ArCH2X (Ar=aro-
naphthylmethyl oxygen (C O) bond cleavage of naphthyl-
methoxy compounds in the Tn states by using the stepwise
two-laser irradiations.[8–11] The occurrence of the C O-bond
cleavage was found to depend on both the excitation energy
and crossing between potential surfaces of the Tn state and
the C O-bond dissociation. The excitation energy dissipa-
tion or electronic delocalization of the Tn state including the
C O bond is important for the bond cleavage. Herein, we
report the C O-bond cleavage of esters having a naphthyl
group, such as NpCO-OR and RCO-ONp (Np=a- and b-
ꢀ
[a] Dr. X. Cai, M. Sakamoto, Dr. M. Fujitsuka, Prof. Dr. T. Majima
The Institute of Scientific and Industrial Research (SANKEN)
Osaka University, Mihogaoka 8-1, Ibaraki
Osaka 567-0047 (Japan)
ꢀ
Fax : (+81)6-6879-8499
E-mail: majima@sanken.osaka-u.ac.jp
ꢀ
[b] Dr. M. Yamaji
ꢀ
Department of Chemistry, Gunma University
Kiryu, Gunma 376-8515 (Japan)
Chem. Eur. J. 2007, 13, 3143 – 3149
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3143

b
naphthyl (aNp and Np, respectively), R=Ph and Me) in the
Tn states by the two-color two-laser flash photolysis techni-
ꢀ
ques. The substituent effects on the C O-bond cleavage in
the Tn states are discussed. Figure 1 shows the structures of
the esters substituted by Np used in this study.
Figure 1. Structures of esters having a naphthyl group (Np) used in this
study.
Results and Discussion
ꢀ
C O-Bond cleavage of esters with a naphthyl group in the
S1 state, but not in the T1 state: It is well known that the
photo-Fries rearrangement (or [1,3]-shift) reaction occurs
ꢀ
following the C O-bond cleavage of carboxylic acids and
their derivatives in the S1 state.[3,12–16] Therefore, it is expect-
ꢀ
ed that the C O-bond cleavage of esters such as NpCO-OR
and RCO-ONp in the S1 state occurs during the laser flash
ꢀ
photolysis. The C O-bond cleavage in NpCO-OR(S1) and
RCO-ONp(S1) was confirmed by the detection of the transi-
ent absorption of the two fragment radicals (Figure 2). For
Figure 2. Transient absorption spectra observed during the 266 nm laser
flash photolysis of a) aNpCO-OPh, b) bNpCO-OPh, c) aNpCO-OMe, d)
bNpCO-OMe, e) PhCO-OaNp, f) PhCO-ObNp, g) MeCO-ObNp, h) and
MeCO-ObNp in argon-saturated acetonitrile at room temperature. The
ground-state absorbance of the samples was adjusted to be 1.0 at 266 nm.
The transient absorption spectrum was obtained at 100 ns after the
266 nm laser flash. The circles show the absorption positions of the frag-
ment radicals or the T1 state.
C
example, the absorption bands of benzoyl radical (PhCO )
a
C
and a-naphthoxy radical ( NpO ) were observed around 325
and 397 nm,[17–19] respectively, during the 266 nm laser flash
photolysis of PhCO-OaNp (Figure 2e). On the other hand,
ꢀ
no C O-bond cleavage occurred in NpCO-OR(T1) and
RCO-ONp(T1). For example, the T1-state transient absorp-
tion was observed and decayed during the 355 nm laser flash
photolysis of PhCO-OaNp in the presence of benzophenone
(BP) used as the triplet sensitizer without formation of any
new absorption band (Figure 3e, dotted line). The transient
absorption of PhCO-OaNp(T1) was similar to that of naph-
thalene(T1), indicating that the T1-state energy of PhCO-
OaNp(T1) is localized on the Np moiety.[3,20–22]
The lmax(T1) at 460 nm for aNpCO-OPh(T1), 426 nm for
a
bNpCO-OPh(T1), 440 nm for NpCO-OMe(T1), and 422 nm
b
for NpCO-OMe(T1), which were broader and shifted to the
longer wavelength side compared with that of naphthale-
ne(T1) at 415 nm.[20] On the other hand, the lmax(T1) at
405 nm for PhCO-OaNp(T1), 412 nm for PhCO-ObNp(T1),
406 nm for MeCO-OaNp(T1), and 412 nm for MeCO-
ObNp(T1), which were shifted to the shorter wavelength side
compared with that of naphthalene(T1) at 415 nm.[20] It is
suggested that the energy gap (DET1-Tn) between the T1 and
Tn states becomes smaller when Np is connected to the
carbon atom of the esters, NpCO-OR, while it becomes
larger when Np is connected to the oxygen atom of the
esters, RCO-ONp. The T1-state transient absorption of
esters having a a-Np group was broader and shifted to the
longer wavelength side compared with those of esters
The T1-state transient absorption: The transient absorption
spectra of the used compounds in the T1 state were similar
to that of naphthalene(T1) as shown in Figure 3, dotted line.
The T1-state transient absorption peaks (lmax(T1)) and the
extinction coefficients of the T1-state transient absorption
peaks (emax) which calculated based on e525 =6250mꢀ1 cmꢀ1 of
benzophenone(T1) as the reference[20] were shown in
Table 1.
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Chem. Eur. J. 2007, 13, 3143 – 3149

Two-Laser Flash Photolysis
FULL PAPER
Table 1. T1-state transient absorption peaks (lmax(T1)) and the extinction
coefficients of the T1-state transient absorption peaks (emax).
Esters
lmax(T1) [nm]
emax [mꢀ1 cmꢀ1
]
aNpCO-OPh
bNpCO-OPh
aNpCO-OMe
bNpCO-OMe
PhCO-OaNp
PhCO-ObNp
MeCO-OaNp
MeCO-ObNp
460
426
440
422
405
412
406
412
9380
13800
6700
10700
13400
18300
14300
20100
photolysis of NpCO-OR(T1) and RCO-ONp(T1) (Figure 3,
solid line). The DDOD increased with increasing the 425 nm
laser intensity. No recovery of the T1 state after the second
laser flash indicated that decomposition of the T1 state upon
second laser photolysis occurred in the Tn state.[8,9,11] Based
on the transient absorption measurement, the decomposi-
[8,9,11]
ꢀ
tion is assigned to the C O-bond cleavage.
In the case of aNpCO-OPh, with a decrease of the absorp-
tion intensity of NpCO-OPh(T1), absorption bands of phe-
a
C
noxyl radical (PhO ) at 362 and 398 nm were observed (Fig-
ure 3a, solid line).[17–19] Absorption bands of the counter rad-
a
C
ical, a-naphthoyl radical ( NpCO ) with small and broad
peaks in the region of 400–500 nm overlapped with the
a
strong absorption of NpCO-OPh(T1). The absorption bands
a
C
C
of PhO around 362 and 398 nm and those of NpCO in the
region of 400–500 nm were obvious after subtraction of the
ꢀ
broken line, showing that the C O-bond cleavage occurred
a
C
C
from the Tn state to give NpCO and PhO (Figure 3a,
inset). Compared with the absorption peak of aNpCO-
OPh(T1) at 460 nm, that of NpCO-OPh(T1) at 426 nm shift-
b
ed to the shorter wavelength side. The absorption bands of
C
PhO around 362 and 398 nm and that of b-naphthoyl
Figure 3. Transient absorption spectra observed during the two-color two-
laser flash photolysis of a) aNpCO-OPh, b) bNpCO-OPh, c) aNpCO-
OMe, d) bNpCO-OMe, e) PhCO-OaNp, f) PhCO-ObNp, g) MeCO-OaNp,
h) and MeCO-ObNp in argon-saturated acetonitrile (6.0”10ꢀ3 m) in the
presence of BP(6.0”10 ꢀ3 m) as the triplet sensitizer. The transient ab-
sorption spectra were observed during the 355 nm laser irradiation (at
500 ns after the laser flash) (dotted line) and the successive irradiation
with the 355 and 425 nm two-lasers (at 100 ns after the second 425 nm
laser flash; delay time between the two lasers, 400 ns) (solid line). The
broken lines in a), b), e)–h) were obtained from the transient absorption
of the T1 state multiplied by a factor for OD at the peak fitted to the ex-
perimental value, showing the T1-state absorption spectra without forma-
tion of any new absorption band. Inset, the transient absorption spectra,
showing the spectral changes immediately after the second 425 nm laser
irradiation, obtained by subtraction of the broken line from the solid
line.
b
C
( NpCO ) in the region of 450–550 nm were obviously ob-
served after the second 425 nm laser irradiation of NpCO-
OPh(T1) (Figure 3b, inset). Therefore, the C O-bond cleav-
age occurred in NpCO-OPh(Tn) to give PhO and NpCO
during the two-color two-laser flash photolysis, see Equa-
tions (1–3).
b
ꢀ
C
C
h
n355=BP
NpCO-OPh
NpCO-OPhðT Þ
ð1Þ
ð2Þ
ð3Þ
ƒƒƒƒ!
1
hn425
ƒƒ!
NpCO-OPhðT Þ
NpCO-OPhðT Þ
1
n
CꢀO cleavage
C
NpCO-OPhðT Þ
NpCOC þ OPh
ƒƒƒƒƒƒƒ!
n
In the case of NpCO-OMe, no depletion of NpCO-OMe(T1)
was observed during the two-color two-laser flash photolysis
having a b-Np group, suggesting that the interaction be-
tween the ester moiety and a-Np is more efficient than for
b-Np. In other words, the DET1-Tn is smaller for the esters
having a a-Np group than for those having a b-Np group.
ꢀ
(Figure 3c and d), indicating that no C O-bond cleavage oc-
curred in NpCO-OMe(Tn).
In the case of PhCO-ONp, appearance of the absorption
a
bands of PhCO around 325 nm,[17–19] and NpO around
C
C
b
[17–19]
ꢀ
C
C O-Bond cleavage in the Tn states: Except for NpCO-
397 nm[17–19] or NpO radicals around 382 and 465 nm
OMe(T1), the clear depletion of the T1-state absorption
were observed during the second 425 nm laser flash photoly-
sis of PhCO-ONp(T1) (Figure 3e and f, and insets). The
(DDOD) was observed during the second 425 nm laser flash
Chem. Eur. J. 2007, 13, 3143 – 3149
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3145

T. Majima et al.
ꢀ
C O-bond cleavage occurred in PhCO-ONp(Tn) during the
two-color two-laser flash photolysis, see Equation (4).
have listed the energy levels of the S1, T1, and Tn states (ES1,
ꢀ
ET1, and ETn, respectively) and C O-bond dissociation ener-
gies (BDE) of the compounds used in the present work in
Table 2.
CꢀO cleavage
C
PhCO-ONpðT Þ
PhCOC þ ONp
ð4Þ
ƒƒƒƒƒƒƒ!
n
In the case of MeCO-ONp, the
C
C
C
absorption bands of MeCO
ꢀ
Table 2. F value and the calculated C O bond dissociation energy (BDE) together with the S1-state energy
(ES1), the T1-state energy (ET1), and the Tn-state energy (ETn).
a
around 340 nm and
NpO
b
around 397 nm[17–19] or NpO
[a]
[b]
Esters
ES1[a] [kJmolꢀ1
]
ET1 [kJmolꢀ1
]
ETn [kJmolꢀ1
]
BDE[c] [kJmolꢀ1
]
F[d]
around 382 and 465 nm[17–19]
were observed during the
second 425 nm laser flash pho-
tolysis of MeCO-ONp(T1) (Fig-
ure 3g and h, and insets). The
aNpCO-OPh
bNpCO-OPh
aNpCO-OMe
bNpCO-OMe
PhCO-OaNp
PhCO-ObNp
MeCO-OaNp
MeCO-ObNp
MeO-CH2aNp[e]
319
326
332
352
362
359
362
367
366
365
240
241
241
248
ꢁ250
ꢁ250
249
243
244
452
488
452
490
499
515
505
515
508
509
214
215
251
263
191
211
193
215
256
261
0.029
0.039
!0.001
!0.001
0.038
0.047
0.059
<0.01
0.048
!0.001
ꢀ
C O-bond cleavage occurred in
MeCO-ONp(Tn), see Equation
(5).
b
MeO-CH2 Np[e]
245
C-O cleavage
[a] The ES1 and ET1 values of NpCO-OR and RCO-ONp were calculated based on the fluorescence and phos-
ð5Þ phorescence spectra, respectively. The fluorescence spectra were measured in acetonitrile at 295 K and the
phosphorescence spectra were measured in a mixture of methanol-ethanol (1:1 v/v) at 77 K. Because the phos-
phorescence spectra of PhCO-OaNp and PhCO-ObNp were absent, ET1 values of PhCO-OaNp and PhCO-
ObNp were assumed to be equal to those of a- and b-methoxynaphthalene, respectively.[20] [b] The ETn values
MeCO-ONpðT Þ
ƒƒƒƒƒƒ!
n
MeCOC þ ONp
C
were estimated from the edge of the T1-state absorption peak at the longer wavelength side.[24] [c] BDE was
The different DDOD values for
the different esters during the
second 425 nm laser flash pho-
tolysis at the same laser intensi-
ty indicated the substituent
effect on the C O-bond cleav-
age in the Tn states. The DDOD
value of bNpCO-OPh was
calculated based on the heat of formation (DHf) for the esters and the fragment radicals using a semi-empirical
PM3 program contained in MOPAC’97.[5] [d] The quantum yield of the C O-bond cleavage was calculated
ꢀ
from the bleaching of the T1-state absorption (DDODT1) and the number of the second 425 nm laser photons
absorbed by the T1 state using an actinometry of the T–T absorption of zinc tetraphenylporphyrin at 470 nm in
cyclohexane (eTFT =50000mꢀ1 cmꢀ1 at 470 nm).[25] The data error was in 10%. [e] Ref. [9].
ꢀ
larger than that of aNpCO-OPh. That of PhCO-ObNp was
larger than that of PhCO-OaNp. In the case of MeCO-ONp,
the DDOD value of MeCO-OaNp(T1) was larger than that
of MeCO-ObNp(T1). Effects of the substituent (Ph, Me), a-
ꢀ
Np, and b-Np of the esters on the C O-bond cleavage are
discussed below.
The transient absorption spectra observed during the
266 nm laser flash photolysis of NpCO-OR and RCO-ONp
(Figure 2) were essentially equivalent with those observed
as new peaks during the two-color two-laser (355 and
425 nm) flash photolysis of NpCO-OR and RCO-ONp
(Figure 3, inset). Since same fragment radicals are produced
Figure 4. Plots of DDODT1 versus 425 nm laser intensities, a) aNpCO-
b
a
b
~
~
*
*
OPh ( ), b) NpCO-OPh ( ), c) PhCO-O Np ( ), d) PhCO-O Np ( ),
and e) MeCO-OaNp (&).
Except for ET1 of NpCO-OMe(T1) which is smaller than
BDE, ET1 values of the esters are larger than the corre-
ꢀ
from the S1 and Tn states, the C O-bond cleavage occurs in
ꢀ
NpCO-OR(Tn) and RCO-ONp(Tn).
sponding BDE value. Absence of the C O-bond cleavage in
NpCO-OR(T1) and RCO-ONp(T1) indicates that a large
ꢀ
C O-Bond cleavage quantum yield (F) and mechanisms: It
was observed that DDOD values increased with increasing
the second 425 nm laser intensity. The quantum yields (F)
energy barrier (DET1) exists between the T1(p,p*) state and
G
[9,10]
ꢀ
the C O-bond dissociation potential surfaces,
Figure 5.
ꢀ
On the other hand, the occurrence of the C O-bond cleav-
age in the Tn state indicates that a small energy barrier
ꢀ
of the C O-bond cleavage in the Tn states were determined
by the procedure reported previously.[9,10,23] Figure 4 shows
plots of DDOD for NpCO-OPh, PhCO-ONp, and MeCO-
OaNp versus the second 425 nm laser intensity. The F value
for MeCO-ObNp was estimated to be less than 0.01 and
those for NpCO-OMe were less than 0.001 (Table 2). It is
shown that an excited-state energy and a bond dissociation
energy are closely correlated and crucial factors for the oc-
currence of the bond cleavage in the excited states.[4] We
(DETn) exists between the Tn state and the C O bond disso-
ꢀ
ciation potential surfaces. During the second 425 nm laser ir-
radiation of the T1 state, the Tn
duced together with many other Tn states, having lower exci-
tation energies than that of the Tn(p,p*) state via the rapid
A
AHCTREUNG
ꢀ
internal conversion (IC), from which the C O-bond cleav-
age occurs. The electronic delocalization including Np and
ester groups must be involved in such Tn state (reactive Tn
3146
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Chem. Eur. J. 2007, 13, 3143 – 3149

Two-Laser Flash Photolysis
FULL PAPER
electronic delocalization in NpCO-OPh(Tn) including Ph is
possible because of the p electrons of Ph. On the other
hand, such electronic delocalization in NpCO-OMe(Tn) in-
cluding Me is impossible because of no p electron of Me.
Therefore, whether R is Ph or Me with or without p elec-
ꢀ
trons, respectively, is important for the occurrence of the C
O-bond cleavage in NpCO-OR(Tn). In the case of NpCO-
OPh, the broad and red-shifted absorption of aNpCO-
OPh(T1) compared with that of bNpCO-OPh(T1) was ob-
served. The DET1-Tn of aNpCO-OPh is smaller than that of
NpCO-OPh. Since the C O-bond cleavage in the Tn state
is a competitive process with the internal conversion from
b
ꢀ
ꢀ
Figure 5. An energy diagram during the C O-bond cleavage of esters
(R1CO-OR2) during the two-color two-laser flash photolysis; hn2: the
second 425 nm laser excitation, IC: internal conversion, DET1: an energy
ꢀ
ꢀ
the Tn state to the T1 state, the F of the C O-bond cleavage
in the Tn state should depend on the Tn-state lifetime. The
Tn-state lifetime and the relationship with the DET1-Tn value
have been reported for the substituted naphthalenes in the
Tn states.[22] The smaller DET1-Tn value corresponds to the
shorter lifetime of the Tn-state and the faster internal con-
version from the Tn to T1 states. Consequently, the F of the
barrier between the potential surfaces of the T1(p,p*) state and the C O
bond dissociation, DETn: an energy barrier between the potential surfaces
N
1
2
ꢀ
C
C
of the Tn state and the C O-bond dissociation, R CO and OR : the frag-
ment radicals.
C O-bond cleavage in aNpCO-OPh(Tn) is expected to be
ꢀ
state). In other words, the reactive Tn state has not only
b
ꢀ
p,p* character but also s,s* character of the C O bond.
smaller than that of NpCO-OPh(Tn).
ꢀ
Therefore, this reactive Tn state correlates with the C O-
Similar to NpCO-OPh(Tn), the electronic delocalization
between Np and Ph moieties in PhCO-ONp(Tn) is possible
because of p electrons of Ph. The broader and red-shifted
absorption of PhCO-OaNp(T1) than that of PhCO-ObNp(T1)
ꢀ
bond dissociation potential surface leading to the C O-bond
ꢀ
cleavage. Accordingly, absence of C O-bond cleavage in
NpCO-OMe(Tn), indicates that DETn for NpCO-OMe is
very large.
indicates that the DET1-Tn of PhCO-OaNp is smaller than
b
ꢀ
that of PhCO-O Np. Consequently, the F of the C O-bond
Substituent effects on the C O-bond cleavage: For the oc-
cleavage in PhCO-OaNp(Tn) is expected to be smaller than
that of PhCO-ObNp(Tn).
ꢀ
ꢀ
currence of the C O-bond cleavage in the Tn state, three
key factors are probably considered. Firstly, the ETn should
On the other hand, in the case of MeCO-ONp and
NpCO-OMe, such electronic delocalization in MeCO-
ONp(Tn) and NpCO-OMe(Tn) including Np and Me moiet-
ies is impossible because of no p electron of Me. Therefore,
ꢀ
be larger than BDE. Secondly, since the C O-bond cleavage
in the Tn state is a competitive process with the internal con-
version from the Tn state to the T1 state,[8,9,11] the lifetime of
the Tn state must be enough long to correlate with a disso-
ꢀ
the mechanism of the occurrence of the C O-bond cleavage
ꢀ
ciative potential surface leading to the C O-bond cleavage.
in MeCO-ONp(Tn) is different from those in PhCO-
Thirdly, the electronic delocalization in the Tn state is neces-
ONp(Tn) and NpCO-OPh(Tn). In the case of MeCO-ONp,
ꢀ
ꢀ
sary for the occurrence of the C O-bond cleavage. The data
in Table 2 show that all ETn values are much larger than
BDEs, suggesting that the ETn must be a key factor, but not
the C O-bond cleavage mechanism could be similar to that
of methoxymethylnaphthalene (MeO-CH2Np).[8,9,11] The F
a
ꢀ
value of the C O-bond cleavage in MeO-CH2 Np(Tn) was
0.048, while no C O-bond cleavage occurred in MeO-
CH2 Np(Tn). The different F values for MeO-CH2 Np and
MeO-CH2 Np should be due to the electronic character and
ꢀ
ꢀ
the most important one for the occurrence of the C O-bond
b
a
cleavage in the Tn state.
b
For the second factor, the lifetimes of several substituted
naphthalenes in the Tn states have been reported in a time
domain of several tens picoseconds.[22] Since the T1-state
energy is mainly localized on the Np moiety, the NpCO-OR
and RCO-ONp esters can be identified as the substituted
naphthalenes. The Tn-state lifetimes of NpCO-OR(Tn) and
RCO-ONp(Tn) are expected to be similar to those of the re-
conformation difference for a-Np and b-Np. Because the
methyl carbon of CH2Np is sp3 carbon atom, the s C O
ꢀ
bond can interact with p electrons of Np. Such interaction is
expected to be larger in MeO-CH2 Np(Tn) than that in
a
b
MeO-CH2 Np(Tn) because of the different substitution posi-
tions of the sp3 carbon atom in MeO-CH2Np(Tn). Conse-
ꢀ
ꢀ
ported substituted naphthalenes (10–60 ps). Since the C O-
bond cleavage occurs in the fs–ps time scale in the ground
quently, the C O-bond cleavage occurs in MeO-
CH2 Np(Tn), but not in MeO-CH2 Np(Tn).
a
b
and excited states,[3,4,14] such lifetimes of 10–60 ps for the Tn
The sp3 carbon atom connects on Np in MeO-CH2Np,
while the sp3 oxygen atom connects on Np in MeCO-ONp.
Since the sp3 oxygen atom has non-bonding electrons, the in-
ꢀ
states would be enough long for the occurrence of the C O-
bond cleavage.
ꢀ
ꢀ
For the third factor, it is very important whether the C O
teraction between the s C O bond and Np moiety in
MeCO-ONp is expected to be stronger than that in MeO-
bond is involved in the electronic delocalization in NpCO-
OR(Tn) or RCO-ONp(Tn), and it is clear that R, a-Np, and
b-Np have serious effects. In the case of NpCO-OPh, the
ꢀ
CH2Np. Therefore, the s C O-bond cleavage occurs more
efficiently in MeCO-ONp(Tn) than in MeO-CH2Np(Tn)
Chem. Eur. J. 2007, 13, 3143 – 3149
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3147

T. Majima et al.
a
(F=0.059, <0.01, 0.048, and !0.001 for MeCO-OaNp,
more efficiently in MeO-CH2 Np(Tn) and MeCO-OaNp(Tn)
a
b
b
MeCO-ObNp, MeO-CH2 Np, and MeO-CH2 Np, respective-
than in MeO-CH2 Np(Tn) and MeCO-ObNp(Tn).[9]
ly, Table 2). Such electronic interaction is expected to be
The electronic interaction between Np and R moieties in-
a
more efficient in MeO-CH2 Np(Tn) and MeCO-OaNp(Tn)
volving the C O bond of NpCO-OR and RCO-ONp also
ꢀ
b
than in MeO-CH2 Np(Tn) and MeCO-ObNp(Tn), because of
causes a change of the BDE value. The BDE values of
NpCO-OR and RCO-ONp, R = Ph with p electrons, are
the different substitution positions of the sp3 carbon and
oxygen atoms in MeO-CH2Np(Tn) and MeCO-ONp(Tn), re-
spectively.
smaller than those of
R = Me without p electrons
(Table 2). A relationship between the BDE versus F values
indicates that the smaller BDE value correlates to the larger
F value (Figure 6). In another words, the large electronic in-
teraction between Np and R moieties of NpCO-OR(Tn) and
RCO-ONp(Tn) leads to the small BDE value and the large
F value.
The electronic interaction between Ar and R moieties in-
ꢀ
volving the C O bond of ArCO-OR (Ar = Ph, R = Np, or
ꢀ
Ar = Np, R = Ph) in the Tn states causes the C O-bond
cleavage. Although the T1 state is localized on Np moiety,
the Tn state could be delocalized on Np and Ar moieties in-
ꢀ
ꢀ
cluding the C O bond in ArCO-OR(Tn). Therefore, the C
ꢀ
O bond has the antibonding character, leading to the C O-
bond cleavage. On the other hand, in the cases of NpCO-
ꢀ
OMe, MeCO-ONp, and MeO-CH2Np, the C O-bond cleav-
age depends on the interaction between the Tn state of Np
ꢀ
and the antibonding orbital of the C O bond. In NpCO-
OMe, the T1 state of Np is delocalized including the carboxy
group (NpCO moiety), based on the broad and red-shifted
absorption of the T1 state (Figure 3c and d). The Tn state of
the planar NpCO moiety can not interact electronically with
Figure 6. Plots of F versus BDE values of esters in Table 2.
ꢀ
ꢀ
the C O bond, because the C O bond orbital is almost per-
pendicular to the p electronic orbital of the NpCO moiety
ꢀ
or the interaction between the C O-bond orbital and the p-
electronic orbital of the NpCO moiety is negligible. There-
ꢀ
fore, no C O-bond cleavage occurs in NpCO-OMe(Tn). In
MeCO-ONp and MeO-CH2Np, the Tn-state orbital of Np
Conclusion
ꢀ
ꢀ
can not strongly interact with the C O bond because of no
C O-Bond cleavage of several esters in the Tn states was
observed during the two-color two-laser flash photolysis at
ꢀ
p electron of Me group, but weakly interact with the C O
ꢀ
ꢀ
bond. Therefore, the C O bond has some antibonding char-
acter, and the F values of the C O-bond cleavage are small
in MeCO-ONp(Tn) and MeO-CH2Np(Tn).
The obvious effects of a- and b-Np on the C O-bond
cleavage in MeCO-ONp(Tn) and MeO-CH2Np(Tn) should
be due to the difference of the electronic interaction be-
tween the s C O bond and the a- and b-Np moieties. It is
room temperature. The quantum yields (F) of the C O-
ꢀ
bond cleavage were estimated based on the plots of the dis-
appeared T1 state versus the second 425 nm laser intensities
and listed in Table 2. The F value seems to depend on the
ꢀ
ꢀ
characters of the substituent groups on the esters. The C O-
b
bond cleavage in NpCO-OPh(Tn) occurred more efficiently
than in NpCO-OPh(Tn) and that in PhCO-ObNp(Tn) occur-
a
ꢀ
known that absorption of naphthalene(T1) is due to two
red more efficiently than in PhCO-OaNp(Tn). In the case of
!
!
ꢀ
transitions, the short-axis polarized transition (3A1g
MeCO-ONp, the F value for MeCO-OaNp(Tn) was larger
!
3B2u+) and the long-axis polarized transition (3B3g
3B2u
than that for MeCO-O Np(Tn). No C O-bond cleavage oc-
curred in NpCO-OMe(Tn). The electronic delocalization in
the Tn state is necessary for the occurrence of the C O-
bond cleavage in NpCO-OR(Tn) or RCO-ONp(Tn). There-
ꢀ
+
b
ꢀ
ꢀ
).[26–28] In a-Np, the short-axis polarized transition (3A1g
3B2u+) is enhanced, the absorption of a-Np(T1) becomes
ꢀ
broader and red-shifted than that of naphthalene(T1), while
!
3B2u+) is
ꢀ
in b-Np, the long-axis polarized transition (3B3g
fore, R, Ph or Me with or without p electrons, respectively,
ꢀ
enhanced. The absorption of b-Np(T1) resembles that of
naphthalene(T1).[22,26–28] It is suggested that the electronic
delocalization in a-Np(T1) is larger than that in b-Np(T1).
Such larger electronic delocalization is expected in a-Np(Tn)
than that in b-Np(Tn). Because of this electronic delocaliza-
tion difference in a-Np(Tn) and b-Np(Tn), the more interac-
is one of key factors for the occurrence of the C O-bond
cleavage in the Tn states. When R is Ph with p electrons, the
ꢀ
C O bond is involved in the electronic delocalization be-
tween Ph and Np moieties in the Tn state, resulting in the
occurrence of the C O-bond cleavage in NpCO-OPh(Tn)
and PhCO-ONp(Tn). In the case of NpCO-OMe(Tn) and
ꢀ
ꢀ
ꢀ
tion between the s C O bond and Np moiety in MeO-
MeCO-ONp(Tn), R is Me without p electrons, the C O-
bond cleavage depends strongly on the electronic interaction
a
CH2 Np(Tn) and MeCO-OaNp(Tn) than that in MeO-
b
b
ꢀ
ꢀ
CH2 Np(Tn) and MeCO-O Np(Tn) is reasonable. The C O
between the C O bond and Np moiety. The larger electron-
a
bond has more antibonding character in MeO-CH2 Np(Tn)
ic interaction in MeCO-OaNp(Tn) than in MeCO-ObNp(Tn)
b
and MeCO-OaNp(Tn) than in MeO-CH2 Np(Tn) and
results in the F value larger for aNp compounds(Tn) than
b
b
ꢀ
MeCO-O Np(Tn). Therefore, the C O-bond cleavage occurs
for Np compounds(Tn).
3148
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 3143 – 3149

Two-Laser Flash Photolysis
FULL PAPER
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Materials: a- and b-Naphthoyl phenyl esters (aNpCO-OPh and bNpCO-
OPh) and a- and b-naphthoyl methyl esters (aNpCO-OMe and bNpCO-
OMe) were synthesized by reactions of naphtholyl chloride with phenol
and methanol, respectively. Benzoyl a- and b-naphthyl esters (PhCO-
OaNp and PhCO-ObNp) and acetyl a- and b-naphthyl esters (MeCO-
OaNp and MeCO-ObNp) were synthesized by reaction of a- and b-naph-
thol with benzoic anhydride and acetic anhydride, respectively. Benzo-
phenone from Nacalai Tesque Inc was purified from ethanol before use
as the triplet sensitizer. Acetonitrile (spectral grade) from Nacalai
Tesque Inc was used as received. All sample solutions were freshly pre-
pared and deoxygenated by bubbling with argon before irradiation. The
ground-state absorbance of the naphthyl derivatives was adjusted to be
1.0 at 266 nm during the 266 nm laser flash photolysis. BP(6.0”10 ꢀ3 m)
and naphthyl derivatives (6.0”10ꢀ3 m) were used during the two-color
two-laser flash photolysis. All experiments were carried out at room tem-
perature.
Two-color two-laser flash photolysis: Third and fourth harmonics (355
and 266 nm, respectively) from a Nd/YAG laser (Brilliant, Quantel; 5 ns
full width at half maximum (fwhm)) and the laser light at 425 nm
(7 mJpulseꢀ1) from an OPO laser (Continuum, Surelite OPO) which was
pumped by another Nd:YAG laser (Continuum, Surelite II-10; 5 ns
fwhm) were used as excitation light sources. Two laser flashes were
synchronized by a pulse generator with a delay time of 10 ns–10 ms. The
probe light was obtained from a 450 W Xe lamp (Osram XBO-450). The
transmitted probe light was focused on a monochromator (Nikon G250).
The output of the monochromator was monitored using a photomultiplier
tube (PMT, Hamamatsu Photonics, R928). The signal from the PMT was
recorded on a transient digitizer (Tektronix TDS 580D). A multichannel
analyzer system (Hamamatsu Photonics, C5967) was used for the mea-
surement of the transient absorption spectra. The total system was con-
trolled with a personal computer via GP-IB interface. To avoid any
damage of the sample solution by the probe light, a suitable cutoff filter
was used in front of the sample. The eight time shots to one sample at
each wavelength were carried out to get one set of experimental data,
and then the sample was replaced by the fresh one. The experiments
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Acknowledgements
This work has been partly supported by a Grant-in-Aid for Scientific Re-
search (Project 17105005, Priority Area (417), 21 st Century COE Re-
search, and others) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT) of Japanese Government. The authors
also thank to JSPS for a fellowship for X.C.
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Received: June 12, 2006
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Published online: January 10, 2007
Chem. Eur. J. 2007, 13, 3143 – 3149
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3149