Solvent dependence of triplet energy transfer from triplet benzophenone to naphthols and methoxynaphthalenes

Article abstract of DOI:10.1039/a707820j

Solvent effects on triplet-triplet energy transfer (TET) from triplet benzophenone (³BP*) to naphthol (NpOH) competing with hydrogen atom abstraction (HA) of ³BP* from NpOH and methoxynaphthalene (NpOMe) without HA have been studied in fluid media by 355 nm laser flash photolysis at 295 K. The efficiency (ψ_TET) and rate constant (k_TET) of TET in these systems were obtained. It was shown that the value of k_TET was dependent not only on the solvent viscosity (η) but also on the dielectric constant (κ) of the solvents, and ψ_TET and k_TET increased with increasing κ, contrary to the Dexter prediction. An increase in k_TET with increasing κ may be caused by the contribution of the dipole-dipole interaction (by the Foerster theory) due to perturbation of the ¹(π, π*) state to the lowest triplet state ³(n, π*) of benzophenone, in addition to the electron-exchange mechanism (by the Dexter theory).

Full text of DOI:10.1039/a707820j

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Solvent dependence of triplet energy transfer from triplet
benzophenone to naphthols and methoxynaphthalenes
Takuya Tanaka, Minoru Yamaji and Haruo Shizuka*
Department of Chemistry, Gunma University, Kiryu, Gunma 376 8515, Japan
Solvent e†ects on tripletÈtriplet energy transfer (TET) from triplet benzophenone (3BP*) to naphthol (NpOH) competing with
hydrogen atom abstraction (HA) of 3BP* from NpOH and methoxynaphthalene (NpOMe) without HA have been studied in
Ñuid media by 355 nm laser Ñash photolysis at 295 K. The efficiency (t ) and rate constant (k ) of TET in these systems were
TET
TET
obtained. It was shown that the value of k
was dependent not only on the solvent viscosity (g) but also on the dielectric
TET
constant (i) of the solvents, and t
and k increased with increasing i, contrary to the Dexter prediction. An increase in k
TET
TET
TET
with increasing i may be caused by the contribution of the dipoleÈdipole interaction (by the Forster theory) due to perturbation
of the 1(p, p*) state to the lowest triplet state 3(n, p*) of benzophenone, in addition to the electron-exchange mechanism (by the
Dexter theory).
Triplet energy transfer (TET) is such an important elementary
process, both in chemistry and biochemistry, that a large
number of theoretical and experimental studies have been
reported1h26 since the Ðrst report on the triplet sensitization
of naphthalene by benzophenone in a rigid matrix by Terenin
and Ermolaev.27 Triplet sensitization techniques have been
applied to efficient production of triplet states and evaluation
of the molar absorption coefficients of triplet molecules in
Ñuid media using the assumption that the transfer efficiency is
unity.14 Moreover, until recently, the bimolecular TET rate
constant was regarded as equal to that of quenching for
various triplet-sensitization systems.
In this decade, however, we have shown that the TET effi-
ciencies are not always unity in the quenching of triplet ben-
zophenone (3BP*) by naphthalene derivatives (NpX).28h37
When the efficiency is not unity, the residual efficiency should
be considered as due to a radiationless decay process induced
by NpX, called “induced quenching (IQ)Ï.28h37 The quenching
processes of 3BP* by NpX are illustrated by Scheme 1.33 For
example, when 1-naphthol (1NpOH) was employed as a quen-
cher for 3BP*, it was found that TET to 1NpOH competed
with HA from 1NpOH and IQ by 1NpOH (Scheme 2).30,31
The rate constant and efficiency of TET depend signiÐcantly
on the solvent polarity. In polar media, TET takes place effi-
ciently while in non-polar media, HA is predominant.34 In
general, it is known that the photoreactivity of HA of aro-
matic carbonyl triplets depends on the solvent polarity and
substituents which sensitively a†ect the electronic character of
the lowest triplet state.6,38h48 The 3(n, p*) state of aromatic
carbonyl compounds efficiently abstracts hydrogen atom from
H-donor molecules, such as alcohols and hydrocarbons,
whereas the 3(p, p*) state only does so inefficiently.6,38h48 The
HA reactivities of triplet acetophenones45 and xanthone47 are
known to be dependent on solvent polarity.
On the other hand, the solvent dependence of TET from
3BP* to 1NpOH has not yet been solved. In general, the TET
mechanism is interpreted by the electron-exchange mechanism
of Dexter, which requires overlap between the electronic
obitals of the donor and the acceptor.1 The probability of
TET was found to depend on the dielectric constant (i) of the
media, i.e. it is proportional to i~2.1 However, our data
showed that the efficiency and rate constant for TET from
3BP* to 1NpOH increased, contrary to the prediction of the
Dexter theory.34
In the present work, in order to make clear the solvent
dependence of the TET process in Ñuid media, laser photolysis
studies have been carried out on the BPÈnaphthol and
Èmethoxynaphthalene systems at 295 K.
Experimental
Benzophenone (BP), 1-naphthol (1NpOH), 2-naphthol
(2NpOH) and 2-methoxynaphthalene (2NpOMe) were puri-
Ðed twice by vacuum sublimations. 1-Methoxynapthalene
(1NpOMe) was puriÐed by vacuum distillation. All the
samples employed in the present study were commercially
available. Acetonitrile (ACN), acetone (AC) and benzonitrile
(PhCN) were puriÐed by distillation. Carbon tetrachloride
(CCl ; Spectrosol from Dojin) was used as supplied. Deion-
4
ized water was distilled. ACN, CCl , AC, PhCN and mixtures
4
of ACNÈH O, CCl ÈACN and CCl ÈAC were used as sol-
vents. They were inert to HA of 3BP*.
2
4
4
The concentration of BP was usually 6.7 ] 10~3 mol dm~3.
The concentrations of NpOH and NpOMe were less than
3.0 ] 10~3 mol dm~3. All samples in a quartz cell of 10 mm
path length were thoroughly degassed by freezeÈpumpÈthaw
cycles on a high-vacuum line.
Scheme 1
The triplet energies (E ) of BP in polar and non-polar
T
media were determined from the 0È0 band of the phosphor-
escence spectrum in methanolÈethanol (1 : 1 v/v) and
methylcyclohexaneÈisopentane (3 : 1 v/v) glass at 77 K, respec-
tively.
Laser Ñash photolysis in the sample solutions was carried
out at 295 K by using third harmonics (355 nm) of an
Nd3` : YAG laser from J.K. Lasers (HY-500). The pulse dura-
tion and energy were 8 ns and 40 mJ pulse~1, respectively.
Scheme 2
J. Chem. Soc., Faraday T rans., 1998, 94(9), 1179È1187
1179

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The detection system has been described elsewhere.36 The
transient data obtained by laser photolysis were analysed by
the least-squares best-Ðt method with errors ^5%.
(31NpOH*) or triplet 2NpOH (32NpOH*). Therefore, it can
be said that the spectral changes in Fig. 1(a) and (b) indicate
that TET from 3BP* (E \ 69.1 kcal mol~1 in polar media) to
T
In order to determine the quantum yields (U ) for inter-
NpOH occurs e†ectively in ACN to produce triplet NpOH
ISC
system crossing (ISC) of NpOH and NpOMe, time-resolved
(3NpOH*; E \ 58.5 kcal mol~1 for 31NpOH* and 60.2 kcal
T
thermal lensing techniques which require Ñuorescence
mol~1 for 32NpOH* in polar media).
quantum yields (U ) and E values of NpOH and NpOMe
3BP* ] NpOH ] BP ] 3NpOH*
(I)
f
T
were employed.48h50 The U values of NpOH and NpOMe
f
On the other hand, the reaction of 3BP* with NpOH in
were determined by comparison with that of 1NpOMe in
CCl was quite di†erent from that in ACN. Fig. 1(c) and (d)
ACNÈH O (4 : 1 v/v) (U \ 0.36).51 The values of E , U and
4
2 f
are listed in Table 1.
The molar absorption coefficients (eThT) of the tripletÈtriplet
(TÈT) absorptions of NpOH and NpOMe at a peak wave-
length (j) were determined with the use of a 308 nm laser
pulse from an XeCl excimer laser (Lambda Physik Las-
ertechnik LEXTRA 50) as follows. After 308 nm laser pulsing
in the solution of NpOH or NpOMe in a cell with 10 mm
T
f
show the time-resolved transient absorption spectra obtained
U
ISC
by 355 nm laser photolysis in the BP (6.7 ] 10~3 mol dm~3)È
j
1NpOH(2.0 ] 10~3 mol dm~3) and È2NpOH (2.0 ] 10~3
mol dm~3) systems in CCl , respectively. With a decrease in
4
intensity of 3BP* at 530 nm,53,56,57 the absorption band due
to the benzophenone ketyl radical (BPK) at 545 nm14,56,58
appears together with an absorption peak of the 1-naphthoxyl
radical (1NpO~) at 395 nm30,59 in Fig. 1(c), or of the 2-
naphthoxyl radical (2NpO~) at 470 nm59,60 in Fig. 1(d). It was
path length, the initial absorbance change (*AThT) at j due to
j
the formation of triplet NpOH (3NpOH*) or NpOMe
found that, in CCl , HA of 3BP* from NpOH to produce
(3NpOMe*) is given by
4
BPK and naphthoxyl radicals (NpO~) occurred efficiently
*AThT \ U eThTI N~1
(1)
)
rather than TET.
j
ISC j abs A
where N is AvogadroÏs number. The number of photons (I
absorbed by NpOH or NpOMe at the excitation wavelength
3BP* ] NpOH ] BPK ] NpO~
(II)
A
abs
In order to obtain kinetic data for interactions between
was determined by using the TÈT absorption (*ABP ) of BP at
520
3BP* and NpOH, the decay rates of 3BP* (K ) were mea-
520 nm in ACN as an actinometer32
obsd
sured in various solvents by 355 nm laser Ñash photolysis. Fig.
*ABP \ UBP eBP I N~1
520 ISC 520 abs
(2)
2(a) and (b) show the plots of k
and [2NpOH], respectively, in ACN and CCl . Since all plots
as a function of [1NpOH]
A
obsd
where UBP and eBP stand for the triplet yield and molar
4
ISC
520
give straight lines, k
can be expressed by
absorption coefficient at 520 nm which are known to be 1.052
and 6500 dm3 mol~1 cm~1 in ACN,53 respectively. By com-
obsd
k
\ k ] k [NpOH]
(3)
obsd
0
q
paring eqn. (1) with eqn. (2), the eThT values of NpOH and
where k and k are the decay rate constant of 3BP* in the
j
0
q
NpOMe were determined and are listed in Table 1.
absence of NpOH and the quenching rate constant of 3BP*
by NpOH, respectively. From the intercept and slope of the
line, the values of k and k were obtained. The experimental
Results
0
q
data under various conditions are listed in Table 2.
The quantum yields (U and U ) for TET and HA can
TET competition with HA in the BP–NpOH system
TET HA
be deÐned by eqn. (4) and (5), respectively.
Since NpOH and NpOMe have no absorbance at 355 nm in
the solvents used, upon 355 nm laser irradiation in the
BPÈNpOH and BPÈNpOMe systems, only BP is photoexcit-
ed to form 3BP* within a picosecond timescale via fast
ISC18,54 according to the El-Sayed rule.55 Fig. 1(a) and (b)
show the time-resolved transient absorption spectra obtained
by 355 nm laser Ñash photolysis in the BP (6.7 ] 10~3 mol
dm~3)È1NpOH(2.0 ] 10~3 mol dm~3) and È2NpOH(2.0 ]
10~3 mol dm~3) systems in ACN, respectively. The TÈT
absorption spectrum of BP with a peak at 520 nm53,56,57
decreases according to Ðrst-order kinetics, accompanying an
increase in that of 1NpOH at 430 nm30 in Fig. 1(a) or of
2NpOH at 430 nm in Fig. 1(b). The absorption band at 430
nm in Fig. 1(b) was assigned to the TÈT absorption spectrum
of 2NpOH, since it was the same as that observed upon direct
laser excitation of 2NpOH at 308 nm, and was quenched
e†ectively by dissolved oxygen. The decay rate of 3BP* was in
good agreement with the rise rate of triplet 1NpOH
U
\ [3NpOH*] N I~1
(4)
(5)
TET
net A abs
U
\ [BPK] N I~1 \ [NpO~] N I~1
HA
net A abs
net A abs
where [3NpOH*] , [BPK] and [NpO~] are the net con-
net
net
net
centrations of 3NpOH* produced by TET from 3BP*, BPK
and NpO~ by HA, respectively. The absorbed photon Ñux of a
laser pulse (I ) at 355 nm was determined by eqn. (2) as
described above. The value of the absorbance change (*A ) at
wavelength (j) can be expressed by eqn. (6).
abs
j
~
*A \ eThT[3NpOH*] ] (eBPK ] eNpO )[BPK]
j
j
net
j
j
net
~
\ eThT[3NpOH*] ] (eBPK ] eNpO )[NpO~]
(6)
j
net
j
j
net
where *A at j is the net maximum absorbance of 3NpOH*
(430 nm), BPK (545 nm), 1NpO~ (395 nm) or 2NpO~ (470 nm)
j
after the decay of 3BP*. The molar absorption coefficients of
~
3NpOH* (eThT), BPK (eBPK) and NpO~ (eNpO ) at j are sum-
j
j
j
marized in Table 3.
Table 1 Triplet energy (E ), quantum yields of Ñuorescence (U ) and intersystem crossing (U ) and molar absorption coefficients (eThT) of the
T
f
ISC
j
TÈT absorption at a peak wavelength (j ) of NpOH and NpOMe
max
E /kcal mol~1
U a
U
eThT a (j )/103 dm3 mol~1 cm~1
T
f
ISC
j
max
non-polarb
polarc
CHd
ACN
CHd
ACN
CHd
ACN
1NpOH
2NpOH
1NpOMe
2NpOMe
58.5
61.9
60.2
62.2
58.5
60.2
59.7
61.9
0.23
0.30
0.33
0.36
0.41
0.33
0.36
0.33
0.61e
0.65e
0.64e
0.63e
0.46e
0.51e
0.64f
0.65f
7.1(430)
10.3(430)
6.2(435)
7.3(430)
10.9(430)
6.5(435)
12.5(430)
11.0(430)
a See text for determination. b Determined from the 0È0 band of the phosphorescence spectrum in methylcyclohexaneÈisopentane (3 : 1 v/v) glass
at 77 K. c Determined from the 0È0 band of the phosphorescence spectrum in methanolÈethanol (1 : 1 v/v) glass at 77 K. d Cyclohexane.
e Determined by the thermal lensing method at 77 K. Errors ^ 2%. f From ref. 33.
1180
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Fig. 1 Transient absorption spectra observed after 355 nm laser pulsing in BP solution (a) with 1NpOH(2.0 ] 10~3 mol dm~3) at (1) 35, (2) 80,
(3) 150 and (4) 250 ns in ACN; (b) with 2NpOH(2.0 ] 10~3 mol dm~3) at (1) 40, (2) 80, (3) 120 and (4) 300 ns in ACN; (c) with
1NpOH(2.0 ] 10~3 mol dm~3) at (1) 30, (2) 60, (3) 100 and (4) 200 ns in CCl ; (d) with 2NpOH(2.0 ] 10~3 mol dm~3) at (1) 40, (2) 80, (3) 160
4
and (4) 400 ns in CCl
4
By using *A and the molar absorption coefficients of tran-
and t values can also be given by
j
HA
sient species, the concentration of [3NpOH*] , [BPK]
net
net
U
\ t k [NpOH]UBP (k ] k [NpOH])~1
TET q ISC 0
(7)
(8)
and [NpO~] can be estimated from eqn. (6). The U
and
TET
q
net
TET
U
values were determined experimentally using eqn. (4)È(6).
U
\ t k [NpOH]UBP (k ] k [NpOH])~1
HA
HA
HA q ISC 0
q
On the basis of the values of U
and U , the efficiencies
TET
HA
Since the employed [NpOH] were of the order of 10~3 mol
(t
and t ) for TET and HA were determined. The t
TET
HA
TET
dm~3, an approximation, k [NpOH] A k is achieved for
q
0
k B 109 dm3 mol~1 s~1 and k B 105 s~1 (see Table 2).
q
0
Hence, the values of the efficiencies could be regarded as equal
to those of the quantum yields. The efficiency (t ) of the (IQ)
IQ
of 3BP* by NpOH was determined by
t
\ 1 [ t [ t
(9)
IQ
TET
HA
With the use of the determined efficiencies, the rate constants
(k , k and k ) for TET, HA and IQ were estimated from
TET HA
IQ
eqn. (10)È(12), respectively.
k
\ k t
(10)
(11)
(12)
TET
q
TET
HA
IQ
k
\ k t
HA
q
k
\ k t
IQ
q
The rate constants obtained are listed in Table 2.
TET process in the BP–NpOMe system without HA
It has been shown that the interaction between 3BP* and
NpOMe in ACN is TET.29,33 In CCl , it was also observed
4
from the transient absorption spectral changes in the BPÈ
NpOMe system that TET from 3BP* (E \ 69.1 kcal mol~1
T
in polar media and 68.5 kcal mol~1 in non-polar media) to
NpOMe occurred efficiently to produce triplet NpOMe
(3NpOMe*; E \ 59.7 kcal mol~1 in polar media and 60.2
Fig. 2 First-order decay rate (k
)
of 3BP* in ACN(L) and
T
obsd
kcal mol~1 in non-polar media for 31NpOMe*; E \ 61.9
CCl (|) as a function of (a) [1NpOH] in the BP(6.7 ] 10~3 mol
T
4
kcal mol~1 in polar media and 62.2 kcal mol~1 in non-polar
dm~3)È1NpOH system and (b) [2NpOH] in the BP(6.7 ] 10~3 mol
dm~3)È2NpOH system
media for 32NpOMe*). No HA of 3BP* from NpOMe was
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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Table 2 Kinetic parameters and efficiencies of TET, HA and IQ in the BPÈNpOH systema
sample
solvent
kb
gc,d
eThT e
k
f
k
g
t
t
t
k
g
k
g
k
g
j
0
q
TET
HA
IQ
TET
HA
IQ
1NpOH
CCl
2.2h
7.9
12.7
15.6
20.7h
7.7
13.4
19.2
30.0
37.5h
2.2h
7.9
12.7
20.7h
7.7
9.00h
6.58
5.42
4.45
3.07h
7.88
6.81
6.04
4.35
3.41h
9.00h
6.58
5.42
3.07h
7.88
6.81
6.04
4.35
3.41h
7.10
7.16
7.21
7.22
7.30
7.13
7.16
7.20
7.26
4.7
5.5
5.5
3.6
4.3
4.4
3.7
4.3
3.7
2.8
4.7
4.6
3.9
4.1
4.4
3.7
4.3
3.7
2.4
6.1
5.6
6.1
6.6
7.8
7.1
6.6
6.6
7.7
9.4
5.8
5.0
5.5
7.2
5.1
5.1
5.3
6.8
8.3
0.03
0.35
0.46
0.50
0.65
0.20
0.30
0.44
0.52
0.73
\0.01
0.53
0.60
0.85
0.44
0.50
0.56
0.67
0.82
0.73
0.43
0.26
0.20
0.08
0.61
0.41
0.34
0.23
0.08
0.57
0.43
0.35
0.11
0.54
0.40
0.36
0.26
0.09
0.24
0.22
0.28
0.30
0.27
0.19
0.29
0.22
0.25
0.19
0.42
0.04
0.05
0.04
0.02
0.10
0.08
0.07
0.09
0.18
2.0
2.8
3.3
5.0
1.4
2.0
2.9
4.0
6.9
0.03
2.6
3.3
6.1
2.3
2.6
3.0
4.6
6.8
4.5
2.4
1.6
1.3
0.6
4.3
2.7
2.3
1.8
0.8
3.3
2.1
1.9
0.6
2.8
2.1
1.9
1.8
0.8
1.5
1.2
1.7
2.1
2.2
1.4
1.9
1.5
1.9
1.8
2.4
0.2
0.3
0.2
0.1
0.5
0.4
0.5
0.8
4
CCl ÈAC(3 : 1 v/v)
4
CCl ÈAC(1 : 1 v/v)
4
CCl ÈAC(1 : 2 v/v)
4
AC
CCl ÈACN(10 : 1 v/v)
4
CCl ÈACN(4 : 1 v/v)
4
CCl ÈACN(2 : 1 v/v)
4
CCl ÈACN(1 : 2 v/v)
4
ACN
CCl
7.30
2NpOH
10.30
10.48
10.64
10.90
10.39
10.49
10.59
10.77
10.90
4
CCl ÈAC(3 : 1 v/v)
4
CCl ÈAC(1 : 1 v/v)
4
AC
CCl ÈACN(10 : 1 v/v)
4
CCl ÈACN(4 : 1 v/v)
13.4
19.2
30.0
37.5h
4
CCl ÈACN(2 : 1 v/v)
4
CCl ÈACN(1 : 2 v/v)
4
ACN
a Errors ^ 5%. b The i value of the mixed solvent was assumed to be proportional to the molar fractions of those of the original solvents. c The
g value of the mixed solvent was measured by the Ostwald method at 25 ¡C on the basis of those of ACN, AC and CCl. d Units: 10~4 Pa s.
e Units: 103 dm3 mol~1 cm~1. See text for details. f In units: 105 s~1. g In units: 103 dm3 mol~1 s~1. h From ref. 52.
Table 3 Molar absorption coefficients (e ) of benzophenone ketyl
observed. By analogy with the BPÈNpOH system, the quen-
j
radical (BPK), naphthoxyl radicals (NPOÕ) and triplet naphthols
ching rate constant (k ) of 3BP* by NpOMe was determined
q
(3NpOH*) at a wavelength (j)a
and listed in Table 4.
The quantum yield (U ) of TET for the BPÈNpOMe
system was also determined from eqn. (13).
1NpOH
2NpOH
TET
~
~
j/nm
eBPK
eNpO
eThT
eNpO
eThT
j
j
j
j
j
U
\ [3NpOMe*] N I~1
TET
net A abs
395
430
475
545
310
450
780
4100
1400
570
2200
1350
2500
È
5880b(5720)c
7300b(7100)c
3600b(2500)c
460b(450)c
4200b(3800)c
10 900b(10 300)c
700b(660)c
\ *AThTN eThT~1I~1
(13)
j
A j
abs
where [3NpOMe*]
is the net concentration due to
3220
1200
È
(È)
net
3NpOMe* produced by TET of 3BP*, and *AThT denotes the
j
a Units: dm3 mol~1 cm~1. b In ACN. c In cyclohexane.
maximum net absorbance of 3NpOMe* at a peak wavelength
Table 4 Kinetic parameters and efficiencies of TET and IQ in the NpOMeÈBP system
1NpOMe
2 NpOMe
solvent
ib
gc,d
k
e
eThT f
k
g
t
t
k
g
k
g
eThT h
k
g
t
t
k
g
k
g
0
j
q
TET
IQ
TET
IQ
j
q
TET
IQ
TET
IQ
CCl
2.2h
7.9
9.00h
6.58
2.2
1.9
6200
6290
5.4
6.2
0.85
0.91
0.15
0.09
4.6
5.7
0.8
0.5
12 500
12 040
4.9
5.4
0.56
0.67
0.44
0.33
2.7
3.6
2.2
1.8
4
CCl ÈAC
4
(3 : 1 v/v)
CCl ÈAC
15.6
4.45
1.9
6420
7.7
0.95
0.05
7.3
0.4
11 400
6.6
0.81
0.19
5.4
1.2
4
(1 : 2 v/v)
AC
20.7i
7.7
3.07i
7.88
2.2
2.3
6500
6250
9.2
5.6
0.93
0.89
0.07
0.11
8.6
5.0
0.6
0.6
12 270
12 030
8.0
5.0
0.83
0.60
0.17
0.40
6.6
3.0
1.4
2.0
CCl ÈACN
4
(10 : 1 v/v)
CCl ÈACN
13.4
19.2
30.0
6.81
6.04
4.35
2.2
2.0
1.6
6300
6340
6440
5.8
6.2
8.0
0.88
0.89
0.90
0.12
0.11
0.10
5.1
5.6
7.2
3.4
0.7
0.6
0.8
0.5
11 800
11 320
11 000
5.3
5.7
7.3
0.62
0.68
0.75
0.38
0.32
0.25
3.3
3.9
5.4
2.0
1.8
1.9
4
(4 : 1 v/v)
CCl ÈACN
4
(2 : 1 v/v)
CCl ÈACN
4
(1 : 2 v/v)
PhCN
ACN
25.2i
37.5i
40.1
12.3i
3.41i
3.86
4.5
2.2
2.2
6500
6500
6500
3.9
10.0
11.0
0.88
1.0
0.93
0.12
0
0.07
11 000
11 000
11 000
3.6
8.8
9.6
0.69
0.94
0.88
0.31
0.06
0.12
2.5
8.2
8.5
1.1
0.6
1.1
10.0
10.3
\ 0.01
ACNÈH O
0.7
2
(9 : 1 v/v)
ACNÈH O
44.5
46.8
51.0
58.4
5.02
5.67
6.57
7.14
1.4
1.5
1.6
1.5
6500
6500
6500
6500
12.4
11.3
11.6
12.1
0.95
0.99
0.94
0.92
0.05
0.01
0.06
0.08
11.7
11.2
10.9
11.1
0.7
0.1
0.7
1.0
11 000
11 000
11 000
11 000
2
(4 : 1 v/v)
ACNÈH O
10.3
11.1
0.93
0.85
0.07
0.15
9.6
9.4
0.7
1.7
2
(3 : 1 v/v)
ACNÈH O
2
(2 : 1 v/v)
ACNÈH O
2
(1 : 1 v/v)
a Errors ^ 5%. See text for determination. b The i value of the mixed solvent was assumed to be proportional to the molar fractions of those of
the original solvents. c The g value of the mixed solvent was measured by the Ostwald method at 25 ¡C on the basis of those of ACN, AC and
CCl . d Units: 10~4 Pa s. e Units: 105 s~1. f Units: dm3 mol~1 cm~1 at 435 nm. g Units: 109 dm3 mol~1 s~1. h Units: dm3 mol~1 cm~1 at 430
4
nm. i From ref. 52.
1182
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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(j). The absorbed photon Ñux (I ) of a laser pulse at 355 nm
abs
was determined from eqn. (2). The value of the molar absorp-
tion coefficient (eThT) of 3NpOMe* from Table 4 was used.
j
By similar treatment to that of the BPÈNpOH system, the
efficiency (t ) for TET in the BPÈNpOMe system was
TET
obtained. The efficiency (t ) of IQ of 3BP* by NpOMe was
IQ
determined from eqn. (14).
t
\ 1 [ t
TET
(14)
IQ
The t
TET
and t values obtained are listed in Table 4. The
IQ
rate constants (k
and k ) for TET and IQ in the BPÈ
TET
IQ
NpOMe system was determined from eqn. (10) and (12), and
are also listed in Table 4.
Discussion
TET process competing with HA in the BP–NpOH system
It was found by laser photolysis in the BPÈNpOH system that
TET competed with HA in Ñuid media, and the former was a
predominant process in polar ACN, whereas the latter was
predominant in non-polar CCl . Fig. 3(a) and (b) show the
4
efficiencies (t , t and t ) as a function of i for the BPÈ
TET HA
IQ
1NpOH and PBÈ2NpOH systems, respectively.
Fig. 3 Efficiencies for TET (…), HA (|) and IQ (K) as a function of
i of the solvent for (a) the BPÈ1NpOH and (b) the BP-2NpOH
systems. The dashed lines were drawn along the experimental points.
See text for details.
The t
value gradually increases up to 0.75 and 0.85 with
an increase in i in the BPÈ1NpOH and È2NpOH systems,
TET
respectively. In contrast, the t value decreases from 0.73 to
0.04 in BPÈ1NpOH and from 0.57 to 0.09 with an increase in
HA
i in the BPÈ2NpOH systems. The t value seems to be inde-
IQ
pendent of i. Thus, it can be said that both TET and HA
BPÈ1NpOH system and from 3.3 ] 109 to 8 ] 108 dm3
between 3BP* and NpOH are signiÐcantly dependent on the
mol~1 s~1 in the BPÈ2NpOH system. However, the k value
IQ
solvent polarity. The rate constants (k , k , k and k ) are
seems to be almost independent of i. These plots show that
q
TET HA
IQ
shown in Fig. 4(a) and (b) as a function of i.
the k
(or t ) value increases with increasing solvent
TET
TET
With increase in i, the k
value increases from 1.8 ] 108
polarity while the k (or t ) decreases. The k values in
TET
HA
HA
HA
to 6.9 ] 109 dm3 mol~1 s~1 in the BPÈ1NpOH system and
from 3 ] 107 to 6.8 ] 109 dm3 mol~1 s~1 in the BPÈ2NpOH
non-polar media were ca. 4È6 times greater than those in
polar media in the BPÈNpOH system. In the course of our
study on HA from phenol (PhOH) by 3BP* without TET, it
system. In contrast, with an increase in i, the k
value
HA
decreases from 4.5 ] 109 to 8 ] 108 dm3 mol~1 s~1 in the
was found that the k value (2.0 ] 109 dm3 mol~1 s~1) in
HA
Fig. 4 Rate constants k (L), k
(…), k (|) and k (K) as a function of i of the solvent in (a) the BPÈ1NpOH, (b) the BPÈ2NpOH systems
q
TET
HA
IQ
and the reciprocal of solvent viscosity (g) in (c) the BPÈ1NpOH and (d) the BPÈ2NpOH systems. The solid and dashed lines were drawn using
eqn. (15) and along the experimental points, respectively. See text for details.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1183

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CCl was ca. 32 times greater than that (6.2 ] 107 dm3 mol~1
4
s~1) in ACN.¤
A similar solvent e†ect in HA from PhOH by neutral
alkoxyl radicals has been reported by Ingold and co-
workers.60 They interpreted the solvent e†ect as being due to
hydrogen bond formation between PhOH and the solvents.
The stronger the hydrogen bond between PhOH and the
solvent, the smaller the absolute rate constants for HA from
PhOH by alkoxyl radicals.60 According to their conclusion,
the reactivity of HA from NpOH by 3BP* may also depend
on the hydrogen-bonding ability of the solvents. However,
considering the electronic structure, the HA character of 3BP*
is di†erent from those of the alkoxyl radicals. It is known that
the electronic character of the lowest triplet state of BP is
3(n, p*) which is perturbed by 3(p, p*) through vibronic coup-
ling.62 In general, carbonyl compounds having the 3(n, p) state
as the lowest triplet state can abstract a hydrogen atom from
H donors efficiently, whereas the 3(p, p*) state only does this
inefficiently.6,38h47 With an increase in solvent polarity, the
energy level of 3(n, p*) increases while that of 3(p, p*)
decreases, which leads to further perturbation of the 3(p, p*)
state to 3(n, p*). Consequently, there is less HA reactivity of
BP in polar media. Therefore, the solvent dependence of HA
from NpOH of 3BP* as well as the case of the BPÈPhOH
system should be explained by two possible mechanisms, in
terms of hydrogen bonding and solvent polarity. The HA
dependence of 3BP* will be discussed in the near future.
Fig. 5 Efficiencies for TET (L) and IQ (K) as a function of the i of
the solvent in (a) the BPÈ1NpOMe and (b) the BPÈ2NpOMe systems.
The dashed lines were drawn along the experimental points. See text
for details.
The k values of 3BP* by NpOH in all the solvents used
q
were of the order of 109 dm3 mol~1 s~1. Since the values of k
q
were close to those of di†usion-controlled processes, it can be
With increasing i, the t
value in BPÈ1NpOMe (or
È2NpOMe) increases slightly from 0.85 to 1.0 (or from 0.56 to
TET
said that the quenching of 3BP* by NpOH occurs in a colli-
sion process. The rate constants of TET, HA and IQ were
0.94). In contrast, with increasing i, the t value decreases
IQ
smaller than the di†usion limit of the solvent (k ), indicating
slightly. It was concluded that the t
and t values in the
dif
TET
IQ
that these processes took place in an encounter between 3BP*
BPÈNpOMe system depend on i, considering the experimen-
tal errors (^5%).
and NpOH. The k value is proportional to the reciprocal of
dif
solvent viscosity (g) according to DebyeÏs equation.12 The vis-
Fig. 6(a) and (b) show the plots of k , k
and k values
q
TET
IQ
cosity of the TET, HA and IQ processes in the BPÈNpOH
system was elucidated as follows. Fig. 4(c) and (d) show plots
of the k , k , k and k values as a function of g~1 for the
obtained by laser photolysis at 355 nm in the BPÈ1NpOMe
and È2NpOMe systems, respectively, as a function of i. With
increasing i, the k
value increases from 4.6 ] 109 to
q
TET HA
IQ
TET
BPÈ1NpOH and È2NpOH systems, respectively. The values
1.2 ] 1010 dm3 mol~1 s~1 for the BPÈ1NpOMe system and
from 2.7 ] 109 to 9.6 ] 109 dm3 mol~1 s~1 for the BPÈ
of k and k
increase with decrease in g, except for those in
q
TET
ACNÈH O mixtures, as shown in Table 4, while they are con-
2NpOMe system. In the BPÈNpOMe system, the k
value
2
TET
siderably smaller than k estimated from a slightly modiÐed
increases appreciably with increasing i, contrary to the pre-
diction of the Dexter theory. In contrast, the k values
dif
DebyeÏs equation.12
IQ
decrease slightly with increasing i. In Fig. 6(a) and (b), com-
k
\ (8RT /2000)g~1
(15)
paring the k
values in CCl ÈACN (Group B) with those in
dif
TET
4
CCl ÈAC (Group A) in the region of i \ 25, the latter values
are markedly greater than the former. Furthermore, the k
value in PhCN (Group C) with a relatively large i value (25.2)
are the smallest found. Thus, it can be said that the variation
It is unexpected that the k value will increase with increase
in g. The k value for the BPÈNpOH system is controlled by
solvent polarity rather than g.
4
HA
TET
HA
The TET mechanism can be interpreted by an electron-
exchange interaction which requires sufficiently close
approach between the triplet energy donor and acceptor to
allow electronic overlap between them. The Dexter mecha-
nism in rigid media shows that the probability of TET is pro-
in k
values in Ñuid media should be caused mainly by
TET
another factor rather than i.
Fig. 6(c) and (d) show plots of k , k
of g~1 in the BPÈ1NpOMe and È2NpOMe systems, respec-
tively. Both k and k
except for those in ACNÈH O mixtures [Group D in Fig. 6(c)
and (d)]. However, their values are distinctly smaller than the
and k as a function
q
TET
IQ
values increase with increasing g~1,
portional to i~2.1 However, the t
and k
values in the
q
TET
TET
TET
BPÈNpOH system in the liquid phase increased with increas-
ing i, contrary to the prediction of the Dexter theory.
2
di†usion rates calculated by eqn. (15). The k
value increases
TET
with decreasing of g, but the k
value is not a linear function
TET process for the BP–NpOMe system without HA
TET
of g~1. Similar behaviour has been observed by Wagner and
Kochevar whose results provide evidence that the observed
rate constants for exothermic TET from 3(n, p*) ketones to
dienes are not di†usion-controlled values but typically about
half these values in solvents with low viscosity.12 Therefore, it
In order to investigate the solvent dependence of TET without
HA, we used NpOMe which is almost isoelectronic in struc-
ture to NpOH. Since the k values obtained in the BPÈ
q
NpOMe system are ca. 109 dm3 mol~1 s~1 in all solvents, it is
concluded that the quenching of 3BP* by NpOMe occurs via
a collision process as for BPÈNpOH. The plots of the effi-
ciencies as a function of i are shown in Fig. 5(a) and (b) for
1NpOMe and 2NpOMe, respectively.
seems that the viscosity e†ect on the k
value in the BPÈ
TET
NpOMe system is more dominant than the solvent polarity.
In contrast, the k
value in ACNÈH O mixtures [Group D
TET
2
in Fig. 6(c) and (d)] decreases with increasing g~1. For the
TET process from 3BP* to NpOMe in a highly polar solvent
¤ Unpublished data.
such as ACNÈH O, it is considered that i is a main factor
2
1184
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Fig. 6 Rate constants k (L), k
(…) and k (K) as a function of i of the solvent in (a) the BPÈ1NpOMe, (b) the BPÈ2NpOMe systems and
q
TET
IQ
the reciprocal of solvent viscosity (g) in (c) the BPÈ1NpOMe and (d) the BPÈ2NpOMe systems. The solid and dashed lines were drawn using eqn.
(15) and along the experimental data, respectively. Group A, B, C and D were circled by the k
values obtained in CCl ÈAC, CCl ÈACN, PhCN
TET
4
4
and ACNÈH O solvents, respectively. See text for details.
2
rather than g. The collision complex between 3BP* and
NpOMe has more or less charge-transfer character since they
behave as an electron-acceptor and election-donor, respec-
tively. It is known that the energy of the charge-transfer state
is inÑuenced by solvent polarity. If a charge-transfer inter-
action contributes to the exchange term, through wavefunc-
tions of the collision complex, by the magnitude of interaction,
the TET rate may be a†ected by solvent polarity. It has been
reported by calculation, that Coulombic interaction contrib-
utes to the Davydov splitting of triplets in naphthalene and
anthracene crystals, and that conÐguration interaction
between the neutral and charge-resonance triplet exciton
states increases the splitting by ca. 25È30%.64 However, in the
present work, even in the presence of a triplet-energy acceptor,
no change in the transient absorption spectrum of 3BP* was
observed. As a result, it seems that the contribution of Cou-
lombic terms to intermolecular electron-exchange interactions
in solution is negligible. The i e†ect on TET in solution will
be rationalised in the next section.
theory.1 In Ñuid media, the TET process occurs via a collision
process between the triplet energy donor and acceptor mol-
ecules, therefore, there is no solvent molecule(s) between them.
On the other hand, the triplet donor and acceptor are separat-
ed by solvent molecules in rigid media with dielectric constant
i where the probability of TET is proportional to i~2 accord-
ing to the Dexter theory.1 For the TET process from 3BP* to
NpOH or NpOMe, the i e†ect on the t
and k
values
TET
TET
was found in Ñuid media, as stated above. In order to explain
the results, we proposed the following explanation for the
polarity e†ect on TET in a previous paper.34 The solvent
dependence of TET should be interpreted in terms not only of
the electron-exchange mechanism (Dexter mechanism)1 but
also of the dipoleÈdipole interaction (Forster mechanism).62
In a polar solvent (especially a protic solvent),63 the energy
level of the 3(n, p*) state of BP becomes higher compared to
that in a non-polar solvent, whereas that of the 1(p, p*) state is
relatively lower in a polar media. As a result, the energy gap
between 3(n, p*) and 1(p, p*) of BP in polar media becomes
less than that in non-polar media, and the degree of pertur-
bation by 1(p, p*) increases in the lowest triplet state 3(n, p*)
of BP with an increase in the solvent polarity. In other words,
the 1(p, p*) character in the 3(n, p*) state of BP increases
through spinÈorbital coupling61 in a polar solvent, resulting
in an increase in the possibility for TET from 3BP* to NpOH
or NpOMe by dipoleÈdipole interaction, in addition to the
electron-exchange mechanism.
Solvent polarity e†ect on TET with and without HA
competition
In the BPÈNpOMe system without HA, the t
and k
TET
TET
values increased slightly with increasing i. Since the triplet
state of NpOH has a structure that is almost isoelectronic
with NpOMe, it was expected that TET in the BPÈNpOH
system should have slight solvent dependence as it does in the
BPÈNpOMe system. However, the solvent polarity depen-
dence of TET competing with HA in the BPÈNpOH system
was considerably larger than that without HA in the BPÈ
NpOMe system. This can be explained by a considerable
Conclusion
By means of nanosecond laser Ñash photolysis in BPÈNpOH
and ÈNpOMe, the solvent polarity e†ect on TET was
decrease in k
increase in the k
As described above, the value of k
increasing solvent polarity in both the BPÈNpOH and
value with increasing i, resulting in an
observed in various solvents. The efficiencies (t ) and rate
HA
TET
constants (k ) for TET in the present systems increase with
TET
value.
TET
increased with
increasing relative permittivity (i), against the prediction of
TET
the Dexter theory. The solvent polarity e†ect of k
in the
TET
BPÈNpOH system is considerably larger than that in the BPÈ
ÈNpOMe systems, contrary to the prediction of the Dexter
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1185

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H. Katayama, A. Tsuchida, S. Ito and M. Yamamoto, J. Phys.
Chem., 1993, 97, 9270.
25 P. J. Wagner, G. M. El-Taliawi, J. Am. Chem. Soc., 1992, 114,
8325; P. J. Wagner, B. P. Giri, H. W. Frerking Jr. and J. DeFran-
cesco, J. Am. Chem. Soc., 1992, 114, 8326.
26 P. S. Engel, D. W. Horsey, J. N. Scholz, T. Karatsu and A. Kita-
mura, J. Phys. Chem., 1992, 96, 7524.
27 A. Terenin and V. Ermolaev, T rans. Faraday Soc., 1956, 52, 1042.
28 H. Shizuka and M. Fukushima, Chem. Phys. L ett., 1983, 101, 598.
29 H. Shizuka, H. Hagiwara, H. Satoh and M. Fukushima, J. Chem.
Soc., Chem. Commun., 1985, 1454.
NpOMe system. It is concluded that the solvent dependence
of TET from 3BP* to NpOH in competition with HA is
enhanced by a decrease in k
with increasing solvent
HA
polarity. The TET process in Ñuid media occurs upon colli-
sions when there is no solvent molecule between the donor
and acceptor. The Dexter equation (the TET probability pro-
portional to i~2) is applicable for the TET system in rigid
media where the triplet energy donor and acceptor are
separated by solvent molecules. An increase in k
with
TET
increasing i in the present system can be explained by the
contribution of the Forster mechanism (the dipoleÈdipole
interaction) due to perturbation of the 1(p, p*) state to the
lowest triplet state 3(n, p*) of BP in polar media (especially for
protic media).
30 H. Shizuka, H. Hagiwara and M. Fukushima, J. Am. Chem. Soc.,
1985, 107, 7816.
31 S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chem., 1991, 95,
5489.
32 S. Kaneko, M. Yamaji, M. Hoshino and H. Shizuka, J. Phys.
Chem., 1992, 96, 8028.
33 M. Yamaji, T. Sekiguchi, M. Hoshino and H. Shizuka, J. Phys.
Chem., 1992, 96, 9353.
We thank Associate Prof. Y. Kajii of Tokyo University and
Prof. K. Obi of Tokyo Institute of Technology for their tech-
nical support of the thermal lensing experiments. This work
was supported by the Ministry of Education, Science, Sports
and Culture of Japan, a Grant-in-Aid for ScientiÐc Research
on Priority Area (Photoreaction Dynamics), 06239101.
34 M. Yamaji, T. Tanaka and H. Shizuka, Chem. Phys. L ett., 1993,
202, 191.
35 M. Yamaji, T. Kiyota and H. Shizuka, Chem. Phys. L ett., 1994,
226, 199.
36 M. Yamaji, Y. Aihara, T. Itoh, S. Tobita and H. Shizuka, J. Phys.
Chem., 1994, 98, 7014.
37 T. Kiyota, M. Yamaji and H. Shizuka, J. Phys. Chem., 1996, 100,
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