Internal conversion of o-aminoacetophenone in solution

Article abstract of DOI:10.1039/b105292f

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laser-single photon counting system and time-resolved thermal lensing (TRTL) method. The fluorescence quantum yield (φ_f) and lifeti

Full text of DOI:10.1039/b105292f

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Internal conversion of o-aminoacetophenone in solution
Toshitada Yoshihara, Hirofumi Shimada, Haruo Shizuka and Seiji Tobita*
Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan
Received 15th June 2001, Accepted 10th September 2001
First published as an Advance Article on the web 29th October 2001
The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a
femtosecond laserÈsingle photon counting system and time-resolved thermal lensing (TRTL) method. The
Ñuorescence quantum yield (U ) and lifetime (q ) of o-AAP depend strongly on the nature of the solvent. In
f
f
nonpolar solvents, o-AAP gives very small U values (U \ 2.4 ] 10~4 in n-hexane) and remarkably short
f
f
Ñuorescence lifetimes (q \ 9.4 ps in n-hexane), suggesting the presence of very fast nonradiative deactivation
processes. The measurements of the quantum yield (U ) of intersystem crossing based on the energy transfer
and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal
f
isc
conversion. In aprotic solvents, the rate (k ) of internal conversion for o-AAP decreases signiÐcantly with
ic
increasing solvent polarity (k \ 1.0 ] 1011 s~1 in n-hexane, k \ 2.4 ] 109 s~1 in acetonitrile). In protic
ic
ic
solvents, the k value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The
internal conversion rate in aprotic solvents is scarcely a†ected by deuterium substitution of the NH group in
o-AAP, while a large isotope e†ect is found for o-AAP in deuteriated protic solvents. It is concluded that the
ic
2
efficient S ] S internal conversion in nonpolar aprotic solvents arises from vibronic interactions between
1
0
close-lying 1(p,p*) and 1(n,p*) states (the proximity e†ect), and in protic solvents intermolecular
hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.
substantiated by the observation of a large Stokes-shifted Ñuo-
rescence band [l(St) P 10 000 cm~1].
1. Introduction
The photophysical and photochemical properties of aromatic
carbonyl compounds are strongly a†ected by the presence of a
substituent on the aromatic ring. In general, the p,p* state is
stabilized by introducing an electron-donating substituent on
the aromatic ring, while the location of the n,p* state is only
slightly modiÐed by the electron-donating substituent. Thus,
the Ñuorescence properties of the compounds with close-lying
1(n,p*) and 1(p,p*) states are expected to depend markedly on
the nature of the solvent, such as its polarity and hydrogen-
bonding ability.
For o-aminoacetophenone (o-AAP), it is expected that the
(n,p*) and (p,p*) states are closely located in the lowest excited
singlet states because of the presence of a strong electron-
donating substituent. Owing to the proximity of two elec-
tronic levels the photophysical properties of o-AAP would be
very sensitive to environment, such as solvent polarity and
temperature. Until recently, only a few studies have been
reported on the photophysics of o-AAP in solution.7h9 Previ-
tali et al.7 have reported that the Ñuorescence of o-AAP was
extremely weak in cyclohexane at room temperature, while in
isopropanol the Ñuorescence was enhanced signiÐcantly. From
the relatively small Stokes shift, they concluded that ESIPT
was not involved in o-AAP. Lui and McGlynn8 have carried
out a comparative study of the emission spectra of o-, m- and
p-AAP in an ethanol glass at 77 K, and determined the emis-
sion lifetimes and quantum yields at 77 K.
The objective of our study is to investigate quantitatively
the photophysical parameters such as rates of internal conver-
sion and intersystem crossing of o-AAP in various solvents at
room temperature and to clarify the proximity e†ects of 1(n,
p*) and 1(p,p*) states on the photophysical properties. To
determine the Ñuorescence lifetimes in the picosecond time
region, a time-correlated single photon counting system com-
bined with a femtosecond excitation laser pulse is used. The
quantum yield of intersystem crossing is determined by using
the time-resolved thermal lensing technique.
It is well known that the lowest and second excited singlet
states of acetophenone (AP) have n,p* and p,p* character,
respectively. Since the energy gap between the S (n,p*) and
1
S (p,p*) states is sufficiently large (B7300 cm~1),1 this order is
2
maintained in the lowest excited triplet states. As a result, the
S state of AP undergoes fast intersystem crossing to produce
1
possibly T (p,p*) states according to El-SayedÏs rule2 and
2
relaxes to the T (n,p*) state by internal conversion. Because of
the T (n,p*) character, triplet AP shows hydrogen atom
abstraction reactions from various substrates. In o-
1
1
methylacetophenone, electronic perturbations due to the
methyl group are relatively small, so that o-
methylacetophenone exhibits very fast intersystem crossing
from the S (n,p*) state to the triplet state, and intramolecular
1
hydrogen atom abstraction takes place in the T (n,p*)
1
state.3 In contrast with o-methylacetophenone, o-
hydroxyacetophenone, which contains
a strong electron-
donating OH group and also an intramolecular hydrogen
bond between the OH group and the carbonyl oxygen, the
2. Experimental
electronic character of the S state becomes p,p* in both non-
Materials
1
polar and polar solvents. From the S (p,p*) state, o-
1
hydroxyacetophenone shows
a
very fast excited-state
Acetophenone (AP; Wako) and o-methoxyacetophenone (o-
MAP; Acros) were puriÐed by vacuum distillation. o-
Aminoacetophenone (o-AAP; Tokyo Kasei) was puriÐed by
intramolecular proton transfer (ESIPT) reaction.4h6 The
occurrence of the intramolecular proton transfer reaction is
4972
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978
This journal is ( The Owner Societies 2001
DOI: 10.1039/b105292f

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column chromatography on a silica-gel column using benzene
as the eluent. o-AAP was deuteriated by shaking with
source. The transient signal was recorded on a digital storage
oscilloscope (Tektronix, TDS-744 500 MHz 2G Samples/s),
and transferred to a personal computer (NEC, PC-9821Ap)
for analyses.
methanol-d (Aldrich), letting stand overnight, and evapo-
1
rating the solvent under vacuum. The procedure was repeated
several times to ensure deuteriation. The deuterium content in
For TRTL measurements, the third harmonic (355 nm) of a
nanosecond Nd3` : YAG laser (Spectra Physics, GCR-130,
pulse width 6 ns) was utilized for the excitation source. A
HeÈNe laser beam (NEC, GLG2026, 633 nm) was used as
monitoring light. The probe light was introduced into a
monochromator (Jobin-Yvon, HR-10) after passing it through
an optical Ðlter and a small pinhole (Corion, 2401; 300 lm
diameter). The intensity change of the probe beam was
detected by a photomultiplier (Hamamatsu, R928). The TRTL
signals were recorded on a digitizing oscilloscope (Tektronix,
TDS-744). The detection system of the TRTL signals has been
reported elsewhere.17 The TRTL signals were averaged over
Ðve hundred laser shots to improve the signal-to-noise ratio.
The absorbance of the sample solutions for the TRTL mea-
surements was adjusted to ca. 0.10 at excitation wavelength.
The TRTL experiments were carried out at 293 K.
the amino group of o-AAP-d was estimated to be [95.0% by
2
NMR spectroscopy. o-Acetylaminoacetophenone was synthe-
sized by N-acetylation of o-AAP with acetic anhydride. The
product was puriÐed by recrystallization from n-hexane. Ben-
zophenone (BP; Wako) and 9,10-dichloroanthracene (DCA;
Tokyo Kasei) were puriÐed by recrystallization from ethanolÈ
water and methylcyclohexane, respectively. n-Hexane (n-Hex;
Kanto) and cyclohexane (CH; Aldrich, spectrophotometric
grade) were puriÐed by passing through a dry alumina
column. Acetonitrile (MeCN; Kanto), propionitrile (EtCN;
Merck), butyronitrile (PrCN; Kanto), ethanol (EtOH; Kanto)
and methanol (MeOH; Kanto) were distilled. Diethyl ether
(DEE; Kanto, spectroscopic grade), dimethyl sulfoxide
(DMSO; Kishida, spectroscopic grade) and 2,2,2-tri-
Ñuoroethanol (TFE; Kishida, spectroscopic grade) were used
without further puriÐcation. Methanol-d (MeOD; Aldrich,
1
99.5% D) and ethanol-d (EtOD; Wako, 99% D) were used as
received.
3. Results and discussion
1
3.1. Absorption and Ñuorescence spectra
Methods
Figs. 1(a)È(c) show the absorption spectra of AP, o-MAP and
o-AAP in n-Hex. The S (n,p*) and S (p,p*) bands of AP can
Absorption and Ñuorescence spectra were measured with a
Ubest-50 UV/vis spectrophotometer and a Hitachi F-4010
spectroÑuorometer, respectively. Fluorescence quantum yields
were determined by comparing the integrated intensities of the
Ñuorescence spectra with that of standard quinine bisulfate in
1
2
be seen at around 319 nm (31 400 cm~1) and 278 nm (36 000
cm~1), respectively. The molar absorption coefficient of each
band was determined as e \ 45 M~1 cm~1 and e \ 880
319
278
M~1 cm~1, showing their forbidden characters.
It is known that the electronic energy gap (*E B 7300
cm~1) between the S (n,p*) and S (p,p*) states of AP is rela-
0.5 M aqueous sulfuric acid solution (U \ 0.546).10 The Ñuo-
rescence lifetimes were measured by using the single photon
f
1
2
counting method. For nanosecond lifetime measurements, a
pulsed discharge lamp (pulse width 1 ns, repetition rate 40
kHz) Ðlled with hydrogen gas was used as excitation light
source. The picosecond lifetime measurements were carried
out by using a self-mode-locked Ti : sapphire laser (Spectra-
Physics Tsunami, center wavelength 800 nm, pulse width ca.
70 fs, repetition rate 82 MHz) pumped by a Nd3` : YAG laser
(Spectra-Physics Millennia V, 532 nm, 4.5 W). The generation
of the second harmonic (400 nm, pulse width ca. 200 fs) was
performed in an LBO crystal. The third harmonic (266 nm,
pulse width ca. 250 fs) was generated by a sum frequency
mixing of the fundamental and the second harmonic of the
Tsunami laser system. The repetition frequency of the excita-
tion pulse was reduced to 4 MHz by using a pulse picker
(Spectra-Physics Model 3980). The second harmonic (400 nm)
in the output beam was used as trigger pulse. The Ñuorescence
from the sample solution was observed through a polarizer at
the magic angle (54.7¡) with respect to the polarization direc-
tion of the excitation laser pulse. The emission light was
detected by
a microchannel plate (MCP; Hamamatsu,
R3809U-51) after passing through a monochromator (Oriel
Model 77250). The instrument response function had a half-
width of 20È25 ps. The analyses of the Ñuorescence decay
curves were carried out using the deconvolution method.
The quantum yield of intersystem crossing (U ) of o-AAP
isc
in each solvent was determined by means of the triplet energy
transfer method11,12 or the time-resolved thermal lensing
(TRTL) method.13h15 For the triplet energy transfer method,
BP (U B 1.0)16 in MeCN was used as a reference. DCA was
isc
used as an energy acceptor because of its low molar absorp-
tion coefficient at the excitation wavelength (308 nm), and low
triplet energy. The small contribution of the triplet DCA
formed by direct absorption of the 308 nm light was taken
into account for the analyses. The solutions used in the experi-
ments were degassed by freezeÈpumpÈthaw cycles on a high
vacuum line. A XeCl excimer laser (Lambda Physik, Lextra
50, pulse width 17 ns, 308 nm) was employed for the excitation
Fig. 1 Absorption spectra of (a) AP and (b) o-MAP in n-Hex.
Absorption, Ñuorescence (solid lines) and Ñuorescence excitation
(broken line) spectra of o-AAP in (c) n-Hex, (d) DEE, (e) MeCN and
(f) DMSO at 293 K.
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978
4973

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tively large in a nonpolar solvent.1 In o-MAP, the S (p,p*)
of ESIPT systems reported so far, consistent with the absence
of ESIPT in o-AAP.
2
band is ca. 3000 cm~1 red-shifted compared with that of AP
and the molar absorption coefficient (e \ 3500 M~1 cm~1)
Fig. 2 shows the absorption and Ñuorescence spectra of
o-AAP in protic solvents (EtOH, MeOH and TFE). It can be
seen that the Ðrst absorption maximum of o-AAP in TFE
shifts remarkably to shorter wavelengths compared to those in
EtOH and MeOH. In general, aromatic carbonyl compounds
with amino group(s) such as aminoanthraquinones and
aminoÑuorenones can form hydrogen bonds with protic sol-
vents at di†erent sites in both the ground and excited
states.19h25 In the case of o-AAP, there are at least three sites
for hydrogen-bond formation with a protic solvent molecule
as shown in Fig. 3: the hydrogen bonds between (1) the lone
pair electrons in the amino group and the hydrogen atom of a
protic solvent molecule, (2) the carbonyl oxygen atom and the
hydrogen atom of a protic solvent molecule, and (3) the
hydrogen atom of the amino group and the oxygen atom of a
protic solvent molecule.
In acidic alcohol TFE (hydrogen-bonding donor character
a \ 1.51),26 hydrogen bonds of type (1) should be predomi-
nantly formed in the ground state. Upon excitation of o-AAP,
intramolecular charge transfer takes place from the amino
group to the carbonyl moiety. This electron migration would
cause rearrangement of hydrogen-bond interactions from type
(1) to type (2) and/or type (3) in the excited state. As a result,
the Ðrst absorption maximum shifts to the blue, and the Ñuo-
rescence maximum shifts to the red in TFE, resulting in the
large Stokes shift (7300 cm~1). Since MeOH (a \ 0.93)27 has a
slightly larger proton-donating ability compared to EtOH
(a \ 0.83),27 a similar tendency can be observed when one
compares the absorption and Ñuorescence spectra of o-AAP in
EtOH and MeOH.
298
increases remarkably, because the energy of the S (p,p*) state
2
is stabilized due to introduction of an electron-donating sub-
stituent such as the methoxy group. On the other hand, the
S (n,p*) band of o-MAP is located at a similar energy to that
1
of AP. For o-AAP [Fig. 1 (c)], a strong absorption band
appears at 353 nm (28 300 cm~1), which can be assigned to
the p,p* band of o-AAP because of the large molar absorption
coefficient (e \ 4800 M~1 cm~1). The p,p* band of o-AAP
353
is signiÐcantly red-shifted relative to that of AP and o-MAP
due to the presence of a strongly electron-donating amino
group and the formation of an intramolecular hydrogen bond
between a hydrogen atom of the amino group and carbonyl
oxygen. The p,p* bands of o-MAP and o-AAP have intramol-
ecular charge transfer character from the electron-donating
group to the carbonyl moiety, resulting in an increase of the
molar absorption coefficient. It should be noted here that the
electronic energy gap between the 1(n,p*) and 1(p,p*) states of
o-AAP is remarkably decreased compared with those of AP
and o-MAP in n-Hex.
Figs. 1(c)È(f) show the absorption and Ñuorescence spectra
of o-AAP in aprotic solvents (n-Hex, DEE, MeCN and
DMSO). The Ðrst absorption band has a large molar absorp-
tion coefficient in all solvents. Therefore, the band can be
assigned to p,p* transition having intramolecular charge
transfer character. The Ðrst 1(p,p*) absorption maximum
[l (abs)] is found to undergo a red-shift with increasing
max
solvent polarity, resulting in an increase of the energy gap
between the 1(n,p*) and 1(p,p*) states. The Ñuorescence
maximum of o-AAP [l (Ñu)] is red-shifted with an increase
max
of solvent polarity, showing the intramolecular charge transfer
character of the S state. The values of the Stokes shift [l(St)]
1
were 4300È5100 cm~1 for o-AAP in aprotic solvents, as
shown in Table 1.
In order to examine the occurrence of ESIPT in o-AAP,
we
measured
the
Ñuorescence
spectrum
of
o-
acetylaminoacetophenone, which has a more acidic NÈH
group, in n-Hex at 293 K. o-Acetylaminoacetophenone
exhibited dual Ñuorescence with maxima at 415 and 525 nm.
The large Stokes-shifted Ñuorescence band was attributable to
the Ñuorescence from a proton-transferred form. The relatively
small Stokes-shift values [l(St) \ 4300È5100 cm~1] of o-AAP
in various solvents show that o-AAP does not undergo ESIPT
in the S state. The absence of ESIPT in o-AAP may be due
1
to the fact that the NÈH proton is only weakly hydrogen-
bonded to the carbonyl oxygen as compared to o-
acetylaminoacetophenone. The hydrogen-bond energy in
o-AAP can be estimated to be D1.5 kcal mol~1 in CDCl 18
3
from proton NMR chemical shifts of the amino group of
o-AAP (d \ 6.27 ppm) and aniline (d \ 3.55 ppm). The intra-
molecular hydrogen bond in o-AAP is much weaker than that
Table 1 Absorption maximum l (abs), molar absorption coefficient
max
Fig. 2 Absorption, Ñuorescence (solid lines) and Ñuorescence excita-
tion (broken line) spectra of o-AAP in (a) EtOH, (b) MeOH and (c)
TFE at 293 K.
e
at l (abs), Ñuorescence maximum l (Ñu) and Stokes shift l(St)
max
max
max
of o-AAP in solution at 293 K
l
(abs)/
103 cm~1
e
/
l
(Ñu)/
l(St)/
max
max
max
Solvents
M~1 cm~1 103 cm~1 103 cm~1
1
2
3
4
5
6
7
8
9
n-Hex
CH
DEE
PrCN
EtCN
MeCN
DMSO
EtOH
MeOH
28.3
28.3
27.9
27.9
27.9
27.9
27.4
27.4
27.5
28.3
4800
4400
5500
5000
5300
5200
5800
4400
5000
3300
23.9
23.9
23.6
23.3
23.0
22.8
22.4
21.9
21.7
21.0
4.6
4.6
4.3
4.6
4.9
5.1
5.0
5.5
5.8
7.3
Fig. 3 Hydrogen-bonded forms of o-AAP in the ground state in
alcohols.
10 TFE
4974
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978

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3.2. Fluorescence quantum yields and lifetimes of o-AAP
Fluorescence quantum yields (U ) and lifetimes (q ) of o-AAP
3.3. Radiationless processes: internal conversion and
intersystem crossing
f
f
in various solvents at 293 K are listed in Table 2. In nonpolar
Since the contribution of photochemical reactions to radi-
ationless deactivation of o-AAP was negligible in all the sol-
vents employed, the radiationless processes can be attributed
to internal conversion or intersystem crossing. Therefore, we
measured the quantum yield of intersystem crossing (U ) of
o-AAP in MeCN by using the triplet energy transfer
solvents such as n-Hex and CH, the Ñuorescence of o-AAP is
extremely weak as seen from the very small U values
f
(D10~4). In aprotic solvents, the U values tend to increase
f
remarkably with increasing dielectric constant of the solvent.
isc
In contrast, in protic solvents, such a tendency is not seen, and
the U values decrease with an increase of hydrogen-bonding
donor character of the solvent.
The Ñuorescence decay proÐles of o-AAP were measured by
the picosecond single photon counting method. Typical decay
method.11,12 Fig. 5(a) shows the transient absorption spectra
obtained by 308 nm laser photolysis of the BP (2.5 ] 10~2
M)/DCA (1.0 ] 10~4 M) system in MeCN at 293 K. The
absorption band with maximum at 520 nm observed 200 ns
after laser pulsing is ascribable to the tripletÈtriplet (T ^ T )
f
proÐles in n-Hex and MeCN are shown in Fig. 4. The q value
f
n
1
of o-AAP in n-Hex and MeCN can be determined to be 9.4 ps
absorption of BP in a polar solvent.28 With the passage of
(s2 \ 1.093) and 384 ps (s2 \ 1.004), respectively, based on
deconvolution analyses. As summarized in Table 2, the Ñuo-
rescence lifetimes of o-AAP increase signiÐcantly with increas-
ing solvent polarity in aprotic solvents, while the values
decrease with the increase of the hydrogen-bonding donor
time, the intensity of the T ^ T absorption band at around
n
1
520 nm decreases according to Ðrst-order kinetics, and a new
absorption band appears at 418 nm, with an isosbestic point
at 435 nm. The absorption band at 418 nm can be assigned to
the T ^ T absorption due to DCA. The decay rate at 520
n
1
ability of the solvent. The radiative rate constants (k ) of
f
o-AAP calculated from q and U in various solvents are in the
f
f
range of (2.4È6.6) ] 107 s~1, showing the p,p* character of the
Ñuorescent state. In contrast, the radiationless deactivation
rates from the S state of o-AAP are inÑuenced strongly by
1
solvent properties.
Fig. 5 (a) Transient absorption spectra observed at 0.2, 0.8 and 2.4
ls after 308 nm laser pulsing of the BP (2.5 ] 10~2 M)/DCA
(1.0 ] 10~4 M) system in MeCN at 293 K. Insets are the time proÐles
of the transient absorption spectra monitored at 520 nm (upper) and
418 nm (lower). (b) Transient absorption spectra obtained at 0.3, 1
and 4 ls after 308 nm laser photolysis of the o-AAP (2.5 ] 10~3
M)/DCA (1.0 ] 10~4 M) system in MeCN at 293 K. Inset is the time
proÐle monitored at 418 nm.
Fig. 4 Fluorescence decay curves of o-AAP in (a) n-Hex and (b)
MeCN at 293 K.
Table 2 Photophysical parameters of o-AAP in solution at 293 K
Solvents
e
q /ps
U
U
a
U
k /107 s~1
k
/108 s~1
k /1010 s~1
f
f
isc
ic
f
isc
ic
1
2
3
4
5
6
7
8
9
n-Hex
CH
DEE
PrCN
EtCN
MeCN
DMSO
EtOH
MeOH
TFE
1.89
2.02
4.34
20.3
29.7
35.9
46.5
24.6
32.7
26.7
9.4
10
143
294
320
2.3 ] 10~4
4.1 ] 10~4
5.1 ] 10~3
1.2 ] 10~2
1.2 ] 10~2
1.7 ] 10~2
8.1 ] 10~2
4.5 ] 10~2
1.4 ] 10~2
1.0 ] 10~2
\0.02
\0.02
0.03
0.05
0.06
[0.98
[0.98
0.96
0.94
0.93
0.91
0.72
0.91
0.95
2.4
4.1
3.6
4.1
3.8
4.4
5.0
3.2
2.2
6.6
21
20
2.1
1.7
1.9
1.8
1.2
0.36
0.64
1.3
10
9.8
0.67
0.32
0.29
0.24
0.044
0.065
0.15
0.64
384
0.07(0.07b)
1650
1390
629
0.20
0.05
0.04
0.02
10
152
0.97
a The U values determined by using the TRTL method. b The U value determined by the triplet energy transfer method.
isc isc
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978
4975

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nm was in good agreement with the rise rate monitored at 418
nm as shown in the inset of Fig. 5(a), indicating that energy
transfer from triplet BP (3BP*) to DCA occurs in MeCN to
produce triplet DCA (3DCA*).
In a similar manner, Fig. 5(b) shows the transient absorp-
tion spectra obtained by 308 nm laser photolysis of the o-AAP
(2.5 ] 10~3 M)/DCA (1.0 ] 10~4 M) system in MeCN at 293
K. With a decrease of the weak absorption band at around
440 nm, a strong absorption peak appears at 418 nm. This
shows that energy transfer from triplet o-AAP (3o-AAP*) to
DCA takes place in MeCN.
The maximum concentration of 3DCA* ([3DCA*]@ ) pro-
max
duced by energy transfer from 3BP* to DCA is expressed as:
*A@
418
[3DCA*]@
max
\
\ a@U@ [1BP*]
(1)
isc
0
e418
DCA
where *A@
418
and e418 are the maximum absorbance at 418
DCA
nm due to 3DCA* formation by energy transfer from 3BP* to
DCA, and the molar absorption coefficient of 3DCA* at 418
nm, respectively, and a@, U@ , and [1BP*] are the efficiency of
isc
0
energy transfer from 3BP* to DCA, the quantum yield for
intersystem crossing of BP (B1.0),16 and the initial concentra-
tion of 1BP*. The a@ value can be formulated by eqn. (2) with
the use of the kinetic parameters (k@ and k@ ):
Fig. 6 Time-resolved thermal lensing signals of o-AAP in (a) n-Hex,
(b) DEE, (c) MeCN, (d) DMSO, (e) EtOH and (f) TFE at 293 K.
0
q
signal (U ) is given by:
total
k@ [DCA]
q
a@ \
(2)
U
U
U E
k@ ] k@ [DCA]
slow
total
isc
T
\
(7)
0
q
E
[ U SE T
ex
f
s
where k@ and k@ are the decay rate constant of 3BP* in the
0
q
where E and E are the excitation photon energy (28 170
absence of DCA and the quenching rate constant of 3BP* by
ex
T
cm~1, 80.5 kcal mol~1) and the 0È0 transition energy of the
5
DCA. The k@ and k@ values are obtained to be 3.2 ] 10 s~1
0
q
lowest triplet state, and SE T is the average energy dissipated
and 8.0 ] 109 M~1 s~1, respectively. When o-AAP is used as
s
1
by Ñuorescence from the S state. The E value of o-AAP was
a triplet donor molecule, the maximum concentration of
T
determined to be 19 700 cm~1 (56.3 kcal mol~1) from the 0È0
band of the phosphorescence spectrum of o-AAP at 77 K
observed by acetone photosensitization experiments.
3DCA* ([3DCA*] ) is expressed similarly by:
max
*A
418
[3DCA*]
max
\
\ aU [1o-AAP*]
(3)
isc
0
In n-Hex [Fig. 6(a)], the slow rise component is extremely
small, indicating a negligible formation yield of the triplet
state. The slow rise component, which corresponds to the slow
thermal release from the lowest triplet state, increases grad-
ually with increasing solvent polarity in aprotic solvents [see
e418
DCA
where *A is the maximum absorbance at 418 nm due to
418
3DCA* formation, and a, U , and [1o-AAP*] are the effi-
ciency of energy transfer from 3o-AAP* to DCA, the quantum
yield for intersystem crossing of o-AAP, and the initial con-
centration of 1o-AAP* in MeCN. The a value can be given by
eqn. (4) with the use of the kinetic parameters (k and k ):
isc
0
Figs. 6(a)È(d)]. The U values of o-AAP in each solvent were
isc
estimated by eqn. (7), and U values were calculated by eqn.
ic
(6). The quantum yields (U and U ) are listed in Table 2
together with U . The U value in MeCN determined by the
TRTL method gives the same value as that determined by the
energy transfer method, demonstrating that the TRTL method
0
q
isc
ic
k [DCA]
f
isc
q
a \
(4)
k ] k [DCA]
0
q
is applicable to the determination of U in o-AAP.
where k and k are the decay rate constant of 3o-AAP* in the
isc
0
q
The rate constants of intersystem crossing (k ) and internal
absence of DCA and the quenching rate constant of 3o-AAP*
by DCA. The k and k values are obtained to be 2.0 ] 105
isc
conversion (k ) of o-AAP in each solvent were calculated from
ic
0
q
the data of U , U and q by eqns. (8) and (9):
s~1 and 9.5 ] 109 M~1 s~1, respectively. Hence, U
of
isc ic
f
isc
o-AAP can be expressed by eqn. (5) with the use of eqns. (1)
k
\ U q~1
(8)
(9)
and (3):
isc
isc f
k \ U q~1
ic
ic f
a@ [1BP*]
a [1o-AAP*] *A@
*A
0
418
418
U
\ U@
isc
(5)
The results are shown in Table 2. For o-AAP in nonpolar
solvents (n-Hex and CH), the rate of internal conversion
(D1011 s~1) is signiÐcantly larger than those of intersystem
crossing (D109 s~1) and Ñuorescence (D107 s~1), and the
internal conversion becomes the most dominant relaxation
process (U [ 0.98) from the S state. This is in marked con-
isc
0
Based on these analyses, the U value of o-AAP in MeCN
isc
was determined to be 0.07. From the U and U values, the
isc
ic
f
quantum yield of internal conversion (U ) can be calculated
by:
ic
1
trast with the relaxation property of AP, in which intersystem
U \ 1 [ U [ U
ic
(6)
f
isc
crossing is the major deactivation process from the S state. In
1
The triplet energy transfer method using BP could not be
used in the other solvents employed in this work because the
photochemical reactions of 3BP* with the solvent molecules
aprotic solvents, both internal conversion and intersystem
crossing rates slow down considerably with an increase of
solvent polarity. As described in section 3.1, the Ðrst 1(p,p*)
absorption band of o-AAP is shifted to the red with increasing
solvent polarity, which results in larger energy gap between
the 1(n,p*) and 1(p,p*) states. Therefore, the remarkable
solvent e†ects on the rate of internal conversion and inter-
system crossing suggest that vibronic interactions between the
were not negligible. Therefore, the U values in other solvents
isc
were determined by using the time-resolved thermal lensing
(TRTL) method. The TRTL signals of o-AAP in typical sol-
vents are shown in Fig. 6. The intensity ratio (U /U
)
slow total
between the slow rise component (U ) and the total TRTL
slow
4976
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978

View Article Online
close-lying 1(n,p*) and 1(p,p*) states play an important role in
the photophysics of o-AAP in solution. In protic solvents, the
rates of internal conversion and intersystem crossing are
reduced in comparison with those in aprotic solvents with
comparable polarity.
to the energy gap law32 on the radiationless transitions of aro-
matic molecules, the S ] S internal conversion is negligible
1
0
if the energy di†erence [*E(S ,S )] between the S and S
1 0
1
0
states is larger than D60 kcal mol~1.33 Since the magnitude
of *E(S ,S ) for o-AAP is D75 kcal mol~1 in a nonpolar
1 0
solvent, the usual internal conversion mechanism cannot be
3.4 Isotope e†ect on the radiationless rate of o-AAP
applied to the occurrence of the extremely fast internal con-
version in o-AAP.
The strong polarity dependence of the internal conversion
The deuterium isotope e†ects on the radiationless rate of
o-AAP were examined to reveal the mechanism of the radi-
ationless transition in solution. It is well known that the
replacement of hydrogen by deuterium leads to a decrease in
decay rate via a decrease of FranckÈCondon factor.29,30 The
rate (k ) suggests that the stabilization of the excited o-AAP
ic
molecule due to solvation is related to the fast internal con-
version. A plot of log k vs. the solvent parameter f (e) given
ic
by eqn. (11) is shown in Fig. 7(a).
nonradiative rate constant (k ) from the initial electronic state
nr
o sT to the Ðnal electronic state o lT is given by:
e [ 1
2e [ 1
f (e) \
(11)
2n
+
k
\
oSs o v o lT o2 oSs o s To2o
(10)
nr
s
l
In eqn. (11), e denotes the static dielectric constant of the
solvent (see Table 2). Fig. 7(a) shows a tendency such that k
where Ss o ¿ o lT is an electronic matrix element, Ss o s T is a
ic
s
l
decreases with an increase of f (e). According to the Onsager
vibrational overlap integral whose square is the FranckÈ
Condon factor, and o is the state density of the Ðnal state.
Since for large molecules the value of o should be sufficiently
large in solution, in general, the FranckÈCondon factor
becomes the most important factor to determine the non-
radiative rate. In order to clarify the e†ects of intramolecular
hydrogen-bonding and NÈH vibrations on the radiationless
deactivation of o-AAP, the Ñuorescence lifetimes of deuterium-
theory, the solvation energy (E ) of a solute molecule with
solv
Onsager radius a and dipole moment k is given by:
M
2k2
M
E
\
f (e)
(12)
solv
a3
Therefore, one can deduce from the result in Fig. 7(a) and eqn.
(12) that the fast internal conversion is induced by vibronic
interactions between close-lying S (p,p*) and S (n,p*) states
labeled o-AAP (o-AAP-d ), where hydrogen atoms in the
2
1
2
amino group were replaced by deuterium atoms, was mea-
(so-called proximity e†ect)34, and the rate of internal conver-
sion is decreased by solvation stabilization of the Ñuorescent
state.
sured in several solvents. If intramolecular NÈHÉ É ÉO
hydrogen-bonding interactions are responsible for the fast
internal conversion of o-AAP, one can expect that the deu-
teriation of the NÈH group reduces the FranckÈCondon
factor in eqn. (10) and results in an increase of the Ñuorescence
In Fig. 7(b), log k is plotted as a function of the electronic
ic
energy gap [*E(S ,S )] between the S (p,p*) and S (n,p*)
1 2
1
2
states of o-AAP. The *E(S ,S ) values are estimated by using
the following equation:
1 2
lifetime in o-AAP-d .21,31 The results are summarized in
2
Table 3.
*E(S ,S ) \ [E (AP) ] *E] [ E (o-AAP)
(13)
In aprotic solvents, n-Hex, MeCN and DMSO, the isotope
1 2
s
s
e†ects due to deuteriation of the NH group in o-AAP are
where E (AP) stands for the 0È0 transition energy (27 550
2
negligibly small, although a slight isotope e†ect (kH /kD \
nr nr
s
cm~1) of the S (n,p*) ^S transition of AP in n-Hex, which
1
0
1.19) is seen in DMSO. The isotope e†ect observed in DMSO
corresponds to the 0È0 transition energy of the S (n,p*) state
2
is probably due to hydrogen-bonding interactions between the
NÈH moiety and the DMSO molecule. Thus, the intramolecu-
lar hydrogen-bonding interaction cannot be the main cause of
the fast radiationless processes of o-AAP in aprotic solvents.
In contrast, relatively large solvent isotope e†ects are recog-
nized in EtOH and MeOH. This Ðnding suggests that the
intermolecular hydrogen-bonding interactions between solute
and protic solvent molecules are responsible for the enhance-
ment of the nonradiative transition.
of o-AAP, and E (o-AAP) is the S (p,p*) energy of o-AAP
s
1
determined from the intersection of the absorption and Ñuo-
rescence bands in each solvent. Since the 1(n,p*) band of AP is
slightly blue-shifted with an increase of the solvent polarity,
the E (AP) value is corrected for the peak shift (*E )of the 1(n,
s
s
p*) band of AP in each solvent, with respect to that in n-Hex.
It can be seen from Fig. 7(b) that the log k value of o-AAP
ic
tends to increase linearly with a decrease of *E(S ,S ). This
1 2
3.5 Fast internal conversion of o-AAP in solution
As described in section 3.3, we found that the S state of
1
o-AAP deactivated very rapidly via fast internal conversion,
and the rate depended strongly on the solvent character, such
as its polarity and hydrogen-bonding donor ability. According
Table 3 Deuterium isotope e†ects on the radiationless rate (k ) of
nr
o-AAP at 293 K
k
a/109 s~1
nr
Solvents
o-AAP
o-AAP-d
kH /kD
2
nr nr
n-Hex
106
104
2.64
0.47
1.0
0.97
1.2
MeCN
DMSO
EtOH
EtOD
MeOH
MeOD
2.56
0.56
0.69
0.17
0.43
4.1
3.7
1.57
Fig. 7 Plots of log k vs. (a) f (e) and (b) *E(S ,S ). Each solvent is
a Calculated from the equation k \ (1 [ U )/q .
nr
ic
1 2
f
f
numbered as shown in Table 1.
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978
4977

View Article Online
ported by Grants-in-Aid for Science Research from the Minis-
try of Education, Culture, Sports, Science and Technology of
Japan (No. 11640496).
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Fig. 8 shows a schematic representation of the solvent polarity
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1
0
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3
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4. Conclusion
The relaxation processes of excited o-AAP were investigated
by means of time-resolved and steady-state Ñuorometry and
the time-resolved thermal lensing (TRTL) technique. The Ñuo-
rescence quantum yield and lifetime of o-AAP depended
markedly on the solvent polarity in aprotic solvents and on
the hydrogen-bonding donor ability in protic solvents. The
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isc
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cantly with increasing solvent polarity (k \ 1.0 ] 1011 s~1 in
ic
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nr
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1
2
(so-called proximity e†ect) play an important role in the radi-
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Acknowledgements
The authors would like to thank Dr. S. Leach of Observatoire
de ParisÈMeudon for helpful discussions. This work was sup-
35 N. Kanamaru and E. C. Lim, J. Chem. Phys., 1975, 62, 3252.
4978
Phys. Chem. Chem. Phys., 2001, 3, 4972È4978