Femtosecond fluorescence study of the substitution effect on the proton transfer in thermochromic salicylideneaniline crystals

Article abstract of DOI:10.1021/jp9620126

The substitution effect on the dynamics of the proton transfer in thermochromic salicylideneanilines in the crystalline phase was investigated using femtosecond time-resolved fluorescence spectroscopy. From the deuteration effect and low-temperature measu

Full text of DOI:10.1021/jp9620126

644
J. Phys. Chem. A 1997, 101, 644-649
Femtosecond Fluorescence Study of the Substitution Effect on the Proton Transfer in
Thermochromic Salicylideneaniline Crystals
Taro Sekikawa and Takayoshi Kobayashi*
Department of Physics, UniVersity of Tokyo, Tokyo 113, Japan
Tamotsu Inabe
Department of Chemistry, Hokkaido UniVersity, Sapporo 060, Japan
ReceiVed: July 8, 1996; In Final Form: October 1, 1996X
The substitution effect on the dynamics of the proton transfer in thermochromic salicylideneanilines in the
crystalline phase was investigated using femtosecond time-resolved fluorescence spectroscopy. From the
deuteration effect and low-temperature measurement, it is concluded that the proton tunnels the potential
barrier quantum-mechanically in the excited state. The rate of proton transfer was found to decrease with the
energy separation between the excited enol and keto forms due to the different potential-barrier heights. The
change in the potential-barrier height is qualitatively explained by the electron-donating or -accepting property
of the substituent around the hydrogen bond. The thermalization of the vibrational states of the excited enol
form was also observed.
I. Introduction
The photoinduced proton transfer either from one moiety to
another in a molecule or between two molecules is of great
interest from both basic and application viewpoints.1-4 It is
regarded as one of the simplest chemical reactions and is related
to some biological processes. There have been continuing
extensive arguments as to whether or not the transfer is taking
place by a quantum-mechanical tunneling process.1 In terms
of application, the proton transfer might be used for optical
memory or as a switch utilizing the large spectral changes
induced by the configurational rearrangement of the π electrons.
Since the proton transfer changes the electronic state of a
molecule, it is expected that the substituent groups near the
hydrogen bond may change the potential of the proton in the
hydrogen bond. Thus, the dynamics of the proton transfer
strongly depends on the distribution of the π electrons around
the intramolecular hydrogen bond. In the present paper, we
focus on the substitution effect on the dynamics of the
photoinduced proton transfer in salicylideneanilines (SAs) in
the crystalline phase. Although the development of the fem-
tosecond pulse laser makes it possible to trace the process of
the proton transfer, most of the previous studies have been
performed in the liquid5-14 or in the gas phase.15 In the liquid
phase, the hydrogen-bond length might be different even in the
nonpolar solvent with lack of planarity of a molecule and the
viscosity of the solvent influences the proton dynamics as shown
by many previous studies.4,8,11,16-20 Thus, the investigation in
the crystal phase is expected to provide information on the
intrinsic dynamics of the proton.
Figure 1. Stationary fluorescence and excitation spectra at various
temperatures of MClSA. The excitation energy of the fluorescence
Among the many systems with an intramolecular hydrogen
bond, the thermochromic SAs in the crystalline phase22-29 are
picked in the present study. According to previous studies,22-29
a schematic energy diagram of the thermochromic SA is
considered to be the inset of Figure 1. In the ground state, the
enol form is more stable than the keto form. The energy
separation between these tautomers in the ground state is small
enough for the keto form to be thermally populated. Thus, the
absorption spectrum changes drastically with temperature. On
spectra was 3.10 eV. The probe energy of the excitation spectra was
2.18 eV. The inset shows the energy diagram and the consitutional
formulas of a thermochromic SA.
the other hand, in the excited state, the keto form is more stable
than the enol. Thus, the proton moves spontaneously to the
opposite side in the hydrogen bond very rapidly by photoex-
citation, which is the origin of the large Stokes shift of the
fluorescence. Consequently, the thermochromic SA is consid-
ered to perform a closed reaction cycle with the enol-keto-
type proton transfer by photoexcitation without cis-trans
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5639(96)02012-9 CCC: $14.00 © 1997 American Chemical Society

Proton Transfer in Salicylideneaniline
J. Phys. Chem. A, Vol. 101, No. 4, 1997 645
isomerization, unlike the photochromic SA.16,30-35 It is known
that the thermochromic and the photochromic properties are
exclusive in SAs.22 The difference between the thermochromic
SAs and photochromic ones is in the crystal structure: Since
the photochromic SAs are nonplanar and have rather loosely
packed crystals, the cis-trans isomerization can be caused by
photoexcitation.23,24
TABLE 1: Energy Parameters (eV) in ClSA, MClSA, and
MSAa
E(S1) - E(S0) E(S′1) - E(S′0) E(S0) - E(S′0) E(S1) - E(S′1)
ClSA
MClSA
MSA
2.496
2.637
2.570
2.421
2.507
2.479
0.031
0.100
0.068
0.044
0.030
0.023
a E(Sn) is the energy of the Sn state.
In order to reveal the substitution effects, three thermochromic
SAs were investigated: (1) N-5′-chlorosalicylideneaniline (ClSA),
(2) 4-methyl-N-5′-chlorosalicylideneaniline (MClSA), and (3)
4-methyl-N-salicylideneaniline (MSA). The comparison among
these three samples is expected to reveal the substitution effects
on the dynamics of the proton transfer.
In the present paper, we have measured the lifetimes of the
excited enol states of the thermochromic SAs, which are
determined by the proton-transfer time, using femtosecond
fluorescence spectroscopy.36 The rate of proton transfer was
found to change drastically by the substitution. The change in
the proton dynamics is attributed to the different barrier heights
of the proton potential in the excited state. The effect of the
substituent is discussed in terms of electronic theory.
crystal thin enough to be used for the measurement of the
absorption spectrum.
Figure 1 shows the fluorescence excited at 3.10 eV and
excitation spectra probed at 2.18 eV at various temperatures of
MClSA. The excitation spectra did not depend on the probed
fluorescence energy. The fluorescence and excitation spectra
of ClSA and MSA were similar to those of MClSA. Compared
with the absorption and fluorescence spectra in solution, no
prominent structures, which are expected to be originated from
excitonic effects in crystal, were observed. Two common
features of the thermochromic SAs were observed: One is that
the absorption of the thermochromic band around 2.4 eV in the
excitation spectra increased with temperature above 70 K,
indicating that the keto form is thermally populated. The other
is that the large Stokes shift of the fluorescence was observed
below 70 K. Since the fluorescence band has a mirror symmetry
with the thermochromic absorption band in the excitation
spectrum and since the fluorescence could be observed by the
selective excitation of the thermochromic absorption band, the
stationary fluorescence originates mainly from the keto form.
Thus, the large Stokes shift of the fluorescence below 70 K
shows that the excited enol form relaxes to the excited keto
form, indicating that the photoexcitation causes the proton
transfer.
Using the excitation and fluorescence spectra, the energy
separations between the states, S1 and S0 (Ee), S′1 and S′0 (Ek),
and S0 and S′0 (Ea), are estimated experimentally. The values
of Ee and Ek were estimated from the lowest edges of the
excitation spectra and the highest edges of the fluorescence
spectra, respectively, at the lowest temperature to eliminate the
thermal effects. In the present study, the edge of each spectrum
is defined as the photon energy where the intensity becomes
1/20th of the peak intensity in each spectrum to remove the
ambiguity. The value of Ea was evaluated from the temperature
dependence of the thermochromic band in the excitation
spectrum by assuming the Boltzmann distribution of the
population of the keto form. As a result, the energy separation
between S1 and S′1 (Eg) was obtained. The results are listed in
Table 1. The value of Eg was found to reduce in the order of
ClSA, MClSA, and MSA, which is correlated with the dynamics
of the proton transfer, as will be discussed later.
Time-Resolved Fluorescence. (1) Time-Resolved Fluo-
rescence. Parts a, b, and c of Figure 2 show the time
dependence of the fluorescence intensities of ClSA, MClSA,
and MSA, respectively, at the probe photon energies at room
temperature.
In ClSA, the kinetics of the fluorescence is strongly dependent
on the probe photon energy. On the higher energy side of the
fluorescence spectrum, the rapid fluorescence decay is observed,
while the fluorescence decays relatively slowly on the lower
energy side. In the intermediate region, the decay function is
biexponential with the lifetimes of subpicoseconds and several
hundred picoseconds with varying fractions depending on the
photon energy, indicating the existence of two fluorescent
species. The solid lines in Figure 2a represent the fitted curves
by the least-squares method with the response function convo-
luted. The decay rate of the fast decay component was
II. Experiment
ClSA, MClSA, and MSA were prepared by the condensation
of chlorosalicylaldehyde and aniline, by the condensation of
chlorosalicylaldehyde and p-methylaniline, and by the conden-
sation of salicylaldehyde and p-methylaniline in refluxed
methanol, respectively. Single crystals of these samples were
grown from the methanol solution by slow evaporation.
The steady-state fluorescence and excitation spectra were
measured by a spectrofluorometer. The light source was a Xe
lamp. All excitation spectra were corrected for the intensity of
the light source using a thermopile. A continuous liquid He
flow cryostat was used to measure the fluorescence and
excitation spectra at various temperatures.
The fluorescence decay in the femtosecond region was
measured using a femtosecond fluorescence up-conversion
system. A fundamental titanium (Ti):sapphire laser at 770 nm
(1.61 eV) with an average power 600 mW and a repetition rate
of 100 MHz was used to produce second harmonics (SH) at
385 nm (3.22 eV) with an average power of 10 mW in a 3-mm
LBO crystal (type I). The SH was used to excite the samples
after the separation from the fundamental by a dichroic mirror.
The polarization of the excitation light was set parallel to the
long axis of the molecule in the crystal. The average excitation
power was less than 1 mW. The remaining fundamental was
used as a probe beam to up-convert the fluorescence from the
sample in a 0.5-mm BBO crystal (type I) by angle tuning. Thus,
the polarizations of the probed fluorescence and the excitation
pulse are parallel. The cross correlation between the SH and
the fundamental had a full width at half-maximum of 260 fs
when an objective lens was used for collecting the fluorescence.
The measured cross correlation was chosen as a response
function of the system. The up-converted signal was detected
by a photomultiplier with a photon-counting system after passing
through a monochromator. A continuous liquid N2 flow cryostat
was used to measure the fluorescence decay at 77 K.
III. Results
Stationary Fluorescence and Excitation Spectra. The
stationary fluorescence and excitation spectra of the samples
were measured at various temperatures to clarify the electronic
structures of the samples. The absorption spectra of all samples
could not be measured, since it was not possible to prepare a

646 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Sekikawa et al.
Figure 2. (a, left) Fluorescence decay at 2.53, 2.48, 2.34, 2.25, 2.18, and 2.10 eV excited at 3.22 eV of ClSA at 293 K from the top to the bottom.
(b, middle) Fluorescence decay at 2.70, 2.58, 2.48, 2.38, 2.30, 2.21, and 2.14 eV excited at 3.22 eV of MClSA at 293 K from the top to the bottom.
(c, right) Fluorescence decay at 2.58, 2.48, 2.38, 2.30, 2.21, and 2.14 eV excited at 3.22 eV of MSA at 293 K from the top to the bottom.
Figure 3. Probe photon energy dependence of the fast decay rate of
the fluorescence from MClSA at 293 K (open squares) and 77 K (open
triangles) and from MSA at 293 K (open circles).
determined to be 1.28 ( 0.04 ps-1. Since the time-integrated
intensity of the slow decay component was about 500 times
larger than that of the fast decay component and since the slow
decay component was observed in the same region of the
stationary fluorescence spectrum, the slow decay component
contributes dominantly to the stationary fluorescence spectrum.
Hence, the slow decay component is attributed to the fluores-
cence of the excited keto form. The origin of the fast decay
component will be discussed later.
The fluorescence dynamics of the deuterated (ClSA-d)
species, in which the hydrogen of interest is replaced by a
deuterium atom, was also investigated to gain insight into the
potential shape in the excited state. A similar time dependence
was observed. However, the decay rate of the fast decay
component was 1.00 ( 0.04 ps-1, which is smaller than that of
ClSA. The deuteration effect was observed.
The bimodal fluorescence decays in MClSA and MSA were
also fitted by the biexponential function with decay times of
subpicoseconds and several hundred picoseconds. The results
Figure 4. (a) Absorption and fluorescence spectra of MClSA 293 K.
(b) Fluorescence decays of MClSA at 2.38 eV excited at 3.22 eV
are shown by the solid curves in parts b and c of Figures 2.
Since the time-integrated fluorescence intensity of the slow
decay component is dominant, the slow decay component is
also ascribed to the fluorescence of the excited keto form, as in
the case of ClSA. However, the fluorescence kinetics was
different from that of ClSA in the following point: The fast
decay rate becomes larger at the higher probed photon energy
from 0.31 to 1.6 ps-1 in MClSA and from 0.23 to 0.85 ps-1 in
MSA. Figure 3 shows that the fast decay rate strongly depends
on the fluorescence photon energy. At the lower energies, the
decay rate becomes nearly constant at 0.36 ( 0.04 and at 0.28
( 0.06 ps-1 in MClSA and in MSA, respectively. The decay
rate of MClSA was faster than that of MSA at all probe photon
(upper) and 2.95 eV (lower) at 293 K. (c) Time dependence of the
fluorescence of ClSA at 77 K. The fluorescence energies are 2.53
(upper) and 2.21 eV (lower).
energies. These differences among the three samples may be
attributed to substitution effects.
(2) Selective Excitation of the Keto Form. In order to
clarify the origin of the fast decay components of the present
systems, MClSA was also excited at 2.95 eV by tuning the Ti:
sapphire laser: The keto form is excited more efficiently than
the enol form as shown in the absorption spectrum of Figure
4a, in which the absorption band between 2.5 and 2.9 eV is the
thermochromic band.

Proton Transfer in Salicylideneaniline
J. Phys. Chem. A, Vol. 101, No. 4, 1997 647
Parts b and c of Figure 4 show that the dynamics of the
fluorescence decay at 2.38 eV strongly depends on the excitation
energy; no fast decay components were observed when the
thermochromic absorption band was excited efficiently at 2.95
eV, as shown in the lower part of Figure 4b. On the other hand,
the bimodal fluorescence decay was observed when MClSA was
excited at 3.22 eV, as shown in the upper part of Figure 4b.
This indicates that the fast decay corresponds to the relaxation
of the excited enol form.
Compared with the absorption spectrum, the absorbance in
the excitation spectrum at 297 K increases with the photon
energy at 3.3 eV. This may be due to the difference in the
experimental conditions. Powderlike samples made of many
small crystals were used for the measurement of the excitation
spectra. The excitation spectrum does not agree with the
absorption spectrum of the sample of larger size. The efficiency
of the excitation is reduced because of the strong scattering of
the excitation light at shorter wavelengths. The light intensity
from the Xe lamp strongly depends on the photon energy
between 2.5 and 3 eV, and it becomes lower rapidly beyond 3
eV, which makes it difficult to obtain the correct excitation
spectrum. Thus, it is difficult to compare the absorbance of
the enol form and that of the keto form using the excitation
spectrum, although it is enough to identify the enol and keto
forms in the excitation spectrum.
the observed decay rate is much higher than those of the
radiative decay and the intersystem crossing, the almost constant
decay rate at the lower energy is considered to be due to the
proton transfer from S1 to S′1. Thus, the rates of the proton
transfer in MClSA and MSA are determined to be 0.36 ( 0.04
and 0.28 ( 0.06 ps-1, respectively. Two decay components at
lower fluorescence energies suggest the existence of the potential
barrier between the excited enol and keto forms: If there were
no barriers, the fluorescence would decay exponentially with
the continuous elongation of the decay time from the higher to
lower energies in a fluorescence spectrum. In addition, the rate
of proton transfer should be much larger in the barrierless
potential as observed in 2-(2′-hydroxy-5′-methylphenyl)benzo-
triazole (TIN)6,8,9 and 3-hydroxyflavone,11 in which the rate of
proton transfer is 6.7 and 4.2 ps-1, respectively. These proton-
transfer rates are considered to be determined by the frequency
of the vibrational mode involving the hydrogen bond.6,8,9
The low-temperature experiment offers information to clarify
the mechanism of the proton transfer. At 77 K, the rate of
proton transfer was not much different from that at room
temperature. Since the rate of proton transfer by the thermal
activation mechanism is expected to become exponentially lower
with temperature, it is concluded that quantum-mechanical
proton tunneling through the barrier is taking place.
The existence of the barrier is supported by the deuteration
effect on the dynamics of the proton transfer; the rate of proton
transfer in ClSA becomes smaller by deuteration from 1.28 to
1.00 ps-1. In the potential with a barrier, the rate of proton
transfer correlates with the frequency of the O-H stretching
mode. Thus, the reduction of the rate by deuteration indicates
the existence of the barrier. This contrasts with the previous
results of the proton transfer in the femtosecond region,9,15 which
might be due to the difference in phase of the investigated
system: In the liquid or gas phase, a molecule can be more
deformed to shorten the hydrogen-bond length for lack of inter-
molecular interactions, resulting in the barrierless potential. It
is, however, necessary to investigate the potential shape of the
proton transfer further.
(3) Low-Temperature Measurement. The fluorescence
dynamics of ClSA and MClSA were investigated at 77 K to
gain insight into the process of the proton transfer. Since the
keto form is thermally populated at room temperature, it is
difficult to observe the rise time of the fluorescence of the keto
form. On the other hand, the evolution of the fluorescence of
the keto form is expected to be observed at 77 K. The time
dependence of the fluorescence of ClSA at 77 K at 2.53 and
2.21 eV is shown in Figure 4c. The lifetime at 2.53 eV was
0.76 ( 0.05 ps, which agrees with the rise time of 0.72 ( 0.05
ps at 2.21 eV. This agreement strongly suggests that the fast
decay component is the precursor to the slow component. In
MClSA, the fluorescence also decays biexponentially, similar
to the measurements at 293 K. The fast decay rate at the probe
photon energy of MClSA is also plotted in Figure 3.
In MClSA and MSA, the decay rate of the fluorescence of
the enol form was found to be smaller at the lower fluorescence
energy. Two fluorescence processes can be tentatively con-
sidered to be the origin of the change in the decay rate of the
fluorescence: (1) the fluorescence of the nonthermally excited
S1 vibrational levels and (2) the fluorescence of the higher
energy states (Sn). In the first process, the probe energy
dependence of the fast decay rate is due to the repopulation
among the S1 vibrational levels: Since the present molecular
system was excited with an excess energy with respect to the
S1 state, the SA molecule is expected to be in a vibrationally
nonthermalized state after the femtosecond pulse excitation. The
nonthermally vibrational distribution makes it possible to emit
fluorescence with higher energy. The fluorescence decay rate
is determined by the rate of thermalization of a molecule and
is expected to depend on the probed fluorescence energy: The
decay rate becomes smaller in the lower energy region of the
fluorescence spectrum. In the second process, the decay rate
is determined by the rate of internal conversion. Thus, the decay
rate of the short-wavelength fluorescence is larger.
IV. Discussion
Dynamics of Proton Transfer. From the femtosecond
fluorescence spectroscopy, the lifetime of the excited enol state
was found to be less than a few picoseconds. In addition to
the proton transfer, three other possible decay mechanisms of
the excited enol state can be considered: (1) intersystem crossing
to a triplet state and (2) radiative and (3) nonradiative decays
to the ground enol state. Since the radiative lifetime of the
excited state of an organic molecule is generally a few
nanoseconds, the second mechanism can be ruled out. In
addition, the time of singlet-triplet intersystem crossing of an
aromatic compound is longer than several hundred picosec-
onds.37 Hence, the first mechanism does not contribute
dominantly to the decay of the excited enol state. If the
nonradiative decay to the ground enol state is the main pathway,
the sample would hardly fluoresce. On the other hand, the three
samples are highly fluorescent. Therefore, it can be concluded
that the third mechanism is much less important than the proton
transfer in the present system and that the decay rate of the
excited enol state almost corresponds to the proton-transfer rate.
Consequently, the rate of the proton transfer in ClSA is
determined to be 1.28 ( 0.04 ps-1. In MClSA and MSA, the
decay rate of the fluorescence of the enol form becomes constant
at the lower fluorescence energy, as shown in Figure 3. Since
The present experimental result can be attributed not to
process 2 but to process 1 for the following two reasons: Firstly,
the fast decay component was observed below the absorption
edge of the enol form determined by the stationary excitation
spectra at 10 K. This suggests that the fluorescence is emitted
from S1; the fluorescence from Sn is expected to be observed
even above the absorption edge of the enol form. Secondly,

648 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Sekikawa et al.
the observed decay rate is much lower than that expected for
the internal conversion: In MSA, the shortest decay time of
the fluorescence observed is 1.2 ps, which is too long to be an
internal conversion time between the excited states with the same
spin manifold.38 On the other hand, the observed decay rate is
reasonable as the rate of thermalization. In the present system,
the thermalization process is considered to be competing with
the proton transfer, 0.36 and 0.28 ps-1 in MClSA and MSA,
respectively. These rates are consistent with the rate of
thermalization, 0.2 ps-1, estimated in TIN.13 In another large
aromatic molecule, R-terthiophene, it was also found that the
intramolecular vibrational population redistribution occurs in a
few picoseconds.39
ClSA is larger than that of MSA, it is considered that the
chlorine atom stabilizes the keto form more efficiently in the
present case.
The value of Eg in MClSA does not have the largest value
among the three, indicating that the energy stabilization is not
the simple summation of the chlorine and methyl substitution
effects. This may be explained as follows: The chlorine atom
also donates the π electron by the resonance effect; the wave
functions of the π electrons from the chlorine atom may interfere
with that of the methyl group, resulting in the reduction of the
resonance effect of the methyl group. Thus, these two substit-
uents cannot simply give additional effects.
V. Summary
Therefore, the fluorescence kinetics in the present SAs can
be considered as follows: Photoexcitation transfers the enol
form to the vibrationally excited S1 states via vertical transition.
The vibrationally hot states thermalize within a few picoseconds.
At the same time, the proton is transferred to the excited keto
state with the rates of 1.28, 0.36, and 0.28 ps-1 in ClSA, MClSA,
and MSA, respectively, and then the molecules relax to the
ground state. Here, since the energy separation between the
excited enol (S1) and keto states (S′1) is less than 0.04 eV, the
thermally populated excited enol state is expected at higher
temperatures after the proton transfer. Thus, strictly speaking,
the decay rate of the slow component corresponds to the
weighted average of the relaxation rates of the S1 and S1′ states
at room temperature.
The substitution effect on the dynamics of the proton transfer
in the thermochromic SA in the crystalline phase was investi-
gated using femtosecond time-resolved fluorescence spectros-
copy. It is concluded that the proton transfer takes place by
the quantum-mechanical tunneling in the excited state from the
deuteration effect and the low-temperature experiment. The rate
of proton transfer was found to increase in the order of MSA,
MClSA, and ClSA, which can be explained by the different
barrier heights of the potential in the excited state indicated by
the stationary fluorescence and excitation spectra. The change
in the potential is qualitatively explained by the electron-
donating or -accepting property of the substituent around the
hydrogen bond. In ClSA and MSA, the thermalization of the
vibrationally hot states in S1 was observed.
Substitution Effects. The following two prominent changes
in the fluorescence dynamics were observed by substitution:
(1) The rate of the proton transfer becomes smaller in the order
of ClSA, MClSA, and MSA. (2) The decay rate of the
fluorescence of the enol form becomes smaller at the lower
fluorescence energies in MClSA and MSA, although it was not
observed in ClSA.
Acknowledgment. We are grateful to Profs. U. Nagashima
and T. Mitani for valuable discussions. We thank Dr. K.
Misawa and Ms. A. Yang for their technical support. The work
was partly supported by a Grant-in-Aid for specially Promoted
Research from the Ministry of Education, Science, and Culture
(No. 0510200021).
The difference in the dynamics of the proton transfer among
ClSA, MClSA, and MSA can be explained in terms of the
change in the barrier height of the potential-energy surface in
the excited state. The energy separation (Eg) between the
excited enol (S1) and keto (S′1) forms is obtained from the results
of the stationary fluorescence and excitation spectra as listed
in Table 1. The values of Eg of ClSA, MClSA, and MSA are
44, 30, and 23 meV, respectively, while the proton-transfer rates
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in ClSA, MClSA, and MSA are 1.28, 0.36, and 0.28 ps-1
,
respectively. If it is assumed that the hydrogen-bond lengths
are not different among the three systems, both the height and
thickness of the potential barrier increase with a decrease in
Eg, resulting in the lower probability of the proton tunneling.
Hence, the rate of proton transfer becomes lower with a
decreasing Eg.
In the case of ClSA, the thermalization process was not
observed. This is because the rate of proton transfer, 1.28 ps-1
,
is much higher than that of the thermalization, 0.2 ps-1: The
proton transfer finishes before the thermalization. The absence
of the thermalization in ClSA is also attributed to the lower
barrier height of the potential in the excited state.
The difference in Eg among the samples may be ascribed to
the substitution effect as follows: Since the chlorine atom
attracts the electrons of the oxygen atom through the σ bond
by the inductive effect, it decreases the strength of the O-H
bond. Thus, the chlorination reduces the barrier height between
the enol and keto forms. The methyl group is a π-electron
donor. It enhances the proton-accepting property of the nitrogen
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