Excited-state double proton transfer in 3-formyl-7-azaindole: role of the nπ* state in proton-transfer dynamics

Article abstract of DOI:10.1021/jp001807g

3-Formyl-7-azaindole (3FAI) and its derivatives have been synthesized to study the role of the nπ^* state in the excited-state double proton transfer (ESDPT) reaction. In 3FAI monomer as well as its associated hydrogenbonded complexes the lowest excited singlet state has been concluded to be in the nπ^* configuration. The association constants incorporating the hydrogen bonding formation were determined to be 1.9 × 10⁴ (313 K), 2.2 × 10⁴ (298 K) and 1.8 × 10⁵ M^-1 (298 K) for 3FAI dimer, 3FAI/azacyclohexanone and 3FAI/acetic acid, respectively, in cyclohexane. In alcohols, the rate of solvent (e.g., methanol) diffusional migration, forming a 'correct' precursor for ESDPT, is concluded to be much slower than the rate of S_ππ(*) → S_nπ(*) internal conversion which has been deduced to be 4.37 × 10¹² s^-1. ESDPT is prohibited in the S_nπ(*) state of which the relaxation dynamics are dominated by the rate of S_nπ(*) → T_ππ(*) intersystem crossing. In contrast, for 3FAI dimer or 3FAI/acetic acid complex possessing intact dual hydrogen bonds the intrinsic ESDPT is competitive with the rate of S_ππ(*) → S_nπ(*) internal con version, resulting in a prominent imine-like tautomer emission. The results provide the first model among 7AI analogues in which the fast rate of S_ππ(*) → S_nπ(*) internal conversion serves as an internal clock to examine the mechanism of guest molecules (including the bulk alcohols) assisted ESDPT.

Full text of DOI:10.1021/jp001807g

J. Phys. Chem. A 2000, 104, 8863-8871
8863
Excited-State Double Proton Transfer in 3-Formyl-7-azaindole: Role of the nπ* State in
Proton-Transfer Dynamics
Pi-Tai Chou,* Guo-Ray Wu, Ching-Yen Wei, Mei-Ying Shiao, and Yun-I Liu
Department of Chemistry, The National Chung-Cheng UniVersity, Chia Yi, Taiwan R.O.C.
ReceiVed: May 17, 2000; In Final Form: July 25, 2000
3-Formyl-7-azaindole (3FAI) and its derivatives have been synthesized to study the role of the nπ* state in
the excited-state double proton transfer (ESDPT) reaction. In 3FAI monomer as well as its associated hydrogen-
1
bonded complexes the lowest excited singlet state has been concluded to be in the nπ* configuration. The
association constants incorporating the hydrogen bonding formation were determined to be 1.9 × 104 (313
K), 2.2 × 104 (298 K) and 1.8 × 105 M-1 (298 K) for 3FAI dimer, 3FAI/azacyclohexanone and 3FAI/acetic
acid, respectively, in cyclohexane. In alcohols, the rate of solvent (e.g., methanol) diffusional migration,
forming a “correct” precursor for ESDPT, is concluded to be much slower than the rate of Sππ* f Snπ*
internal conversion which has been deduced to be 4.37 × 1012 s-1. ESDPT is prohibited in the Snπ* state of
which the relaxation dynamics are dominated by the rate of Snπ* f Tππ* intersystem crossing. In contrast, for
3FAI dimer or 3FAI/acetic acid complex possessing intact dual hydrogen bonds the intrinsic ESDPT is
competitive with the rate of Sππ* f Snπ* internal conversion, resulting in a prominent imine-like tautomer
emission. The results provide the first model among 7AI analogues in which the fast rate of Sππ* f Snπ*
internal conversion serves as an internal clock to examine the mechanism of guest molecules (including the
bulk alcohols) assisted ESDPT.
1. Introduction
7AI and its analogue 7-azatryptophan have been successfully
applied to probe the solvation and/or protein dynamics.11-15 The
The dual hydrogen-bonding (HB) dimer of 7-azaindole (7AI)
has long been recognized as a simplified model for the HB base
pair of DNA,1-3 which upon electronic excitation undergoes
the double proton-transfer reaction, resulting in a large Stokes
shifted tautomer emission (e.g., λmax ∼ 480 nm in cyclohexane).
At the molecular level, such an excited-state double proton
transfer (ESDPT) process provides one possible mechanism for
the mutation which has been proposed to be, in part, due to a
“misprint” induced by the proton-transfer tautomerism of a
specific DNA base pair during replication, recording an error
message.3-5 Recently, much research has focused on the
femtosecond dynamics of double proton transfer in the 7AI
dimer.6-10 In the gas phase, the femtosecond TOF-MS result6
has led to an establishment of a sequential ESDPT mechanism
which was further supported by the arrest of a 7AI dimer
cationic species via a coulomb explosion technique.7 On the
basis of femtosecond fluorescence upconversion in combination
with transient absorption techniques, Fiebig et al.10 have resolved
excitation-wavelength-dependent multiexponential tautomer rise
times of the 7AI dimer in nonpolar solvents, which are the basis
of a nonconcerted double-proton transfer. Consequently, they
concluded that on the global potential energy surface both
trajectories of the symmetric and asymmetric vibrational motion
coupled with the solvent dynamics must be considered for the
proton-transfer reaction. On the other hand, via the formation
of a 7AI(host)/guest HB complex the dynamics of ESDPT
incorporating guest molecules have also received considerable
attention.11-17 Studies of 7AI complexed to single molecules
of carboxylic acids and lactams have revealed rapid ESDPT
reaction,16 especially for the catalytic type of reaction where
the guest molecule (e.g., acetic acid) remains unchanged during
the reaction. In alcohols and water the dynamics of ESDPT in
proton transfer dynamics of 7AI in linear-carbon-chain monoal-
cohols have been interpreted in terms of a two-step model
incorporating solvent rearrangement to a “correct” configuration,
followed by a rapid proton transfer which is possibly dominated
by a tunneling mechanism. In water solvents the explanations
are somewhat scattering in terms of the fraction of correctly
solvated 7AI undergoing ESDPT and the relaxation pathways
of the tautomer species.14,15,16a However, an even slower rate
of the solvent reorganization at the first step is generally
accepted in water and polyalcohols (vide infra).
More recently, studies have been extended to the dimers of
7AI analogues of biological importance. Among which, purines
possessing a similar electronic moiety with respect to 7AI have
received particular interest.18,19 On the basis of a synthetic
approach it has been shown that the number as well as the
position of the nitrogen atom in either five- or six-member ring
system of 7AI alters the relative ESDPT thermodynamics,
especially in the noncatalytic type of the proton-transfer reaction.
Further focuses on the chemical modification of 7AI are of
particular interest to study the substituent effect on the proton-
transfer reaction. In this paper we report the spectroscopic and
dynamic studies of 3-formyl-7-azaindole (3FAI, see Figure 1)
of which the lowest excited singlet state is concluded to be in
the 1nπ* configuration in its monomer as well as in HB
complexes. Consequently, the fast rate of Sππ* f Snπ* internal
conversion (IC) serves as an internal clock to examine the
mechanism of guest molecules coupled ESDPT reaction.
2. Experimental Section
2.1. Materials. 7-Azagramine (C10H13N3), a precursor for
synthesizing 3-formyl-7-azaindole, was synthesized according
10.1021/jp001807g CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/13/2000

8864 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Chou et al.
deuterated 3FAI (3FAI(D)) and lactam (lactam (D)) was
synthesized by dissolving 3FAI (or lactam) in CH3OD. CH3-
OD was then gradually evaporated in the vacuum line. This
procedure was repeated three times and the product was stored
in the N2 purged drybox where the sample preparation was also
performed. The formation of 3FAI(D) and lactam(D) was
checked by proton NMR where ∼92% and 90% of the N-H
proton for 3FAI and lactam, respectively disappeared after
deuteration. The deuterated acetic acid (CH3COOD, 98%,
Arcose) was purchased from Aldrich and used without further
purification.
2.2. Measurements. Steady-state absorption and emission
spectra were recorded by a Varian (Cary 3E) spectrophotometer
and a Hitachi (F4500) fluorimeter, respectively. Both wavelength-
dependent excitation and emission response of the fluorimeter
have been calibrated according to a previously reported method.16
Room-temperature fluorescence quantum yields were measured
using quinine sulfate/1.0 N H2SO4 as reference, assuming a yield
of 0.564 with 365 nm excitation.21 For the phosphorescence
measurement an Nd:YAG (fourth harmonic, 266 nm) was used
as a excitation source. The emission was collected at a right
angle with respect to the excitation source and detected by an
intensified charge coupled detector (ICCD, Princeton Instrument,
model 576G/1). Typically, 100 laser shots were collected and
averaged in each data acquisition period. Naphthalene was
used as a standard to measure the relatively phosphorescence
yield, of which the phosphorescence yield was known to be
0.1 in a 77 K EPA (v/v 2:2:5, ether-isopentane-ethyl alcohol)
glass.22
Nanosecond lifetime measurements have been previously
described.16d Picosecond lifetime measurements were achieved
by using either second (380-420 nm) or third (260-275 nm)
harmonic of the femtosecond Ti-Sapphire oscillator (82 MHz,
Spectra Physics) as an excitation source. An Edinburgh OB
900-L time-correlated single photon counter was used as a
detecting system. Since the fwhm of the excitation pulse is
typically ∼120 fs, the resolution is limited by the detector
response of ∼30 ps. The fluorescence decays were analyzed
by the sum of exponential functions with an iterative convolution
method reported previously.21 This procedure should allow
partial removal of the instrument time broadening and conse-
quently renders a temporal resolution of ∼15 ps.
Figure 1. Structures of 7AI, 3FAI and the corresponding derivatives
and HB complexes.
to Robison and Robison.20 The crude product was purified by
column chromatography using ethyl acetate as an eluent to
obtain 7-azagramine. NMR analyses: 1H NMR (400 MHz,
CDCl3) δ 2.266 (s, 6H), 3.608 (s, 2H, CH2), 7.079 (m, 1H),
7.256 (s, 1H), 8.037 (d, J ) 6.32 Hz, 1H), 8.306 (d, J ) 3.2
Hz, 1H), 10.22 (b, 1H).
3FAI was prepared through the hydrolysis of 7-azagramine.20
The crude product was purified by recrystallization from water
followed by column chromatography using ethyl acetate as an
1
eluent. H NMR (300 M Hz, CDCl3) δ 7.31 (m, 1H), 8.02 (s,
The nanosecond pump-probe transient absorption experiment
was performed by using a modified flash lamp (EG&G model
LS-1130, ∼50 ns pulse width) as a white-light probe pulse. A
cylindrical lens was used to shape the pump Nd:YAG 355 nm
(∼8 ns, Continuum Surlite II) pulse to a rectangular size of 40
× 2 mm2. The white-light probe pulse was then collected by
an optical fiber, collimated by a 5 cm focal length lens, and
skimmed by a slit to obtain a 2 × 2 mm2 rectangular shape
before entering the sample cell. Both pump and probe pulses
were crossed by a 90° angle with an overlapping distance of 4
cm. The probe white light pulse, after passing through the sam-
ple solution, was focused on the entrance of the slit (300 µm)
of the ICCD system. With an average of 100 shots an absorbance
as low as 10-3 can be detected in the spectral range of 300-
700 nm.
Details of the theoretical approach (6-31G(d,p)) for the
thermodynamics of 3FAI HB complexes in the ground state
has been previously described.23 The calculation of the lower
lying excited singlet states was performed by either Cis (singlet,
Nstats ) 6)24 or Td (singlet, Nstats ) 6)25-27 method incorpo-
rating B3LYP/6-311++G(2d,p) basis sets. However, both
methods give negligible differences regarding the lowest energy
level of the singlet excited state in 3FAI.
1H), 8.45 (m, 1H), 8.65 (m, 1H), 10.05 ( s, 1H, CHO), 10.89
(b, 1H, NH).
1-Methyl-3-formyl-7-azaindole (1MFAI) was synthesized by
adding sodium hydride (57%, 60 mg) to the THF solution
containing 3FAI (0.15 g), followed by the addition of methyl
1
iodide (30 mg). H NMR (CDCl3, 200 MHz): δ 3.99 (s, 3H),
7.22 (t, J ) 3.2 Hz, 1H), 7.82 (s, 1H), 8.40 (d, J ) 3.2 Hz,
1H), 8.52 (d, J ) 6.4 Hz, 1H), 9.92 (s, 1H).
3-Formyl-7-methyl-7H-pyrrolo[2,3-b]pyridine (3FMPP, see
Figure 1) was synthesized by the reaction of 3FAI (0.15 g) and
CH3I (0.70 g) in THF under N2. NaOH (2.5 N, 10 mL) was
then added and the mixture was stirred for ∼20 min to obtain
3FMPP (60 mg). 1H NMR (CDCl3, 200 MHz): δ 4.44 (s, 3H),
7.22 (t, J ) 8.0 Hz, 1H), 7.83 (d, J ) 6.4 Hz, 1H), 8.44 (s,
1H), 8.87 (d, J ) 7.8 Hz, 1H), 10.03 (s, 1H).
Cyclohexane and various alcohols are of spectrograde quality
(Merck Inc.) and used right after received. Acetic acid (Merck
Inc.) was purified through a fractional distillation. Azacyclo-
hexanone (Aldrich) was recrystallized twice from methanol.
Since the stable conformer of 2-azacyclohexanone is in the
lactam form we thus use the abbreviation “lactam” in order to
distinguish it from the “lactim” form (see Figure 1). The N(1)

Double Proton Transfer in 3-Formyl-7-azaindole
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8865
C0
1
1
4
)
+
(1)
A - ꢀMC0
C
ꢀD - 2ꢀM
Ka(ꢀD - 2ꢀM)
0
In eq 1 ꢀM and ꢀD are molar extinction coefficients of the
monomer and dimer monitored at a specific wavelength, e.g.,
300 nm, where an appreciable percentage of absorbance can be
attributed to the self-associated species. Under a sufficiently
low 3FAI concentration where only monomer exists, ꢀ3M00 was
calculated to be ∼930 M-1 cm-1. A plot of C0/(A - ꢀMC0) at
300 nm as a function of 1/C0 shown in the inset of Figure 2
reveals a sufficiently linear behavior, supporting the assumption
of dimeric formation in 3FAI. Accordingly, by fixing the ꢀM
value, a linear least-squares fit using eq 1 gives a Ka values to
be 1.9 × 104 M-1 (313 K) in cyclohexane.
Figure 3 shows the absorption spectra of 3FAI upon adding
the acetic acid in cyclohexane. In this experiment, the initial
concentration of 3FAI, C0, was prepared to be as low as 9.5 ×
10-6 M to avoid the self-dimerization. The formation of 3FAI/
acetic acid HB complexes can be clearly shown by the growth
of an absorption band at ∼297 nm accompanied by an
appearance of isosbestic point at ca. 285 nm throughout the
titration, indicating the existence of equilibrium with a common
intermediate. On the basis of the assumption of 3FAI complexed
by stoichiometrically equivalent guest molecules (e.g., acetic
acid), the relationship between the measured absorbance as a
function of the initially prepared acetic acid concentration, Cg,
can be expressed by19
Figure 2. (____) Concentration-dependent absorption spectra of 3FAI
in cyclohexane, in which 3FAI (C0) was prepared at (a) 2.8 × 10-5
,
(b) 5.6 × 10-5, (c) 1.7 × 10-4, (d) 3.8 × 10-4, and (e) 8.6 × 10-4 M.
The absorption spectra are normalized at 260 nm. (- - -) Absorption
spectrum of 1MFAI (1.0 × 10-5 M) in cyclohexane. Inset: Plot of
C0/(A300 - ꢀM300C0) values versus 1/C0 for a-e and a best nonlinear
least-squares fitting curve using eq 1.
The following sections are organized according to a sequence
of steps where we first performed detailed absorption and
fluorescence titration experiments to determine the thermody-
namic and ESDPT properties of 3FAI HB complexes. The
results in combination with triplet-triplet transient absorption
measurement and low-temperature phosphorescence experiments
lead us to deduce the competitive rate of Snπ* f Sππ* IC versus
the rate of proton-transfer reaction. On the basis of this proposed
IC/ESDPT mechanism proton-transfer tautomerism mediated by
various types of the guest molecule was discussed in details.
A0
ꢀM
1
)
+ 1
(2)
(
)
[
]
A - A0
ꢀC - ꢀM KaCg
where ꢀM and ꢀC are molar extinction coefficients of the 3FAI
monomer and HB complex, respectively at a specific wave-
length. The straight-line plot of A0/(A - A0) as a function of
1/Cg at a selected wavelength of 300 nm (see inset of Figure 3)
supports the validity of the assumption of a 1:1 3FAI/acetic
acid complex formation. Consequently, a best linear least-
squares fit using eq 2 deduces ꢀ3M00 and Ka to be 2742 M-1
cm-1 and 1.8 × 105 M-1, respectively. Remarkable 1:1 3FAI/
lactam HB association was also observed in the absorption
titration study, and a Ka value was determined to be 2.2 × 104
M-1 (298 K, see Table 1). In comparison, although 1MFAI
provides one hydrogen-bonding site (i.e., the pyridinal nitrogen)
negligible spectral change was observed upon adding either
acetic acid or lactam with concentrations up to 10-3 M. The
result unambiguously supports the formation of cyclic dual
hydrogen bonds in both 1:1 3FAI/acetic acid and 3FAI/lactam
complexes.
3. Results
3.1. HB Association. When the concentration was prepared
to be as low as 8.0 × 10-6 M, 3FAI exhibits the first π f π*
absorption band maximum at 282 nm (ꢀ282 ∼ 6214 M-1 cm-1).
The spectral feature is similar to that of 1-methyl-3-formyl-7-
azaindole (1MFAI) that is generally treated as a nonproton-
transfer model due to the lack of an N-H proton. Thus, the
assignment of the 282 nm band to the amine-like monomer
species is unambiguous. Upon increasing the concentration,
changes in the spectral features were observed in which a red
shift of the spectra appears in the spectral range of >280 nm
(see Figure 2a-e). Note that due to the sparse solubility in
nonpolar solvents the titration experiment was performed in
cyclohexane at 313 K (40 °C). In comparison, 1MFAI revealed
an unchanged absorption profile in the same range of concentra-
tions as 3FAI. The results suggest the formation of 3FAI dimer
and/or higher-order aggregates through a hydrogen bonding
effect. Similar to that proposed in 7AI,16 we assume that the
geometrical structure of the 3FAI dimer possesses a cyclic dual
hydrogen bonding configuration (see Figure 1) where the dual
HB sites, i.e., pyrrolic proton (N(1)H) and the pyridinal nitrogen
(N(7)) in 3FAI, act as a proton donor and acceptor, respectively.
Consequently, a competitive equilibrium between the monomer
and its self-associated dimer incorporating dual hydrogen bonds
can be established. When the experimental condition was
manipulated so that the fraction of 3FAI dimer formation is
less than 15%, the absorbance of the mixture at a given
wavelength as a function of initially prepared C0 can be
expressed by19
3.2. Emission Properties of the 3FAI. Table 1 lists the
steady-state emission properties as well as relaxation dynamics
for 3FAI and its HB species in cyclohexane. In contrast to the
prominent normal monomer emission in the case of 7AI (λmax
∼ 320 nm with Φf ∼ 0.22 in cyclohexane13,14), an attempt to
detect the fluorescence associated with 3FAI monomer (<1.0
× 10-5 M in cyclohexane) in the spectral range of <350 nm
failed. Similar nonluminescence behavior was observed for
1FMAI in room temperature cyclohexane. Upon increasing the
3FAI concentration, a unique long-wavelength emission maxi-
mum at 437 nm gradually appears (see Figure 4). At an initially
prepared concentration C0 of < 10-3 M where the dimer
concentration is significantly lower than that of the monomer,
2
a plot of the inverse of 437 nm emission intensity versus 1/C0
reveals a straight line behavior. The results suggest the precursor
of the 437 nm band to be the 3FAI dimer. Further supporting

8866 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Chou et al.
(3)
the relation depicted in eq 3
1
1 1
Ka Cg
) (2.3RI0ꢀcφobsC0)-1 1 +
(
)
Fp
where Cg represents the added acetic acid (or lactam) concentra-
tion, Fp denotes the integrated fluorescence intensity, Φobs is
the measured tautomer fluorescence quantum yield, I0 and R
denote the instrument factor (such as sensitivity, alignment, etc.)
and the excitation source intensity, respectively. During the
titration experiment where the detection geometry, excitation
intensity and the initially prepared 3FAI concentration remain
unchanged, the plot of 1/Fp versus 1/Cg shown in the inset of
Figure 5 exhibits good linear behavior, reconfirming the
formation of a 1:1 3FAI/acetic acid dual HB complex. A best
linear least-squares fit gives the slope and intercept to be 3.1 ×
10-7 and 0.034, respectively. Consequently, a Ka value of 1.1
× 105 M-1 was obtained. On the basis of the same approach a
Ka value was deduced to be 2.8 × 104 M-1 for the 3FAI/lactam
complex. Both results, within experimental error, are consistent
with that obtained from the absorption titration study.
Deuterium isotope effect in the steady-state emission intensity
was observed among the three studied 3FAI/guest HB com-
plexes. The tautomer emission intensity of 3FAI(D) dimer,
3FAI(D)/acetic acid(D) and 3FAI(D)/lactam(D) was found to
be 0.64, 0.71, and 0.40 relative to their corresponding nondeu-
terated HB complexes. Note that such a significant isotope-
dependent tautomer emission intensity was not observed in the
cases of three corresponding 7AI HB complexes in cyclohexane.
Details will be elaborated in the discussion section.
Figure 3. Concentration-dependent absorption spectra of 3FAI (9.4
× 10-6 M) in cyclohexane by adding various acetic acid concentrations
(Cg) of (a) 0, (b) 1.7 × 10-6, (c) 3.3 × 10-6, (d) 5.0 × 10-6, (e) 6.7 ×
10-6, (f) 1.0 × 10-5, (g) 1.5 × 10-5, (h) 2.0 × 10-5, (i) 4.0 × 10-5, (j)
7.6 × 10-5, and (k) 2.0 × 10-4 M. Inset: Plot of A0/(A - A0) at 300
nm as a function of 1/Cg in curves b-k and a best least-squares fitting
curve using eq 2.
evidence can be given by the excitation maximum of 290 nm
which is ∼8 nm red shifted with respect to that of the absorption
spectrum mainly attributed to the monomer (see Figure 4).
Consequently, the 437 nm emission band with a Stokes shift of
∼104 cm-1 (peak-to-peak) leads us to conclude the occur-
rence of ESDPT in the 3FAI dimer, resulting in a tautomer
emission. This viewpoint can be further supported by the
observation of a 458 nm emission (τf ∼ 12.3 ns) for 3FMPP in
cyclohexane (Figure 4). Note that 3FMPP generally serves as
a model of the 3FAI tautomeric form (see Figure 1). The dimeric
tautomer emission shows an unresolved rise time followed by
a single-exponential decay rate of 1.3 × 108 s-1 (τf ∼ 7.7 ns),
indicating that the rate of ESDPT for the 3FAI dimer should
be >15 ps-1 of our system response time (see the Experimental
Section).
Figure 5 shows the fluorescence spectra of 3FAI as a function
of acetic acid concentrations in cyclohexane. Upon excitation
in the region of the isosbestic point of ∼285 nm where both
3FAI and 3FAI/acetic acid complex absorb, an enormously large
Stokes shifted fluorescence was observed maximum at ∼445
nm. The dynamics of relaxation of this 445 nm emission band
were found to be excitation-wavelength independent and
revealed a single-exponential decay rate of 8.9 × 107 s-1
(τf ∼ 11.2 ns) in the spectral region of 400-550 nm. The
large Stokes shift of >10 000 cm-1 leads to the assignment of
the 445 nm band to the tautomer emission unambiguously.
Furthermore, a careful lifetime analysis shows an unresolved
rise of the tautomer emission under a system response time
of ∼15 ps. Similarly, a unique tautomer fluorescence (440
nm, τf ∼ 7.8 ns) was also observed upon exciting the 3FAI/
lactam dual HB complex (see Table 1). However, the ob-
served fluorescence yield is only ∼6.7% of the 3FAI/acetic
acid complex. Note that for the three studied HB complexes
normal emission was not detected in the spectral range of
300-400 nm. Detailed discussion regarding mechanism of IC/
ESDPT competing dynamics will be presented in the later
section.
In alcohols differences between 3FAI and 7AI are even more
remarkable. In methanol 3FAI exhibits an extremely weak
tautomer emission maximized at 460 nm. By integrating the
emission intensity and comparing with that of the 7AI tautomer
in methanol of which Φobs was known to be 2.4 × 10-3 14b
,
one
obtained an Φobs value of 9.5 × 10-5 (see Table 1). Extremely
weak tautomer emission was also observed in other linear-chain
alcohols such as ethanol and propanol. For all alcohols studied
no emission band ascribed to the normal emission was detectable
in the spectral region of 300-400 nm. In tert-butyl alcohol and
polyalcohols such as ethylene glycol the tautomer emission was
too weak to be detected. When the fluorimeter was operated
under the same amplification and optical configuration through-
out the study the sensitivity limit defined as the signal/noise
ratio to be <3 was estimated to be ∼3.0 × 10-5. We thus
concluded that the yield of tautomer fluorescence of 3FAI in
tert-butyl alcohol, if there is any, should be as low as 3.0 ×
10-5. In comparison, prominent dual emission consisting of
normal and tautomer fluorescence was observed for 7AI in linear
long-chain monoalcohols as well as polyalcohols.1-3,11-15
4. Discussion
4.1. Relaxation Pathways in 3FAI. The nonluminescent
behavior of 3FAI in the monomeric form is intriguing. In
comparison, a high quantum yield (Φf ∼ 0.22) of the monomer
emission has been reported for 7AI in cyclohexane.13,14 For the
case of 7AI monomer, there exist two close low-lying excited
singlet states possessing the ππ* character, which are generally
assigned to 1La and 1Lb states. This assignment is analogous to
those defined in the linear condensed polyacenes28 and indoles29
where La and Lb states are different in the nodal distributions
of their wave functions, resulting in a difference in the dipole
Because of the lack of normal fluorescence for both 3FAI
monomer and its associated HB complexes, the tautomer
emission in 3FAI/acetic acid and 3FA/lactam HB complexes is
well resolved. Hence, the association constant can be obtained
more precisely from the fluorescence titration studies based on
1
moment. A semiempirical approach suggests the Lb state to
be the lowest excited singlet state in 7AI, while the 1La state is

Double Proton Transfer in 3-Formyl-7-azaindole
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8867
TABLE 1: Thermodynamic and Photophysical Properties of 3FAI and 3FAI HB Complexes and Its Methylated Derivatives in
Cyclohexane
absorption λmax (nm)
florescence λmax (nm)
Φobs
NA
τf (ns)
Ka (M-1) (298 K)
3FAI
282, 255
286, 260g
290a
NA
460
437
437
445
445
440
440
NA
458
NA
9.5 × 10-5
0.021
0.013
0.042
0.030
0.0028
0.0011
NA
∼8.3b
3FAI dimer
7.7
1.9 × 104 d
3FAI(D) dimer
239a
7.8
3FAI/ACIDc
292a
11.2
11.3
7.8
7.8
NA
1.8 × 105
1.1 × 105 e
2.2 × 104
2.8 × 104 e
3FAI(D)/ACID(D)
3FAI/LACTAM
3FAI(D)/LACTAM(D)
1MFAI
292a
292a
292a
290
3FMPP
375, 425f
0.085
12.3
0.07g
11.3g
a Values were obtained from the fluorescence excitation spectrum. b This value is subject to a larger uncertainty due to the extremely weak
emission intensity. c ACID denotes acetic acid. d Values were obtained at 313 K. e Data were obtained from the fluorescence titration experiment.
f The first vibronic peak of the S0-S1 (ππ*) transition. g Data were obtained in methanol. NA: not available.
Figure 4. (____) A. Concentration-dependent fluorescence spectra of
Figure 5. Fluorescence spectra as a function of the acetic acid
3FAI in cyclohexane, in which 3FAI was prepared at (a) 7.1 × 10-6
,
concentration (Cg) which was prepared to be (a) 0, (b) 1.7 × 10-7, (c)
3.3 × 10-7, (d) 6.7 × 10-7, (e) 1.0 × 10-6, (f) 1.7 × 10-6, (g) 2.3 ×
10-6, (h) 3.3 × 10-6, (i) 5.6 × 10-6, (j) 1.1 × 10-5, and (k) 2.5 ×
10-5. Inset: Plot of 1/Fp of the integrated 450 nm emission band as a
function of 1/Cg in curves b-k and a best least-squares fitting curve
using eq 3.
(b) 5.6 × 10-5, (c) 2.5 × 10-4, (d) 5.7 × 10-4, and (e) 8.6 × 10-4 M.
B. Excitation spectrum of sample c. monitored at 440 nm. (‚‚‚) The
absorption spectrum of sample c. (- - -) Absorption and fluorescence
spectra of 3FMPP in cyclohexane (1.5 × 10-5 M). Inset: Time-
dependent fluorescence of 3FAI (1.0 × 10-3 M) monitored at 450 nm
(λex: 300 nm) fitted by one single-exponential decay (τ ) 7.7 ns, ø2 )
1.005).
ππ* state should carry oscillator strength much higher than that
1
1
of the nπ* level. Consequently, a fast ππ* f nπ* IC should
take place prior to the 1Lb (or 1La) f S0 relaxation, resulting in
a ∼unity population in the Snπ* state. Owing to the orbital
forbidden Snπ* f S0 transition the deactivation of the Snπ* state
is expected to be dominated by intersystem crossing (ISC). Thus,
an appreciable population in the triplet state is expected, of
which the relaxation dynamics are commonly dominated by
radiationless pathways in solution phase at ambient temperature.
Such a consequence rationalizes the nonluminescence behavior
of 3FAI.
higher in energy by ∼2000 cm-1 in the gas phase.30 In the
solution phase, due to the solvation effect these two excited
sates are believed to be located even more closely since the 1Lb
and 1Lb bands are essentially unresolved on the basis of a steady-
state fluorescence excitation anisotropy experiment.15f The
carbonyl substitution at the C(3) position of 7AI, forming 3FAI,
does not likely change the location of La and Lb state
significantly, as indicated by the similar absorption spectrum
in the region of 270-300 nm between 7AI and 3FAI. However,
a salient absorption feature maximized at ca. 255 nm for 3FAI
is absent in the case of 7AI monomer (see Figure 2). In light of
the structural difference between 7AI and 3FAI being the formyl
functional group at the C(3) position the 255 nm absorption
band is then tentatively assigned to be a highly excited S0 f
Sn (n > 1, ππ*) transition in 3FAI incorporating the carbonyl
π electron. Accordingly, the carbonyl oxygen should also
introduce an n f π* transition which is plausibly in the lowest
excited singlet state. This proposal can be qualitatively supported
by an ab initio approach based on the B3LYP/6-311++G(2d,p)
basis sets (see the Experimental Section) which estimates the
lowest singlet excited state to be a forbidden transition (oscillator
strength f ∼ 0) located at ∼327 nm in the gas phase. In contrast,
the allowed Lb band was calculated to be the lowest singlet
excited state in 7AI.31 Upon a Franck-Condon excitation, the
To verify this proposed relaxation mechanism, several at-
tempts focusing on identifying the triplet state were carried out.
In a 77 K methylcyclohexane glass, 3FAI exhibits a phospho-
rescence maximum at 525 nm (Figure 6) of which the emission
yield (Φp) and lifetime (τp) were measured to be ca. 10-3 and
1.5 µs, respectively. We also carried out a nanosecond transient
absorption measurement. Inset of Figure 6 reveals the transient
absorption spectrum of 3FAI in degassed cyclohexane upon 266
nm excitation. The decay of the transient absorption, within
experimental error, follows single exponential kinetics with a
lifetime of ca. 800 ns. The rise time is too fast to be resolved
by our current transient absorption system of ca. 30 ns. The
decay dynamics are strongly quenched by the addition of O2
with a quenching rate of ∼2.2 × 109 M-1 s-1 which is

8868 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Chou et al.
SCHEME 1
has originally been proposed to be due to the slow solvent
reorganization to an appropriate precursor favorable for a relayed
type of double proton transfer. The results have been subse-
quently reconfirmed through more detailed studies based on a
time-resolved photon counting technique.13-15 Consequently, a
two-step coupling proton-transfer mechanism has been proposed
and depicted in Scheme 1 where * denotes the excited singlet
state. The initially predominant species populated in the excited
state is ascribed to the A* state which represents a nonspecific
HB configuration except for the cyclic dual HB type (i.e., the
B* state). The first proton-transfer step incorporates a slow rate
of solvent reorganization k1 to the B* state which possesses a
correct geometry for the proton transfer to proceed, coupled
with a reverse rate of solvent randomization k-1 to any com-
plex formation unfavorable for the proton transfer to proceed.
Once in the B* state a competitive rate of intrinsic proton-
transfer reaction kpt may take place, forming the tautomer
species. Although the solution of the coupling kinetic expression
shown in Scheme 1 may not be straightforward, simplification
can be made based on the much slower solvent reorganization
rate k1 relative to the k-1 and/or kpt. As a result, a steady-state
approach can be made for the B* state, and the overall rate of
excited-state relaxation dynamics, krxn, in the A* state can be
expressed as
Figure 6. Time-dependent spectral evolution of the phosphorescence
of 3FAI in a 77 k methylcyclohexane glass (λex ∼ 266 nm) at (a) 320
ns, (b) 350 ns, (c) 450 ns, (d) 550 ns, (e) 650 ns, (f) 1 µs, (g) 1.5 µs,
(h) 2.5 µs, (i) 4.5 µs, and (j) 10 µs. Note that an extremely low
concentration of 1.0 × 10-7 M was used in this study to minimize the
low-temperature aggregation of 3FAI. Inset: the transient absorption
spectrum of 3FAI (1.0 × 10-5 M, λex ∼ 266 nm) in the degassed
cyclohexane. The spectrum was obtained at a pump-probe delay time
of 200 ns.
approximately one-ninth of the diffusion-controlled rate in
cyclohexane. Since the triplet state with its energy of >8000
cm-1 is commonly deactivated by O2 through an energy transfer
3
1
mechanism, i.e., T1 + O2 f S0 + O2, the observed transient
absorption with nearly diffusion-controlled O2 quenching rate
can be reasonably ascribed to the triplet-triplet transition.32 On
the basis of the electronic configuration and the group-theoretical
argument in combination with numerous experimental results,
El-Sayed34 concluded that the rate of ISC should qualitatively
be 3 orders of magnitude larger between states with different
configuration (such as S1 (nπ*) f T(ππ*) or S1 (ππ*) f
T(nπ*)) than the same configuration (such as S1 (nπ*) f T(nπ*)
or S1(ππ*) f T(ππ*)). Probable estimates are 109-108 s-1 and
106-105 s-1 for the former and latter cases, respectively. If the
rate of ISC is the dominant deactivation process in the Snπ* state,
the result of a , 30 ns rise time for the triplet-state population
suggests a fast Snπ* f Tππ* ISC.
k1kpt + kAk-1 + kAkpt
krxn
)
(4)
k-1 + kpt
In the case of 7AI kA is much smaller than k1, k-1 and kpt,
Consequently, eq 4 is dominated by the rate of proton-transfer
process, which can be further discussed based on two extreme
cases. When the intrinsic proton-transfer rate kpt is much faster
than k-1 (case 1), the overall proton-transfer rate can be
expressed as
4.2. Photophysics in Hydrogen-Bonded Complexes. The
1n f π* transition normally corresponds to a hypsochromic
shift when the corresponding nonbonding valence electrons are
directly involved in the HB formation. Such an effect can be
rationalized by the stabilization of lone pair electrons upon
forming a hydrogen bond. However, for 3FAI/guest HB
complexes the dual hydrogen bonding formation mainly incor-
porates pyrrolic N-H (proton donor) and pyridinal nitrogen
(proton acceptor), while the carbonyl oxygen does not play a
role in the HB complex formation. Accordingly, it is reasonable
to assume that the Snπ* configuration remains in the lowest
excited singlet state even upon forming 3FAI/guest HB com-
plexes. On the basis of this proposed mechanism we first focus
on the dynamics of ESDPT of 3FAI in alcohols. Subsequently,
the discussion regarding the relaxation dynamics of 3FAI dimer
and 3FAI/guest HB complexes is presented.
3FAI in Alcohols. The dynamics of excited-state proton
transfer of 7AI in alcohols have been extensively studied. Using
a picosecond transient Kerr-gating technique McMorrow and
Aartsma11 have concluded that the proton-transfer reaction time,
depending on various alcohols, is in the range of few hundred
picoseconds. Such a time scale is exceedingly slower than the
subpicosecond proton-transfer time of the 7AI dimer possessing
intact cyclic dual hydrogen bonds.9,10 The slow reaction rate
krxn ) k1
(case 1)
(5)
In this case the overall rate of proton-transfer reaction is mainly
determined by the slow solvent reorganization dynamics.
Conversely, for case 2 it is assumed that the proton-transfer
rate kpt is relatively slower than k-1. Thus, krxn can be expressed
as
k1
krxn
)
kpt
(case 2)
(6)
k-1
In this case, the solvation thermodynamics (i.e., Keq ) k1/k-1
)
coupled with an intrinsic proton transfer rate describe the overall
reaction dynamics. Both cases explain the slow solvent-
dependent proton-transfer dynamics in various alcohols equally
well. However, more detailed isotope, solvent polarity experi-
ments revealed certain discrepancies between two proposed
mechanisms.14,15 More recently, Mentus and Maroncelli14c
applied a computer simulation on the fractional population of
A* and B* states as well as the corresponding molecular
dynamics of solvation. The results lead to a conclusion toward
the solvation equilibrium controlled mechanism. They further

Double Proton Transfer in 3-Formyl-7-azaindole
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8869
estimated the intrinsic proton transfer kpt rate to be ca. 0.3 ps-1
in methanol, which is on the same order of the magnitude as
that obtained experimentally from the 7AI dimer possessing
intact dual hydrogen bonds.9,10 Nevertheless, both mechanisms
derived above are in agreement regarding a slower specific
solvent reorganization rate which is normally on the order of
several hundred picoseconds and is even slower (∼1 ns) in water
and polyalcohols.
an unusually large kA value can be reasonably attributed to a
fast rate of Sππ* f Snπ* IC.
The above kinetic derivation is base on a stance that the
proton-transfer reaction is prohibited in the Snπ* state for the
case of 3FAI HB complexes. Although this happens to be the
first case in 7AI analogues, the proposed unfavorable proton
transfers in the Snπ* state is not uncommon. Cases of the nπ*
and ππ* states mixing based on the theoretical approach have
been reported in several excited-state intramolecular proton
transfer (ESIPT) molecules. For example, the OCCCN ring in
1-amino-3-propenal has been treated as a model for the 2-(2′-
hydroxyphenyl) benzothiazole (HBT) and 2-(2′-hydroxyphenyl)-
benzoxazole (HBO) which undergo excited-state intramolecular
proton transfer (ESIPT) reaction.35 Ab initio calculations give
an nπ* and an ππ* excited states that are nearly isoenergetic
for geometry identical to that of the enol ground state.
Optimization of geometry in the excited-state results in an enol
minimum of the nπ* state with a high barrier for proton transfer.
As for another example, ab initio calculations of o-hydroxy-
benzaldehyde36 and o-hydroxyacetophenone37 have predicted no
barrier of ESIPT in the first excited Sππ* state. Whereas the
ESIPT in the Snπ* state possesses a substantial barrier. For the
Because of the extremely weak tautomer emission for 3FAI
in various alcohols, the possibility of ground-state thermally
assisted formation of the double-hydrogen-bonded alcohol
complex of 3FAI (such as in structure B of Scheme 1) which,
upon direct excitation, gives rise to the tautomer emission has
to be considered. However, such a possibility may be discounted
by the deuterated 3FAI (3FAI(D)) experiment. In the deuterium
isotope experiment we found that the tautomer emission intensity
of 3FAI(D) titrated by CH3OD in cyclohexane is ∼40% of the
nondeuterated 3FAI. If fast ESDPT takes place through the
direct excitation of structure B, Φpt is expected to be ∼1 for
both 3FAI and 3FAI(D). Assuming similar tautomer emission
efficiency (Φc) for 3FAI(D) and 3FAI in cyclohexane (vide
infra), the observed tautomer emission yield Φobs ()ΦptΦc) is
expected to be independent of deuterium substitution. This
proposed mechanism contradicts the observed deuterium isotope
effect on the tautomer emission intensity. Alternatively, the
extremely weak tautomer emission for 3FAI in various alcohols
can be rationalized based on the same framework of the above-
proposed mechanism (see Scheme 1 and eq 4). However, the
rate of nonproton-transfer decay channel (i.e., kA ) in state A*
is no longer negligibly slow due to a fast Sππ* f Snπ* IC. Thus,
the mechanism of ESDPT of 3FAI in alcohols is mainly based
on a slower solvent reorganization rate in competition with a
much faster Sππ* f Snπ* IC. The fraction of proton-transfer
process versus the overall relaxation dynamics, Φpt, is deduced
to be either Φpt ) k1/(k1 + kA) ∼ k1/kA or Φpt ) [(k1/k-1)kpt]/
[(k1/k-1)kpt + kA] ∼ [(k1/k-1)kpt]/kA, depending on which model,
i.e., solvation dynamics or thermodynamics, used to elaborate
the ESDPT dynamics in alcohols. For the convenience of the
following discussion we simply use a notation kpt′ to represent
the rate of overall proton-transfer reaction. As a result, Φpt can
be generally expressed as kpt′/kA in the case of 3FAI in alcohols.
The experimentally observed fluorescence yield of the tautomer
emission (i.e., C*) can be expressed as Φobs ) ΦptΦc where
Φc is the fluorescence quantum yield of the tautomer emission
under the condition of Φpt ) 1. Although Φc was experimentally
unobtainable, a reasonable approach can be made by assuming
that both 3FAI tautomer and 3FMPP have the same radiative
decay rate. The emission yield and decay rate of 3FMPP were
measured to be 0.07 and 8.8 × 107 s-1, respectively in methanol
(see Table 1), hence a radiative decay rate of 6.2 × 106 s-1
was deduced. The tautomer decay rate of 3FAI in methanol
was measured to be 1.2 × 108 s-1 (see Table 1) we then
calculate Φc to be 0.052 for 3FAI(tautomer) in methanol. Giving
Φobs ) ΦptΦc where Φobs and Φc were 9.5 × 10-5 and 0.052,
respectively, a value of Φpt of ∼1.83 × 10-3 was then deduced.
In another approach, both 7AI and 3FAI possess a similar
molecular structure on the proton donating and accepting sites
where the dynamics of ESDPT are rate limited by the solvation
thermodynamics (or dynamics) in alcohols. Thus, it is reasonable
to take the solvent incorporated proton-transfer rate of 8 × 109
s-1 (∼125 ps14) for 7AI in methanol to be the kpt′ value for
3FAI in methanol. Since Φpt can be expressed as kpt′/kA, a kA
value, i.e., the decay rate attributed to the nonproton-transfer
processes, is deduced to be 4.37 × 1012 s-1 (∼230 fs-1). Such
1
case of 3FAI, the nπ* configuration is expected to increase
the electron donating ability of the carbonyl oxygen, hence
increases the electron density in the pyrrolic system. The
consequence may lead to a decrease of the N(1)-H acidity,
resulting in the frustration for ESDPT. Further theoretical
approaches will be of interest to resolve this issue. The decay
dynamics in the Snπ* state are thus dominated by intersystem
crossing and internal conversion, rationalizing the lack of normal
fluorescence for 3FAI in alcohols.
3FAI/Guest Dual HB Complexes. In contrast to the extremely
weak or none luminescence behavior of 3FAI in alcohols, the
observation of a unique, relatively much stronger tautomer
emission among 3FAI dimer, 3FAI/acid and 3FAI/lactam
complexes is intriguing. Because of the nearly unaffected
carbonyl oxygen in the dual hydrogen bonding formation it is
reasonable to conclude that Snπ* is also in the lowest excited
singlet state for three studied 3FAI HB complexes. As concluded
in the absorption and emission titration experiments these three
3FAI HB complexes should possess intact cyclic dual hydrogen
bonds. This viewpoint can be supported by a theoretical
approach (6-31G(d,p)) in which the calculation based on a dual
HB geometry estimates a large formation enthalpy of -10.79,
-12.86 and -11.74 kcal/mol for 3FAI dimer, 3FAI/acetic acid
and 3FAI/lactam, respectively. Note that for the 7AI dimer
possessing intact cyclic dual hydrogen bonds ESDPT takes place
with a negligible geometry adjustment, resulting in an ultrafast
intrinsic proton-transfer reaction time of e1 ps.9,10 Accordingly,
a large-amplitude reorientation of the guest molecule is not
necessary upon executing the ESDPT reaction, and hence
Scheme 1 can be simplified to Scheme 2 for the three studied
HB complexes (only the 3FAI dimer is shown in Scheme 2).
In Scheme 2 whether kpt incorporates concerted or sequential
process is not known at this stage and is irrelevant to the
following discussion. Upon an allowed π-π* excitation to the
1
1
Sππ* state (the La or Lb band, or a state mixing between La
and Lb) competitive relaxation pathways take place between a
fast Sππ* f Snπ* IC and an intrinsic rate of proton-transfer
reaction, kpt. The fraction for each individual deactivation
process can be qualitatively estimated as follows. The observed
yields of tautomer emission, Φobs, for 3FAI dimer, 3FAI/acetic
acid and 3FAI/lactam complexes were measured to 0.021, 0.042
and 0.0028, respectively. Similar to that derived in methanol it

8870 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Chou et al.
SCHEME 2
For all studied 3FAI HB species appreciable population in
the normal triplet state is expected. Accordingly, a key question
regarding the double proton transfer in the triplet state, i.e., T
f T′ (prime denotes the tautomer species) is intriguing. We
have attempted to perform the triplet-triplet absorption mea-
surement in the concentrated 3FAI. Unfortunately, since the
spectral profiles remain unchanged during the time-dependent
spectral evolution this issue cannot be resolved at this stage.
On one hand, the result may indicate the prohibition of the T
f T′ proton transfer reaction. On the other hand, it might
indicate that ESDPT in the triplet state takes place at a time
scale much shorter than the system response time of 30 ns so
that the observed transient spectra mainly correspond to the T1′
f Tn′ transition. Further studies focusing on the dynamics of
proton transfer reaction in the triplet state for 3FAI HB
complexes are currently in progress.
is also reasonable to assume a similar radiative decay rate
between 3FMPP and 3FAI(tautomer) HB complexes in cyclo-
hexane. Giving the emission yield and decay rate of 3FMPP to
be 0.085 and 8.1 × 107 s-1 (see Table 1), a radiative decay
rate kr of 6.9 × 106 s-1 was then deduced in cyclohexane. The
lifetime of the tautomer emission for each 3FAI/guest complex
has been measured and shown in Table 1. Consequently, Φc
was calculated to be 0.053, 0.077, and 0.054 for 3FAI dimer,
3FAI/acetic acid and 3FAI/lactam, respectively. The relatively
smaller Φc value with respect to that of 3FMPP can be
rationalized by the HB effect in 3FAI(tautomer)/guest com-
plexes, which may induce additional radiationless decay chan-
nels. On the basis of the relation of Φpt ) Φobs /Φc, Φpt was
then calculated to be 0.40 and 0.55 for 3FAI dimer and 3FAI/
acetic acid complex, respectively. Φpt can be further expressed
as Φpt ) kpt/(kpt + kA) according to Scheme 2. Since the rate of
Snπ* f Sππ* IC is commonly independent of solvents it is
reasonable to adopt same kA (IC) rate of 4.37 × 1012 s-1
estimated in methanol for three studied 3FAI HB complexes.
Accordingly, an intrinsic proton-transfer rate kpt of 2.91 × 1012
s-1 (343 fs-1) and 5.34 × 1012 s-1 (187 fs-1) was deduced for
3FAI dimer and 3FAI/acetic acid complex, respectively. These
values are qualitatively in agreement with the rate of proton
transfer of g1.0 × 1012 s-1 reported in the case of 7AI dimer.9,10
Assuming similar rate of IC and Φc upon the N-H deuterium
substitution, the deuterium isotope-dependent tautomer emission
intensity can thus be rationalized by the slower deuterium
transfer rate relative that of proton. As a result, kpt for deuterium
transfer is less competitive with respect to the rate of IC. For
example, giving Φobs of 0.03 (vide supra) for the 3FAI(D)/acetic
Conclusion
In summary the 1nπ* configuration is concluded to be in the
lowest excited singlet state for both normal monomer and HB
complexes in 3FAI. An Sππ* f Snπ* IC rate has been estimated
to be ∼230 fs-1 in methanol. ESDPT of 3FAI in alcohols,
similar to that of the 7AI/alcohols complex, requires a geo-
metrical adjustment of the guest molecule on a time scale of
few hundred picoseconds and thus can hardly compete with the
Sππ* f Snπ* IC. On the other hand, the intact dual HB formation
in the cases of 3FAI dimer, 3FAI/acetic acid and 3FAI/lactam
complexes leads to a fast ESDPT which is comparable with
the rate of Sππ* f Snπ* IC. Accordingly, the steady-state
tautomer emission intensity reveals a remarkable deuterium
isotope effect. The introduction of a carbonyl functional group
at the C3 position thus presents the first case among 7AI
analogues to demonstrate the competitive IC/ESDPT reaction
in which the fast Sππ* f Snπ* IC serves as an internal clock to
examine the dynamics of ESDPT.
Acknowledgment. This work was supported by the National
Science Council, Taiwan, R.O.C. (NSC87-2119-M-194-002).
References and Notes
(1) Taylor, C. A.; El-Bayoumi, A. M.; Kasha, M. Proc. Natl. Acad.
Sci. U.S.A. 1969, 65, 253
(2) Ingham, K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1971, 93,
5023.
(3) Ingham. K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1974, 96,
1674.
(4) Watson, J. D.; Crick, F. H. C. Nature (London) 1953, 171, 964.
(5) Watson, D. G.; Sweet, R. M.; Marsh, R. E. Acta Crystallogr. 1965,
19, 573.
(6) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature 1995, 378.
(7) Folmer, D. E.; Poth, L.; Wisniewski, E. S.; Castleman, A. W., Jr.
Chem. Phys. Lett. 1998, 287, 1.
(8) Chachisvillis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. J. Phys.
Chem. A. 1998, 102, 669.
(9) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1998, 102, 7740.
(10) Fiebig, T.; Chachisvillis, M.; Manger, M.; Zewail, A. H.; Douhal,
A.; Garcia Ochoa, I.; de La Hoz Ayuso, A.J. Phys. Chem. A 1999, 103,
7419.
(11) McMorrow, D.; Aartsma, T. Chem. Phys. Lett. 1986, 125, 581.
(12) Moog, R. S.; Bovino, S. C.; Simon, J. D. J. Phys. Chem. 1988, 92,
6545.
acid(D) complex, kD was then estimated to be ∼360 fs-1
.
pt
Similar method gives a kD value of 675 fs-1 for the 3FAI(D)
pt
dimer. Note that in this study negligible deuterium isotope
effect was observed in the steady-state tautomer emission
intensity for the 7AI dimer where kpt is apparently .kA, al-
though significant deuterium transfer dynamics have been
reported.10
Interestingly, Φobs for the 3FAI/lactam complex is only ca.
6.7% of that measured in 3FAI/acetic acid (see Table 1). If the
assumption of a similar Φc holds among three studied 3FAI
HB complexes, a smaller kpt value of 2.4 × 1011 s-1 (4.2 ps-1
)
was then estimated for the 3FAI/lactam HB complex. The
ESDPT reaction in 3FAI/lactam can be ascribed as a noncata-
lytic type16c,d where both host and guest should undergo
tautomerism simultaneously during ESDPT. The more ender-
gonic reaction may empirically lead to a higher formation free
energy of the activated complex, i.e., an existence of higher
energy barrier. Further investigation focusing on the ultrafast
time-resolved ESDPT dynamics is necessary to resolve this
issue.
(13) Koijnenberg, J.; Huizer, A. H.; Varma, C. A. O. J. Chem. Soc.,
Faraday Trans. 2 1988, 84 (8), 1163.
(14) (a) Moog., R. S.; Maroncelli, M. J. Phys. Chem. 1991, 95, 10359.
(b) Chapman, C. F.; Maroncelli, M. J. Phys. Chem. 1992, 96, 8430. (c)
Mentus, S.; Maroncelli, M. J. Phys. Chem. A 1998, 102, 3860.
(15) (a) Negreie, M.; Bellefeuille, S. M.; Whitham, S.; Petrich, J. W.;
Thornburg, R. W. J. Am. Chem. Soc. 1990, 112, 7419. (b) Negrerie, M.;
Gai, F.; Bellefeuille, S. M.; Petrich, J. W. J. Phys. Chem. 1991, 95, 8663.
(c) Negrerie, M.; Gai, F.; Lambry, J.-C.; Martin, J.-L.; Petrich, J. W. J.
Phys. Chem. 1993, 97, 5046. (d) Chen, Y.; Rich, R. L.; Gai, F.; Petrich, J.

Double Proton Transfer in 3-Formyl-7-azaindole
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8871
W. J. Phys. Chem. 1993, 97, 1770. (e) Chen, Y.; Gai, F.; Petrich, J. W. J.
Am. Chem. Soc. 1993, 115, 10158. (f) Rich, R. L.; Chen, Y.; Neven, D.;
Negrerie, M.; Gai, F.; Petrich, J. W. J. Phys. Chem. 1993, 97, 1781. (g)
Gai, F.; Rich, R. L.; Petrich, J. W. J. Am. Chem. Soc. 1994, 116, 735. (h)
Smirnov, A. V.; English, D. S.; Rich, R. L.; Lane, J. Teyton, L.;
Schwabacher, A. W.; Luo, S.; Thornburg, R. W.; Petrich, J. W. J. Phys.
Chem. 1997, 101B, 2758 and references therein.
(16) (a) Chou, P. T.; Martinez, M. L.; Cooper, W. C.; McMorrow, D.;
Collin, S. T.; Kasha; M. J. Phys. Chem. 1992, 96, 5203. (b) Chang, C. P.;
Hwang, W. C.; Kuo M. S.; Chou, P. T.; Clement, J. H. J. Phys. Chem.
1994, 98, 8801. (c) Chou, P. T.; Wei, C. Y.; Chang, C. P.; Chiu, C. H. J.
Am. Chem. Soc. 1995, 117, 7259. (d) Chou, P. T.; Wei, C. Y.; Chang, C.
P.; Kuo, M. S. J. Phys. Chem. 1995, 99, 11994. (e) Chou, P. T.; Yu, W. S.;
Chen, Y. C.; Wei, C. Y.; Martinez, S. S. J. Am. Chem. Soc. 1998, 120,
12927.
(23) Chou, P. T.; Wei, C. Y.; Hung, F. T. J. Phys. Chem. B 1997, 101,
9119.
(24) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. J.
Phys. Chem. 1992, 96, 135.
(25) Petersson, G. A.; Malick, D. K.; Wilson, W. G.; Ochterski, J. W.;
Montgomery, J. A, Jr.; Frisch, M. J. J. Chem. Phys. 1998, 109, 10570.
(26) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454.
(27) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
Phys. 1998, 108, 4439.
(28) Platt, J. R. J. Chem. Phys. 1949, 17, 484.
(29) Valeur, B.; Weber, G. Photochem. Photobiol. 1977, 25, 441.
(30) Ilich, P. J. Mol. Struct. 1995, 354, 37.
(31) Note that the La band has been reported to be in the lowest excited
singlet state upon forming HB complexes such as the 7AI dimer.9
(32) Appreciable amount of the singlet oxygen (1∆g f 3Σ-g(0,0)) 1273
nm emission sensitized by 3FAI was detected. A detailed description of
the near-IR detecting system has been given in ref 33.
(33) Chou; P. T.; Chen, Y. C.; Wei, C. Y.; Chen, S. J.; Lu, H. L. J.
Phys. Chem. A 1997, 101, 8581.
(17) Chaban, G. M.; Gordan, M. S. J. Phys. Chem. A 1999, 103, 185.
(18) Chou, P. T.; Wei, C. Y.; Wu, G. R.; Chen, W. S. J. Am. Chem.
Soc. 1999, 121, 12186.
(19) Chou, P. T.; Wu, G. R.; Wei, C. Y.; Cheng, C. C.; Chang, C. P.;
Hung, F. T. J. Phys. Chem. B 1999, 103, 10042.
(34) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834.
(20) Robsion, M. M.; Robison, B. L. J. Am. Chem. Soc. 1955, 77, 457.
(21) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
(22) Olness, D.; Sponer, H. J. Chem. Phys. 1963, 38, 1799.
(35) Sobolewski, A. L.; Domcke, W. Chem. Phys. Lett. 1993, 211, 82.
(36) Sobolewski, A. L.; Domcke, W. Chem. Phys. 1994, 184, 115.
(37) Vener, M. V.; Scheiner, S. J. Phys. Chem. 1995, 99, 642.