Time-dependent density functional theory study on the hydrogen ...

Computational and Theoretical Chemistry 972 (2011) 57–62
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Computational and Theoretical Chemistry
journal homepage: www.elsevier.com/locate/comptc
<strong>Time</strong>-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong> <strong>theory</strong> <strong>study</strong> on the hydrogen bonding in
electronic excited states of 6-amino-3-((thiophen-2-yl) methylene)-phthalide
in methanol solution
Hong Dong, Ce Hao ⇑ , Jingwen Chen, Jieshan Qiu
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China
article
info
abstract
Article history:
Received 27 March 2011
Received in revised form 17 June 2011
Accepted 17 June 2011
Available online 25 June 2011
Keywords:
Hydrogen bonding
Electronic excited state
<strong>Time</strong>-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong> <strong>theory</strong>
Hydrogen bond strengthening and cleavage
In this work, we have applied the time-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong> <strong>theory</strong> (TDDFT) method to investigate
the excited-state hydrogen bonding dynamics of 6-amino-3-((thiophen-2-yl) methylene)-phthalide
(6-ATMPH) in methanol (MeOH) solution. In the hydrogen-bonded complex, the intermolecular hydrogen
bond (C@OHAO) can be formed between the 6-ATMPH and the MeOH molecules. The 6-ATMPH monomer
and hydrogen-bonded dimer can be photoexcited initially to the S 1 state. We calculated the geometric
structures and energies of the hydrogen-bonded complex and the isolated 6-ATMPH in different
electronic states at the level of B3LYP with the TZVP basis sets. We found that the bond lengths of the
C@O and OAH groups increased after formation of the intermolecular hydrogen bond in the ground state.
Furthermore, the calculated hydrogen bond binding energy increased to 31.5 kJ/mol in the electronically
excited state from 23.6 kJ/mol in the ground state. These results clearly indicate the intermolecular
hydrogen bond of the hydrogen-bonded complex is strengthened in the electronically excited state.
The hydrogen bond of the hydrogen-bonded 6-ATMPH–MeOH complex strengthening in the electronically
excited state was also confirmed by theoretically monitoring the spectra shift of the stretching
vibrational modes of the C@O and OAH groups. Our theoretical <strong>study</strong> results have clarified the dispute
regarding the intermolecular hydrogen bond cleavage or strengthening in the electronically excited state.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Molecular photochemistry is of has a fundamental importance
in modern chemistry. It is affected significantly by the intermolecular
interactions between solute and solvent in solution [1–4]. Asa
site-specific local interaction between hydrogen donor and acceptor
molecules, intermolecular hydrogen bonding is an important
type of solute–solvent interaction. Intermolecular hydrogen bonding
also plays a fundamental role in the chemical structure and
reactivity of water, proteins, and DNA, the building blocks of life
[5–10]. Photoexcitation can induce charge redistribution of the
hydrogen bond in the electronically excited state, a process defined
as hydrogen-bonding dynamics [6,11–25]. Until now, many experimental
and theoretical methods have been applied to investigate
intermolecular hydrogen bonding [26–34]. However, relatively
few studies have been carried out on excited-state hydrogenbonding
dynamics. In very recent years, Zhao et al. have made a
significant contribution to the systematic <strong>study</strong> of the excited-state
⇑ Corresponding author. Tel./fax: +86 411 84706323.
E-mail address: haoce_dlut@126.com (C. Hao).
hydrogen bonding dynamics with a variety of experimental and
theoretical studies [35–41].
To obtain a better understanding of hydrogen bonding dynamics
in the electronically excited state, diverse experimental and
theoretical methods have been applied [11,12,41–44]. Femtosecond
time-resolved vibrational spectroscopy has shown potential
to provide good insight into the microscopic dynamics and information
on local structures [11,12]. The time-<strong>dependent</strong> <strong>density</strong>
<strong>functional</strong> <strong>theory</strong> (TDDFT) method has also been used to <strong>study</strong>
the hydrogen-bonding dynamics in the electronically excited state
[24,25,39–44]. Nibbering et al. have used site-specific vibrational
spectroscopy in the femtosecond domain to investigate the ultrafast
response of Coumarin 102 (C102) in the hydrogen-donating
solvent CHCl 3 or phenol [11]. They found that the vibrational
absorption of the C@O stretching showed an ultrafast blue-shift
within a 200 fs time scale in the electronically excited state [11].
Thus, they proposed that the intermolecular hydrogen bond
C@OHAO formed between C102 and phenol can be cleaved upon
electronic excitation [11]. Palit and co-workers then followed this
mechanism of hydrogen bond cleavage upon photoexcitation of
chromophores in hydrogen-donating solvents [22]. But it has been
demonstrated that the hydrogen-bonding dynamics and electronic
state transition take place in the same time scale [11]. Thus they
2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.comptc.2011.06.013

58 H. Dong et al. / Computational and Theoretical Chemistry 972 (2011) 57–62
should be coupled to one other. Furthermore, Zhao et al. have demonstrated
that the blue-shift of the spectra should be attributed to
the transition from the ground state to the electronically excited
state [41]. For the first time, Zhao et al. also demonstrated that
the intermolecular hydrogen bond between C102 and phenol can
be strengthened significantly in early photoexcitation to the electronically
excited state [41]. These two conclusions about the
hydrogen-bonding dynamics in the electronically excited state of
the C102 in phenol solvent are completely different. We therefore
need to further investigate other systems to obtain information as
to the hydrogen bond in the electronically excited states.
6-amino-3-((thiophen-2-yl) methylene)-phthalide (6-ATMPH),
as one of the derivatives of 3-phenylmethylene-1(3H)-isobenzofuranone
(benzylidene-phthalide, BPH), is an organic luminophore
with high photostability and fluorescence quantum yield in solution
[45–50]. Nikolov et al. have investigated the properties of
6-ATMPH in a series of nonpolar to polar solvents [46]. They confirmed
that the intermolecular hydrogen bonds between 6-ATMPH
and solvent molecules can be formed only in protic solvents [46].
Moreover, they found that the shapes and position of the absorption
spectra of the 6-ATMPH in protic alcohol solvents were changed
significantly [46]. They attributed these changes to the
formation of the intermolecular hydrogen bond (C@OHAO) between
the carbonyl group of the 6-ATMPH molecule and the hydroxyl
group of the protic solvents [46]. However, we do not
have sufficient information to describe the intermolecular hydrogen
bond of the hydrogen-bonded 6-ATMPH–MeOH complex in
the electronically excited state. Few theoretical works have been
conducted to investigate the intermolecular hydrogen bonding of
the hydrogen-bonded 6-ATMPH–MeOH complex upon its photoexcitation
to the excited state.
In the present work, we have employed the TDDFT method to
calculate the excited-state geometric structures, energies and
vibrational spectrum of the isolated 6-ATMPH and the hydrogenbonded
complex. The TDDFT method has been confirmed as a
reliable method to investigate hydrogen-binding dynamics by
monitoring the infrared spectra of the groups involved in the formation
of intermolecular hydrogen bonds in the electronically excited
state [24,25,39,41]. Only the solvent molecules in the first
solvation shell can contribute to the hydrogen-bonding dynamics
[35,41,51–56]. So we focused our attention on investigating the
solvent molecule directly hydrogen-bonded with the 6-ATMPH
molecule without consideration of the bulk effort of the outer
solvation shells. The excited-state hydrogen-bonding dynamics,
which occurs at ultrafast timescales, can be studied by monitoring
the vibrational motions of the hydrogen donor and acceptor groups
[39–41,57–64]. We have demonstrated theoretically that the intermolecular
hydrogen bond C@OHAO of the hydrogen-bonded 6-
ATMPH–MeOH complex can be strengthened in the electronically
excited-state. The calculated hydrogen bond binding energies,
the spectral shift of the stretching mode of the C@O and OAH
groups and the hydrogen bond lengths of different electronic states
confirmed the mechanism of the hydrogen bond strengthening in
the electronically excited state.
2. Computational methods
In this work, the isolated 6-ATMPH monomer and the hydrogen-bonded
6-ATMPH–MeOH solute–solvent complex were investigated
using the <strong>density</strong> <strong>functional</strong> <strong>theory</strong> (DFT) with Becke’s
three-parameter hybrid exchange function with Lee–Yang–Parr
gradient-corrected correlation <strong>functional</strong> (B3-LYP) in the ground
state [65]. For the excited state, however calculations were performed
using the time-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong> <strong>theory</strong>
(TDDFT) method with B3-LYP hybrid <strong>functional</strong> [65–67]. The
triple-f valence quality with one set of polarization function (TZVP)
was chosen for basis sets throughout [66]. It is difficult to calculate
the electronic structures of the electronic excited state for complex
molecular systems. As we know, several quantum chemistry computation
methods can be used to optimize the ground-state electronic
structures for hydrogen-bonded systems. The configuration
interaction with single substitute (CIS) and the complete active
space self-consistent field (CASSCF) methods are very popular
computational methods for the excited-state electronic structures
[68–71]. It is noted that the CIS method is less expensive but gives
less accurate results [68,69]. On the contrary, the CASSCF method
can give accurate results but is quite expensive for large molecular
systems [70,71]. The time-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong> <strong>theory</strong>
(TDDFT) method becomes a good candidate for computing the
electronic excited states of complex molecular systems because
of its moderate efficiency and accuracy [35–41]. The intermolecular
hydrogen bonds in the electronic excited state of other systems
in the hydrogen-bonding environment have been investigated theoretically
by Zhao et al. using the TDDFT method [51–60]. Fine
quadrature grids 4 were employed. Harmonic vibrational frequencies
in the ground and excited state were determined by diagonalization
of the Hessian [72]. The excited-state Hessian was obtained
by the numerical differentiation of analytical gradients using central
differences and default displacements of 0.02 Bohr [73]. The
infrared intensities were determined from the gradients of the dipole
moment [72,73]. The artificial curves of the infrared spectra
were determined by fitting a Lorenz curve with half-peak width
of 15 nm. All the electronic structure calculations were carried
out using the TURBOMOLE program suite [65–67,72,73].
3. Results and discussion
The optimization geometric structures of the isolated 6-amino-
3-((thiophen-2-yl) methylene)-phthalide (6-ATMPH) molecule and
the hydrogen-bonded 6-ATMPH–MeOH complex in the ground
state are shown in Fig. 1. A theoretical discussion of the intermolecular
hydrogen bond formed between the 6-ATMPH and the
methanol molecule is firstly given here. One can find that the intermolecular
hydrogen bond may be formed between the nitrogen of
the amino group or the oxygen of the carbonyl group in the molecule
of 6-ATMPH and the methanol solvent, respectively [46].
Nikolov et al. have attributed the shapes and position of the
absorption spectral changes of the 6-ATMPH in protic alcohol solvents
to the formation of the intermolecular hydrogen bond
(C@OHAO) [46]. In terms of our experimental approach, we only
<strong>study</strong> hydrogen bonds of the type C@OHAO here. In our hydrogen-bonded
complex model, only the solvent molecules in the inner
solvation shell are involved. We focused our attention on
investigating the solvent molecule directly hydrogen-bonded with
the 6-ATMPH molecule without consideration of the bulk effort of
the outer solvation shells. It has been demonstrated that the electronic
excited-state hydrogen-bonding dynamics for chromophores
in solution can be studied reliably using this type of
hydrogen-bonded complex model [51–56]. So the hydrogenbonded
complex proposed here is a good model for <strong>study</strong>ing the
hydrogen-bonding dynamics in solution.
From Fig. 1 we can see the 6-ATMPH molecule has a planar conformation.
For the hydrogen-bonded complex, the hydrogen bond
C@OHAO remains in the plane of the 6-ATMPH molecule. At
the same time, the methyl group of the methanol molecule resides
throughout the plane. These results are in accordance with other
hydrogen-bond complexes studied by Zhao et al [35–41]. The calculated
length of the hydrogen bond C@OHAO between the H
and the O atoms is 2.020 Å; the calculated hydrogen bond binding
energy is 23.6 kJ/mol. We find that the lengths of the C@O and

H. Dong et al. / Computational and Theoretical Chemistry 972 (2011) 57–62 59
Fig. 1. The optimized geometric structures of the hydrogen-bonded complex as well as the isolated 6-ATMPH in the ground state. Dotted lines denote the intermolecular
hydrogen bonds.
HAO groups are increased because of the formation of the hydrogen
bond C@OHAO. The length of the C@O bond increases from
1.197 Å to 1.207 Å and the length of the OAH bond increases from
0.963 Å to 0.970 Å. Therefore, we can predict that the C@O and
OAH groups will be continue to be lengthened into a stronger
hydrogen bond. The angles formed by C@OH and OHAO are
108° and 171°, respectively. The dihedral angle formed between
the plane of the 6-ATMPH molecule and CAO group of the methanol
is 170°, while the OAH group remains on the plane of the
6-ATMPH molecule. The NAH bond resides out of the plane of
the 6-ATMPH molecule after the intermolecular hydrogen bond
formed and the dihedral angle changes to 141°. When the methanol
molecule forms the hydrogen-bonded complex, the piramidalization
of the amino group may result form the formation of a
hydrogen bond. This is the only distinctly geometrical structural
change between the monomer and the hydrogen-bonded complex.
All the results indicate that the geometric conformation of the 6-
ATMPH molecule does not change remarkably after the hydrogen
bond formed in the ground state.
The electronic excitation energies and corresponding oscillation
strengths of the low-lying electronically excited states for the
hydrogen-bonded complex and the isolated 6-ATMPH monomer
are calculated using the TDDFT method. According to the Franck–
Condon principle, the electronic excitation energies and fluorescence
energies given in Tables 1 and 2 are vertical energies
(without optimizing the geometric conformation of the excited
state). It should be noted that both the isolated 6-ATMPH and
the hydrogen-bonded complex have the largest oscillator strength
in the S 1 state. Thus, the 6-ATMPH monomer and the hydrogenbonded
dimer can be photoexcited initially to the S 1 state. According
to Table 1, the S 1 state mainly corresponds to the orbital
Table 1
The calculated electronic excitation energies (nm) and corresponding oscillator
strengths (in the parenthesis) of the hydrogen-bonded complex and the isolated 6-
ATMPH molecule in the low-lying electronically excited states. The contributions of
orbital transition for the S 1 state are also shown.
6-ATMPH
6-ATMPHAMeOH
S 1 389 (0.506) 401 (0.453)
H ? L 90.4% H ? L 91.6%
S 2 336 (0.416) 338 (0.418)
S 3 307 (0.074) 314 (0.022)
S 4 285 (0.017) 312 (0.068)
S 5 274 (0.001) 299 (0.015)
H: The highest occupied molecular orbital (HOMO).
L: The lowest unoccupied molecular orbital (LUMO).
Table 2
The calculated electronic excitation energies (nm) and the fluorescence energies of
the hydrogen-bonded complex and the isolated 6-ATMPH molecule in the S 1 state, the
experimental results are shown in the parenthesis.
6-ATMPH
6-ATMPHAMeOH
Abs. 389 (385) 401 (387)
Flu. 468 (482) 471 (483)
transition from the highest occupied molecular orbital (HOMO)
to the lowest unoccupied molecular orbital (LUMO). We also find
that the excitation energy and the oscillation strength of the isolated
6-ATMPH are very close to that of the hydrogen-bonded complex
in the S 1 state.
We list the calculated electronic excitation energies and fluorescence
energies of the hydrogen-bonded complex and the isolated
6-ATMPH molecule in the S 1 state in Table 2 with experimental results
in parentheses. The calculated excitation energy of the
6-ATMPH monomer is 389 nm in the ground state. For the hydrogen-bonded
dimer, the calculated result is 401 nm. These results
are in good agreement with the experimental data of 385 nm and
387 nm [46]. In the S 1 state, the calculated fluorescence energies
are 468 nm and 471 nm for the 6-ATMPH monomer and hydrogen-bonded
dimer, respectively. These results also coincide with
the experimentally observed results 482 nm and 483 nm, respectively
[46]. After the intermolecular hydrogen bond formed, the
excitation energy of the hydrogen-bonded dimer is decreased correspondingly
compared with that of the 6-ATMPH monomer in the
ground state. But the fluorescence energy of the 6-ATMPH monomer
is nearly the same as that of the hydrogen-bonded dimer. This
may be because the hydrogen-bonding dynamics and the electronic
state transition take place at the same time scale [11]. They
couple to one other and weaken the effect to the hydrogen-bonded
dimer in the excited state.
Molecule orbitals (MOs) analysis can provide insight directly
into the nature of the excited state [22–24,51–61]. We display
the frontier molecular orbitals of the hydrogen-bonded complex
in Fig. 2. According to our calculated results in Table 1, the S 1 state
of the hydrogen-bonded dimer corresponds to the orbital transition
from HOMO to LUMO. So only the HOMO and LUMO orbitals
of the hydrogen-bonded complex are shown here. The p character
for the HOMO and the p ⁄ character for the LUMO can be seen
clearly from Fig. 2. So the S 1 state of the hydrogen-bonded complex
has the p–p ⁄ feature. As the methanol molecule lacks p orbitals,
both the electronic densities of the HOMO and LUMO are localized
on the 6-ATMPH moiety. Thus, the S 1 state of the hydrogen-bonded

60 H. Dong et al. / Computational and Theoretical Chemistry 972 (2011) 57–62
Fig. 2. Frontier molecular orbital (MOs) of the hydrogen-bonded complex.
dimer is a locally excited (LE) state on the 6-ATMPH moiety. This
may be another reason for the fluorescence energy of the 6-ATMPH
monomer being nearly the same as that of the hydrogen-bonded
dimer. In the S 1 state, it is apparent that the transition from HOMO
to LUMO involves an intramolecular charge redistribution from the
N atom to the carbonyl group. The electronic <strong>density</strong> of the C@O
group increases after the transition. These results have been proposed
by Zhao and coworkers [35–41]. The charge of electronic
<strong>density</strong> in the C@O group can influence the C@O and HAO groups
directly, that is to say the intermolecular hydrogen will be changed
in the S 1 state.
From what has been discussed above, we know that the hydrogen-bonded
complex is directly photoexcited to the S 1 state. Geometric
optimization of the hydrogen-bonded dimer in the S 1 state
has been carried out using the TDDFT method. After that, we calculated
the excited-state infrared spectra of the hydrogen-bonded
complex at the same level. The calculated IR spectra in the electronically
excited state are difficult and very time-consuming
[37–41]. Because the changes in site-specific hydrogen bonding
interaction can induce spectral shifts of some characterized vibrational
modes involved in the formation of the hydrogen bond [35–
41], we give the stretching vibrational frequencies of C@O and
OAH groups in different electronic states in Fig. 3. It is clear that
the stretching mode of the C@O group of the 6-ATMPH monomer
is red-shifted 288 cm 1 from 1841 cm 1 in the ground state to
1553 cm 1 in the S 1 state. We also find that the C@O stretching
vibrational frequency of the hydrogen-bonded dimer is down
shifted 40 cm 1 compared with that of the isolated 6-ATMPH in
the ground state. Therefore both the electronic excitation and
intermolecular hydrogen bond formation can lead to the stretching
vibrational frequency of the C@O group red-shift. Compared with
the C@O stretching vibrational frequency of isolated 6-ATMPH in
the S 1 state, we find that the C@O stretching vibrational frequency
of the hydrogen-bonded 6-ATMPH complex in the S 1 state is nearly
unchanged. The stretching mode of the C@O group is therefore not
good enough to monitor the hydrogen-bonding dynamics of the
hydrogen-bonded 6-ATMPH–MeOH complex.
The stretching mode of the OAH group has a red-shift of
125 cm 1 after the formation of the intermolecular hydrogen bond
C@OHAO in the ground state. However, an additional shift of
36 cm 1 is found for the hydrogen-bonded complex in the S 1 state.
This result provides collateral evidence for the presumption that
the intermolecular hydrogen bond of the hydrogen-bonded dimer
is strengthened in the S 1 state. As we have confirmed, the methanol
moiety remains in its electronic ground state while the hydrogenbonded
complex is in the S 1 state. That may be the reason why the
change of the stretching vibrational frequency of the OAH group
reflects the hydrogen-bonding dynamics of the hydrogen-bonded
6-ATMPH–MeOH complex and why the C@O group vibrational frequency
is nearly unchanged in the excited state.
The excited-state geometric optimization for the hydrogenbonded
dimer has been performed using the TDDFT method. We
list the calculated hydrogen bond binding energies and
corresponding group bond lengths, as well as the bond length of
the hydrogen bond in different electronic states in Table 3. For
the intermolecular hydrogen bond C@OHAO, it is clear that
the hydrogen bond length of OH decreases from 2.020 Å in the
ground state to 1.995 Å in the excited state. On the other hand,
we can find that the lengths of the C@O and OAH groups are
slightly increased to 1.198 Å and 0.970 Å after the intermolecular
hydrogen bond formation in the ground state. For the excited state,
the bond lengths of the hydrogen-bonded C@O and OAH groups
are increased continually to 1.212 and 0.972 Å. All these bond
length changes of the hydrogen-bonded complex indicate that
the intermolecular hydrogen bond C@OHAO is strengthened in
the excited state. We have indicated that the S 1 state of the hydrogen-bonded
dimer is in the locally excited (LE) state on the
6-ATMPH moiety, so the hydrogen bond binding energy can be easily
calculated by subtracting the energy of the isolated 6-ATMPH in
its S 1 state and the energy of the methanol in its ground state from
Fig. 3. The calculated vibrational absorption spectra of the isolated 6-ATMPH and the hydrogen-bonded complex in different electronic states. The HAO stretching vibrational
band of the free methanol in the ground state is also shown.

H. Dong et al. / Computational and Theoretical Chemistry 972 (2011) 57–62 61
Table 3
The calculated corresponding hydrogen bonding energies E HB (in kJ/mol) and lengths
(in Å) of the hydrogen bonds and the hydrogen-bonding groups in different electronic
states.
the energy of the hydrogen-bonded complex in the S 1 state. The
basis set superposition error (BSSE) of the calculated binding energy
at this level is less than 1–2 kJ/mol and is much smaller than
the hydrogen bond binding energies. The calculated hydrogen
bond binding energy for the intermolecular hydrogen bond is increased
to 31.5 kJ/mol in the excited from 23.6 kJ/mol in the
ground state. Thus, we can confirm that the intermolecular hydrogen
bond C@OHAO of the hydrogen-bonded 6-ATMPH–MeOH
complex is strengthened in the electronically excited state. At the
same time, the mechanism of hydrogen bond strengthening in
the electronically excited state first proposed by Zhao et al. in a
previous <strong>study</strong> is supported by our theoretical calculation results.
4. Conclusion
MeOH 6-ATMPH 6-ATMPHAMeOH
L OAH L C@O E HB L OAH L OH L C@O
S 0 0.963 1.197 23.6 0.970 2.020 1.207
S 1 1.198 31.5 0.972 1.995 1.212
In this work, the electronic excited-state hydrogen-bonding
dynamics of the 6-ATMPH chromophore in methanol solvent was
studied theoretically using the time-<strong>dependent</strong> <strong>density</strong> <strong>functional</strong>
<strong>theory</strong> (TDDFT) method at the level of B3LYP with the TZVP basis
sets. The geometric conformations of the isolated 6-ATMPH
molecule as well as the hydrogen-bond complex in the ground
state and the S 1 state are discussed here. A type of hydrogen bond
C@OHAO can be formed between the 6-ATMPH and the methanol
molecule in the ground state. The frontier molecular orbitals
analysis demonstrates that the S 1 state of the hydrogen-bonded
complex is an LE state localized on the 6-ATMPH moiety. Our calculated
results indicate that the bond lengths of C@O and OAH,
which are involved in the hydrogen bonding, are increased owing
to the formation of the intermolecular hydrogen C@OHAO in
the ground state. Moreover, the lengths of the bond C@O and
OAH groups have a larger enlargement in the excited state compared
with that in the ground state. Correspondingly, the length
of the intermolecular hydrogen bond is decreased in the excited
state. In addition, the calculated hydrogen bond binding energy
is increased to 31.5 kJ/mol in the excited state from 23.6 kJ/mol.
These results have demonstrated clearly that the intermolecular
hydrogen bond C@OHAO between 6-ATMPH and methanol is
strengthened upon photoexcitation. Using the TDDFT method, we
also calculated the vibrational absorption spectra of the hydrogen-bonded
groups in the ground and the excited state. By monitoring
the infrared spectral shift of OAH, we can confirm that the
intermolecular hydrogen bond of the hydrogen-bonded 6-
ATMPH–MeOH complex is strengthened in the excited state. The
hydrogen bond cleavage mechanism proposed in some other studies
regarding the hydrogen-bonding dynamics is excluded from
our theoretically calculated results. Our results are in accordance
with the mechanism of the intermolecular hydrogen bond
strengthening in the electronically excited state that was first demonstrated
by Zhao and coworkers.
Acknowledgement
This work was supported by the National Natural Science Foundation
of China (Grant No. 21036006).
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