Time-dependent density functional theory study on the electronic excited-state hydrogen-bonding dynamics of 4-aminophthalimide (4AP) in aqueous solution: 4AP and 4AP–(H2O)1,2 clusters

Time-Dependent Density Functional Theory Study on the

Electronic Excited-State Hydrogen-Bonding Dynamics of

4-Aminophthalimide (4AP) in Aqueous Solution: 4AP and

4AP–(H2O)1,2 Clusters

RUI WANG,1CE HAO,

1PENG LI,

2NING-NING WEI,

1JINGWEN CHEN,

1JIESHAN QIU

1

1State Key Laboratory of Fine Chemicals, School of Environmental and Biological Science andTechnology, Dalian University of Technology, Dalian 116024, China

2Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

Received 21 July 2009; Revised 12 December 2009; Accepted 21 December 2009DOI 10.1002/jcc.21504

Published online 10 March 2010 in Wiley InterScience (www.interscience.wiley.com).

Abstract: The time-dependent density functional theory (TDDFT) method has been carried out to investigate the

excited-state hydrogen-bonding dynamics of 4-aminophthalimide (4AP) in hydrogen-donating water solvent. The

infrared spectra of the hydrogen-bonded solute2solvent complexes in electronically excited state have been calcu-

lated using the TDDFT method. We have demonstrated that the intermolecular hydrogen bond C¼¼ O��H��O and

N��H��O��H in the hydrogen-bonded 4AP2(H2O)2 trimer are significantly strengthened in the electronically

excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen-bonding

groups in different electronic states. The hydrogen bonds strengthening in the electronically excited state are con-

firmed because the calculated stretching vibrational modes of the hydrogen bonding C¼¼O, amino N��H, and H��O

groups are markedly red-shifted upon photoexcitation. The calculated results are consistent with the mechanism of

the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen

bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electroniclly

excited state of chromophores in hydrogen-donating solvents exists in many other systems in solution.

q 2010 Wiley Periodicals, Inc. J Comput Chem 31: 2157–2163, 2010

Key words: excited state; hydrogen-bonding dynamics; TDDFT; electronic spectra; infrared spectra

Introduction

Molecular photochemistry in solution is greatly affected by

intermolecular interactions between the solute and the solvent

molecules.1–3 Intermolecular hydrogen bonding, which has been

investigated extensively by a variety of experimental and theo-

retical methods, is a significant type of solute2solvent interac-

tion.4,5 When a solute with a polarizable functional group is dis-

solved into a protic solvent, the solute and the solvent molecules

form an intermolecular hydrogen bond.6 Upon photoexcitation,

as the result of difference in charge distribution of the different

electronic states, the hydrogen bond formed by solute and sol-

vent molecules in excited states undergo reorganization and

greatly changing.7 Therefore, intermolecular hydrogen bonding

has a considerable effect on behavior of solvent photochemistry

in protic solvents.

Hydrogen-bonding dynamics plays an important role on the

electronically excited-state dynamics of the hydrogen-bonded

complexes.8–25 However, knowledge about the process of

excited-states hydrogen-bonding dynamics is rather limited

because of the extremely short time scale involved. Ultrafast

spectroscopy was adopted by most researchers to explore the

knowledge of hydrogen-bonding dynamics. Whereas because of

the limited spectral resolution for the femtosecond laser pulses,

it is inadequate to use ultrafast spectroscopy solely.26–35 Fortu-

nately, theoretical calculations can provide us a clear picture of

the early time hydrogen bond response to electronic excitations.

Using the time-dependent density functional theory (TDDFT)

Contract/grant sponsor: The National Natural Science Foundation of

China; contract/grant number: 20773018

Contract/grant sponsor: The Key Laboratory of Industrial Ecology and

Environmental Engineering, China Ministry of Education

Correspondence to: P. Li; e-mail: [email protected] or C. Hao; haoce@

dlut.edu.cn

q 2010 Wiley Periodicals, Inc.

method, Zhao et al. studied the ground and excited states vibra-

tional spectra of coumarin 102 (C102) in hydrogen-donating sol-

vents in their benchmark study.5 By theoretically monitoring the

spectral shift of some characteristic hydrogen-bonding vibra-

tional modes involved in the formation of hydrogen bonds, they

demonstrated for the first time that the intermolecular hydrogen

bond was significantly strengthened in the electronically excited

state, which has been a milestone on the study of the excited-

state hydrogen bonding.5 However, hydrogen bond cleavage

mechanism in excited state was proposed by some other people

for a long time.27–30 The stretching vibrational frequency of the

C¼¼O band for the C102 in CHCl3 or phenol solvents was blue-

shifted within a 200-fs time scale in their study. They thought

that the spectral blueshift should be attributed to the ultrafast

cleavage of hydrogen bond C¼¼O��H in the excited state. How-

ever, it was demonstrated for the first time by Zhao et al. that

the transient spectral blueshift was due to the electronic state

hopping from the ground to the S1 state.5 However, the debate

between the hydrogen bond strengthening and cleavage in the

electronically excited state is mostly focused on the coumarin

102. Will the hydrogen bond in other related systems strengthen-

ing or cleavage? To clarify this problem, more studies on other

hydrogen-bonded complexes in excited states are needed.

4-aminophthalimide, which is often used in study of nanoma-

terials, microorganizations, and biological systems, is known as

an excellent probe for their fluorescence lifetimes, spectra and

quantum yields are affected greatly by the environment proper-

ties.31–47 It has been reported that the bathochromic shift of fluo-

rescence spectrum in protic solvent is much greater than in the

aprotic ones, and the greatest bathochromic shift of 4AP fluores-

cence spectrum is observed in water solvent. This indicates that

there is a strong interaction between the 4AP and water mole-

cule. Topp and coworkers studied the properties of 4AP and the

hydrogen-bonded 4AP2(H2O)1,2 complexes under jet-cooled

conditions and showed the results of infrared double-resonance

experiments on these complexes. The structures of the hydro-

gen-bonded 4AP2(H2O)1,2 clusters were given.48 However, the

detail of the hydrogen bond between 4AP and water molecules

was still not clear. Thus, we are motivated to theoretically study

the excited-state hydrogen-bonding dynamics of the hydrogen-

bonded complexes between 4AP and H2O molecules in electron-

ically excited states. In this article, we have investigated the

hydrogen-bonded 4AP2(H2O)1,2 complexes using the TDDFT

method and focused our attention on the transient changes of

intermolecular hydrogen bonds in the early time of electronic

excitation. The TDDFT method have been demonstrated to be a

reliable tool for the calculation of the infrared spectra in the

electronically excited state.49–53 Therefore, the IR spectrum of

the hydrogen-bonded 4AP2(H2O)1,2 complexes in different elec-

tronic states are also calculated by the TDDFT method. In this

work, we have theoretically demonstrated that the bond lengths

of the intermolecular hydrogen bond C¼¼O��H��O and

N��H��O��H in the hydrogen-bonded 4AP2(H2O)2 trimer in

electronically excited state are shortened. Meanwhile, the calcu-

lated vibrational absorption spectra of isolated 4AP and the

hydrogen-bonded 4AP2(H2O)1,2 complexes show that the

stretching vibrational modes of hydrogen binding involved in

the formation of hydrogen bonds are significantly red-shifted

upon electronic excitation to the excited state. The results indi-

cate that the intermolecular hydrogen bond C¼¼O��H��O and

N��H��O��H in the hydrogen-bonded 4AP2(H2O)2 trimer are

strengthened in the electronically excited state. The calculated

result is consistent with the mechanism of the hydrogen bond

strengthening in the electronically excited state proposed by

Zhao et al. in their previous article.

Theoretical Methods

The ground state geometric optimization was performed using

the density functional theory (DFT) method with Becke’s three-

parameter hybrid exchange function with Lee-Yang-Parr gradi-

ent-corrected correlation functional (B3LYP) functional.54 The

excited state electronic structures were calculated using TDDFT

method with B3LYP functional. Geometric optimizations

in ground state and excited state were also calculated by

the second-order coupled-cluster singles-and-doubles (CC2)

method.55,56 The resolution of the identity (RI) was used to

make the calculations feasible. The excited state infrared spectra

were calculated by use of the optimized excited state structures.

The triple-f valence quality with one set of polarization func-

tions (TZVP) was chosen as basis sets. At the same time, four

fine quadrature grids were used.57 Harmonic vibrational frequen-

cies in the ground state and the excited state were determined

by diagonalization of the Hessian.58 The excited-state Hessian

was obtained by numerical differentiation of analytical gradients

using central differences and default displacements of 0.02 bohr.

The infrared intensities were determined from the gradients of

the dipole moment.59 All the electronic structure calculations

were carried out using the TURBOMOLE program suite.

Results and Discussion

Ground-State Geometric Conformations

To depict the intermolecular hydrogen-bonding interactions

between the 4AP and H2O molecules, the 4AP2(H2O)2 trimer

and two conformers, which are denoted as 4AP2H2Oa and

4AP2H2Ob, respectively, of 4AP2H2O dimer are discussed

here. The structures of the hydrogen-bonded 4AP2(H2O)1,2complexes are obtained with DFT method by using the struc-

tures showed by Topp et al. as initial geometry. The optimized

ground-state conformations of the hydrogen-bonded 4AP–

(H2O)1,2 complexes and the 4AP monomer are shown in Figure

1. The optimized conformation of the isolated 4AP shows that

only the two hydrogen atoms on the amino are not on the plane

of the 4AP molecule. The dihedral angle between the plane of

4AP molecule and N4��H group is 208. In both the conforma-

tions of the hydrogen-bonded 4AP2H2Oa and 4AP2H2Ob com-

plexes, the H��O group of the hydrogen bond C¼¼O��H��O

remains in the plane of the 4AP molecule and the other O��H

group of the water molecule resides out of plane. However, the

two water molecules in the hydrogen-bonded 4AP2(H2O)2trimer reside out of the plane. The bond lengths of free C¼¼O

and imino N��H groups in 4AP monomer are calculated to be

1.208 and 1.009 A, respectively, while the bond lengths in the

2158 Wang et al. • Vol. 31, No. 11 • Journal of Computational Chemistry

Journal of Computational Chemistry DOI 10.1002/jcc

hydrogen bond C1¼¼O��H��O and N2��H��O��H are lengthened

to 1.219 and 1.018 A for the C1¼¼O and N4��H groups in the

hydrogen-bonded 4AP2H2Oa complex, respectively. Thus, the

bond lengths of the hydrogen-bonded groups are slightly length-

ened due to the formation of intermolecular hydrogen bond. The

lengths of the hydrogen bond C1¼¼O��H��O between O and H

atoms and the hydrogen bond N2��H��O��H between H and O

atoms are calculated to be 1.970 and 2.056 A in the hydrogen-

bonded 4AP2H2Oa complex. For the hydrogen-bonded

4AP2H2Ob complex, the lengths of the hydrogen bond

C¼¼O��H��O and N��H��O��H are almost the same as the

hydrogen-bonded 4AP2H2Oa complex. The length of the hydro-

gen bond C3¼¼O��H��O between O and H atoms is calculated

to be 1.895 A in the hydrogen-bonded 4AP2(H2O)2 complex,

which is shorter than C¼¼O��H��O in 4AP2H2O dimer. The

calculated hydrogen bond length of N4��H��O��H between H

and O is 2.087 A. Herein, the lengths for hydrogen bond

C¼¼O��H��O and N4��H��O��H suggest that the intermolecular

hydrogen bond N4��H��O��H is weaker than the hydrogen

bond C¼¼O��H��O.

Calculated Electronic Spectra

To understand the nature of the electronically excited states for

the hydrogen-bonded 4AP2(H2O)1,2 complexes, electronic exci-

tation energies and corresponding oscillator strengths of the low-

lying excited states are calculated using the TDDFT method. The

S1 absorption peak is calculated to be at 344 nm for the isolated

4AP. For the hydrogen-bonded 4AP2H2Oa and 4AP2H2Ob, the

S1 absorption peaks are 343 and 345 nm, respectively. One can

find that there are no significant difference in electronic excita-

tion energies between the 4AP monomer and the hydrogen-

bonded 4AP2H2O dimer. This means that one water molecule

almost has no effect on the electronic excitation energies. While

for the hydrogen-bonded 4AP2(H2O)2 trimer, S1 absorption peak

is large red-shifted to 374 nm due to the solute2solvent intermo-

lecular hydrogen-bonding interactions. The experimental results

are also correspondingly listed in Table 1.48 It can be found that

the TDDFT calculated results are in good agreement with their

experimental values. Moreover, we also calculated the fluores-

cence emission energies for the isolated 4AP and its hydrogen-

bonded complexes. The fluorescence maximum is located at 402

nm for the isolated 4AP while it is at 453 nm for the hydrogen-

bonded 4AP2(H2O)2 trimer. The large redshift is due to the for-

mation of hydrogen bond C3¼¼O��H��O and N4��H��O��H.

Frontier Molecular Orbitals

The frontier molecular orbitals (MOs) of the hydrogen-bonded

4AP2(H2O)2 complex are shown in Figure 2. Herein, we only

Figure 1. Geometric structures of 4AP monomer and hydrogen-bonded 4AP2(H2O)1,2 complexes.

2159Excited-State Hydrogen Bonding Dynamics of 4-Aminophthalimide (4AP) in Aqueous Solution


show the HOMO and LUMO orbitals, since the S1 state of the

hydrogen-bonded complexes corresponds to the orbital transition

from HOMO to LUMO. One can find that the HOMO and

LUMO orbitals are the p and p* character, respectively. There-

fore, it is evident that the S1 state is the pp* feature. In addition,

the electron densities of both the HOMO and LUMO are strictly

localized on the 4AP moiety. This indicates that only the 4AP

moiety has been electronically excited in the S1 state, while the

water molecules moiety should be located in its electronic

ground state. Thus, the S1 state of the hydrogen-bonded

4AP2(H2O)2 complex should be assigned as locally excited

(LE) state on 4AP molecule. Further observation indicates the

transition from HOMO to LUMO involves the intramolecular

charge redistribution from the aromatic moiety to the carbonyl

groups. So the electron density of the C¼¼O group increases af-

ter the transition from HOMO to LUMO. That means the elec-

tron density in the carbonyl group can directly influence the

intermolecular hydrogen bond C¼¼O��H��O.

Calculated Vibrational Absorption Spectra

To depict the transient change of the intermolecular hydrogen

bond in the early time of electronic excitation, the infrared spec-

tra of the ground-state and S1 state for the 4AP monomer and

the hydrogen-bonded 4AP2(H2O)1,2 complexes are calculated

using the DFT and TDDFT method. The calculated IR spectra

of isolated 4AP in different electronic state are shown in Fig-

ure 3. The calculated vibrational absorption spectra are in good

agreement with the experimental results (3453, 3494, and 3548

cm21) recorded in the ground-state infrared spectra of jet-cooled

for 4AP in the 3100–3700 cm21 region.48 Moreover, the two

C¼¼O groups stretching band of 4AP monomer are markedly

red-shifted by 83 and 50 cm21 from 1792 cm21 in ground state

to 1709 and 1742 cm21 in S1 state. The stretching vibrational

frequency of the imide NH group is slightly red-shifted from

3630 to 3607 cm21 due to the electronic excitation. However,

the stretching mode of NH2 group is nearly unchanged upon

electronic excitation. The calculated vibrational absorption spec-

tra of the hydrogen-bonded 4AP2H2Oa and 4AP2H2Ob com-

plexes in different electronic states are shown in Figure 4.

Herein, the stretching modes of imide NH group in both the

hydrogen-bonded 4AP2H2Oa and 4AP2H2Ob are slightly

shifted from ground state to excited state. So, we suppose that

the intermolecular hydrogen bond N2��H��O��H is not strength-

ened in the excited state.

Figure 5 shows the calculated IR spectra of ground state and

excited state for the hydrogen-bonded 4AP2(H2O)2 trimer. One

can find that the stretching vibrational frequency of the donor

C¼¼O group is slightly red-shifted by 31 cm21 from 1792 to

1761 cm21 due to formation of the intermolecular hydrogen

bond C¼¼O��H��O. However, as discussed earlier, the stretching

mode of C¼¼O group is significantly red-shifted by 83 cm21 due

to the electronic excitation, that is, the C¼¼O stretching mode is

not a sensitive vibrational mode to monitor the hydrogen-bond-

ing dynamics, which is similar to the case of hydrogen-bonded

Fluorenone2MeOH complex.6 The calculated IR spectrum of

hydrogen-bonded 4AP2(H2O)2 trimer in the spectral region of

the O��H stretching band are also shown in Figure 5. The

ground-state infrared spectra of jet-cooled for the hydrogen-

bonded 4AP2(H2O)2 complex shows six characteristic peaks

(3392, 3450, 3494, 3501, 3542, and 3724 cm21) in the 3100–

3900 cm21 region.48 Our calculated result is in accordance with

the experimental results. The wave number 3532 cm21 is the do-

Table 1. Calculated Electronic Excitation Energies (nm) and

Corresponding Oscillator Strengths of Isolated 4AP as well as

Hydrogen-Bonded 4AP2(H2O)1,2 Complexes

4AP 4AP2(H2O)a 4AP2(H2O)b 4AP2(H2O)2

S1 Abs. 344(0.070) 343(0.074) 345(0.060) 374(0.080)

Exp. 345 351 349 378

Flu. 402 408 416 453

S2 315(0.000) 315(0.000) 315(0.000) 309(0.000)

S3 279(0.022) 280(0.012) 280(0.008) 282(0.007)

S4 271(0.000) 271(0.000) 271(0.000) 276(0.000)

Figure 2. Frontier molecular orbitals (MOs) of the hydrogen-

bonded 4AP2(H2O)2 complex.

Figure 3. Calculated vibrational absorption spectra of the 4AP

monomer in different electronic states. Ground-state assignments:

1792 (C¼¼O stretch); 3584 (NH2 symm. stretch); 3630 (imide NH

stretch); and 3691 (NH2 antisymm. stretch).



nor O��H stretching band of the hydrogen bond C¼¼O��H��O.

Upon electronic excitation, the donor O��H stretching band is

drastically red-shifted by 195–3337 cm21. From the earlier dis-

cussion, the S1 state of the hydrogen-bonded 4AP2(H2O)2 com-

plex is LE states. The water molecules remain in its electronic

ground state when the hydrogen-bonded complexes excited to

the S1 state. Therefore, the O��H stretching band can be slightly

influenced by the electronic excitation. So the drastically redshift

of the O��H stretching band should be attributed to the hydro-

gen bond C¼¼O��H��O strengthening in the excited state. At the

same time, the stretching mode of the free O��H group is

unchanged upon electronic excitation to the S1 state of the

hydrogen-bonded 4AP2(H2O)2 complex. This suggests that the

electronic excitation to the S1 state of the hydrogen-bonded

4AP2(H2O)2 complex has no influence on the water molecules,

which is consistent with the LE nature of the S1 state for the

hydrogen-bonded 4AP2(H2O)2 complex. In addition, the

stretching mode of the amino donor NH group in ground state is

red-shifted by 113 cm21 due to the formation of intermolecular

hydrogen bond N4��H��O��H and a larger redshift of 191 cm21

was found when the hydrogen-bonded 4AP2(H2O)2 complex

excited to the S1 state. The additional redshift of 78 cm21

should also be attributed to the hydrogen bond N4��H��O��H

strengthening in the excited state of the hydrogen-bonded

4AP2(H2O)2 complex.

Excited-State Hydrogen Bond Strengthening

The calculated hydrogen bond lengths and the bond lengths

of hydrogen-bonded groups for the hydrogen-bonded

4AP2(H2O)a,b complexes in the ground and excited states are

listed in Table 2. It can be seen that the length of the hydrogen

bond N2��H��O��H between H and O atom is almost uncharged

from ground state to excited state. As discussed earlier, the

stretching mode of imide NH group in both the hydrogen-

bonded 4AP2H2Oa and 4AP2H2Ob complexes are slightly

shifted from ground state to excited state. So the hydrogen bond

N2��H��O��H in the hydrogen-bonded 4AP2H2Oa and

4AP2H2Ob complexes is not strengthening in the excited state.

However, the hydrogen bond length of C¼¼O��H��O between

oxygen and hydrogen atom is significantly shortened in the S1state. Furthermore, both the C¼¼O and O��H groups are slightly

increased in the excited state. All the calculated results show

Figure 4. Calculated vibrational absorption spectra of the hydrogen-

bonded 4AP2H2Oa (a) and 4AP2H2Ob (b) in different electronic

states. Ground-state assignments: (a): 1767, 1816 (C¼¼O stretch);

3483 (imide NH donor); 3586 (NH2 symm. stretch); 3623 (OH do-

nor); and 3693 (NH2 antisymm. stretch); 3870 (free OH), (b): 1769,

1815 (C¼¼O stretch); 3488 (imide NH donor); 3584 (NH2 symm.

stretch); 3633 (OH donor); and 3871 (free OH). [Color figure can be

viewed in the online issue, which is available at www.interscience.-

wiley.com.]

Figure 5. Calculated vibrational absorption spectra of the hydrogen-

bonded 4AP2(H2O)2 complex in different electronic states. Ground-

state assignments: 1761 (donor C¼¼O); 1813 (free C¼¼O); 3471

(amino donor NH); 3532 and 3574 (OH donor in chain); 3629 (im-

ide free NH); 3677 (amino free NH); and 3860 (free OH). [Color

figure can be viewed in the online issue, which is available at

www.interscience.wiley.com.]



that the intermolecular hydrogen bond C¼¼O��H��O in the

hydrogen-bonded 4AP2H2Oa and 4AP2H2Ob complexes are

significantly strengthened in the excited state.

In Table 3, the calculated hydrogen bond lengths as well as

the bond lengths of hydrogen-bonded groups for the hydrogen-

bonded 4AP2(H2O)2 trimer in the ground and excited states are

shown. It can be seen that the length of the hydrogen bond

C3¼¼O��H��O between O and H atoms is significant shortened

from 1.895 to 1.779 A due to the electronic excitation to the S1state of the hydrogen-bonded 4AP2(H2O)2 complex. At the

same time, the bond lengths of both the C¼¼O and O��H groups

are slightly increased during the hydrogen-bonded complexes

are excited to the S1 state. Moreover, the relatively weak inter-

molecular hydrogen bond N4��H��O��H is also strengthened

because the hydrogen bond length of N4��H��O��H between H

and O atoms is decreases from 2.087 A in the ground state to

1.958 A in the excited state. Meanwhile, the bond lengths of

both the N4��H and O��H groups are also slightly increased in

the S1 state. From our calculated results, both the intermolecular

hydrogen bond C¼¼O��H��O and N4��H��O��H are strength-

ened in the electronically excited state. To clarify the reliability

of the TDDFT calculated results, geometric optimization was

also carried out by the second-order coupled-cluster singles-and-

doubles (CC2) method. The RI was used to make the calcula-

tions feasible. The hydrogen bond lengths and the bond lengths

of hydrogen-bonded groups for the hydrogen-bonded

4AP2(H2O)1,2 complexes in the ground and excited states cal-

culated by RI–CC2 method are also shown in Tables 2 and 3.

We have found that the RI–CC2 calculated lengths of hydrogen

bonds are somewhat longer than TDDFT calculated. As Kohn

reported that CC2 tends to give too long bond distances, espe-

cially if multiple bonds are involved.60 However, the CC2

results also reconfirm that hydrogen bond length is shortened in

excited state and hence the hydrogen bond becomes strengthened

in the excited state by comparison with the ground state hydro-

gen bond.

Conclusions

We have reported here the results of the theoretical study on the

excited-state hydrogen-bonding dynamics of 4AP chromophore

in hydrogen-donating water solvent using the TDDFT method.

The geometric structures and energies of the hydrogen-bonded

4AP2(H2O)1,2 complexes as well as the isolated 4AP in the

ground state and the S1 state have been calculated. Meanwhile,

the IR spectra of the 4AP monomer and 4AP2(H2O)1,2 com-

plexes in different electronic states have been calculated. Three

types of intermolecular hydrogen bond between 4AP and water

molecules have been studied. The intermolecular hydrogen

bonds N2��H��O��H in the hydrogen-bonded 4AP2H2Oa and

4AP2H2Ob are almost not changed in excited state. However,

the intermolecular hydrogen bond C¼¼O��H��O and

N4��H��O��H in the hydrogen-bonded 4AP2(H2O)2 complex

are significantly strengthened in the electronically excited state

upon photoexcitation. From our calculated results, we have also

been demonstrated that the S1 state of the hydrogen-bonded

4AP2(H2O)1,2 complexes are LE states on 4AP moiety. The

water molecules are located in its electronic ground state. Our

calculated results are in accordance with the hydrogen bond

strengthening mechanism in the electronically excited state,

while in contrast with the hydrogen bond cleavage mechanism.

This work shows that the hydrogen bond strengthening behavior

in electronically excited states may widely exist in many other

systems.

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Table 2. Calculated Lengths of Hydrogen Bonds (in A) and Hydrogen Bonding Groups for Isolated 4AP and

Hydrogen-Bonded 4AP2(H2O)a,b Dimers in Different Electronic States

4AP 4AP2H2Oa 4AP2H2Ob

LC1¼¼O LN2��H LC3¼¼O LC1¼¼O LO��H LH��O LN2��H LH��O LC3¼¼O LO��H LH��O LN2��H LH��O

S0 1.208 1.009 1.208 1.219 1.970 0.974 1.018 2.056 1.218 1.975 0.974 1.018 2.056

(1.222) (1.012) (1.223) (1.232) (1.983) (0.973) (1.020) (2.004) (1.232) (1.991) (0.973) (1.020) (2.011)

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(1.244) (1.015) (1.247) (1.256) (1.856) (0.980) (1.024) (2.018) (1.261) (1.833) (0.981) (1.024) (2.007)

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Table 3. Calculated Lengths of Hydrogen bonds (in A) and Hydrogen

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Different Electronic States

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LC3¼¼O LO��H LH��O LN4��H LH��O LO��H

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(1.233) (1.903) (0.977) (1.018) (2.043) (0.978)

S1 1.256 1.779 0.992 1.026 1.958 0.983

(1.268) (1.743) (0.993) (1.034) (1.826) (0.986)

The corresponding RI-CC2 calculated results are shown in parentheses.



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Documents

Time-dependent density functional theory study on the electronic excited-state hydrogen-bonding dynamics of 4-aminophthalimide (4AP) in aqueous solution: 4AP and 4AP–(H2O)1,2 clusters