Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON THE ELECTRONIC EXCITED-STATE HYDROGEN BONDING DYNAMICS OF METHYL ACETATE IN AQUEOUS SOLUTION

The time-dependent density functional theory (TDDFT) method has been carried out to investigate the hydrogen-bonding dynamics of methyl acetate ( CH 3 CO 2 CH 3) in hydrogen-donating water solvent. The ground-state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low-lying electronically-excited states for the isolated CH 3 CO 2 CH 3 and H2O monomers, the hydrogen-bonded CH3CO2CH3-(H2O)1, 2 complexes have been calculated using DFT and TDDFT methods respectively. One intermolecular hydrogen bond C=O⋯H–O is formed between CH3CO2CH3 and one water molecule in CH3CO2CH3-H2O dimer. Meanwhile, in CH3CO2CH3-(H2O)2 trimer, two intermolecular hydrogen bonds C=O⋯H–O are formed between CH3CO2CH3 and two water molecules. By theoretically monitoring the excitation energy changes among the CH3CO2CH3 monomer, the CH3CO2CH3-H2O dimer, and the CH3CO2CH3-(H2O)2 trimer, we have demonstrated interestingly that in some electronically-excited states, the intermolecular hydrogen bonds are strengthened inducing electronic spectral redshifts, while in others weakened with electronic spectral blueshifts. The phenomenon that hydrogen bonds are strengthened in some electronic states while weakened in others can arouse further probe into CH3CO2CH3-(H2O)1, 2 complexes.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON DYNAMICS OF HYDROGEN BONDING IN EXCITED STATES OF TRANS-ACETANILIDE IN METHANOL SOLVENT

The hydrogen-bonding dynamics in both singlet and triplet excited states of the trans-acetanilide ( AA ) in methanol ( MeOH ) solvent was investigated using the time-dependent density functional theory (TDDFT) method. Geometric optimizations of the hydrogen-bonded AA–MeOH complexes considered here as well as the isolated AA and MeOH molecules were performed using density functional theory (DFT) method. At the same time, the TDDFT method was performed to calculate the electronic transition energies and corresponding oscillation strengths of all the compounds in the low-lying electronically excited states. In this study, only the intermolecular hydrogen bonds C=O⋯H–O and N–H⋯O–H can be formed. A theoretical forecast that changes of hydrogen bonds in the low-lying electronic excited states was proposed. We discussed not only ground-state geometric structures and electronic excitation energies but also frontier molecular orbitals and electron density transition. The intermolecular hydrogen bonds between AA and MeOH molecules play an important role in the geometric structures and electronic excitation energies. Zhao et al. have put forward the relationship between the electronic spectra and hydrogen bonding dynamics for the first time. According to Zhao's rule, a redshift of the relevant electronic spectra will appear if hydrogen bond is strengthened, while the hydrogen bond weakening can make an electronic spectra shift to blue.

Hydrogen bonding in the electronic excited state

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.

Theoretical reconsideration on the hydrogen bonding and coordination interactions of chlorophyll a in aqueous solution

In the present work, explicit water molecule and solvent-field effects on the absorption spectrum of chlorophyll a have been studied using time-dependent density functional theory (TDDFT) method. Calculated results show that the one complex and two water coordinated complexes formed by concerted coordination and hydrogen-bonding interactions would be the most preferable conformations of chlorophyll a in aqueous surroundings. Moreover, four obvious absorption bands are assigned by comparing the theoretically simulated absorption spectra with the experimental ones. The theoretical study shows that the explicit water molecule interactions slightly influence the first absorption band. However, the water coordination and hydrogen-bonding interactions can significantly affect the second absorption band which has a strong red-shift. The solvent-field effect due to the polarity of water on absorptions in Q-bands is relatively smaller than that on absorptions in B-bands. As a consequence, our theoretical study on the absorption spectra in the 350–400 nm region presents that the absorption strength in this region was influenced by the explicit coordination and hydrogen bonding interactions from water molecules, significantly.

THEORETICAL STUDY OF THE HYDROGEN BOND CHARACTER BETWEEN THE FNO AND HO2 RADICAL

The hydrogen-bonding characters between FNO and HO2 radical are studied systematically with the MPWB1K method and 6-311++G (d, p), aug-cc-PVDZ as well as aug-cc-PVTZ basis sets. The relevant geometrical characteristics, energy properties, and the characters of the intramolecular hydrogen bonds have been reported in this work. In addition, the changes of electrostatic potential density are presented for further understanding the nature of hydrogen bonds and the preference of F atom as the hydrogen-bonding site.

Experimental and theoretical study of the exited state of aminostyryl terpyridine derivatives: hydrogen-bonding effects

The synthesis, structure and photophysical behavior of the aminostyryl terpyridine derivatives, named 4'-(4-{2-[4-(N,N-dimethylaniline)]vinyl}phenyl)-2,2':6',2″-terpyridine (M(1)) and the related model compounds 4'-(4-{2-[4-(N,N-diphenylammino)phenyl]vinyl}phenyl)-2,2':6',2″-terpyridine (M(2)), respectively, are reported. Large solvatochromic shifts of the first excited-state fluorescence maximum suggest the intramolecular charge transfer characters for both compounds. In addition, with N,N-dimethyl substituents, its fluorescence is quenched a lot in protic solvents. This is consisted with the decay of its S(1) state, through nonradiative internal conversion, to the ground state, which is facilitated by the formation of the hydrogen bond between M(1) and alcohols. Whereas, the introduction of N,N-diphenyl substituents has been proved to be a hydrogen-bond-free case with the unchanged Φ(f). Furthermore, the formation of TICT state via diffusive twisting motion of the dimethyl/phenylamino group is the major relaxation process, which is proved by the ultrafast relaxation dynamics experiment and theoretically conformational optimization of the first excited-state of both the compounds.

Excited-state hydrogen bonding and deprotonation of esculetin in solution: a DFT/TDDFT study

The combined density functional theory (DFT) and time-dependent density functional theory (TDDFT) method was used to study the electronic spectral properties of different deprotonated forms of esculetin. By comparing the experimental absorption and fluorescence bands with the calculated electronic spectra, it is evidently demonstrated that the minor absorption and fluorescence bands observed at slightly longer wavelengths than the principal bands in experiments are predominantly from the de-H3 form of the esculetin monomer. Furthermore, we clarified the relationship between electronic spectral shifts and electronic excited-state intramolecular hydrogen bonding changes: the strengthening of intramolecular hydrogen bond can induce an electronic spectral blueshift while the intramolecular hydrogen bond weakening can result in an electronic spectral redshift.

Excited-state proton transfer via hydrogen-bonded acetic acid (AcOH) wire for 6-hydroxyquinoline

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogen-bonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N- group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N- group in 6HQ upon excitation to the S(1) state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogen-bonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N- group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.

Time-dependent density functional theory study on excited-state dihydrogen bonding O-H···H-Ge of the dihydrogen-bonded phenol-triethylgermanium complex

Intermolecular dihydrogen bond O-H···H-Ge in the electronically excited state of the dihydrogen-bonded phenol-triethylgermanium (TEGH) complex was studied theoretically using time-dependent density functional theory. Analysis of the frontier molecular orbitals revealed a locally excited S(1) state in which only the phenol moiety is electronically excited. In the predicted infrared spectrum of the dihydrogen-bonded phenol-TEGH complex, the O-H stretching vibrational mode shifts to a lower frequency in the S(1) state in comparison with that in ground state. The Ge-H stretching vibrational mode demonstrates a relatively smaller redshift than the O-H stretching vibrational mode. Upon electronic excitation to the S(1) state, the O-H and Ge-H bonds involved in the dihydrogen bond both get lengthened, whereas the C-O bond is shortened. With an increased binding energy, the calculated H···H distance significantly decreases in the S(1) state. Thus, the intermolecular dihydrogen bond O-H···H-Ge of the dihydrogen-bonded phenol-TEGH complex becomes stronger in the electronically excited state than that in the ground state.