Dapeng Yang

A DFT/TDDFT investigation of the excited state proton transfer reaction of fisetin chromophore.

In the present work, 3, 3, 4, 7-tetrahydroxyflavone (fisetin), as one of the most extensive distributed flavonoids, has been investigated on the excited state proton transfer (ESPT) based on the time-dependent density functional theory (TDDFT) method. The calculated absorption and fluorescence spectra based on the TDDFT method are in agreement with the experimental results. Two kinds of structures of fisetin chromophore are found in the first excited (S1) state, which may be due to the proton transfer reactive. Hydrogen bond strengthening has been testified in the S1 state based on comparing staple bond lengths and bond angles involved in hydrogen bonding between the S0 state and the S1 state. In addition, the calculated infrared spectra at the O-H stretching vibrational region and calculated hydrogen bond energy also declare the phenomenon of hydrogen bond strengthening. The frontier molecular orbitals (MOs) analysis and Natural bond orbital (NBO) manifest the intramolecular charge transfer of fisetin chromophore, which reveals the tendency of proton transfer. The potential energy surfaces of the S0 and S1 states are constructed to explain the mechanism of the proton transfer in excited state in detail.

Elaborating the excited-state proton transfer behaviors for novel 3H-MC and P2H-CH

In the present work, we theoretically study and elaborate the excited state proton transfer (ESPT) behavior of two novel polyimides (3H-MC and P2H-CH) reported in the previous work [K. Kanosue, T. Shimosaka, J. Wakita and S. Ando, Macromolecules, 2015, 48, 1777; K. Kanosue, R. Augulis, D. Peckus, R. Karpicz, T. Tamulevicius, S. Tamulevicius, V. Gubinas and S. Ando, Macromolecules, 2016, 49, 1848]. Using density functional theory (DFT) and time-dependent DFT (TDDFT) methods with electrostatic potential surfaces and the reduced density gradient (RDG), we affirm that an intramolecular hydrogen bond should be formed in the ground state for these two systems. Comparing bond lengths, bond angles and the corresponding infrared (IR) vibrations of intramolecular hydrogen bonds for 3H-MC and P2H-CH, we clarify that hydrogen bonds should be strengthened in the S1 state, which provides the possibility for the excited state intramolecular proton transfer (ESIPT) reaction. Analyses of frontier molecular orbitals (MOs) theory prove that the ESIPT process could be facilitated by charge transfer upon excitation. Based on constructing potential energy surfaces (PESs), we provide the excited state dynamical overall perspective about ESIPT reactions for both 3H-MC and P2H-CH. We not only clarify the ESPT mechanism of these two systems, but also make contributions for the applications of such kinds of systems in the future.

A competitive excited state dynamical process for the 2,2′-((1E,1′E)-((3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)-bis(azanylylidene))bis(methanylylidene))-diphenol system

By applying density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we theoretically investigate the excited state dynamical process for the 2,2′-((1E,1′E)-((3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)-bis(azanylylidene))bis(methanylylidene))-diphenol (YT) system. Our results show that two intramolecular hydrogen bonds in YT strengthen in the S1 state, which may trigger an excited state proton transfer (ESPT) process. By exploring the frontier molecular orbitals (MOs), we confirm that charge redistribution indeed has effects on excited state dynamical behavior. Furthermore, this implies the tendency of the ESPT reaction. Analyzing the constructed S1-state potential energy surface (PES), we find three excited state potential barriers which are low enough for a complete excited state double proton transfer (ESDPT) process. Comparing barriers, we confirm a competitive process for stepwise and simultaneous ESDPT pathways.

A theoretical study about the excited state intermolecular proton transfer mechanisms for 2-phenylimidazo[4,5-b]pyridine in methanol solvent

In this study, within the framework of density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we theoretically investigated the novel system 2-phenylimidazo[4,5-b]pyridine (PIP) with respect to the dynamical behavior of its excited state in methanol (MeOH) solvents. Herein, two hydrogen-bonded networks have been discussed between PIP and MeOH, and it has been found that two MeOH connected to PIP (PIP–2MeOH) should be the best arrangement in both S0 and S1 states. Investigations on the electronic spectra of PIP–2MeOH have verified this point. Via analysis of hydrogen bond wires and corresponding infrared (IR) vibrational spectra, we have found that N1–H2⋯O3 of PIP–2MeOH undergoes the biggest change upon photoexcitation that reflects the tendency of the excited state intermolecular proton transfer (ESIPT) process. According to the results of our theoretical potential energy curves along different coordinates, we confirmed that ESIPT reaction should occur along the hydrogen bond wire N1–H2⋯O3 first. After the ESIPT reaction, proton transfer of PIP–2MeOH-PT* could proceed via intersystem crossing (ISC) process from S1 state to T1 state with a negligible energy gap 0.031 eV. Due to this non-radiation process, the fluorescence peak of PIP–2MeOH-PT* could be quenched. Our study not only explains previous successful experiment, but also proposes a new excited state dynamical mechanism for the PIP system.

Theoretical investigation on ESIPT mechanism of a new fluorescent sensor in different solvents

In the present work, a new phenylbenzimidazole derivatized fluorescent sensor (L) (J. Lumin. 147 (2014) 179), has been investigated on the excited state proton transfer (ESPT) based on the time-dependent density functional theory (TDDFT) method. The calculated absorption and fluorescence spectra based on the TDDFT method are in agreement with the experimental results. Two kinds of structures of L chromophore are found in the first excited (S1) state, which may be due to the proton transfer reactive. Hydrogen bond strengthening has been testified in the S1 state based on comparing staple bond lengths and bond angles involved in hydrogen bonding between the S0 state and the S1 state. In addition, the calculated infrared spectra at the N-H stretching vibrational region and calculated hydrogen bond energy also declare the phenomenon of hydrogen bond strengthening. The frontier molecular orbitals (MOs) and Mullikens charge distribution analysis method as well as natural bond orbital (NBO) demonstrate the charge distribution, which provides the tendency of ESIPT reaction. The potential energy surfaces of the S0 and S1 states are constructed to explain the mechanism of the proton transfer in the excited state in detail. In addition, the ESIPT process of sensor L is dependent on different solvents.

Modulation of the 4-aminophthalimide spectral properties by hydrogen bonds in water.

TD-DFT and DFT calculations have been performed to examine the relationship between the spectral shifts of 4-aminophthalimide (4AP) and the formation of hydrogen bonds in water solution. The computations of the S0 state are at the IEFPCM-B3LYP/6-311++G(d, p) level and the S1 state at the TD-IEFPCM-B3LYP/6-311++G(d, p) level. The eleven structures of the hydrogen-bonded 4AP clusters formed with different number water molecules in both S0 and S1 states were optimized. The absorption, fluorescence and infrared spectra were calculated. The results of the hydrogen bond energy and length reveals that the hydrogen bonds formed by the nitrogen atom of the amine group with water molecule (A type) are significantly weakened from states S0 to S1. In contrast, the hydrogen bonds formed by the oxygen atoms of the two carbonyl groups (B type) with water molecules and those formed by the two hydrogen atoms of the amine group (C type) with water molecules are remarkably strengthened. Comparing with the 4AP monomer spectra, the weakening for the hydrogen bond of A type could be responsible for the blueshifts of the electronic absorption spectra and the stretching vibrational spectra of the two NH groups in 4AP from states S0 to S1. The significant redshifts of the electronic spectra and the S0-S1 downshifts of the stretching vibrational modes of the two NH groups and the two carbonyl groups in 4AP could be attributed to the strengthening of hydrogen bonds for B and C types.

Theoretical explorations about the excited state behaviors for two novel high efficient ESIPT compounds

Two high efficient excited state intramolecular proton transfer (ESIPT) compounds (i.e., 3-(5-([1,1′-biphenyl]-4-yl)oxazol-2-yl)-4′-(N,N-diphenylamino)-[1,1′-biphenyl]-4-ol (1) and 4′-(N,N-diphenylamino)-3-(5-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)oxazol-2-yl)-5-methyl-[1,1′-biphenyl]-4-ol (2)) are explored theoretically. Based on DFT and time-dependent DFT (TDDFT) methods, we investigate the hydrogen bonding interactions and ESIPT mechanism. Via B3LYP/TZVP/IEFPCM (toluene) theoretical level, we reappear the experimental steady-state spectra, which demonstrate that the theoretical manner is reasonable and effective. Based on reduced density gradient (RDG) versus sign(λ2)ρ analyses, we confirm intramolecular hydrogen bond for both 1-enol and 2-enol. Investigating geometrical parameters and infrared (IR) vibrational spectra, we verify the O-H···N should be strengthened in the S1 state for 1-enol and 2-enol systems. Exploring frontier molecular orbitals (MOs) and charge density difference (CDD) maps, we find charge redistribution provides the tendency of ESIPT. The constructed potential energy curves demonstrate that the proton transfer should happen in the S1 state. Particularly, the low potential energy barriers of forward and backward ESIPT process for both 1 and 2 systems, the dynamical equilibrium could be verified, which means 1 and 2 systems should be potential for novel white light LEDs materials. This work not only explores and explains previous experimental phenomenon, but also makes a reasonable assignment about the ESIPT mechanism.

Theoretical research on excited-state intramolecular proton coupled charge transfer modulated by molecular structure

At the TD-B3LYP/TZVP/IEFPCM theory level, we have theoretically studied the excited-state intramolecular proton coupled charge transfer (ESIPCCT) process for both 4′-N,N-diethylamino-3-hydroxyflavone (3HFN) and 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) molecules. Our calculated hydrogen bond lengths and angles sufficiently confirm that the intramolecular hydrogen bonds O1–H1⋯O2 and O1–H1⋯N1 formed at the S0 states of 3HFN and diCN-HBO should be significantly strengthened in the S1 state, which is further supported by the results obtained based on the analyses of infrared spectra shifts, molecular orbitals and charge density differences maps. The significant strengthening of intramolecular hydrogen bonds O1–H1⋯O2 and O1–H1⋯N1 upon photoexcitation should facilitate the ESIPCCT process of the two title molecules. The scanned potential energy curves and confirmed excited-state transition states for both 3HFN and diCN-HBO show that the proton can be easily transferred from O1 to O2 (N1 for diCN-HBO) through the strengthened intramolecular hydrogen bonds upon photoexcitation to the S1 state.

Theoretical insights into the excited state hydrogen bond and ESIPT reaction for 2-amino-3-(2’-benzoxazolyl)quinoline and 2-amino-3-(2’-benzothiazolyl)-quinoline

Two N-H type excited state intramolecular proton transfer (ESIPT) systems (i.e., 2-amino-3-(2’benzoxazolyl)quinoline (ABO) and 2-amino-3-(2’-benzothiazolyl)-quinoline (ABT)) have been investigated. Adopting DFT and TDDFT methods coupling with B3LYP functional and TZVP basis set, our simulations about ABO and ABT molecules have successfully reappeared experimental results, based on which the rationality of our calculations is confirmed. Using Atoms in Molecules (AIM) analytical method, we firstly explore the interactions about chemical bond and verify the formation of hydrogen bond N-H•••N for ABO and ABT in the S0 state. Investigating the primary geometrical parameters involved in N-H•••N, we find it should be strengthened in the S1 state. Upon photoexcitation, charge transfer phenomenon is found via frontier molecular orbitals (MOs), and charge redistribution provides the tendency of ESIPT reaction for ABO and ABT. According to our constructed potential energy curves of both S0 and S1 states for ABO and ABT using two kinds of methods (i.e., the elongation of N-H single bond and the weakening of H•••N hydrogen bond), we clarify the ESIPT mechanisms and explain the recovery of four-level reaction cycle. Our searching transition state (TS) structures and simulated intrinsic reaction coordinate (IRC) path further confirm the ESIPT reaction.

Modulating N-H-based Excited-State Intramolecular Proton Transfer by Different Electron-Donating/Withdrawing Substituents in 2-(2’-aminophenyl)benzothiazole Compounds

At the B3LYP/6-311+G(d, p)/IEFPCM (in dichloromethane) theory level, the N-H-based excited-state intramolecular proton transfer (N-H-based ESIPT) process of 2-(2’-aminophenyl)benzothiazole (PBT-NH2) and its three derivatives 2-(2’-methylaminophenyl)benzothiazole(PBT-NHMe), 2-(2’acetylaminophenyl)benzothiazole (PBT-NHAc) and 2-(2’-tosylaminophenyl) benzothiazole (PBT-NHTs) have been explored by the time-dependent density functional theory (TD-DFT) method. Our calculated hydrogen bond lengths and angles sufficiently confirm that the intramolecular hydrogen bonds N1-H•••N2 formed at the S0 states of the four compounds should be significantly strengthened in the S1 state, which are further supported by the results obtained based on the analyses of infrared spectra shifts. The scanned potential energy curves reveal that the energy barriers of the first singlet excited state of the four titled compounds along the ESIPT reactions are predicted at 8.74, 8.98, 6.72 and 1.69 kcal/mol, respectively, suggesting that the inclusion of a strong electron-withdrawing tosyl (Ts) group can remarkably facilitate the occurrence of the ESIPT reaction, while the involvement of an electron-donating methyl group has slight opposite effect on the ESIPT process of the amino-type hydrogen-bonding system.