Yufang Liu

Time-Dependent Density Functional Theory Study on Electronically Excited States of Coumarin 102 Chromophore in Aniline Solvent : Reconsideration of the Electronic Excited-State Hydrogen-Bonding Dynamics

The time-dependent density functional theory method was performed to investigate the electronically excited states of the hydrogen-bonded complex formed by coumarin 102 (C102) chromophore and the hydrogen-donating aniline solvent. At the same time, the electronic excited-state hydrogen-bonding dynamics for the photoexcited C102 chromophore in solution was also reconsidered. We demonstrated that the intermolecular hydrogen bond CO...H-N between C102 and aniline molecules is significantly strengthened in the electronically excited-state upon photoexcitation, since the calculated hydrogen bond energy increases from 25.96 kJ/mol in the ground state to 37.27 kJ/mol in the electronically excited state. Furthermore, the infrared spectra of the hydrogen-bonded C102-aniline complex in both the ground state and the electronically excited state were also calculated. The hydrogen bond strengthening in the electronically excited-state was confirmed for the first time by monitoring the spectral shift of the stretching vibrational mode of the hydrogen-bonded N-H group in different electronic states. Therefore, we believed that the dispute about the intermolecular hydrogen bond cleavage or strengthening in the electronically excited-state of coumarin 102 chromophore in hydrogen donating solvents has been clarified by our studies.

Time-dependent density functional theory study on the electronic excited-state geometric structure, infrared spectra, and hydrogen bonding of a doubly hydrogen-bonded complex.

The geometric structures and infrared (IR) spectra in the electronically excited state of a novel doubly hydrogen‐bonded complex formed by fluorenone and alcohols, which has been observed by IR spectra in experimental study, are investigated by the time‐dependent density functional theory (TDDFT) method. The geometric structures and IR spectra in both ground state and the S1 state of this doubly hydrogen‐bonded FN‐2MeOH complex are calculated using the DFT and TDDFT methods, respectively. Two intermolecular hydrogen bonds are formed between FN and methanol molecules in the doubly hydrogen‐bonded FN‐2MeOH complex. Moreover, the formation of the second intermolecular hydrogen bond can make the first intermolecular hydrogen bond become slightly weak. Furthermore, it is confirmed that the spectral shoulder at around 1700 cm−1 observed in the IR spectra should be assigned as the doubly hydrogen‐bonded FN‐2MeOH complex from our calculated results. The electronic excited‐state hydrogen bonding dynamics is also studied by monitoring some vibraitonal modes related to the formation of hydrogen bonds in different electronic states. As a result, both the two intermolecular hydrogen bonds are significantly strengthened in the S1 state of the doubly hydrogen‐bonded FN‐2MeOH complex. The hydrogen bond strengthening in the electronically excited state is similar to the previous study on the singly hydrogen‐bonded FN‐MeOH complex and play important role on the photophysics of fluorenone in solutions.

A TD‐DFT study on the hydrogen bonding of three esculetin complexes in electronically excited states: Strengthening and weakening

Time‐dependent density functional theory (TD‐DFT) method was used to study the excited‐state hydrogen bonding of three esculetin complexes formed with aprotic solvents. The geometric structures, molecular orbitals (MOs), electronic spectra and the infrared (IR) spectra of the three doubly hydrogen‐bonded complexes formed by esculetin and aprotic solvents dimethylsulfoxide (DMSO), tetrahyrofuran (THF) and acetonitrile (ACN) in both ground state S0 and the first singlet excited state S1 were calculated by the combined DFT and TD‐DFT methods with the COSMO solvation model. Two intermolecular hydrogen bonds can be formed between esculetin and the aprotic solvent in each hydrogen‐bonded complex. Based on the calculated bond lengths of the hydrogen bonds and the groups involved in the formation of the intermolecular hydrogen bonds in different electronic states, it is demonstrated that one of the two hydrogen bonds formed in each hydrogen‐bonded complex is strengthened while the other one is weakened upon photoexcitation. Furthermore, it is found that the strength of the intermolecular hydrogen bonds formed in the three complexes becomes weaker as the solvents change from DMSO, via THF, to ACN, which is suggested to be due to the decrease of the hydrogen bond accepting (HBA) ability of the solvents. The spectral shifts of the calculated IR spectra further confirm the strengthening and weakening of the intermolecular hydrogen bonds upon the electronic excitation. The variations of the intermolecular hydrogen bond strengths in both S0 and S1 states are proposed to be the main reasons for the gradual spectral shifts in the absorption and fluorescence spectra both theoretically and experimentally.

TD-DFT Study of the Double Excited-State Intramolecular Proton Transfer Mechanism of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole

The 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPD) is chosen to investigate the excited-state double proton transfer process (ESDPT). The IR spectra, bond distance, and angle analyses show that the two intramolecular hydrogen bonds in the BPD molecule, formed between hydroxyl group and pyridine-type nitrogen atom, are significantly strengthened in the S1 state. The potential energy surfaces in both S0 and S1 states are scanned with varying O-H bond lengths to visually investigate the double proton transfer mechanism. Compared with previous investigations, the proton transfer process can be interpreted in more detail. The hydrogen bond strengthening promotes the proton transfer in the S1 state effectively. The large Stocks shift observed in the experiment can be explained more comprehensively according to the ESDPT mechanism.

Effects of Different-Type Intermolecular Hydrogen Bonds on the Geometrical and Spectral Properties of 6-Aminocoumarin Clusters in Solution

The geometrical and spectral properties of the hydrogen-bonded clusters formed by 6-aminocoumarin (6AC) with solvents of different hydrogen-bonding abilities have been investigated at the CPCM-PBE0/6-311++G(d, p) level of theory. Upon photoexcitation, A type hydrogen bonds will be weakened whereas hydrogen bonds of B and C types should be strengthened. The weakening of hydrogen bond A is responsible for the blue-shifts of the absorption spectra in HFIP and TFE while strengthening of hydrogen bonds B1 and B2 are the reasons for the red-shifts of the absorption spectra in DMSO. The absorption spectra of cluster 6AC–(H2O)3 is in better agreement with the experimental result than 6AC–(H2O)5, which dose not support the conjecture of E. Krystkowiak. Moreover, the stabilization effects of the different types of intermolecular hydrogen bonding on the absorption spectra properties of the hydrogen-bonded 6AC clusters are discussed in detail.Graphical Abstract

Effect of amino group on the excited-state intramolecular proton transfer (ESIPT) mechanisms of 2-(2′-hydroxyphenyl)benzoxazole and its amino derivatives

The excited-state intramolecular proton transfer (ESIPT) reactions of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 5-amino-2-(2′-hydroxyphenyl)benzoxazole (5A-HBO) and 6-amino-2-(2′-hydroxyphenyl)benzoxazole (6A-HBO) were investigated with the time-dependent density functional theory (TD-DFT) method at the B3LYP/6-31G(d,p) theoretical level. The primary bond lengths and infrared (IR) vibrational spectra show that the intramolecular hydrogen bond is significantly strengthened in S1 state. The Mullikens charge distribution and the frontier molecular orbitals (MOs) were analyzed. The result is consistent with the ESIPT mechanism proposed by Han and co-workers. Upon photo-excitation, the intramolecular hydrogen bond of 5A-HBO-enol (1.73 A) and 6A-HBO-enol (1.74 A) in the S1 state is weaker than that of HBO-enol (1.69 A) due to the influence of the amino group in the HBO framework. After vertical excitation to the S1 state, the electronic density redistributes and migrates from the phenol ring to the benzoxazole ring of HBO. While for 5A-HBO and 6A-HBO, it transfers from the amino-benzoxazole moiety to the phenol ring. The analysis of the potential energy curves of HBO, 5A-HBO and 6A-HBO indicates that the ESIPT process of HBO occurs most easily. It is demonstrated that the presence and the position of the amino group in the HBO framework can change the behavior of the intramolecular hydrogen bonds O–H⋯N in the S1 state and thus hinder the ESIPT processes to some extent.

Time-dependent density functional theory study on the hydrogen bonding-induced twisted intramolecular charge-transfer excited states of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine.

In this work, the time‐dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen‐bonded intramolecular charge‐transfer excited state of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine (DMAPIP) in methanol (MeOH) solvent. All the geometric conformations of the ground state and locally excited (LE) state and the twisted intramolecular charge‐transfer (TICT) state for isolated DMAPIP and its hydrogen‐bonded complexes have been optimized. At the same time, the absorption and fluorescence spectra of DMAPIP and the hydrogen‐bonded complexes in different electronic states are also calculated. We theoretically demonstrated for the first time that the intermolecular hydrogen bond formed between DMAPIP and MeOH can induce the formation of the TICT state for DMAPIP in MeOH solvent. Therefore, the two components at 414 and 506 nm observed in the fluorescence spectra of DMAPIP in MeOH solvent were reassigned in this work. The fluorescence peak at 414 nm is confirmed to be the LE state. Furthermore, the red‐shifted shoulder at 506 nm should be originated from the hydrogen‐bonded TICT excited state.

Study on the modulation of spectral properties of the formylperylene–methanol clusters by excited-state hydrogen bonding strengthening

In the present work, the charge transfer (CT) process within the formylperylene (FPe)-methanol (MeOH) systems facilitated by intermolecular hydrogen bonding interactions is theoretically studied in both the ground state S0 and the first singlet excited state S1. The geometric structures, electronic spectra and the infrared spectra of the FPe monomer as well as the various hydrogen-bonded FPe-MeOH complexes in both states were calculated with the density functional theory (DFT) method and time-dependent density functional theory (TD-DFT) methods, respectively. It is demonstrated that the total effect of the intermolecular hydrogen bonding between FPe and the MeOH molecules becomes strengthened in the ground state as the number of the MeOH molecules hydrogen-bonded to the FPe molecule increases from zero to three, which induces large increases in the dipole moment as well as systemic redshifts of the absorption spectra of FPe. Furthermore, upon photoexcitation of the FPe molecule, the intermolecular hydrogen bonds formed in the various hydrogen-bonded FPe-MeOH complexes are further strengthened which leads to even larger dipole moments as well as obvious redshifts of the fluorescence spectra. The calculated electronic spectra of the various hydrogen-bonded FPe-MeOH complexes are in agreement with the steady-state absorption and fluorescence spectra of FPe observed in the binary mixed solvents with different MeOH concentration. The intermolecular hydrogen bonding strengthening in both the ground and excited states are further confirmed by the infrared spectra shifts. Moreover, the vitally important role played by the intermolecular hydrogen bonding interaction and its strengthening upon electronic excitation in the CT process is discussed.

Exploring the interaction of silver nanoparticles with lysozyme: Binding behaviors and kinetics

The role of nanoparticle interaction with biomolecules to form a biocorona is the key to nanoparticle behavior and its consequences in the physiological environment. Since the adsorbed biocorona decides the fate of a nanomaterials in vivo, and thus a comprehensive understanding of the dynamic interactions of the proteins with the nanoparticle is imperative. Herein we investigate the interaction of a model protein, lysozyme with silver nanoparticles (AgNPs) using fluorescence, synchronous fluorescence, UV-vis absorption spectrum and circular dichroism (CD) techniques under the physiological conditions. The results indicated that the binding of AgNPs to lysozyme may be a static quenching mechanism. With the analysis of the fluorescence spectral data, the binding constants and the thermodynamic parameters were determined, which suggests that the binding of AgNPs to lysozyme is a spontaneous process. Moreover, it was demonstrated that the main acting forces between AgNPs and lysozyme may be hydrophobic interactions. At the same time, the conformational change of lysozyme induced by AgNPs was investigated with synchronous fluorescence spectroscopy and CD techniques. The results of kinetic studies reveal that the adsorption of lysozyme on AgNPs surface tends to follow pseudo-second-order kinetic characteristic with obvious hysteresis effect.

Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1

The novel spectral behaviors of Aloesaponarin 1 (AS1) are investigated by studying the dynamics process of excited state intramolecular proton transfer (ESIPT). Two intramolecular hydrogen bonds (HB1 and HB2) are formed between hydroxyl and carbonyl groups of AS1. The calculated potential energy curves of AS1 demonstrate that the ESIPT process along HB1 is energy favorable while not along HB2. The analysis of potential energy curves describes clearly the dynamic behaviors of the proton transfer process from hydroxyl group to carbonyl group along HB1. The infrared spectra of AS1 confirm that the stretching absorption peak of hydroxyl group in HB1 disappears and that a new peak corresponding to hydroxyl group appears in the first excited state, which depicts the ESIPT process indirectly. The fluorescence peaks of AS1 (636nm), AS2 (Aloesaponarin 1 3-O-methyl ether, 629 nm) and AS3 (Aloesaponarin 1 8-O-methyl ether, 522 nm) demonstrate that the fluorescence behavior of AS1 is primarily effected by HB1 rather than HB2. The large Stokes shifts of AS1 (206 nm) indicate that the absorbed energy is partly transferred to non-harmful long fluorescence through ESIPT process, which plays important role in the explanation for the UV protection property of AS1. The inducement and influence factors of ESIPT process of AS1 are illustrated by analyzing electrostatic potential, molecular orbital and natural bond orbital.