Chaozheng Li

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.

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.

A mechanistic study on Decontamination of Methyl Orange Dyes from Aqueous Phase by Mesoporous Pulp Waste and Polyaniline

&NA; The dispersion‐corrected density functional theory (DFT‐D3) is used to investigate the mechanism of mesoporous pulp waste (MPW) and polyaniline (PANI) adsorptive removal methyl orange (MO) dye from their aqueous solutions. The results are absolutely reliable because of the sufficiently accurate method although such big systems are studied. It is demonstrated that hydrogen bond and Van Der Waals interactions play a significant role in MO adsorption by MPW and PANI. For MO adsorption by MPW, hydrogen bond and Van Der Waals interactions are both weakened in S1 state. In contrast, hydrogen bond and Van Der Waals interactions between PANI and MO are both enhanced in S1 state. The thermodynamic parameters such as enthalpy and free energy change reveal that the MO adsorption by MPW and PANI are spontaneous and exothermic. The adsorption of MO on MPW is less favorable in S1 state and the adsorption of MO on PANI is more favorable in S1 state. Therefore, the photoexcitation should be controlled during the MO adsorption by MPW and applied for MO adsorption by PANI. HighlightsThe hydrogen bond and Van Der Waals interactions play a significant role in MO adsorption by MPW and PANI.The influence of photoexcitation on adsorption has been studied firstly in our work.The adsorption of MO on MPW is less favorable in S1 state and the adsorption of MO on PANI is more favorable in S1 state.The MO adsorption by MPW and PANI are spontaneous and exothermic.

Hydrogen bond strengthening induces fluorescence quenching of PRODAN derivative by turning on twisted intramolecular charge transfer

Researchers have proposed different effective mechanisms of hydrogen bonding (HB) on the fluorescence of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and its derivatives. Herein, excited state transition and dynamics analysis confirm that the fluorescence of PD (a derivative of PRODAN with ethyl replaced by 3-hydroxy-2,2-dimethylpropan) emits from the planar intramolecular charge transfer (PICT) state rather than twist ICT (TICT) state, because the fluorescence emission and surface hopping from the TICT state to the twist ground (T-S0) state is energy forbidden. Nevertheless, the strengthening of intramolecular-HB (intra-HB) and intermolecular-HB (inter-HB) of PD-(methanol)2 smooth the pathway of surface hopping from TICT to T-S0 state and the external conversion going to planar ground state by decreasing the energy difference of the two states. This smoothing changes the fluorescence state of PD-(methanol)2 to the TICT state in which fluorescence emission does not occur but surface hopping, leading to the partial fluorescence quenching of PD in methanol solvent. This conclusion is different from previous related reports. Moreover, the inter-HB strengthening of PD-methanol in PICT state induces the cleavage of intra-HB and a fluorescence red-shift of 54nm compared to PD. This red-shift increases to 66nm for PD-(methanol)2 for the strengthening of the one intra-HB and two inter-HBs. The dipole moments of PD-methanol and PD-(methanol)2 respectively increase about 10.3D and 8.1D in PICT state compared to PD. The synergistic effect of intra-HB and inter-HB induces partial quenching of PD in methanol solvent by turning on the TICT state and fluorescence red-shift. This work gives a reasonable description on the fluorescence red-shift and partial quenching of PD in methanol solvent, which will bring insight into the study of spectroscopic properties of molecules owning better spectral characteristics.

The influence of π-conjugation framework on intramolecular proton transfer and Stokes shift in 1,8-Dihydroxydibenzo[a,c]phenazine molecule: a DFT and TD-DFT study

The 1,8-Dihydroxydibenzo[a,c]phenazine (DHBP) is chosen to investigate the relationship among the extension of π-conjugation framework, excited-state proton transfer and the Stokes shift. The IR spectra, bond and angle analyses show that the two intramolecular hydrogen bonds in DHBP molecule are both significantly strengthened in S1 state. The proton donor methanol molecule can inhibits the hydrogen bond strengthening in S1 state. The increment of π-conjugation framework effectively decreases the Stokes shift and promotes the proton transfer. The potential energy surfaces of DHBP indicate that the reverse double-proton transfer can spontaneously happen in S0 state, and the double proton can easily transfer for its relative low potential energy barrier in S1 state.

Photoexcitation effect on the adsorption of hazardous gases on silica surface

There is very little scientific understanding of photoexcitation effect on the adsorption properties of adsorbent. The adsorption of four hazardous gases (SARIN (propan-2-ylmethylphospho-nofluoridate), methyl dichlorophosphate (MDCP), trimethyl phosphate (TMP) and hydrogen sulfide (H2S)) on silica surface is taken as target sample in this work. The adsorption energy order (MDCP<SARIN<TMP) in the ground state is consistent with the strength order of intermolecular hydrogen bond (inter-HB) between hydroxyl group of silica surface and hazardous gas, and the desorption order of the three gases in previous reports. However, with the adsorption energy increase of MDCP and the decrease of SARIN and TMP, this order changes remarkably to SARIN<TMP<MDCP after photoexcitation to excited state by absorbing shortwave ultraviolet irradiation. This change is opposite to the inter-HB weakening of MDCP in the first excited (S1) state and the strengthening of TMP and SARIN in the second excited (S2) state. This opposite change is caused by formation of intermolecular charge transfer state of MDCP and local excitation of SARIN and TMP. The H2S is dissociated after photoexcitation to the S1 state. This work presents photoexcitation as a new standard for the design and detection of adsorption properties of adsorbent for its striking effect on adsorption behaviors.

Asymmetric substitution changes the UV-induced nonradiative decay pathway and the spectra behaviors of β-diketones

Asymmetric substitution has not been termed as an essential factor in studying photo-induced ultrafast dynamics of molecular system. Asymmetric 4-hydroxybut-3-en-2-one (HEO), together with symmetric malonaldehyde (MA) and acetylacetone (AA), have been provided as target sample to study the nonradiative decay (ND) processes of β-diketones. An effective ND pathway of the three molecules is presented that their excited second (S2) states transfer to first (S1) state by nonadiabatic surface hopping, and then transfer to triplet (T1) state by crossing minimum energy crossing point (MECP), after which decay to ground (S0) state through MECP. More importantly, the asymmetric substitution of HEO induces the proton transfer in the S1 state and generates a proton-transferred conformer with lowest energy, which does not occur for MA and AA. This change exploits a new ND pathway that the S1 state decays to the proton transferred T1 state and then undergoes reverse proton transfer to S0 state through the MECPs between the three states. The two pathways of HEO give detailed energy and geometric information on surface hopping of S2/S1 and MECPs of S1/T1/S0, and interpret the reason of the ND pathway while not spectra emission. This result is significantly different from the previous reported ND pathway of photoisomerization or conical intersection between different states. This work shows that asymmetric substitution changes the molecular structure and then changes their spectra behaviors.

Unveiling the mechanism of the promising two-dimensional photoswitch — Hemithioindigo

The control of internal molecular motions by outside stimuli is a decisive task in the construction of functional molecules and molecular machines. Light-induced intramolecular rotations of photoswitches have attracted increasing research interests because of the high stability and high reversibility of photoswitches. Recently, Henry et al. reported an unprecedented two-dimensional controlled photoswitch, the hemithioindigo (HTI) derivative Z1, whose single bond rotation in dimethyl sulphoxide (DMSO) solvent and double bond rotation in cyclohexane solvent can be induced by visible light (J. Am. Chem. Soc. 2016, 138, 12,219). Here we investigate the intramolecular rotations of the HTI and Z1 in different polar solvents by time-dependent density functional theory (TDDFT) and Nonadiabatic dynamic simulations. Due to the steric hindrance between methyl and thioindigo fragment, the rotations of Z1 in the excited state are obstructed. Interestingly, the HTI exhibits two distinct rotation paths in DMSO and cyclohexane solvents at about 50fs. The intermolecular hydrogen bonds between HTI and DMSO play an important role in the rotation of HTI in DMSO solvent. Therefore, the HTI is a more promising two-dimensional photoswitch compared with the Z1. Our finding is thus of fundamental importance to understand the mechanisms of this class of photoswitches and design complex molecular behavior.

The fluorescence quenching phenomenon in newly synthesized blue fluorescence protein molecule caused by anchoring group substitution: a DFT and TD-DFT study

The newly synthesized blue fluorescence protein (BFP) molecule combined with its derivatives were fully investigated using DFT and TD-DFT methods. The frontier molecular orbitals and NBO charges indicate that the intramolecular benzene ring in the BFP molecule can inhibit the charge redistribution after photo-excitation effectively. The substitution of malonic and malononitrile groups can create the fluorescence quenching phenomenon, which may be caused by their participation in the excited state charge redistribution and act as important electron-donating groups. Also, these substitutions can enhance the configuration stability in the S1 state and hinder the formation of a metastable structure. The malononitrile group substitution can significantly decrease the energy barrier in the S1 state and promote proton transfer.