Donglin Li

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.

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.