Xinsheng Xu

Photoinduced electron transfer between 2-methylanthraquinone and triethylamine in an ionic liquid: Time-resolved EPR and transient absorption spectroscopy study

Photoinduced electron transfer between 2-methylanthraquinone (MeAQ) and triethylamine (TEA) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by comparing the time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and the transient absorption spectroscopy. The results of TR-EPR spectroscopy, in which MeAQ was 8 mmol L(-1) and TEA was 150 mmol L(-1), indicated that the transient radical would exist longer time in [bmim][PF6] than in acetonitrile. At the delay time of 8 μs after laser excitation, the TR-EPR signal transformed from an emissive peak into an absorptive peak when the experiment was performed in [bmim][PF6]. The results of the transient absorption spectroscopy, in which MeAQ was 0.1 mmol L(-1) and TEA was 2.2 mmol L(-1), showed that the efficiency and the rate of the photoinduced electron transfer reaction in [bmim][PF6] were obviously lower than that in acetonitrile. It was concluded that various factors, such as concentration, viscosity and local structural transformation of the solution, have an influence on the process of photoinduced electron transfer in [bmim][PF6].

TR-ESR Investigation on Reaction of Vitamin C with Excited Triplet of 9,10-phenanthrenequinone in Reversed Micelle Solutions

Time-resolved electron spin resonance has been used to study quenching reactions between the antioxidant Vitamin C (VC) and the triplet excited states of 9,10-phenanthrenequinone (PAQ) in ethylene glycol-water (EG-H2O) homogeneous and inhomogeneous reversed micelle solutions. Reversed micelle solutions were used to be the models of physiological environment of biological cell and tissue. In PAQ/EG-H2O homogeneous solution, the excited triplet of PAQ (3PAQ*) abstracts hydrogen atom from solvent EG. In PAQ/VC/EG-H2O solution, 3PAQ* abstracts hydrogen atom not only from solvent EG but also from VC. The quenching rate constant of 3PAQ* by VC is close to the diffusion-controlled value of 1.41 × 108 L/(mol ·s). In hexadecyltrimethylammonium bromide (CTAB)/EG-H2O and aerosol OT (AOT)/EG-H2O reversed micelle solutions, 3PAQ* and VC react around the water-oil interface of the reversed micelle. Exit of 3PAQ* from the lipid phase slows down the quenching reaction. For Triton X-100 (TX-100)/EG-H2O reversed micelle solution, PAQ and VC coexist inside the hydrophilic polyethylene glycol core, and the quenching rate constant of 3PAQ* by VC is larger than those in AOT/EG-H2O and CTAB/EG-H2O reversed micelle solutions, even a little larger than that in EG-H2O homogeneous solution. The strong emissive chemically induced dynamic electron polarization of As.− resulted from the effective TM spin polarization transfer in hydrogen abstraction of 3PAQ* from VC.

Molecular insight into the microstructure and microscopic dynamics of pyridinium ionic liquids with different alkyl chains based on temperature response

The temperature dependence of the microstructure and microscopic dynamics of three N-alkylpyridinium tetrafluoroborate ionic liquids, namely, [BPy][BF4], [HPy][BF4], and [OPy][BF4], is investigated via molecular dynamics simulation. Under a temperature change, the microstructure changes of [OPy][BF4] with a longer alkyl chain show a slight difference from that of [BPy][BF4] and [HPy][BF4] in terms of the radial and spatial distribution functions. A temperature increase is found to be advantageous to the aggregation of the polar regions as well as the nonpolar regions in the pyridinium ionic liquids. However, the properties of the microscopic dynamics of the three ionic liquids regularly change under conditions with temperature change. The results show a decreased probability of ions to reverse the direction of their translation motion because the negative parts of the velocity autocorrelation functions increase with the temperature. The temperature dependence of rotational dynamics is more obvious than that of translation. A rise in temperature causes rotational autocorrelation functions to decay quickly, and it considerably reduces relaxation time. With an increase in alkyl chain length, the rotational correlation improves and the relaxation time decreases. The time correlation functions show that the association dynamics of nonpolar regions is faster than that of polar regions over time. The association time is short for long-tailed ionic liquids.

Time-Resolved Electron Paramagnetic Resonance Investigation on the Quenching Reaction of Photo-Induced Excited Triplet Vitamin K3 by Antioxidant Vitamin C in Homogeneous and Micelle Solutions

The reaction mechanism and dynamics of the quenching of the excited triplet of vitamin by vitamin C was studied in both homogenous ethylene glycol-H2O plus hexadecyltrimethylammonium bromide, aerosol OT, sodium dodecyl sulfate, and Triton X-100 micelle solutions by time-resolved electronic paramagnetic resonance. The photolysis of vitamin K3 in EG-H2O solution led to the chemically induced dynamic electron polarization (CIDEP) of the vitamin K3 neutral radical VK3H• and ethylene glycol ketyl radical, which showed that a hydrogen atom abstraction reaction of with the solvent ethylene glycol occurred. The triplet mechanism was the primary mechanism underlying the generation of CIDEP, suggesting that quickly reacted with ethylene glycol before spin-lattice relaxation. During the photolysis of vitamin K3 and vitamin C in ethylene glycol-H2O solution, abstracted hydrogen atoms not only from the solvent ethylene glycol but also from a vitamin C monoanion. The stronger CIDEP signal of the vitamin C monoanion radical indicated that can be rapidly quenched by vitamin C. In hexadecyltrimethylammonium bromide (aerosol OT, sodium dodecyl sulfate)/ethylene glycol-H2O micelle solutions, lipid-soluble needed to diffuse around the micelle surface to react with the water-soluble vitamin C. The spin-lattice relaxation of the parent resulted in weaker CIDEP of the reaction-generated radicals. Furthermore, the attraction between the positive charge layer of the hexadecyltrimethylammonium bromide micelle and vitamin C monoanion made the quenching reaction of by vitamin C rapid, whereas the repulsion between the negative charge layer of aerosol OT (sodium dodecyl sulfate) micelle and vitamin C monoanion made the quenching reaction slow. For Triton X-100 micelle, the coexistence of vitamin K3 and vitamin C in the same polyethylene glycol shell resulted in the fastest quenching reaction and strong CIDEP of the monoanion radical.

Viscosity Effect Study on Quenching of Photoinduced Excited Triplet Duroquinone by TEMPO

ABSTRACT Quenching dynamics of excited quinone molecules are given much attention in photochemistry and biochemistry. In order to study the viscosity effect on the quenching of triplet excited state of duroquinone (3DQ*) by stable radical, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), this study measured chemically induced dynamic electron polarization (CIDEP) spectra and transient absorptive spectra in various solvents. The solvents used were ethylene glycol, 1,2-propanol and their mixtures with different ratio in volume. The Stern-Volmer plot was obtained form CIDEP spectra of photolysis of DQ with different TEMPO concentrations. Combining the slope of the Stern-Volmer plot with lifetime of 3DQ*, determined from the 3DQ* transient absorbance decay curve, the quenching rate constants of 3DQ* by TEMPO were calculated in each solvent. The results indicate that the quenching rate constant is viscosity-dependent, and that it decreases linearly with the increase in solvent viscosity in the range used in our experiment.