Solvent Effect In Several Photochemical Reactions

In most chemical reactions in solutions, the characteristics of solvent generally affact the reaction rates and equilibrium. Based on the laser pump-probe technology,we employ time-resolved transient spectroscopy and DFT calculation to study the solvent effects on energy/electron transfer in some photochemical systems. This dissertation mainly contains three aspects:(1) the photochemical reaction between 1,2-NQ and different bases in binary acetonitrile/water (v/v) solvent; (2) The dynamics of the fluorescence upconversion between I2-Bodipy and Perylene in hexane, heptane, toluene, 1,4-dioxane and dimethyl sulphoxide (DMSO). The details are described as the follows.(1) Photochemical reaction between 1,2-NQ and different bases in different solventsThe photochemical reaction between 1,2-naphthoquinone (NQ) and adenine in binary acetonitrile/water solvent has been investigated by using nanosecond time-resolved laser flash photolysis. With photolysis at 355 nm, NQ generates the singlet state 1NQ*, and the absorption peaks appear red shift with the the increase water ratio in binary acetonitrile/water solvents. Lowest triplet state T1 of NQ (3NQ*)are produced via intersystem crossing. The triplet-triplet absorption of the state in pure contributes three bands at 374, 596 and 650 nm in transient spectra, respectively.And there is hydrogen-atom abstraction between 3NQ* and water in binary acetonitrile/water solvents, generating the NQ+H· and NQ+2H at 343 nm and 420 nm,respectively. The decay rates of 3NQ* decrease with the increase of water ratio. In the presence of adenine, the observation of A·+ cation (at 363nm) and NQ+H· radical (at 343 and 485 nm) with different growing time indicates a multi-step mechanism of electron transfer process followed by a proton transfer between 3NQ* and adenine.Through fitting with the Stern-Volmer relationship, the quenching rate constant kq of 3NQ* by adenine in binary acetonitrile/water (4/1,v/v) is determined as 1.66×109 M-1 s-1. Additionally, no distinct spectral evidence confirms existence of electron transfer between 3NQ* with thymine, cytosine and uracil. According to Rehm-Weller equation,the electron transfer probability between 3NQ* and the four bases generally follows the Eox sequence (A<T<C<U).(2) TTA upconversion of I2-Bodipy and Perylene in different solvents Solvent effect plays a very important role in photochemical reaction and energy transfer process in solution, but it is rarely mentioned in triplet-triplet annihilation(TTA) upconversion fluorescence experiments. In a typical TTA upconversion system of a photosensitizer of diiodo-Bodipy (I2-Bodipy) and a triplet acceptor of perylene,five common inertia solvents, hexane, heptane, toluene, 1,4-dioxane and dimethyl sulfoxide (DMSO), were used to investigate the solvent effect on the overall quantum yield of upconversion fluorescence. Femtosecond and nanosecond time-resolved transient difference absorption spectra were measured to study efficiencies of intersystem crossing (ISC) and triplet-triplet energy transfer (TTET). From the recorded upconversion fluorescence emission spectra, the overall TTA upconversion fluorescence quantum yield was derived. Among the five solvents, the upconversion quantum yield in dioxane is the highest as 19.16%, and more than twice value in toluene, 8.75%. For the solvents of hexane, heptane, toluene and dioxane, the yields generally follow the sequence of polarity and viscosity. However, a very low upconversion quantum yield (1.51%) was observed in DMSO, although the TTET process and fluorescence quantum yield of perylene in DMSO were almost as efficient as it in dioxane. Based on the calculation of density functional theory, a reasonable explanation of solvent effect was proposed.(3) Abnormal TTA-UC process of I2-Bodipy and perylene in DMSO Comparing the fluorescence characteristic in toluene and DMSO, the fluorescence intensity of I2-Bodipy with low concertration in DMSO and the excitation intensity is not a linear relationship as that in toluene. The fluorescence emission was obviously inhibited. The same phenomenon was observer in a similar solvent Tetramethylene sulfone (TMSO). However, in the TTA upconversion involving I2-Bodipy and Perylene, such fluorescence inhibition gradually weaken with the increase of Perylene concertration and when this concertration comes to 1.0×104 M, the fluorescence of I2-Bodipy can fully recover. We identify this as the interaction between I2-Bodipy and DMSO or TMSO, which causes the π-π stacking of I2-Bodipy and the aggregation-caused quenching (ACQ). Additionaly, the interaction between I2-Bodipy and DMSO may also be the reason that the triplet states of I2-Bodipy are not quenched by oxygen.Perylene has a stronger π bond, and it will destroy the interaction between DMSO and I2-Bodipy, resulting the fluorescence of I2-Bodipy back to normal.