S.D. Yao

Reactive intermediates in laser photolysis of guanosine

The photochemical reactions of aqueous solution guanosine with acetone as photosensitizer have been investigated by laser flash photolysis. From detailed kinetic analysis, the reaction mechanism has been derived: dehydrogenated guanosine radical G(-H)* and triplet guanosine 3G* were produced via electron transfer and triplet-triplet energy transfer respectively. The half-life time of 3G* has been determined to be 7.4 μs, and the quenching rate constant of Mn2+ on it was obtained to be 1.9 × 108 M-1 s-1. Based on pH titration, the pKa value of 3G* was also obtained to be 8.7.

Interaction of phenolic antioxidants and hydroxyl radicals

Abstract Based on pulse radiolysis of aqueous solutions of four phenolic antioxidants including green tea polyphenols, quercetin, caffeic acid and sinapic acid the rate constants for reactions of OH and the antioxidants were determined. And green tea polyphenols and quercetin are the strongest antioxidants.

Generation of the C60 radical cation via electron transfer reaction. A pulse radiolysis study

Abstract Using pulse radiolysis techniques the C 60 radical cation has been generated for the first time via electron transfer from the triplet state of C 60 to CCl 4 or CCl 3 CF 3 . The radiation-induced electron transfer rate constant was determined to be 7.8 × 10 6 M −1 s −1 . A novel transient absorption peak at 650 nm of the C 60 cation was observed from pulse radiolysis of benzene containing C 60 and CCl 4 or CCl 3 CF 3 . The extinction coefficient of the C 60 cation at 650 nm was estimated to be ⩾ 4.6 × 10 3 M −1 cm −1 .

The structure characteristics of photoproducts of indolinospirooxazines

Photochromic reactions of four indolinospirooxazine derivatives have been studied using nanosecond laser flash photolysis techniques. Photolysis of the four compounds in acetonitrile and in cyclohexane all leads to the formation of the short-lived CT intermediates as the predominant photoproducts. In certain circumstances few photomerocyanines (PMC) can be observed. From the absorption bands and the lifetimes of CT intermediates, it has been speculated that the substituents at the nitrogen atom of the indole ring influence the structural characteristics of CT intermediates mainly through steric hindrance effects, whereas the substituents at the 5 position of the indoline ring influence mainly through electron effects. A potential energy surface model is established to explain the results.

SYNTHESES AND PHOTOPHYSICAL PROPERTIES OF FULLEROPYRROLIDINES CONTAINING PHOTOACTIVE UNITS

ABSTRACT The syntheses and a systematic spectroscopic study of a series of fulleropyrrolidines containing polycyclic aromatic hydrocarbon are reported. The fulleropyrrolidines were characterized by UV, IR, 1H-NMR, 13C-NMR and ESI-MS. According to the absorption spectra and the electrochemical data, no significant interaction was observed between the polycyclic aromatic rings and fullerene moiety in the ground state of fulleropyrrolidines. However, the fluorescence and nanosecond transient absorption spectra indicate that these fulleropyrrolidines have sensitized fluorescence of fullerene due to the intramolecular energy transfer, and that their excited triplet state production are considerably reduced, which in turn result in a lower optical limiting performance compared with that of C60.

Generation of the C60 radical cation and the radical adduct of trichloromethyl radical to C60. Pulse radiolysis and laser photolysis of C60 in CCl4

Abstract The radical cation of C 60 is generated either via hole transfer from the radical cation of CCl 4 to C 60 or via electron transfer from triplet C 60 to CCl 4 . The radical adduct of . CCl 3 to C 60 , [C 60 CCl 3 ] . , was also produced via two different mechanisms, namely the radical addition reaction and the reaction between triplet C 60 and CCl 4 , respectively.

Electron transfer reactions between nucleic acid bases and tetracyanoethylene in acetonitrile

Abstract The electron transfer reactions between pyrimidines, e.g. thymine (Thy), uracil (Ura) and cytosine (Cyt), and tetracyanoethylene (TCNE) in acetonitrile were studied using nanosecond laser flash photolysis techniques. Laser photolysis of a mixed solution of TCNE and pyrimidines led to the formation of the anion radical of TCNE, which has an absorption band at 350–500 nm ( λ max = 430 nm) and decays following second-order kinetics. The electron transfer reaction occurs through the excited triplet states of the pyrimidines, and the bimolecular quenching rate constants are 7.6 × 10 9 dm 3 mol −1 s −1 for thymine and 1.7 × 10 10 dm 3 mol −1 s −1 for uracil. However, a ground state complex between cytosine and TCNE was found. This complex has an absorption band with λ max at 419, 444 and 472 nm. The photoinduced electron transfer between cytosine and TCNE was not observed on a nanosecond time scale during laser photolysis.

Generation of C60 radical cation and radical adduct of dichloro- or monochloromethyl radical to C60: Pulse radiolysis and laser photolysis of C60 in polar chloromethane

Abstract The radical cation of C 60 is generated either via hole transfer from the radical cation of polar chlorinated methane to C 60 or via electron transfer from the triplet of C 60 to electrophilic polar chlorinated methane. The radical adducts CHCl 2 and CH 2 Cl of C 60 were produced via a similar mechanism to the generation of the radical adduct [C 60 -CCl 3 ] in CCl 4 .

Optical limiting performance of dicyanodihydrofullerene

Abstract The properties of the excited triplet state of dicyanodihydrofullerene [C 60 (CN) 2 ] were determined with nanosecond laser photolysis and a picosecond/nanosecond optical limiting (OL) system. It was found that C 60 (CN) 2 possessed a lower quantum yield and higher extinction coefficient of the excited triplet state, which in turn resulted in its similar OL performance for nanosecond laser pulse compared with pristine C 60 . However, C 60 (CN) 2 shows a better OL performance for picosecond laser pulse, which can be understood according to a three-level model.

Radical adduct of trichloromethyl to C70: A laser flash photolysis of C70 in carbon tetrachloride

Abstract The addition reaction of trichloromethyl radical to C 70 has been investigated by use of laser flash photolysis. The difference absorption spectrum of radical adduct, [C 70 CCl 3 ], exhibits broad absorption band in the visible region. The rate constant for reaction of trichloromethyl radical with C 70 is determined.