Laser photolysis study of photochemical reactions of triplet states of pyridinethiones

Abstract

Photochemical reactions of pyridine-4(1H)-thione (4PyT) and pyridine-2(1H)-thione (2PyT) have been studied by nanosecond laser photolysis and steady photolysis methods. The transient absorption bands at 430 and 460 nm are assigned to 3(4PyT)* and 3(2PyT)*, respectively. The lowest triplet energies, triplet lifetimes and quantum yields of intersystem crossing have been determined. Photoinduced electron-transfer reaction from tetramethylbenzidine (TMB) to 3(4PyT)* occurs with a similar rate to that of 3(2PyT)* in a polar solvent. For dinitrobenzene (DNB), an electron-transfer reaction occurred from 3(4PyT)* or 3(2PyT)* to DNB with a diffusion controlled limit. The reactivity of 3(4PyT)* with H-atom donors is higher than that of 3(2PyT)*. The negative ρ-values of the Hammett plots of hydrogen-abstraction rate constants (kh) from p-XC6H4SH indicate that 3(2PyT)* is more electrophilic than 3(4PyT)*. From the addition reaction of 3(2PyT)* to various alkenes, the more electrophilic character of 3(2PyT)* than 3(4PyT)* is also confirmed. By comparison of these experimental results with MO calculations, the lowest electronic configurations of 3(4PyT)* and 3(2PyT)* are attributed to 3(n,π*) and 3(π,π*), respectively.