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 (k_h) 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.