Laser photolysis study of photochemical reactions of triplet states of pyridinethiones

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) tb (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 p-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,pi*) and (3)(pi,pi*), respectively.