PHOTOCHEMICAL PROPERTIES OF EXCITED TRIPLET-STATE OF 6H-PURINE-6-THIONE INVESTIGATED BY LASER FLASH-PHOTOLYSIS

Photochemical reactions of 6H-purine-6-thione (PuT) via the excited tr iplet state [(3)(PuT)] have been studied by means of laser flash phot olysis in organic solvents. Transient absorption bands at 475 and 690 nm were assigned to (3)(PuT). Intersystem quantum yield and the lowes t triplet energy of (3)(PuT) were evaluated to be 0.99 and 63 kcal/mo l, respectively. The self-quenching rate constant is quite large (2.3 x 10(9) M-1 s(-1) in THF). In photoinduced electron transfer, (3)(PuT) acts as electron acceptor for tetramethylbenzidine, while (3)(PuT)* acts as electron donor for p-dinitrobenzene. Rate constants for H-atom abstraction (k(hT)) Of (3)(PUT) from benzenethiols, tocopherol, and 1,4-cyclohexadiene are on the order of 10(8) M-1 s(-1). From the Hamme tt plots of k(hT) for substituted benzenethiols, a negative rho value indicates that (3)(PuT) has electrophilic character. In the addition reaction of (3)(PuT) toward various alkenes, the electrophilic charac ter of (3)(PuT) was also confirmed. By steady-light photolysis of PuT , purine was produced via (3)(PuT) after H-atom abstraction. On combi nation of these results, the character of the lowest (3)(PuT) was pre sumed.