TRIPLET EXCITED STATE OF COUMARIN AND 4'5‘ DIHYDROPSORALEN: REACTION WITH NUCLEIC ACID BASES AND AMINO ACIDS

Abstract—The triplet‐triplet absorption spectra of coumarin, 5.7 dimethoxycoumarin and the furocoumarin 4'5′ dihydropsoralen. a model for 4'5′ psoralen‐pyrimidine mono adducts, have been determined by the techniques of pulse radiolysis and laser flash photolysis. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 347 nm excitation in water. Reaction rate constants for coumarin and 4'5′ dihydropsoralen triplets with various pyrimidine and purine nucleic acid bases, and amino acids, have been measured. Long‐lived transient absorptions detected after quenching coumarin and 4'5′ dihydropsoralen triplets with tryptophan are assigned to mixtures of the corresponding coumarin radical anion and the tryptophan radical cation. The spectra of the radical anions of coumarin and 4'5′ dihydropsoralen were established using pulse radiolysis of the coumarins in aqueous formate. It is suggested that coumarins and furocoumarin triplets are quenched by nucleic acid bases and amino acids via a chargetransfer mechanism.