Laser flash photolysis study of the photochemistry of isatin and N-methylisatin

Rate constants for the quenching of isatin (1) and N-methylisatin (2) by hydrogen and electron donors were measured by the laser flash photolysis (LFP) technique in benzene solution. For 1, rate constants ranging from 1.5×105 M−1 s−1 (for toluene) to 5.9×109 M−1 s−1 (for 1,4-diazabicyclo[2.2.2]octane, DABCO) were obtained. Slightly lower rate quenching values were obtained for N-methylisatin, which probably indicates an increase in the charge-transfer (CT) character of its triplet state. The lack of dependence of the quenching rate constants of 1 and 2 with the substituent, in the case where substituted phenols were employed as quenchers, indicates that for the two isatins the primary process in the quenching reaction is electron transfer followed by a fast proton transfer, leading to the formation of the corresponding phenoxyl radicals. From the quenching rate constants by trans- and cis-stilbene, a value between 49.3 and 54.3 kcal/mol was estimated for the triplet state of isatin, whereas for N-methylisatin the ET is estimated to be <49.3 kcal/mol.