The Photoreduction of Methylene Blue by Amines—i. A Flash Photolysis Study of the Reaction Between Triplet Methylene Blue and Amines

Flash photolysis was used to study the reduction of the triplet state of methylene blue by both alkyl‐ and aryl‐amines. The extent of the formation of the semireduced form of the dye yielded rate constants of interaction between the triplet state and the amine ( k 5 ). A correlation between log k 5 and ionization potentials for alkylamines (slope = ‐1.75 eV ‐1 ) was interpreted as evidence for the formation of a partial charge‐transfer intermediate. The rate constants ( k 5 ) calculated for aryl‐amines approached the rate of diffusion in many cases. A Hammett plot for a series of N, N‐dimethyl‐anilines yielded a moderately large p value (– 3.28) consistent with the formation of a charge‐transfer intermediate. It was concluded that reaction of amines with triplet methylene blue leads to the formation of a partial charge‐transfer intermediate which may undergo complete electron transfer to yield radicals, or undergo spin inversion and return to the ground state.