Triplet state quenching involving charge transfer interactions between N-methyl indole and aromatic carbonyl compounds in benzene solution

Abstract

In laser photolysis experiments N-methyl indole quenches the triplet states of several aromatic carbonyl compounds including some with lower energy triplet states with rate constants in the range (0.02–10)× 10⁹ dm³ mol^–1 s^–1. Quenching by N-methyl indole is shown to be either exclusively or partially due to electronic energy transfer from either triplet acetophenone or triplet xanthone respectively, and the sensitized spectrum assigned to the triplet state of N-methyl indole is given. Quenching of the other triplet carbonyls by N-methyl indole is shown to depend on the energy predicted for the charge transfer state of each interacting pair of compounds with N-methyl indole as the potential electron donor. In ethanol solution quenching of the triplet state of xanthone by N-methyl indole is demonstrated to occur via electron transfer leading to the formation of separate ions.

Pulse radiolysis of indole, 3-methyl indole and N-methyl indole in benzene solution gives short lived triplet states of these compounds, the spectra of which have been determined. Quenching of triplet N-methyl indole by various compounds with higher and lower triplet states was investigated following pulse radiolysis in benzene solution. The rate constants for quenching of triplet N-methyl indole were also shown to depend on charge transfer interactions and evidence for energy transfer was obtained only in cases where the predicted charge transfer states lie at much higher energies than the locally excited triplet states. The reasons why charge transfer interactions cause rapid relaxation of triplet exciplexes are discussed.