ELECTRON-TRANSFER SINGLET QUENCHING AND EXCIPLEXES IN THE PHOTOREACTION OF SUBSTITUTED ANTHRACENES WITH INDOLE-DERIVATIVES

The effect of substituents on the singlet quenching and photoreaction of anthracene with indole derivatives was investigated in acetonitrile and n-heptane solutions at 298 K. The photoreaction proceeds by the q uenching of the excited singlet of anthracene by the indoles. Bimolecu lar quenching rate constants in acetonitrile follow a Rehm-Weller type correlation. In the nonpolar solvent il-heptane, the rate constants a re of the same order of magnitude than those in acetonitrile. However, methyl substitution at the N heteroatom of the indole greatly reduces the quenching in this solvent. In these cases, exciplex emission was observed. These differences in quenching capacity may be explained by a charge transfer interaction followed by proton transfer in the excit ed state, when it is available at the N-H bond. A nonreactive decay ro ute and exciplex emission is operating in the case of the N-CH3 deriva tives. Photobleaching quantum yields were also determined in heptane. The substitution by methyl or cyano groups at 9 and 10 positions on th e anthracene ring decreases the reaction quantum efficiency. (C) 1998 Elsevier Science S.A. All rights reserved.