Studies on the Photoluminescence of Indoles

Abstract

The literature on the fluorescence of indole, and its application in cone recent studies on the intrinsic fluorescence of proteins, is briefly reviewed. The electronic energy levels of the indole chromophore are described, with a method for analysing the overlapping 1La  and 1Lb transitions, using the differential shifting of these bands caused by methyl substitution. Solvent effects on indole emission are investigated. Non -hydroxylic solvents are found to shift the spectrum in a manner explicable by polarisation interaction. Hydroxylic solvents interact more strongly with the 1La state than with the 1Lb state, causing reversal of these levels in fluorescence from indole or skatole, but not from 5 ~ methylindole. All these indoles form exciplexes with hydroxylic solvents, and the possible structure of the exciplexes is discussed. The interaction of the indole chromophore with the carbonyl group is investigated, and to facilitate this, several indoles were synthesised using the Fischer indole reaction. The ketonic carbonyl quenches by excitation transfer from indole to the ketone, whilst the non - ketonic carbonyl forms a donor - acceptor complex with the excited indole. Quenching is dependent on the electron affinity of the non — ketonic carbonyl group. These studies are compared with reported studies on related benzene derivatives. Comments are made on the determination of quantum yields and lifetimes.