Anthracene singlet quenching by indoles in media of different polarities: mechanism and chemical photoreaction
The quenching of excited singlet anthracene by indole derivatives in solvents of different polarity has been studied using static and time-resolved techniques. The quenching process is dependent on the nature of the indole derivative and on the polar properties of the medium. In non-polar solvents photoreaction takes place with unit efficiency in compounds which have a hydrogen atom bound to the nitrogen. The efficiency decreases in polar solvents and is considerably suppressed in aqueous solvent mixtures. The results here obtained indicate that charge transfer is the primary step for the photoreaction in polar solvents. For compounds with the indolic N—H group, direct hydrogen transfer is the dominant mechanism in non-polar media. The importance of radical ion formation in these anthracene–indole systems is investigated as a function of the indole structure and solvent polarity.