The photophysics of purine-capped Q-CdS has been examined in the presence of certain indoles. The addition of indole does not modify electronic spectrum of purine-capped Q-CdS but it forms a fluorescing charge-transfer intermediate with illuminated US, which has an emissive peak at 495 nm. The intensity and the lifetime of this intermediate are enhanced initially with an increase in concentration of indole. In the presence of other indoles, the fluorescence is simply quenched in a dynamic process without forming any fluorescing intermediate. In contrast, emissive CT intermediate is not formed in the presence of indole or any of its derivatives with adenine-capped Q-CdS. In all the cases the quenching of fluorescence, monitored by steady state and time-resolved methods, follows the Stern-Volmer relationship and takes place with a bimolecular rate constant of similar to10(10) dm(3) mol(-1) s(-1). Purine-capped Q-CdS sensitizes the reactions of the investigated indole(s)-O-2 couple much more efficiently than adenine-capped Q-CdS. The differences in quenching of fluorescence and reactivity of holes between purine-capped Q-CdS and adenine-capped Q-CdS are explained by the difference in the binding of indole to the particle. In the case of purine-capped Q-CdS, specific channels for the binding of the solutes are created through the H-bond with the surface-capped purine. (C) 2003 Elsevier Inc. All rights reserved.