Effect of benzene derivatives bearing electron-releasing and/or electron-withdrawing groups on the fluorescence of CdS-Q clusters

We present in this paper a study of the effect of benzene derivatives bearing electron-releasing and/or electron-withdrawing groups on the fluorescence intensity and lifetimes of three different fluorescence bands of colloidal CdS-Q, with maximum emission wavelength at 460 nm (broad band), 481 nm (excitonic band) or 559 nm (trapped band), generated by photoexcitation of clusters prepared under different experimental conditions. e derivatives bearing electron-withdrawing groups produce static quenching of the luminescence from the several clusters, while in the case of benzene derivatives with electron-releasing substituents the quenching effect is essentially dynamic, in both cases ocurring through an electron-transfer process. veral surface traps in the semiconductor clusters give origin to luminescence bands with different efficiency and lifetime and are also surface sites which can have different affinity for the quencher. We have mentioned in a previous work [J. Photochem. Photobiol. A: Chem. 99 (1996) 177] that the excitonic band originates in the radiative recombination of electrons in the conduction band, after some interval of time residing in shallow surface traps, which are in thermal equilibrium with the conduction band. Broad band and trapped band originate, in turn, in the direct radiative recombination of electrons in deep surface traps, which are not in thermal equilibrium with the conduction band. We propose in this paper a mechanistic scheme, which fits our experimental results, based in the consideration of multiple active surface sites.