The effect of methylviologen on the photoluminescence bands of quantum-sized CdS

Colloidal, quantum-sized CdS (Q-CdS) particles displaying an isolated fluorescence band, with a maximum fluorescence wavelength at 487, 580 or 709 nm, were prepared. From the excitation spectra, the onset of the bands was determined, giving the band gaps of the clusters as 2.88, 2.71 and 2.54 eV respectively. The effect of methylviologen dication (Mv(2+)) On the fluorescence quantum yield and lifetime of the colloidal Q-CdS particles was studied. With the experimental data obtained, an equilibrium constant for the formation of adducts between Mv(2+) and Q-CdS particles was evaluated. Partial quenching of the fluorescence at 487 nm, nearly complete quenching of the fluorescence at 580 nm and total quenching of the 709 nm band were attained. The quenching was dynamic or a combination of static and dynamic, depending on the energy of the emission. Static quenching was only observed for the clusters emitting a fluorescence band with a maximum at 709 nm. For the fluorescence bands with maxima at 487 and 580 nm, it was only possible to observe dynamic quenching. A mechanistic scheme which accounts for the experimental results is given for each cluster system. Within the proposed schemes, radiative processes do not arise from the trapped electrons. Instead, they originate from the interaction between conduction electrons or detrapped electrons and deep trapped holes.