Cadmium benzenethiolate clusters of various size: molecular models for metal chalcogenide semiconductors

The optical and electrochemical properties of three cadmium benzenethiolate clusters, Cd(SPh)4 2-, Cd4(SPh)10 2-, and Cd10 $4(SPh)16 4-, are studied as function of the cluster size. The Cdn clusters (n = 1,4, 10) which may represent molecular models for the semiconductor CdS show structured absorption spectra that are assigned to both ligand-to-metal charge-transfer (LMCT) and intraligand transitions. The absorption and emission bands of Cd10 are red-shifted compared to those of Cd1 and Cd4. The emission of the Cdn clusters (n = 4, 10) is ascribed to a LMCT transition as suggested by the short luminescence lifetimes and the red shift with increasing cluster size. Illumination of the Cdn clusters yields thianthrene, dibenzothiophene, and benzenethiol, the rate of photodegradation depending on the cluster size. Electrochemical studies of Cdn show that, with increasing cluster size, the oxidation potential is shifted negative while the reduction potential is shifted positive. Both Cd4 and Cd10 form charge-transfer complexes with methyl viologen. However, steady-state illumination of these clusters in the presence of methyl viologen does not result in the formation of the methyl viologen radical cation.