Colloidal solutions of CdTe and ZnTe were prepared via precipitation of the metal ions by Na2Te in the presence of polyphosphate. The solutions do not spontaneously fluoresce, but upon illumination or attack by free radicals generated radiolytically the colloids acquire fluorescence, the highest quantum yield being 0.2. ZnTe colloids lose their fluorescence upon standing in the dark but regain it upon exposure to a small dose of light. Co colloids have onsets of absorption and fluorescences that gradually shift with the Cd/Zn ratio. In all cases, the fluorescence band is located at the onset of absorption and the decay of the fluorescence occurs in the 10-ns range. The colloids corrode upon illumination or free radical attack to yield metal, which partially dissolves to produce hydrogen. Tellurium is also formed, colloidal ditelluride formation being an intermediate step.
The optical and electrochemical properties of three cadmium benzenethiolate clusters, Cd(SPh)42-, Cd4(SPh)lo2-, and Cdl,$4(SPh)16e, are studied as function of the cluster size. The Cd, clusters (n = 1,4, 10) which may represent molecular models for the semiconductor CdS show structured absorption spectra that are assigned to both ligand-t*metal charge-transfer (LMCT) and intraligand transitions. The absorption and emission bands of Cdlo are red-shifted compared to those of Cdl and Cd,. The emission of the Cd, 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 Cd, clusters yields thianthrene, dibenzothiophene, and benzenethiol, the rate of photodegradation depending on the cluster size. Electrochemical studies of Cd, show that, with increasing cluster size, the oxidation potential is shifted negative while the reduction potential is shifted positive. Both Cd, and Cdlo 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.
The fluorescence behavior of a new bipyridyl ligand, 2,2'-bipyridyl-3,3'-diol (BPDO), was studied as a function of the metal ion complexed. At pH 7.6, the fluorescence of BPDO is strongly decreased by complexation to Cu(II), but binding to Zn(II) leads to an increase in fluorescence intensity. Time-resolved fluorescence measurements suggest that in the case of Cu(II), a nonfluorescent complex is formed, whereas in the presence of Zn(II), a new, longer decay component appears.
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