Thioxanthyl cation fluorescence

Thioxanthyl cations 6-9 exhibit adiabatic cation formation upon irradiation of the corresponding thioxanthenols in neutral aqueous acetonitrile [25,27]. Shukla and Wan noted an enhanced adiabatic fluorescence intensity for the thioxanthyl cations compared to the xanthyl cations [27]. It was suggested that the thioxanthyl cations are less susceptible to nucleophilic attack by water and thus are not deactivated as readily as the xanthyl systems [27],... 

Table 3 Fluorescence Lifetimes and Quantum Yields for Thioxanthyl Cations...

Measurement of carbocation fluorescence quantum yields and lifetimes as described in the preceding section allowed a detailed analysis of cation photophysical properties. A study of the photophysical parameters for several 9-arylxanthyl and thioxanthyl cations has been recently reported, in which the... 

Samanta et al. calculated a fluorescence rate constant of 2.4 x 10 s for the triphenylmethyl cation, an order of magnitude larger than for the xanthyl or thioxanthyl cations [15]. The fluorescence quantum yield for the triphenylmethyl cation is < 10, suggesting that its nonradiative rate constant is > 10 ... 

The triphenylmethyl, xanthyl and thioxanthyl, 9-phenylxanthyl, and 9-phenylthioxanthyl cations were also generated as stable species on Nafion films by Samanta and colleagues [15]. The carbocations were produced by dipping the Nafion films into n-heptane or acetonitrile solutions of the corresponding alcohols. The carbocation fluorescence spectra and lifetimes on the Nafion films were very similar to those in the ACN-TFA mixtures [15]. Fluorescence from the triphenylmethyl cation was easily observed imder these conditions. 

Several singlet-excited cations have been shown to imdergo photoinduced electron transfer from aromatic donors. Fluorescence from the 9-phenylxanthyl, xanthyl, thioxanthyl, and 9-phenylthioxanthyl cations is quenched in the presence of aromatic compounds [9,10,15], Steady-state fluorescence quenching experiments gave bimolecular excited state rate constants for quenching of the cations by the aromatic donors (Table 7). The quenching rate constants increase... 

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