9-Phenylthioxanthyl cation

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. 

Figure 2 Absorption and fluorescence spectra for the 9-phenylthioxanthyl cation. The fluorescence intensity is in arbitrary units. (From Ref. 30.)...

The triplet-excited 9-phenylthioxanthyl cation and its p-F-substituted analog have been characterized. The cations exhibit weak phosphorescence emission in TFA-TFE or acetonitrile at 77 K, with maxima near 700 nm [13,30]. Triplet energies of 40 and 41 kcal-mol" were estimated based on the wavelength for onset of phosphorescence. Laser excitation at 355 nm of these cations result in transients with at 300 nm and a very weak absorption in the 500-600 nm region attributed to the triplet cations. The triplet-excited 9-phenylthioxanthyl cation has a lifetime of approximately 15 ps at low alcohol concentrations [13]. 

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... 

Triplet-excited xanthyl cations have also been shown to undergo electron transfer processes with aromatic donors [12,13]. The reactivity of the triplet state 9-phenylxanthyl cation, its p-fluoro analog, and the 9-phenylthioxanthyl cation was examined using transient absorption techniques. Irradiation of the 9-phenylxanthyl cation 1 in the presence of biphenyl resulted in an enhanced rate constant for decay of triplet 1, with concomitant production of transients corresponding to the 9-phenylxanthyl radical at 340 nm and the biphenyl radical cation at 670 nm (Fig. 10). Rate constants for triplet decay or radical growth were measured as a function of added quencher concentration for a variety of aromatic donors. Plots of the observed first-order rate constant versus the... 

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