Thioxanthyl cation

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],... 

Stable organic salts have been used to eliminate the requirement for acidic media and to avoid the problems associated with competitive radical generation that occur via homolytic bond cleavage of carbon-heteroatom bonds in laser flash photolytic experiments [28-30]. Treatment of several 9-arylxanthenols and thioxanthenols with fluoroboric acid in propionic anhydride gave crystalline xanthyl and thioxanthyl cation tetrafluoroborate salts [Eq. (5)]. 

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... 

Table 4 Photophysical Parameters for 9-Arylxanthyl and Thioxanthyl Cations...

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 ... 

Protonation of the olefin, or protonation and subsequent dehydration of the parent alcohol, gives cations which are then subjected to laser excitation or steady-state irradiation. Cations generated in this way were identified by their characteristic absorption spectra, which also indicated cation stability over the time scale of the individual experiments by the lack of change in their absorption spectra. Among the numerous cations generated in acidified solution for photochemical studies are the xanthyl and thioxanthyl [7-15], dibenzosuberenyl [10], triphenylmethyl [10,15], a,co-diphenylpolyenyl [16], and 1,1-diarylethyl [17] cations. Media included acetonitrile acidified with trifluoroacetic acid (TFA-ACN) or aqueous sulfuric acid [7-9,11,14,15], TEA in 2,2,2-trifluoro-ethanol (TFA-TFE) [10,12,13], n-heptane acidified with TFA [9], and BFj-etherate in methylene chloride [16]. The absorption spectral data for several cations have been previously reviewed [6]. Characterization of the cation excited states will be discussed in Section III. 

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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