Phenylxanthyl cation

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

Minto and Das observed that the amount of adiabatically produced 9-phenylxanthyl cation increased as the mole fraction of water increased in aque-... 

Berger and Weir reported that 9-phenylxanthen-9-ol adsorption on silica gel results in formation of the 9-phenylxanthyl cation 1, as indicated by a characteristic absorption band at approximately 380 nm using diffuse reflectance spectroscopy [32]. A vacuum-outgassed sample produced a strong luminescence characteristic of the cation. Excitation spectra indicated that the cation luminescence derives from two sources from adiabatic photodehydroxylation of the alcohol and from direct excitation of cation thermally formed on the silica gel surface. 

Numerous values for the fluorescence lifetime and quantum yield have been reported for the 9-phenylxanthyl cation (Table 2). Lifetime values of approximately 27-28 ns have been reported [8,9,11.15,28], whereas measurements in TFA-TFE and for the cation generated on silica gel gave a lifetime of 36-37 ns [10,32]. Similarly, the fluorescence quantum yield is reported as 0.42-0.48 [8,9,15,28], with outlying values of 0.80 and 0.33 in TFA-TFE [10,13]. Trace amounts of water may be responsible for the variation in lifetimes and quantum yields. 

Table 5 also lists rate constants determined using Stem-Volmer fluorescence quenching techniques for water quenching of the 9-phenylxanthyl cation generated from its corresponding alcohol in very strongly acidic aqueous acetonitrile and for the cation tetrafluoroborate salt in acetonitrile [11,28]. Both values are in excellent agreement with the values reported by Minto and Das when the difference in solvent systems is taken into account [8]. 

Figure 9 Transient absorption spectra at (A) 5 ps and (B) 45 ps following 355-nm laser pulse excitation of the 9-phenylxanthyl cation in acetonitrile containing 0.40 M TFA and 10 mM 1-methylnaphthalene. (From Ref. 15.)...

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... 

Figure 10 Transient absorption spectrum measured 7 ps after 3S5-iun laser excitation of the 9-phenylxanthyl cation in the presence of 2 x lO" M biphenyl in TFA-TFE. (From Ref. 13.)...

Note that bleaching can also occur via a nucleophilic attack by solvent or by a nucleophile like N3 and CN on the excited state of a cation. For example, for the excited states of 9-phenylxanthyl cation and related cationic structures, such reactions are shown to have rate constants several orders of magnitude higher... 

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