Dibenzosuberenyl cation

Laser flash photolysis studies of dibenzosuberenyl cations and radical cations504 gj-g further studies in a presently fashionable field of ion and radical intermediate photochemistry. 

The dibenzosuberenyl cation 4 has the advantage that a single laser pulse at 308 nm generates the cation from its corresponding alcohol in TFE and then reexcites it, giving the two-photon phenomenon with one laser [Eq. (4)] [10,19]. 

Figure 3 Transient absorption spectrum of dibenzosuberenyl cation measured 1 ps after 308-nm excitation of 5-dibenzosuberenol in TFE. (From Ref. 19.)...

Johnston reported a rate constant of 1.1 x 10 M" s for quenching of the singlet-excited dibenzosuberenyl cation by azide in 4% aqueous TFE [19]. The cation was generated by laser flash photolysis of 5-dibenzosuberenol, with the rate constant obtained from a plot of the observed rate constant for decay of the transient cation at 525 nm as a function of added azide concentration. The water co-solvent was necessary for solubility of the sodium azide quencher. 

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. 

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