Photoionization of xanthonevia its triplet state or via its radical anion

Abstract

The photoionization of xanthone in methanol–water by light of wavelength 308, 355 and 532 nm was investigated by single-pulse and two-pulse laser-flash photolysis with optical detection of the hydrated electron and the ketone-based intermediates. Kinetic constants and quantum yields were obtained from the intensity dependences of the concentrations. In the absence of an electron donor, the mechanism is sequential. A first photon produces the triplet state, which is then ionized by a second photon with a quantum yield φ_ion of 8 × 10⁻³ at both 308 and 355 nm; at 532 nm photoionization is undetectable (φ_ion < 3 × 10⁻⁶). When an electron donor (triethylamine or DABCO) is added in large excess, excitation of the ketone and quenching are followed by photoionization of the radical anion X˙⁻. The latter regenerates the starting carbonyl compound, so in effect the donor is ionized by a catalytic cycle. The competition of both ionization pathways was studied quantitatively by varying the donor concentration. The ionization of X˙⁻ is monophotonic at 308 and 355 nm with a wavelength-independent quantum yield of 7 × 10⁻². A biphotonic ionization was found at 532 nm; φ_ion of the excited radical anion must be larger than 5 × 10⁻⁵. At that wavelength, nonabsorbing products are also formed with a quantum yield that is higher than φ_ion by a factor of 3.2. The consistently lower quantum yields of photoionization in the absence of a donor were explained by reverse intersystem crossing of the upper excited triplet states as an efficient deactivation channel that is not accessible to the excited radical anions.