Photoionization of xanthonevia its triplet state or via its radical anion

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 φion of 8 × 10−3 at both 308 and 355 nm; at 532 nm photoionization is undetectable (φion < 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; φion 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 φion 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.