Electron spin polarisation in the photoreduction of xanthone in alcohol: effect of concentration and temperature

Time-resolved EPR studies of the hydrogen abstraction reaction of photoexcited xanthone in 2-propanol were carried out as a function of the concentration of xanthone and the sample temperature. The temperature was varied from 22°C to about ?30°C, and the concentration from about 0.2 to 4.0?mM. At low temperature or concentration, the observed spectra of the xanthone ketyl radical and the propan-2-olyl radical could be simulated as a superposition of a hyperfine-independent component due to the emissive triplet mechanism and a hyperfine-dependent component due to the S–T₀ radical pair mechanism. However, with an increase in the concentration of xanthone, the relative contribution of TM decreases, and, concomitantly, the net absorptive component of only the xanthone ketyl radical increases. As the spin polarisation mechanisms do not predict any concentration dependence, this unusual behaviour is explained by invoking the enhancement of the spin–lattice relaxation rates due to Heisenberg spin exchange occurring at high local concentrations of the radicals. The net absorptive signal is attributed to thermally equilibrated radicals. The observed temperature dependence of the spin polarisation behaviour is similarly explained. The origin of the net absorptive signal in the TREPR spectra of the acetone?2-propanol system is also attributed to thermally equilibrated radicals. The self-quenching mechanism of xanthone is proposed to be an electron-transfer reaction from an excited xanthone molecule to another xanthone in the ground state.