Abstract
The transient absorption spectra of 2,3-diphenyl-8,9-dihydro-6H-8λ4-[1,2]oxathiino[4,5-g]quinoxalin-8-one (DPQXS, 35) in various solvent after photolysis at 266 nm or 355 nm are recorded. For both photolysis wavelengths the transient absorption spectra of produced intermediate display similar features with absorption maximum at 310, 390 and 450 nm. A weak broad feature at wavelength longer than 550 nm is also observed. All measured absorption curves display similar bi-exponential decay. The lifetime of the fast component in solvent acetonitrile, n-hexane and toluene is 270, 160 and 170 ns and the slow one is 3.9, 0.8 and 1.2μs, respectively. The quenching rates for the transient species measured for quencher fumaronitrile, N-phenylmaleimide and oxygen are 107M-1s-1, 1.9´108 M-1s-1and 2.5´109 M-1s-1, respectively. Because quenching rate of oxygen is efficient we conclude that the transient product is a triplet biradical. From the obtained Arrhenius plot, the activation energy of fast and slow process is (31±1) kJ/mole and (61±7) kJ/mole. The dissociation mechanism of DPQXS upon excitation in the UV region proposed to undergo fast intersystem crossing to the triplet manifold. Dissociation occurs in the triplet surface for formation the triplet products. Therefore, the transient absorption spectrum is assigned to the triplet biradical form of o-quinodimethane.