Direct and ketone-sensitized photoconversion of 1-nitro-9,10-anthraquinone to 1-amino-9,10-anthraquinone mediated by donor radicals

The full photoreduction of 1-nitro-2-R-9,10-anthraquinone (R = H: N1, methyl: N2) was studied in benzene, acetonitrile and acetonitrile-water mixtures in the presence of 2-propanol and triethylamine (TEA). The major photoproduct is the fluorescing 1-amino-2-R-AQ (A1, A2). The quantum yield of full reduction increases with the donor concentration, approaching ΦNH2=0.1. The intermediates involved are assigned on the basis of spectral and kinetic characteristics. The short-lived triplet state (⩽20 ns) of N2 can be intercepted by 2-propanol or TEA, thereby forming the spectroscopically hidden donor radicals and the nitroAQ radicals which absorb at 400 and 540 nm; the latter band is due to the radical anion. The triplet state of N1 was not observed at room temperature, but the radical properties and decay in the nitrosoAQ are similar for N1 and N2. For donors in lower concentrations ΦNH2 is strongly increased in the presence of benzophenone, acetophenone or acetone, approaching 0.22. The results under direct and sensitized conditions are compared and major dependences and the effects of mixtures of acetonitrile with water are outlined.