Photophysics and photochemistry of nitroanthracenes. Part 1.—Primary processes in the photochemical reactions of 9-benzoyl-10-nitroanthracene and 9-cyano-10-nitroanthracene studied by steady-state photolysis and nanosecond laser photolysis

Abstract

The genuine phosphorescence spectra of the title compounds (XNA) have been clearly observed at 77 K in EPA (diethyl ether–isopentane–ethanol 5 : 5 : 2 volume ratio). Also, the time-resolved absorption spectra obtained at the end of nanosecond laser pulse at 77 K (in EPA) and at room temperature (in ethanol and benzene) have been ascribed to the superposition of the absorptions of the lowest triplet XNA [³XNA(T₁)] on those of the corresponding anthryloxy radicals (XAO˙). The decay of ³XNA(T₁) is not accompanied by the formation of XAO˙ and the following results are obtained. (1) In ethanol at room temperature, the first-order decay of XAO˙ corresponds to hydrogen-atom abstraction by XAO˙ from the solvent, giving rise to the formation of corresponding anthrols (XAOH), while the second-order reaction of XAO˙ with nitrogen(II) oxide (NO˙) yields the corresponding nitrosoanthrones (XNSAN) which decompose very slowly into XAO˙ and NO˙ followed by the rapid formation of XAOH. (2) In benzene at room temperature no formation of XAOH is observed, but there exists an equilibrium between XAO˙(+NO˙) and XNSAN. All these results support our previous conclusion that the nitro → nitrite rearrangement of XNA does not occur for the lowest triplet states of ππ* character but does occur for the higher triplet states of nπ* character, followed by the cleavage of the nitrites to XAO˙ and NO˙.