Intramolecular hydrogen-atom transfer at the lowest excited singlet and triplet states resulting in solvent-dependent novel photochemistry of a sterically strained 1,4-hexyl-bridged anthraquinone
In the picosecond time regime, the lowest excited singlet state of a 1,4-hexyl-bridged anthraquinone (CyAQ) undergoes intramolecular hydrogen-atom transfer giving rise to the formation of the ground-state singlet biradical (1BR) and its methide (MT, i.e. 1,4-hexylidene-bridged 9-hydroxyanthracen-10-one) which exist in equilibrium. The lowest excited triplet state of CyAQ also undergoes intramolecular hydrogen-atom transfer in the submicrosecond time regime. Intersystem crossing from the excited triplet biradical to 1BR is followed by rapid 1BR → MT conversion to reach equilibrium. Although both 1BR and MT produced at 77–140 K are stable, 1BR (and/or MT) produced at room temperature changes to a 1,4-hexylidene-bridged anthraquinone (product 1) in aprotic solvents such as benzene, toluene, methylcyclohexane and 2-methyltetrahydrofuran, or to product 1 (minor) and a 1,4-hexyl-bridged 1,4-diethoxy-9,10-dihydroxyanthracene (product 2, major) in ethanol and EPA (diethyl ether–isopentane–ethanol, 5:5:2 volume ratio). Also, the interconversion of products 1 ⇌ 2 caused by the change of an aprotic solvent ⇌ ethanol (or EPA) and the slow thermal reversion of products 1 and 2 to the original anthraquinone (CyAQ) can be seen at room temperature.