Formation of anthrahydroquinones (XAQH2)via excited singlet charge-transfer complexes (or singlet ion pairs) of anthraquinones with 2,5-dimethylhexa-2,4-diene (DMHD), and photochemical reaction of XAQH2 with DMHD

Abstract

Upon picosecond excitation of a ground-state complex formed between anthraquinone (AQ, a typical planar molecule) and 2,5-dimethylhexa-2,4-diene (DMHD), an excited singlet charge-transfer complex [¹(AQ^δ––DMHD^δ+)*] or a singlet ion pair [¹(AQ˙^––DMHD˙⁺)] is produced within the duration of the excitation light pulse. This transient complex decays following a single-exponential function with a lifetime of 90 ps and the decay process is ascribed to the intracomplex proton transfer, yielding the semiquinone radical of AQ and the 2,5-dimethylhexa-2,4-dienyl radical. A similar result has been obtained for non-planar 1,8-dichloroanthraquinone (cf. Hamanoue et al., J. Photochem. Photobiol. A: Chem., 1993, 76, 7), and so nanosecond laser photolysis of XAQ (anthraquinone or 1,8-dichloroanthraquinone) has also been performed and the second-order decay rate constant of the semiquinone radical (XAQH˙) of XAQ generated in DMHD–toluene is found to be one order of magnitude greater than that generated in ethanol without DMHD. Hence, it is concluded that XAQH˙ abstracts a hydrogen atom from the 2,5-dimethylhexa-2,4-dienyl radical yielding a photoreduced product (XAQH₂, i.e. anthrahydroquinone or 1,8-dichloroanthrahydroquinone) and a biradical of DMHD; probably the reactions of this biradical yield the dimeric and oligomeric compounds of DMHD. Although steady-state photolysis of XAQ in neat DMHD causes the disappearance of the reactant absorption without the accompanying formation of XAQH₂, examination of the effect of DMHD concentration on the formation of XAQH₂ upon steady-state photolysis of XAQ in DMHD–toluene (or benzene) reveals that XAQH₂ disappears by its photochemical reaction with ground-state DMHD: For anthrahydroquinone, the formation of 4′,4′-dimethyl-3′-(2-methylprop-1-en-1- yl)spiro[anthracene-10,2′-oxetan]-9(10H)-one is confirmed.