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

Article abstract of DOI:10.1039/ft9959100607

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-expotential 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(1.) 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(1.) 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-9(10H)-one is confirmed.