Excited-State Dynamics of Ground-State Complexes between Haloanthraquinones and 2,5-Dimethylhexa-2,4-diene, and Quenching of Triplet Haloanthraquinones by 2,5-Dimethylhexa-2,4-diene

Picosecond laser photolysis of haloanthraquinones (XAQ; the bromo and bromochloro compounds) in toluene containing 1 M 2,5-dimethylhexa-2,4-diene (DMHD) gives rise to the appearance of absorption bands due to not only the second and lowest excited triplet states of XAQ but also a transient ionic species (an excited singlet charge-transfer complex or a singlet ion pair) generated by direct excitation of a ground-state XAQ−DMHD complex. In DMHD, similar results are also obtained for α-bromoanthraquinones (the 1-bromo, 1,5-dibromo, and 1,8-dibromo compounds), whereas only the absorption band due to the transient ionic species is observed for the 2-bromo, 1-bromo-5-chloro, and 1-bromo-8-chloro compounds. Although this transient ionic species decomposes into the semiquinone radical of XAQ and 2,5-dimethylhexa-2,4-dien-1-yl radical, nanosecond laser photolysis indicates that a second-order reaction between these two radicals yields the corresponding haloanthrahydroquinone and a biradical (probably 2,5-dimethyl-...