2,5-Dimethyl-2,4-hexadiene induced photodechlorination of 9,10-dichloroanthracene

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Abstract
Photochemical formation of 9-chloroanthracene (MCA) from 9,10-dichloroanthracene (DCA) is observed in the presence of 2,5-dimethyl-2,4-hexadiene (DMH) in acetonitrile (AN). The mechanism of the reaction was investigated using kinetics, deuterium labeling, and quenching techniques. Contrary to conclusions in a recent publication, our work supports the salient features of the mechanism we had proposed earlier. DCA is photostable in degassed AN in the absence of DMH. When DMH is added, irradiation of DCA at 365 or 404 nm converts it quantitatively to MCA. The photoreaction is strongly inhibited when low concentrations of molecular oxygen or 1,2,4,5-tetracyanobenzene are also present. Results from fluorescence quenching studies along with kinetics parameters from the dependence of DCA loss and MCA formation quantum yields on [DMH] implicate participation of the DCA/DMH singlet exciplex, the DCA/(DMH)2 triplex and the DCA radical anion (DCA˙ −) as intermediates in the photodechlorination. Results from experiments using deuterated DMH, deuterated AN, and AN containing D2O or H2O show that the 10-H of MCA is introduced by protonation of DCA˙ −. Contrary to a recent report, there is no radical pathway to MCA via dissociation of DCA˙ − to chloride and MCA˙ radical. Changes in the absorption spectrum of DCA in AN with increasing [DMH] suggest that the static quenching of DCA fluorescence at high [DMH] is due primarily to nearest neighbour quenching instead of DCA/DMH ground state complex formation.