Coexistence of Hydrogen Atom Transfer Reactions through and not through Triplet Ion Pair between p-Chloranil and Durene

Article abstract of DOI:10.1246/bcsj.57.3557

Mechanism of hydrogen atom abstraction reactions by triplet state p-chloranil (³CA) from durene (DH) were studied by picosecond and nanosecond laser photolysis and transient photoconductivity measurements. ³CA was quenched by DH through diffusional encounter to form a triplet ion pair (IP) between CA and DH, p-chloranil semiquinone radical (CAH^.), and 2,4,5-trimethylbenzyl radical (D^.).Ionic dissociation of IP was observed in 1,2-dichloroethane (DCE) as well as in acetonitrile.However, no transient species was observed by direct excitation of a charge-transfer (CT) band of the electron donor-acceptor (EDA) complex between CA and DH.The H-atom transfer leading to production of CAH^. was found to proceed through two distinct mechanisms; H-atom transfer via IP (Mechanism I) and a more rapid transfer competing with IP formation (Mechanism II).The quantum yields of CAH^. produced by Mechanism I and II and the first-order rate constants for proton transfer, ionic dissociation, and intersystem crossing competing with one another in the IP state were estimated to be (0.1 and 0.2) and (2,5, and 13)X10⁶ s^-1, respectively, in DCE at room temperature.