Solvent Effect on Reaction Mechanism and Kinetics of Triplet Ion Pairs between Chloranil and Acenaphthene

Abstract

Abstract A comparative study on decay dynamics of triplet ion pairs has been performed for the chloranil (CA) and acenaphthene (ACN) system in both the nonpolar solvent benzene (BZ) and the moderately polar 1,2-dichloroethane (DCE) by means of nanosecond laser photolysis. Triplet chloranil (3CA) is quenched by ACN to yield the (1:1)triplet ion pair (IP1), 3(CA\ewdot, ACN\underset.+), in BZ, while the (1:2) ion pair (IP2), 3(CA\ewdot, ACN2\underset.+), in DCE as well as IP1 in the concentration range less than 1 M (1 M=1 mol dm−3) of ACN. Temperature dependence of the transient spectra indicates that IP2 is energetically lower than IP1 in DCE. In BZ, IP1 decays through both back electron transfer (back ET) to the ground state of the donor-acceptor pair and intra-ion-pair proton transfer (PT) leading to formation of the chloranil semiquinone radical (CAH·). In DCE, both IP1 and IP2 disappear through the back ET and ionic dissociation (ID) to free anion and cation radicals, while no PT is observed. The efficiency of ID is greater in IP2(0.70) than in IP1(0.33). From temperature dependence on decay profiles, Arrhenius parameters are obtained for each decay process of the ion pairs. The PT process in BZ requires a considerably high activation energy (30 kJ mol−1), while the ID process in DCE proceeds with practically no activation energy for both IP1 and IP2. The activation barrier for the back ET of IP1 in BZ is obtained to be 13 kJ mol−1 which is significantly greater than that (∼0 KJ Jmol−1) in DCE. This solvent effect may be attributed to the difference in the ion pair states: i.e., a contact type in BZ and solvent-separated type in DCE.