Solvent-Dependent Quenching of the Lowest Excited Singlet State of 9,10-Dibromoanthracene by Ground-State 2,5-Dimethylhexa-2,4-Diene Yielding 9-Bromoanthracene or the [4+2] Adduct

Irrespective of the dielectric constant (ε) or the hydrogen-bonding ability of a pure solvent, the lowest excited singlet state (DBA*) of 9,10-dibromoanth racene (DBA) is quenched by ground-state 2,5-dimethylhexa-2,4-diene (DMHD) giving rise to the appearance of an exciplex emission. By means of sub-picosecond and nanosecond laser photolysis as well as steady-state photolysis, however, the following solvent-dependent results are obtained; (1) in acetonitrile (ε=37.5) and acetone (ε=20.7), an exciplex (DBA-DMHD)* formed between DBA* and DMHD generates the DBA radical anion (DBA$\text{}^{•-+}$) as an intermediate for formation of 9-bromoanthracene (BA) from DBA; (2) in ethanol (ε=24.6), 2-propanol (ε=19.9), 1-octanol (ε=10.3), diethyl ether (ε=4.34), and heptane (ε=1.92), a neutral radical species (NR$\text{}^{•}$) generated by decomposition of (DBA-DMHD)* (or by a reaction of DBA* with DMHD) is an intermediate for formation of a dibenzobicyclo[2.2.2]octadiene-type compound (a [4+2] adduct); (3) both DBA$\text{}^{•-}$ and NR$\text{}^{•}$ are generated in methanol (ε=32.7) but the rate (v) of BA formation upon steady-state photolysis of DBA in the presence of 1 M DMHD decreases in the order of v(acetonitrile, ε=37.5) > v(acetone, ε=20.7) > v(methanol, ε=32.7). It thus can be concluded that the mechanism of exciplex decomposition and the reaction of DBA* with DMHD are affected by not only the dielectric constant of a pure solvent but also its hydrogen-bonding ability.