Polar-solvent effect on the photocycloisomerization of symmetrical bis[anthracenes]: A transient ultrafast kinetic study

Bis[anthracenes] are the few among the fluorescing nonconjugated bichromoph ores that possess photoreactive properties. The 9.9'-[methylenebis(oxy)/bis [anthracenes] ] (AOCH(2)OA) exhibit the highest known intramolecular photoc ycloaddition quantum yield from the S-1 state and. moreover, display a high er yield in polar solvents, an unexpected result for symmetrical systems, N o excimer fluorescence was detected in solution at room temperature. The 10 .10'-dimethoxy derivative lb was studied by picosecond (ps) laser spectrosc opy. In nonpolar solvents (methylcyclohexane), S-n <-- S-1 was the only tra nsient absorption detected. whereas, in polar solvents (MeCN), the growth a nd the decay of a second transient were recorded, and the second transient was attributed to a zwitterion A(+.)-A(-.). The kinetics data were derived. and it was concluded that electron transfer and ion recombination should b e at the origin of the observed rate enhancement of cycloadduct formation i n polar solvents.