Polar‐Solvent Effect on the Photocycloisomerization of Symmetrical Bis[anthracenes]: A Transient Ultrafast Kinetic Study

Bisanthracenes] are the few among the fluorescing nonconjugated bichromophores that possess photoreactive properties. The 9,9′‐methylenebis(oxy)]bisanthracenes] 1 (AOCH₂OA) exhibit the highest known intramolecular photocycloaddition quantum yield from the S₁ state and, moreover, display a higher yield in polar solvents, an unexpected result for symmetrical systems. No excimer fluorescence was detected in solution at room temperature. The 10,10′‐dimethoxy derivative 1b was studied by picosecond (ps) laser spectroscopy. In nonpolar solvents (methylcyclohexane), S_n←S₁ was the only transient absorption detected, whereas, in polar solvents (MeCN), the growth and 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 be at the origin of the observed rate enhancement of cycloadduct formation in polar solvents.