The photochemistry of 2-(1-naphthyl)ethyl benzoates: cycloaddition and intramolecular exciplex formation

The photochemistry of the 2-(1-naphthyl)ethyl benzoates 6 and 7 was examined in order to compare them to previously studied 2-arylethyl 4-cyanobenzoates that underwent a Norrish Type II fragmentation. The 1-naphthyl group was incorporated to provide a fluorescent chromophore for probing the intramolecular electron transfer proposed previously for the mechanism. The naphthalene fluorescence was quenched for both 6 and 7 although at very different rates. For 6, with the higher thermodynamic driving force (-68.9 kJ/mol), intramolecular electron transfer was fast in all solvents, independent of their polarity (cyclohexane to methanol). For 7, with the lower driving force (-26.5 kJ/mol) the process was fast only in polar solvents. Exciplex emission, observed for 6 (but not for 7), exhibited a large solvatochromic effect possibly indicating a high dipole moment (28 D) in polar solvents (stretched conformation) but a lower one (17 D) in nonpolar solvents (folded conformation). Finally, the 4-cyanobenzoate 6 was very unreactive photochemically. In contrast, benzoate 7 underwent a 2 + 2 cycloaddition of the ester carbonyl to the naphthalene ring to give products 8 and 9, a process for which we have found no precedent.