Effect of Intramolecular Hydrogen Bonding on Photocleavage Reaction of (3-Benzazolyl-2-hydroxy-5-methylphenyl) Methyl Acetate

Abstract Caged compounds, (3-benzazolyl-2-hydroxy-5-methylphenyl) methyl acetate (HBO-Ac, HBT-Ac, HBI-Ac) were synthesized and their photocleavage reaction were investigated by means of determination of quantum yield of photocleavage reaction as well as absorption and fluorescence spectroscopy and DFT calculation. These compounds are expected to exist as three conformations anti-enol (a), anti-enol (b) and syn-enol and exhibited fluorescence emission due to the tautomer produced by intramolecular hydrogen-atom transfer in anti-enol (a) form. The compounds HBT-Ac and HBI-Ac underwent photocleavage reaction in CH3Cl, CD3OD and aqueous CD3OD, while HBO-Ac underwent photocleavage reaction in CD3OD and aqueous CD3OD but almost no reaction in CDCl3. The photocleavage reaction may take place from anti-enol (a), which is in accordance to the results of DFT calculation and the experimental results that the relative efficiency of photocleavage reaction increased with decreasing the energy level of anti-enol (a) vs. syn-enol in the ground state calculated by DFT. The results obtained in this study suggest that the efficiency of photocleavage reaction depends on the solvent properties and hydrogen-bonding properties of light-absorbing chromophore of caged compounds which control the stability of conformation causing photocleavage reaction.