Photochemistry of 2‘-Hydroxychalcone. One-Way Cis−Trans Photoisomerization Induced by Adiabatic Intramolecular Hydrogen Atom Transfer

Abstract

Intramolecular hydrogen atom transfer and novel one-way photoisomerization of 2‘-hydroxychalcone (2HC) were investigated by quantum yield measurements, transient absorption spectroscopy, and semiempirical calculations. The trans isomer of 2HC (trans-2HC) did not give the cis isomer, while cis-2HC underwent one-way isomerization to give the trans-2HC. The triplet−triplet absorption spectra of cis- and trans-2HC were similar and were assigned to the tautomer produced via the intramolecular hydrogen atom transfer in the excited triplet state. The triplet energies of the normal form and the tautomer form of trans-2HC were estimated by quenching experiments to be 229 and 177.5 kJ mol-1, respectively. Heats of formation, the frontier molecular orbital coefficients, and optimized structures of trans-2HC and cis-2HC in the ground, the excited singlet, and the excited triplet states were calculated by semiempirical (MOPAC93 PM3/4CI) calculations. These calculations agree well with the experimental observations. Based on the experimental observations and theoretical considerations, a novel mechanism and the potential energy surface for the one-way photoisomerization of 2HC were proposed. The photoisomerization of 2HC was induced by hydrogen atom transfer in the excited triplet state. Thus, 2HC can be viewed as a molecule in which the mode of isomerization around the carbon−carbon double bond can be controlled by the remote intramolecular hydrogen bonding. Solvent affected the triplet lifetime without changing the mode of isomerization. On prolonged irradiation, trans-2HC gave flavanone by very low quantum yield (Φ = 1.9 × 10-3 in benzene). The cyclization reaction to form flavanone was studied and the quantum yields were determined.