Enthalpy changes and reaction volumes of photoisomerization reactions in solution: azobenzene and p-coumaric acid

Abstract

The photoisomerizations of azobenzene in ethanol and p-coumaric acid in water were studied by the transient grating and photoacoustic methods. The absorption spectrum of cis- p-coumaric acid and the quantum yield of the isomerization reaction in water were measured by a photostationary absorption method at two wavelengths (Φ iso=0.46±0.05). The diffusion coefficient of p-coumaric acid in water at room temperature is 4.4×10 −10 m 2 s −1. This relatively small value suggests a hydrogen bonding between p-coumaric acid and water. Concomitant with the photoisomerization, the enthalpy changes (Δ H) and reaction volumes (Δ V) of azobenzene and p-coumaric acid were measured. The obtained values (Δ H=43±8 kJ/mol and Δ V=−3±5 cm 3/mol for azobenzene, Δ H=49±7 kJ/mol and Δ V=−0.7±0.5 cm 3/mol for p-coumaric acid) are discussed in terms of structural change in trans → cis isomerization.