Journal of Thermodynamics & Catalysis
Open Access

Abstract

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’-Nitroazobenzene are Highly Influenced by Solvent Polarity

Spurgeon Smith and Fadi Bou-Abdallah

The thermally stable trans isomers of donor-acceptor substituted azobenzene dyes can be easily transformed into unstable cis-form by light. Although azobenzene and its derivatives have been extensively investigated over the years, many aspects of the isomerization reaction kinetics are still unclear or controversial at best. Here, we investigate the kinetics of the thermal cis-trans isomerization of 4-anilino-4'-nitroazobenzene (4A4NAB) in six solvents of different polarities by means of flash photolysis. Using the transition state theory, we were able to determine a number of thermodynamic quantities including the standard enthalpy of activation Δ‡H0, the standard entropy of activation Δ‡S0, the Gibbs free energy of activation Δ‡G0, and the quasi-equilibrium constant K‡ of the isomerization reaction. Higher rates of isomerization and lower activation energies were observed in polar media, consistent with the formation of a charged transition state complex and the presence of solute−solvent interactions such as intermolecular hydrogen bonding. The data suggest that the cis-trans thermal isomerization of 4A4NAB is highly influenced by solvent polarity and the isomerization mechanism is likely to proceed via rotation about the azo double bond.