Isomerization Kinetics of Hydroxy-Substituted Azobenzenes Using a Modified Commercial Flash Photolysis Spectrometer

A laboratory experiment that exposes students to fundamental concepts in kinetics and statistical thermodynamics to study systems of great current interest in nanotechnology by employing a foundational technique in physical chemistry is described. A modified commercial flash photolysis spectrometer was used to study photoinduced isomerization reactions on the millisecond time scale for several hydroxy-substituted azobenzene compounds. The temperature-dependent transient decays were analyzed with an Eyring plot to yield the enthalpy and entropy of activation of the reverse isomerization process. The specific compounds studied were 4-phenylazophenol, 4-methyl-2-(phenylazo)­phenol, and 2,4-dihydroxyazobenzene in ethanol solvent. Instrumental modifications include substitution of a UV-LED for the probe light source and use of a thin polyimide film heater adhered to the side of the sample cuvette to control temperature. Collection of experimental data and the subsequent plotting and analysis are easily accomplished by upper-division chemistry students and the results are consistently good, comparing well with literature values.