A Simple Pulse-Probe Experiment Based on Photochromism

A series of experiments suitable for undergraduates based on photochromism is presented. The experiments involve simple kinetic spectroscopy, transient absorption, NMR, and computational techniques. Specifically, experiments based on the kinetics and transient absorption of 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2-2′-(2H)-indole] (I) in several solvents of differing polarity are presented. The kinetics show a first-order rate law for all solvents. Temperature studies yield enthalpies and entropies that depend on the solvent used. The transient absorption spectra in polar solvents are blue-shifted compared to nonpolar solvents. Additional experiments, using 1H NMR to prove that only one intermediate is formed when the spiro compound is irradiated with UV light are described. Results from Spartan modeling calculations regarding the relative stability of possible intermediates are discussed. The experiments can be extended for semester projects.