Theoretical Study of Photochromic Compounds: Part 3. Prediction of Thermal Stability

Diarylethene derivatives are known to be photochromic, that is, upon irradiation they change their color because of ultrafast photocyclization. Their proposed use as nonvolatile data storage materials requires the cycloreversion process to be very slow in the dark. We apply density functional theory (DFT) methods to predict kinetics of thermal cycloreversion. The cycloreversion occurs through symmetry forbidden conrotatory electrocyclic mechanism with transition state of strong diradical character and requires the use of unrestricted broken-symmetry DFT formalism. Our results suggest that B3LYP and M05-2X are the best functionals to describe kinetics in these compounds (accurate to within 3-4 kcal/mol from the experimental values). The methods validated in this study show great promise as tools in rational design of the improved photochromic materials.