Thermal racemization of spiropyrans: implication of substituent and solvent effects revealed by computational study

The thermal racemization of spiropyrans has been investigated by computational study at the density functional theory and MP2 levels. Two possible reaction mechanisms for the thermal racemization of spiropyrans were found. One reaction mechanism is a three-step reaction, the other represents a four-step reaction. The competition between these two mechanisms is dependent on the strengths of the substituents, as well as the polarities of the solvents. Different substituents and solvents change the rate-determining step and lead to various pathways. The solvent effect is found more pronounced on the electron-withdrawing substituted spiropyrans. In addition, bond length alternation (BLA), a structural parameter, has been employed in this study. It has been proven that BLA is a useful parameter to correlate the effect of substituents and solvents with the racemization mechanism of spiropyrans. The relationship between BLA and activation energies clearly explains the origin of the competition between the two reaction mechanisms. The results from this work would be valuable for selecting substituents and solvents for development of desired optical properties of spiropyrans.