Enhancing the light driven modulation of the refractive index in organic photochromic materials: A quantum chemical strategy

A theoretical investigation based on DFT predictions on 1,2-dithienyl-perfluoro-cyclopentene derivatives is presented. Molecules of this class have been demonstrated to show efficient thermally irreversible photochromism for developing optical switches. In particular, by doping a glassy polymer matrix with 1,2-dithienyl-perfluorocyclopentene derivatives, an optical element with a light induced modulation of the refractive index can be obtained. Here, a simple model is proposed allowing the correlation between the change of the molecular polarizability, upon photoisomerization, to the modulation of the refractive index of the bulk material. Based on this model and on DFT simulations, we suggest criteria for the optimization and design of the bulk material property (i.e. refractive index modulation) by modifying, with suitable chemical substitutions, the molecular and electronic structure of the photochromic species (that is by changing the molecular property, i.e. the molecular polarizability). A systematic analysis of the polarizability changes upon photoisomerization (from the uncolored to the colored form) is carried out for 24 different molecules and correlations with the changes of the electronic and molecular structure are proposed and discussed.