Second-Order Autocatalytic Acceleration in the Thermal Bleaching of Photochromes: Spectroscopic and Computational Analyses with an Isostructural Crystal Library

The photochromic behavior of molecular crystals is highly sensitive to local intermolecular interactions, which challenges quantitative structure-function correlation studies. Our approach, using an isostructural library of salicylidenephenethylamines (SPAs), proposes a FW 2-step type mechanism in which substituent effects and the tautomeric state of neighboring molecules modulate the activation barrier of trans-to-cis thermal relaxation in the solid state. Two distinct types of crystals, Series I and II, showed different bleaching processes, namely simple first-order kinetics and autocatalytic second-order kinetics. Cluster model calculations revealed that Series-I crystals stabilized the transition state (TS) through electrostatic interactions between the NH moiety and the substituent on the salicylidene ring, modulating the activation barrier. By contrast, the Series-II crystals stabilize the transition state through an NH···O hydrogen bond, especially when a molecule is surrounded by the trans-Keto tautomer. This stabilization effect decreases as the bleaching reaction proceeds, increasing the activation barrier. These two stabilization modes link nonlinear kinetics and specific intermolecular contacts.