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.