Comprehensive Computational Analysis of the Polymorph-Dependent Chromism of N‑Salicylideneaniline Using a Cluster Model of Molecular Crystals

In this study, computational analysis of the local molecular environmental model was applied to three salicylideneaniline polymorphs to investigate the relationship between their crystal structures and chromic properties. A molecular cluster model comprising a central molecule and its surroundings was constructed for each polymorph based on the X-ray crystal structure data. The structure was refined by geometric optimization of the central molecule using a two-layer QM/QM scheme implemented by the ONIOM method. The surrounding molecules were then replaced with the optimized central molecule, and these procedures were repeated until the atomic positions and total energies converged sufficiently. Subsequently, structural changes corresponding to tautomerization and cis-to-trans isomerization of the central molecule were introduced into each cluster model. A comparison of the energy profiles of the tautomers and isomers revealed the factors responsible for variations in the chromic properties relevant to the crystal structure. The α2-form showed significant stabilization of the trans-Keto form compared to the α1-form, in agreement with its long lifetime in the photocolored state. Although the β-form showed similar stabilization, it was implied that the excited-state potential surface was unsuitable for efficient photoisomerization, resulting in the absence of photochromism. Furthermore, the partitioned layers in the ONIOM method enabled us to propose novel electrostatic potential maps to evaluate the intermolecular interactions in a visually understandable manner.