Modeling molecular environment of allomorphic N-salicylidene-4-halo-aniline crystals through iterative QM/QM′ structural optimization

Context Two allomorphic crystals, photochromic N -salicylidene-4-bromoaniline (α-SA4B) and non-photochromic N -salicylidene-4-chloroaniline (β-SA4C), were simulated using a hierarchical hybrid quantum mechanical (QM/QM′) method. Despite the similar potential energy surface (PES) profiles observed in their isolated forms, distinct differences emerged in the cluster model simulations of α-SA4B and β-SA4C. This observation suggests that the molecular environment influenced the torsional energy landscape associated with their chromic properties. Method Each crystal was represented by a cluster model consisting of a central molecule and 14 peripheral molecules arranged based on crystallographic symmetry. The central molecule was treated as the high-level QM layer, while the surrounding molecules were kept fixed as the low-level QM′ layer. Following iterative optimization of the cluster model, the PES was calculated to track the cis-trans isomerization process of the central molecule. The iterative optimization was conducted with ONIOM (B3LYP/6-311G**: HF-D3BJ/6-31G*), and the PES was calculated with the same level. For each cluster model, the excited state was calculated with the TD-DFT method. All the quantum chemical calculations were performed using Gaussian 16 software.