Crystal-environment control of excited-state reaction branching in 5-chloro-N-salicylideneaniline

We investigated how crystal packing controls the photochromic reactivity of 5-chloro-N-salicylideneaniline (5CSA) by combining excited-state reaction-path analysis with ab initio molecular dynamics simulations for the isolated molecule and cluster models of its α- and β-phases. For the isolated molecule, the static pathway analysis revealed two competing relaxation channels near the S₁ Franck-Condon region: an ESIPT pathway leading toward trans-keto formation through subsequent C-C twisting, and a competing C-N twist pathway leading toward twist-enol relaxation. The nonplanar α-phase showed branching behavior similar to that of the isolated molecule, whereas the planar β-phase exhibited a strongly biased branching toward ESIPT, indicating dynamic suppression of the competing C-N twist channel. Moreover, although ESIPT occurred efficiently in all environments, the subsequent C-C twisting required for trans-keto formation was strongly hindered in the β-phase. These results show that the phase dependence of photochromism is governed not by the occurrence of ESIPT itself, but by crystal-environment control over excited-state branching and post-ESIPT structural relaxation. The salicylideneaniline planarity rule is therefore understood not merely as a static structure-reactivity relationship, but as a dynamical consequence of crystal-environment-induced restriction of the reaction pathways leading to photoproduct formation.