Decoupling Steric and Charge Effects in Isostructural Salicylideneaniline Polymorphs: A “Spatial Template, Charge Details” Model

Abstract

The regulation of polymorphic behavior by substituents involves a complex interplay of steric and electronic effects that are difficult to decouple. Herein, using −Cl and −CH3 substituents of comparable van der Waals volumes yet opposite electronic characters, we investigated the polymorphic behavior of three salicylideneaniline derivatives. All three compounds form a rare pair of isostructural polymorph families: a yellow, twisted herringbone form and an orange, planar lamellar form, providing an ideal platform for examining charge effects under a constant spatial framework. Within this template, although dispersion is the largest energy contributor in all polymorphs, the dominant factor determining the relative stability difference between the two forms undergoes a fundamental switch: in the chlorine-containing systems, the electrostatic term difference dictates the stability ordering, whereas in the methyl-substituted system, the electrostatic terms are nearly equal and the net dispersive advantage becomes decisive. Solid-state phase-transition temperatures, interfacial crystallization rates, and solid-state fluorescence properties further demonstrate that charge effects extend from static thermodynamics to dynamic kinetics and photophysics. We thus establish, for the chloro- and methyl-substituted salicylideneaniline systems studied here, a “spatial template defines the scaffold, charge effect imparts the details” model, offering an independent electronic dimension for rational polymorph design beyond conventional steric optimization.