Metal-ion-dependent polarity switching and dielectric enhancement in solvent-vapour-responsive mononuclear complexes

Abstract

Design of molecular materials that exhibit multiple cooperative responses to external stimuli is a key challenge in the development of next-generation functional materials. Herein, we report solvent-vapour-responsive mononuclear Fe(III) and Mn(III) complexes, [M(azdp)(acac)(sol)] (M = Fe, Mn; H₂azdp = 2,2′-azodiphenol; sol = MeOH, DMSO), which undergo solvent-vapour-driven structural transformations between polar and nonpolar molecular assemblies through ligand exchange at the solvent coordination site. The Fe(III) complexes exhibited pronounced cooperative aggregation upon DMSO coordination, generating polar molecular packings that showed strong second harmonic generation (SHG), enhanced dielectric constants, and ferroelectric-like relaxor behaviour. By contrast, the Mn(III) analogues form centrosymmetric dimers stabilised by Jahn–Teller distortion, resulting in nonpolar assemblies and SHG inactivity. These findings revealed a direct relationship between the metal-centred electronic structure, ligand geometry, and aggregation-induced functionality. The cooperative transformations integrate optical and dielectric functionalities within a single molecular platform, demonstrating a generalisable strategy for constructing solvent-vapour-responsive polar coordination materials.