Tunable aurophilic aggregates as the basis of mechano- and thermochromism in gold(i) ethynyl complexes

Abstract

Mechanochromic materials that change colour in response to mechanical stimuli hold promise for applications in sensors, data storage, and smart materials. Here we report the mechanochromic and thermochromic behaviour of a systematic isostructural series of gold(I) ethynyl–pyridine complexes, where the position of the pyridine nitrogen atom is varied and compared to a phenyl analogue. All compounds display striking luminescence changes upon mechanical grinding and thermal treatment, with emission consistently shifting to a common green state (520–540 nm). These transformations are reversible upon solvent-vapour exposure and are also retained in PMMA films. Structural (PXRD) and photophysical analyses reveal that these spectral shifts arise from packing-dependent modulation of two accessible excited-state manifolds: a higher-energy ILCT state and a lower-energy state whose stabilization is highly sensitive to aurophilic aggregation and local microstructure. Mechanical grinding disrupts long-range order and generates disordered microdomains that favour this aggregated CT-type excited state and enhance its radiative efficiency, contributing to the strong green mechanochromism. These results demonstrate how subtle ligand variations can direct supramolecular packing and excited-state balance to achieve robust, reversible chromic responses, providing general design principles for next-generation stimuli-responsive Au(I) luminescent materials.