Cooperative Regulation of Photoreactivity in Isostructural 9-Anthracenecarboxylate Complexes by Metal Ions and Benzimidazole Substituents

Abstract

Photochromic materials are capable of reversible or permanent modulation on optical and electronic characteristics under light excitation, thereby holding great application potential in optical switching, data storage, anti-counterfeiting technology and molecular electronic devices. For coordination complexes, the photoresponsive performances can be rationally optimized by screening photoactive ligands, ancillary ligands and central metal ions. In this study, a set of isostructural metal coordination complexes assembled from 9-anthracenecarboxylate and benzimidazole ligands is reported: [M(9-Ac)₂(2-MeBIM)₂] (1; M = Mn; 2; M = Cd; 3; M = Ni) and [M(9-Ac)₂(2-EtBIM)₂] (4; M = Mn; 5; M = Cd; 6; M = Ni), with 9-anthracenecarboxylic acid (9-HAc) served as the photoactive unit and 2-methylbenzimidazole (2-MeBIM) or 2-ethylbenzimidazole (2-EtBIM) adopted as ancillary ligands. Single-crystal X-ray diffraction confirms identical six-coordinate [MN₂O₄] geometries across the series, establishing a well-defined isostructural platform to isolate the effects of alkyl substitution (methyl vs. ethyl) and metal identity on photoresponsive behavior. Upon xenon lamp irradiation, the Mn and Cd complexes undergo distinct radical-mediated photochromism, with crystalline powders changing from pale yellow to yellow or yellowish-brown, as validated by UV−Vis, fluorescence, and electron paramagnetic resonance spectroscopies. In contrast, the Ni analogues remain photoinactive under identical conditions. Notably, comparative studies reveal that methyl substitution promotes stronger fluorescence quenching, whereas ethyl substitution accelerates coloration kinetics. This work provides a straightforward isostructural model to elucidate the cooperative interplay between alkyl substituents and metal ions in governing photochromic performance, thereby expanding the repertoire of 9-HAc-based photoresponsive coordination systems.