Metal-center electron affinity modulates multicolor electrochromism in 2D conjugated metal-organic frameworks

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Two-dimensional conjugated metal-organic frameworks are emerging platforms for electrochromic devices, yet the current stage is limited by monotonous color and an unclear coloration mechanism. While prior studies emphasize the ligand-centered redox process, the role of metal centers is often overlooked. Herein, we report a metal-center electron-affinity mechanism that is critical to electrochromic performance. We first employ an unsupervised machine learning method to screen divalent metal-catecholate (CAT) systems, identifying Zn, Ni, and Cu as optimal candidates. All three systems exhibit multicolor transitions from green to blue and eventually to purple; however, Zn- and Ni-CAT-1 significantly outperform Cu-CAT-1, with nearly threefold faster switching speeds (4.8/5.4 vs. 14.4 s) and cycling stability enhanced by two orders of magnitude ( > 20000 vs. < 300 s). Cu2+ (3d9) possesses a redox-active configuration that favors electron affinity, thereby requiring additional electron participation and a higher energy demand than the relatively inert Ni2+ (3d8) and Zn2+ (3d10). This valence variation also induces stronger metal-oxygen coupling, ultimately leading to slower redox kinetics and structural degradation. As proof of concept, we demonstrate the applicability of these findings to camouflage, displays, and wearables for designing multicolor electrochromic devices. Two-dimensional metal-organic frameworks show promise for electrochromism, but their coloration mechanism remains unclear. Here authors reveal a metal-center electron-affinity mechanism for improving stability and electrochemical kinetics.

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