Tunable redox hopping charge transport and electrochromism in multivariate MOFs: effects of substitution patterns and number of sulfonic acid groups

Abstract

Controlling fundamental electron and ion transport in porous materials is a critical challenge because it dictates the performance, speed, and durability of devices ranging from batteries to high-speed sensors. Electrochromic devices change color under applied voltage, but fast and reversible switching requires efficient electron transfer and ion motion. Metal–organic frameworks (MOFs) offer tunable electron and ion transport, but charge mobility is often limited by slow ion diffusion. Our prior work revealed that sulfonation enables exceptionally fast charge transport and electrochromism. However, developing strategies that allow simultaneous and molecular-level control of electron and ion transport within a single framework remains a central challenge. Here, we demonstrate that systematic sulfonation of MOF linkers provides a direct molecular handle to regulate charge transport in ruthenium-loaded UiO-67 MOFs. By varying the number and position of –SO3H groups on MOF linkers, we isolate sulfonation density and spatial arrangement as independent variables governing charge transport. The 3,3′–SO3H framework exhibited faster charge transport than the 2,2′–SO3H analogue, underscoring the significance of the sulfonation position. Increasing the number of sulfonic acid groups further enhanced performance, with the tetra-substituted 3,3′,5,5′–SO3H variant showing the fastest coloration (1.09±0.05 s) and bleaching (1.5±0.2 s) times and an apparent diffusion coefficient (Dapp) of (5±3)×10-6 cm2/s and a coloration efficiency of (641±12) cm2/C. Raman spectroscopy reveals a mechanism of strong sulfonate–cation interactions, in which sulfonation promotes ion-pair dissociation and facilitates ion motion. Together, these results establish linker sulfonation as a general strategy for controlling both ion–electron transport in MOFs, with enhanced electrochromic performance emerging as a direct manifestation of optimized charge transport.