Janus Subnanochannels in Metalloporphyrin-Based Conjugated Metal-Organic Frameworks Enable Ultrasensitive Humidity Monitoring.

Abstract

Controlling water organization within confined spaces is essential for developing high-performance solid-state protonic and electronic devices. Here, we report a family of metalloporphyrin (MPp) based conjugated metal-organic frameworks (MOFs), MPp-Cu-O (M = Fe, Ni, and Cu), that features Janus-type subnanochannels with alternately arranged hydrophilic CuO₄ linkages and hydrophobic aromatic zones. This channel design enables the chemical and spatial confinement of water molecules into moderately bound hydrogen-bond (H-bond) networks that balance adsorption stability and dynamic exchange. The confined water structures in the MOF channels promote rapid proton hopping while inducing electronic reorganization at CuO₄ nodes, thereby coupling protonic and electronic transport pathways to give ultrasensitive responses to water exposure. Among the three MPp-Cu-O MOFs, NiPp-Cu-O exhibits over 6 orders of magnitude enhancement in conductivity at 90% relative humidity, with rapid response and recovery across a wide humidity range, making it one of the most sensitive chemiresistive humidity sensors reported to date. Combined spectroscopic and computational analyses reveal that nanoconfinement suppresses the formation of overcondensed H-bond networks and that the thus-formed moderately bonded water assembly in the MOF channels has sufficient connectivity for proton conduction but with low reconstruction energy to ensure fast dynamics.