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

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.