Screening-Enabled Chemiresistive Moisture Sensing with Tetrathiafulvalene-Based Electrically Conductive Metal-Organic...

Electrically conductive metal-organic frameworks (MOFs) combine high crystallinity and large surface area with guest-tunable electrical conductivity, making them ideal for chemiresistive gas sensing. Whereas many sensitive and selective MOF-based chemiresistive sensors have been fabricated, their underlying sensing mechanisms often remain elusive, primarily due to the reliance on polycrystalline films. Herein, we overcome this limitation by investigating chemiresistive moisture sensing using single crystals of a series of isostructural MOFs, M₂(TTFTB) (M²⁺ = Mn²⁺, Co²⁺, Zn²⁺, Cd²⁺; TTFTB^4- = tetrathiafulvalene tetrabenzoate). We reveal that electrical conductivity in these materials is strongly affected by Coulombic interactions between TTF^•+ radical defects and charge-balancing anions. In dry conditions, these interactions induce charge trapping. The adsorption of water vapor screens the hole-anion interactions, mitigating charge trapping and thereby significantly enhancing electrical conductivity. This mechanism is particularly pronounced in Zn₂(TTFTB) owing to the high Coulombic potential of Zn²⁺. This study establishes a general methodology for understanding charge transport in MOFs and provides critical guidelines for the rational design of high-performance chemiresistive sensors.