Highly Water-Stable Lanthanide–Oxalate MOFs with Remarkable Proton Conductivity and Tunable Luminescence

Although proton conductors derived from metal–organic frameworks (MOFs) are highly anticipated for various applications including solid-state electrolytes, H₂ sensors, and ammonia synthesis, they are facing serious challenges such as poor water stability, fastidious working conditions, and low proton conductivity. Herein, we report two lanthanide–oxalate MOFs that are highly water stable, with so far the highest room-temperature proton conductivity (3.42 × 10⁻³ S cm⁻¹) under 100% relative humidity (RH) among lanthanide-based MOFs and, most importantly, luminescent. Moreover, the simultaneous response of both the proton conductivity and luminescence intensity to RH allows the linkage of proton conductivity with luminescence intensity. This way, the electric signal of proton conductivity variation versus RH will be readily translated to optical signal of luminescence intensity, which can be directly visualized by the naked eye. If proper lanthanide ions or even transition-metal ions are used, the working wavelengths of luminescence emissions can be further extended from visible to near infrared light for even wider-range applications.