Quantum sensing of microscopic viscosity with a molecular qubit framework

Probing dynamics of guest molecules in nanoscale pores of metal–organic frameworks (MOFs) is essential for optimizing their applications in adsorption, separation, catalysis, etc. Microscopic viscosity, which encompasses both intrinsic macroscopic viscosity of adsorbates and additional frictions exerted by the framework through host–guest interactions, reflects self-diffusion of guest molecules. In this study, we present relaxometric sensing of microscopic viscosity in a molecular qubit framework (MQF), namely a MOF consisting of radical qubits. Guest molecules improve the spin–lattice relaxation time of this material by enhancing its structural rigidity and tweaking local hydrogen bonds. This not only empowers the MQF as a quantum sensor for microscopic viscosity but also demonstrates guest adsorption as a useful strategy to enhance the qubit performance of MQFs.