Molecular Rotors: From Newtonian Fluids to Functional Materials as Viscosity and Temperature Sensing Tools

Molecular rotors (MRs) are a distinctive class of fluorescent molecules whose photophysical properties are influenced by their surrounding environment, making them valuable as viscosity nanosensors. However, interpreting their fluorescence responses in complex systems poses a significant challenge, as these environments can not be adequately described by viscosity alone and require additional or different parameters. This review begins by elucidating the fundamental photophysical mechanisms of MRs' fluorescent sensitivity to viscosity, analyzing the impact of chemical structure on fluorescent properties. It describes the Förster–Hoffman relation, which links viscosity with fluorescent emission, and discusses its limitations. Furthermore, it explores recent applications of MRs, including their use in Newtonian fluids, MR‐based functional materials, protein solutions, and composite materials. It highlights their potential as sensing tools for characterizing polymers, monitoring polymerization in real time, and enabling viscosity mapping. We also examine the thermal sensitivity of molecular rotors, particularly when incorporated into functional materials, positing this characteristic as an advantageous feature rather than a limitation. The review concludes with an overview of structure‐property relationships to aid in the selection and design of new molecular rotors for future applications.