Photophysics of Fluorescent Contact Sensors Based on the Dicyanodihydrofuran Motif

Fluorescent molecular rotors have been used for measurements of local mobility on molecular length scales, for example to determine viscosity, and for the visualization of contact between two surfaces. In the present work, we deepen our insight into the excited-state deactivation kinetics and mechanics of dicyanodihydrofuran-based molecular rotors. We extend the scope of the use of this class of rotors for contact sensing with a red-shifted member of the family. This allows for contact detection with a range of excitation wavelengths up to similar to 600 nm. Steady-state fluorescence shows that the fluorescence quantum yield of these rotors depends not only on the rigidity of their environment, but - under certain conditions - also on its polarity. While excited state decay via rotation about the exocyclic double bond is rapid in nonpolar solvents and twisting of a single bond allows for fast decay in polar solvents, the barriers for both processes are significant in solvents of intermediate polarity. This effect may also occur in other molecular rotors, and it should be considered when applying such molecules as local mobility probes.

Automated summary

Fluorescent molecular rotors have been utilized for measurements of local mobility and contact visualization. This study delves into the excited-state deactivation kinetics and mechanics of dicyanodihydrofuran-based molecular rotors, revealing that fluorescence quantum yield is influenced by both rigidity and polarity of the environment. The findings suggest potential implications for the application of molecular rotors as local mobility probes.