Molecular Rotors Based on the Boron Dipyrromethene Fluorophore

In order to examine how changes in the size of the rotary group affect the efficacy of molecular probes for monitoring changes in local viscosity, a boron dipyrromethene dyebearing a meso‐phenanthrene unit has been synthesized and fully characterized. 19F NMR spectroscopy, together with molecular modelling, indicates that the bulky phenanthryl unit cannot rotate completely around the connecting C–C linkage but can oscillate over a reasonably large dihedral angle. This situation is to be contrasted with the corresponding dye having a meso‐phenylene ring. The latter dye functions as a molecular probe for changes in viscosity of the surrounding solvent but remains essentially insensitive to changes in the polarity of the solvent. The opposite situation is found for the phenanthryl derivative, where a charge‐transfer state lies at higher energy than the emissive π,π* excited state but can be accessed thermally. The results are considered in terms of energy‐level diagrams taking into account rotational freedom. Photophysical properties are reported for both dyes in a range of solvents, and temperature‐dependent studies are described. The design elements for an efficacious, molecular‐scale fluorescent rotor are illustrated by reference to the comparison of two boron dipyrromethene based dyes bearing meso substituents of disparate size. A major issue relates to the distinction between the recognition of viscosity and polarity changes.