Porphyrin-based molecular rotors as fluorescent probes of nanoscale environments

The synthesis, characterization, and photophysical properties of a series of supramolecular triads consisting of a tin(IV) porphyrin with axial ligands of ortho-, meta-, and para-hydroxyphenyl naphthalenediimides are presented. For the meta- and parahydroxyphenyl derivatives, efficient and solvent-viscosity-dependent quenching of porphyrin fluorescence is observed. Experimental and theoretical studies demonstrate that, in these compounds, photoinduced electron transfer from the phenolate to the porphyrin is modulated by large amplitude rotational motions of the naphthalenediimide. These compounds are novel examples of fluorescent molecular rotors, and their potential use as environmental probes of local viscosity and temperature are discussed.