Tuning BODIPY molecular rotors into the red: sensitivity to viscosity vs. temperature

Viscosity variations in the microscopic world are of paramount importance for diffusion and reactions. In the last decade a new class of fluorescent probes for measuring viscosity has emerged termed 'molecular rotors', which allows quantitative mapping of viscosity in microscopically heterogeneous environments. Here we attempt to tune the absorption and emission of one such 'molecular rotor' based on the BODIPY fluorescent core into the red region of the spectrum, to allow better compatibility with the 'tissue optical window' and imaging of cells and tissues. We consequently find that our redemitting BODIPY fluorophores are sensitive to environmental temperature rather than to viscosity, thus suggesting a new prototype for a 'molecular thermometer'.