Fluorescent molecular rotors under pressure: synergistic effects of an inert polymer

Sterically unhindered boron dipyrromethene dyes bearing aryl hydrocarbons at the mesoposition can function as fluorescent probes for monitoring changes in rheology of the surrounding environment. The key aspect of such behaviour relates to the ease of rotation of the aryl ring, which is set in part by frictional forces with nearby solvent molecules. For the target dye under consideration here, gyration of the meso-phenylene ring shows a pronounced temperature dependence but only a modest sensitivity towards applied pressure. Changing the specific viscosity of the solvent by adding a linear polymer has but a small effect on the fluorescence yield of the dye under ambient conditions and thereby indicates that there is little contact between dye and polymer. Under pressure in the presence of polymer, the fluorescence yield increases dramatically and allows design of an effective fluorescence-based pressure sensor. The simplest explanation of this phenomenon has the polymer wrapping around the dye under pressure and curtailing the rotary action. In addition, it has to be considered that the inert polymer renders the chloroform solvent more susceptible to a pressure-induced increase in density by minimising electrostatic repulsion between chlorine lone pairs. In this respect, the polymer acts as a lubricant for compression of chloroform under pressure.