Response of Fluorescent Molecular Rotors in Ternary Macromolecular Mixtures

For a few decades, fluorescent molecular rotors have been commonly employed as local probes of microviscosity in complex materials. However, without proper calibration, relating microviscosity to a physical parameter is unclear, which strongly limits the quantitative use of such probes in biological media, for instance. In this study, the response of a molecular rotor in binary and ternary macromolecular aqueous solutions of poly(ethylene glycol) (PEG) of different molecular weights is investigated in order to better rationalize the sensitivity of rotors to their cybotactic environment. More precisely, for the investigated composition range of ternary mixtures, it is shown that a linear mixing rule applies for the evolution of fluorescence lifetime with the proportion of the two PEGs, and with an increasing ratio of heavy PEG leading to larger lifetimes. These results allow us to test more precisely the free volume theory, which has been proposed in the context of probing glass transition. Analysis shows that while this theory semiquantitatively captures the observation, its precise use raises some questions.