Dynamic Polymer Free Volume Monitored by Single-Molecule Spectroscopy of Dual Fluorescent Flapping Dopant

Polymer free volume is a rational concept for interpreting polymer properties connected to the fraction of unoccupied volume. While arbitrary definition of “free volume” often led to controversy, quantitative evaluation of polymer free volume is important for developing membrane materials as well as packing materials for chromatography that adsorb, diffuse, and separate small molecules. Local free volume moves from moment to moment due to thermal fluctuation of surrounding polymer chains. To evaluate the free volume directly and quantitatively, several approaches have been established such as positron annihilation lifetime spectroscopy (PALS), inverse gas chromatography (IGC), ¹²⁹Xe NMR spectroscopy, and molecular probe. In recent years, some studies have been reported that compare experimental results with molecular dynamics (MD) simulation. However, real-time monitoring of the local free-volume dynamics has only been achieved by single-molecule spectroscopy (SMS) analysis of conformationally flexible probes. In this report, single-molecule fluorescence lifetime has been analyzed because the conformational change of the fluorescent probe, twisted peryleneimide, results in very little spectral difference. This work is an example of the great potential that the SMS technique holds for spatially-resolved dynamic studies of polymers and soft matter in general. Apart from free-volume probing, SMS has been used, e.g., to analyze heterogeneity of local relaxation processes of polymers near glass transition temperature (T_g), to characterize diffusion in solutions or melts, or to monitor polymerization reactions on molecular level. In particular, the SMS method could be an attractive alternative to tracking the dynamics of the polymer free volume because of its high location specificity, suitable dynamic range, non-invasive nature, and relatively affordable and simple optical instrumentation. Here we succeeded in demonstrating such free-volume monitoring by developing a new flexible fluorophore, nitrogen-embedded flapping molecule (N-FLAP), that shows dual fluorescence (FL) spectrum depending on the bent/planar conformations. Compared with the previously reported flapping anthracene, the nitrogen-embedded molecular framework led to much improved FL brightness as well as high photostability, when excited by a visible light. With this unique molecular probe physically doped in polystyrenes, SMS study has been performed to demonstrate that dynamic nanoscale changes of polymer free volume can be monitored in real time by following environment-induced spectral changes of a single N-FLAP molecule. In this way, the probe is capable of visualizing local free volume changes on the order of 200 ų on millisecond time scales for extended periods of time. The use of single-molecule imaging techniques can, in principle, provide also 3-dimensional localization of the probe and enable position-dependent studies of the free volume dynamics.