Single-molecule spectroscopy (SMS) of a dual-fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free-volume dynamics in polystyrenes. The SMS study was realized by structural modification of a previously reported flapping molecule by nitrogen substitution, leading to 20 times increased brightness of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The spectral jumps were statistically analyzed to reveal the dynamics of molecular environment. The analysis together with MD and QM/MM calculations demonstrate that the planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 200 Å^3 is available close to the molecule, and that such free volume lasts for an average of 1.17 seconds.
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