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 improvement of a previously reported flapping molecule
by nitrogen substitution, leading to increased brightness (22 times)
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 observed spectral jumps were statistically analyzed to
reveal the dynamics of the molecular environment. The analysis together
with MD and QM/MM calculations show that the excited-state planarization
of the flapping probe occurs only when sufficiently large polymer
free volume of more than, at least, 280 Å3 is available
close to the molecule, and that such free volume lasts for an average
of 1.2 s.