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

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 angstrom(3) is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

Automated summary

The study utilized a dual fluorescent molecular probe to monitor nanoscale dynamics in polystyrenes, revealing that planarization of the molecule in the excited state only occurs when a large polymer free volume is available nearby, lasting on average 1.2 seconds. This investigation provides insights into the dynamics of molecular environments through statistical analysis and computational simulations.