Dielectric Interfaces Reshape Dynamic Single-Molecule Optical Fingerprints in Nanometer-Thick Polymer Films

Fluorescence lifetimes near dielectric interfaces depend on molecular dipole orientation, but this relationship becomes more complex when emitters are confined within finite nanometer-thick polymer films. Here, we combine radially polarized excitation imaging with time-correlated single-photon counting to follow the orientation and fluorescence lifetime of individual PDI molecules undergoing rotational motion in thin poly(n-butyl methacrylate) films. Orientation-sensitive fluorescence patterns were analyzed using vectorial wave-optical simulations, enabling dipole orientation and excited-state lifetime to be correlated over time for the same emitter. The lifetime–orientation relationships vary systematically with film thickness and between molecules within the same film. Electrodynamic calculations for the glass–polymer–air geometry reproduce the principal trends and show that the observed heterogeneity is consistent with different axial emitter positions. These results establish single molecular dipoles as probes of finite nanometer-thick dielectric environments.