Why Emission Spectral Profile of Diverse Fluoro-phores Look Alike with Thick Polymer Overlayers?

Fluorescence from fluorophore-polymer overlayers reflects an interplay between molecular photo-physics and thickness dependent optical propagation. Combining spectrally resolved single mole-cule measurements, transition dipole moment orientation analysis and ensemble fluorescence, we show that structurally and photophysically distinct fluorophores can exhibit similar red shifted emission profiles under thick polymer overlayers even without additional reflecting surface. This similarity indicates a common optical transfer function imposed by the overlayer rather than a pure-ly fluorophore specific molecular response. Although local molecular interactions, excited state dy-namics and vibronic coupling govern intrinsic emission, the overlayer modifies detected spectra through refractive index contrast, dielectric boundary reflections, cavity effects and collection ge-ometry. Vibronic energy spacing distributions remain largely preserved as thickness changes, whereas spectral positions and relative intensities vary systematically. Thus, polymer thickness pri-marily reshapes detected spectrum through optical propagation and outcoupling while retaining vi-brational modes. Time dependent density functional theory identifies candidate Franck-Condon and Herzberg-Teller active dye modes contributing to spectral fluctuations and dipole reorientation.