Photoluminescence Anisotropy in Conjugated Polymer and Single Molecule Ensembles

The exploration of exciton dynamics in organic semiconductor materials, focusing on conjugated polymers and single molecules has been studied by spectroscopic fluorescence polarization (P) measurements. In this investigation, photoluminescence anisotropy is used to explore exciton migration in ensembles of non-interacting organic semiconductor molecules, such as poly(p-phenylene vinylene) (PPV) and perylene diimide (PDI). Additionally, we used two insulators: polymethyl methacrylate (PMMA) to prevent separate molecules from interacting, and poly (vinyl alcohol) (PVA) as a protective covering layer to slow oxygen diffusion. In partially oxidized poly[2-methoxy-5-(2-ethyloxy)-1,4-phenylenevinylene] (MEH-PPV) ensembles immobilized in PMMA, the result shows a linear decrease in P values with increased emission wavelength, demonstrating exciton migration along the polymer backbone. In contrast, one of the PDI derivatives, 2,9-Dipropyl-anthra[2,1,9-def:6,5,10-d'e'f'diisoquinoline-1,3,8,10-tetrone] (PDI-C3) embodied in PMMA, shows nearly constant P values across wavelengths, indicating that no exciton migration with nearly co-linear absorption and emission transition dipoles. Also, one key challenge encountered was the noise in measurement data, particularly at low laser intensities, which caused emission polarization (P values) fluctuations and reduced data reliability. This study improves the understanding of exciton migration and how it affects the optical properties of organic semiconductors, with important implications for their use in optoelectronic applications.