Solvent-related conformational changes and aggregation of conjugated polymers studied by single molecule fluorescence spectroscopy

Single molecule confocal fluorescence microscopy was used to perform photoluminescence spectroscopy on single, isolated molecules of derivatives of the conjugated polymer poly(p-phenylenevinylene). We show that the fluorescence from single chains of these electroluminescent polymers depends strongly on chain conformation. Extended chains show emission from multiple segments, while tightly-folded chains emit only from few distinct sites. The tight coil in folded chains enables the polymer to funnel excitons to highly aggregated low energy regions via three-dimensional Foerster-type energy transfer. This strong intrachain coupling causes these polymers to mimic the photophysical behavior of single chromophores and leads to localized emission and fluorescence intermittency. Polymer molecules that have specifically designed steric hindrance for backbone contacts show higher resistance to solvent-induced interaction between its segments. Only the introduction of non-solvents to such systems forces the polymer to stack its backbone and form aggregates. The luminescence of these buckled polymer chains shows distinctly red-shifted emission presumably due to excimer-formation. These results have significant implications for photophysics and photochemistry of conjugated polymers and their application in thin film electronic devices.