Effect of Nanoscale Confinement on the Structure and Properties of Single Conjugated Polymer Molecules

The research presented in this thesis was motivated by questions on the effect of nanoscale confinement on molecular conformation and related photophysical properties of conjugated polymers. Using microdroplet techniques as a method of isolating single molecules of various poly[phenylene vinylenes] we discovered that poly[2-methoxy-5- (2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-Methoxy-5-(2'- ethylhexyloxy)-1,4-(cyanovinylene)phenylene] (CN-PPV) can be deposited on precleaned glass substrates with unique transition moment orientation. Structural investigation using a combination of fluorescence emission pattern imaging, atomic force microscopy and polarization anisotropy measurements revealed that individual polymer nanostructures had a high degree of intra-molecular with the long axis of the conjugated segment oriented perpendicular to the substrate. The high degree of internal structural order within individual polymer chains affected the spectral and photophysical properties compared to the bulk polymer. The photochemical stability of z-oriented nanostructures was orders of magnitude higher with ≈30x times more photon count rates than the in- plane oriented species in ambient conditions at similar excitation conditions. Z-oriented nanostructures showed narrow bandwidth spectral emission, which was typically redshifted with respect to the bulk polymer spectra. Investigation of the central frequency distribution of the fluorescence emission spectra of MEH-PPV revealed discrete emission from localized conjugated segments within the nanoparticle. Definitive evidence of single site emission from z-oriented polymer nanostructures was obtained from photon correlation measurements. Fluorescence lifetime and fluorescence quantum yield measurements also point to a transition dipole surrounded by a nanoscale dielectric-in these case-conjugated segments in the polymer chain. With facile sample preparation, high photon count rates and high photochemical stability in the ambient conditions and highly pure single photon emission, z-oriented nanostructures can potentially be used as a source for single photon emission for quantum information processing.