Optical microscopy investigations of the structure-property relationships of conjugated organic molecules: From single molecules to films

We use optical microscopy to investigate the structure-property relationships of a series of phenylenevinylene-based materials. We observe for the first time the single molecule spectroscopy of tetrahedral oligomers comprised of four oligo(phenylenevinylene) "arm" molecules covalently bound to an sp3-hybridized carbon core. The single molecule fluorescence dynamics of the tetrahedral oligomers consist of intensity fluctuations on various timescales and fluctuations in the orientation and amplitude of luminescence polarization, and are consistent with chromophore motion and weak inter-chromophore interactions. The dynamic behavior of the tetrahedral molecule is in contrast to the behavior of single chain "arm" oligomers adsorbed to glass surfaces, where no fluctuations in intensity or in luminescence dipole orientation are observed. We do observe differences in; luminescence polarization amplitude from molecule to molecule.{09}We show that we can identify the conformation of individual oligomer molecules by comparing the fluorescence polarization anisotropy of single oligomers to the distribution of conformers determined by gas-phase ion mobility experiments. We also show that the photophysics of single oligo(phenylenevinylene) molecules depend greatly on the physical characteristics of their environment. The single molecules can be tuned from mostly non-luminescent in non-rigid environments to highly luminescent in rigid environments, with varying degrees of dynamic polarization anisotropy and other photophysical properties. These experiments provide insight into the behavior of individual chromophores in thin films with different physical characteristics.; In addition to single molecules, we have also investigated the nanoscale optical properties of thin films of phenylenevinylene oligomers and polymers. Annealed oligomer films tend to form intricate polycrystalline structures with interesting spatially resolved luminescence features. We find evidence of luminescence from intermolecular states, and we can distinguish different polymorphs based on their luminescence spectra. For polymer films, near-field scanning optical microscopy (NSOM) enables us to probe the nanoscale photoluminescence properties of complex polymer blend films used in organic light-emitting electrochemical cells (LECs). We find varying degrees of admixing between the two polymer phases depending on film preparation.