Conformation and dynamics of conjugated polymers: A single-molecule study

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We use single-molecule spectroscopy to study the conformation and related photophysics of the conjugated polymer MEH-PPV dispersed in spin-coated thin-film matrices. The conformation is measured via the polymer chain absorption anisotropy (absorption ellipsoid ratio r) using a novel fluorescence microscopic method. The method is based on alternative excitation of the sample with far-field epi-fluorescence and near-field total internal reflection modes. Polarization modulated fluorescence from individual molecules is fitted with theoretical expressions to obtain the shape and orientation of the absorption ellipsoid for each chain. A coarse-grain molecular dynamics (MD) is further used to simulate the actual conformations corresponding to the measured absorption ellipsoids. We found that MEH-PPV individual chains can attain a single form of a collapsed conformation in poor-solvent matrices (Zeonex), or can coexist in two forms of extended conformations in good-solvent matrices (PS). The … More MD simulations completely reproduce the experimental results when the factors of solvent, chemical defects and spin-coating conditions are included. The defect cylinder conformation observed in Zeonex explains the photophysical properties of emission intermittency and fluorescence spectra distributions. The two defect-coil-type conformations observed in PS correspond well with the two types of intermittency behavior and with the bi-modal spectral distributions observed. Moment analysis of the MD simulation results provides a link between the two conformation forms and the different photophysical properties. The work on MEH-PPV is further complemented by the study of a graft copolymer consisting of a polythiophene backbone and polystyrene branches. The presence of the branches prevents the molecule from forming a collapsed globule-like conformation. Polythiophenes with grafted PS branches (PT-PS) and without the branches (PT-R) show markedly different photophysical properties. Emission intensity of the PT-PS molecules decays gradually due to photobleaching. In PT-R the emission intensity oscillates between the background and one intensity level before one-step photobleaching takes place. The blinking and bleaching behavior of the PT-R polymer is due to the fact that emission of the chain proceeds from one or a few conjugated segments where the energy is quickly localized after absorption. The continuous decrease of emission intensity in case of PT-PS is a result of each segment emitting independently. The emission properties of the two polymers are further characterized by measuring the single molecule emission anisotropy ratio, the histogram of which shows a peak at 0 anisotropy for PT-PS and minimum at 0 for PT-R, and by studying the time-evolution of fluorescence spectra of individual molecules. Less