We use a novel fluorescence polarization microscope in combination with molecular dynamics calculations to determine the conformation of individual isolated chains of the conjugated polymer MEH-PPV. We found a narrow distribution of defect cylinder conformations in a poor-solvent matrix and two types of defect coil conformations in a good-solvent matrix. The conformations were related to photophysical properties of MEH-PPV by measuring fluorescence intermittency on the same chains. We obtained direct evidence that the photophysics is determined by the chain conformations and that small changes in the polymer microscopic structure can qualitatively affect the photophysical properties.

We report on the photophysical characterization at the single-molecule level of a graft copolymer consisting of a polythiophene backbone and long polystyrene branches. The presence of the branches prevents the polymer chain from forming a collapsed conformational state. The photophysical properties of the resulting solution-like conformation are studied by measuring single-molecule photobleaching dynamics, emission polarization anisotropy and emission spectra. The results are compared with those obtained on the same polythiophene derivative without the branches. It is found that the presence of the branches is a decisive factor in determining the photophysical properties of the polymers on the single-molecule level.

Emission spectra of single molecules of the conjugated polymer MEH-PPV in host matrices of ZEONEX Ò and polystyrene are reported. Depending on the matrix, the polymer chains attain different conformations, from compact defect cylinders to extended defect coils. The conformations are correlated with photophysical properties, such as emission intensity fluctuations, and with shapes and peaks of emission spectra. The observed bimodal distribution of the spectral peaks is related to distribution in conjugated segment lengths.Conjugated polymers are the subject of intense research as active materials for optical and electronic devices.1,2 Photophysical processes play crucial roles in the device function. However, photophysics of conjugated polymers depends strongly on the unique conformation of individual polymer chains. As a result, many questions on the relationship between structure and optical properties remain open. 3,4 The complexity of the study of conjugated polymer photophysics can be partially overcome by the use of single molecule spectroscopy. 5 Recently, we reported a novel fluorescence polarization microscopic method 6 and used it together with molecular dynamics simulations to reveal the conformation of individual polymer chains on a truly single-molecule level.7 This enabled us to study photophysical properties of MEH-PPV on the same single chains and to show that the conformational state is the determining factor for the photophysical processes. Here, we use the previously determined conformations of MEH-PPV in different host matrices and correlate the structural information with single chain emission spectra.Poly[2-methyloxy-5-(2-ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV, Aldrich, M w 200000, PDI 5) was dispersed by spin-coating in thin-film matrices of ZEONEX and of low-molecular weight polystyrene (PS). The film thickness was on the order of 120 nm. The experiments were carried out on an inverted fluorescence microscope 7 using the 488-nm line of Ar þ ion laser for excitation. Time evolving emission spectra were taken with an EM-CCD camera and an imaging spectrograph as a series of tens of images.Single MEH-PPV chains attain compact defect cylinder conformations in the poor-solvent matrix of ZEONEX (Figure 1). The resulting interchain interactions lead to localization of the exciton and to blinking in the emission time-traces (Figure 1a). Single-chain emission spectra measured for several tens of molecules were analyzed and their maxima were plotted in a histogram in Figure 1c. The histogram shows a single-mode Gaussian-like distribution with a center near 550 nm.In the good-solvent matrix of polystyrene, the MEH-PPV is present in two conformations, 7 a compact oblong defect coil (type 1) and an extended disc-like defect coil (type 2), as seen in Figure 2. The two conformations lead to different photophysical behavior. The compact defect coil causes partial localization of the exciton in domains on the chain, 8 resulting in blinking of the emission (Figure 2a). In contrast, the extended coil ...

We propose a simple method for the measurement of the absorption ellipsoid of luminescent nanoparticles. The method is based on a combination of far-field and near-field polarized excitation in a wide-field fluorescence microscope and provides the orientation and axes ratio r of a rotationally symmetric ellipsoid. Potential applications of the method including the study of conjugated polymer conformations are discussed.