Relating Conformation and Photophysics in Single MEH-PPV Chains

Abstract

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.

Conformation and physics of polymer chains: a single-molecule perspective

“…Th is has been demonstrated using a combination of polarization-modulated epi-fl uorescence and total internal refl ection illumination with a widefi eld microscope [20]. It has thus been found that irrespective of conformation, MEH-PPV chains tend to orient parallel with the sample substrate during the spin-coating sample preparation process [15].…”

“…In addition to the projection onto the sample plane, this method uses polarization-modulated total internal refl ection illumination to measure the projection of the ellipsoid in a plane perpendicular to the sample (Figure 3(b)). Th is method has been used to measure the absorption ellipsoids of MEH-PPV in various polymer matrices [15]. Th e conformation of individual chains must still be reconstructed by molecular dynamics simulations, but the conformation is now available for each polymer chain individually, thus making it possible to study the conformation and properties on a truly single-molecule level.…”

“…It was assumed that the observation of fl uorescence blinking must indicate that the chain is collapsed. Recently, a method for 3D determination of absorption ellipsoids was used to examine the relationship between the directly measured conformation and fl uorescence blinking [15]. Th e most important fi nding was that MEH-PPV in low-molecular-weight PS can adopt two diff erent conformations: an extended defect coil and a compact defect coil.…”

Conformation-dependent Room-temperature Emission Spectra of Single MEH-PPV Chains in Different Polymer Matrices

“…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.…”

Connection between the conformation and emission properties of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] single molecules during thermal annealing

“…Previous studies have revealed that there are two main fluorescence time transients in MEH-PPV single molecules, corresponding to collapsed and extended chain conformations. 14,15 A collapsed chain displays discrete fluorescence blinking due to a disruption of the energy transfer pathway to the emitting chromophore by the formation of a long-lived absorbing state or by transient bleaching of the emitting chromophore through said long-lived state. 10 In contrast, an extended chain exhibits continuous photobleaching as one chromophore after another photo-oxidises.…”