Single chain spectroscopy of conformational dependence of conjugated polymer photophysics

Abstract

Single molecule confocal fluorescence microscopy was used to perform photoluminescence spectroscopy on single, isolated molecules of the conjugated polymer poly[2-methoxy,5-(2-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV). We show that the fluorescence from single chains of this electroluminescent polymer depends strongly on chain conformation. The time evolution of the spectra, emission intensity, and polarization all provide direct evidence that memory of the chain conformation in solution is retained after solvent evaporation. Chains cast from toluene solution are highly folded and show memory of the excitation polarization. Exciton funneling to highly aggregated low energy regions causes the chain to mimic the photophysical behavior of a single chromophore. Chains cast from chloroform, however, behave as multichromophore systems, and no sudden discrete spectral or intensity jumps are observed. These also exhibit different spectroscopy from the folded chromophores. R ecent advances in instrumentation have allowed the detection and manipulation of single molecules at room temperature (for recent reviews see, e.g., ref. 1). These techniques have led to the observation of physical effects and properties of individual molecules that could not be probed by previous ensemble-averaging studies. For example, measurements of inter-and intramolecular binding forces and elastic moduli of biological macromolecules by magnetic beads (2, 3), atomic force microscopy (4), hydrodynamic flow (5), or optical tweezers (6) have provided significant insight to the mechanical properties of DNA and proteins. Single molecule fluorescence detection has seen particular interest, because the fluorescence labeling of DNA, RNA, enzymes, and proteins allows for dynamic studies of macromolecular interactions and function, such as complexation (7). The optical visualization of these molecules, however, requires specific labeling with fluorescent dyes or the use of intercalating dyes.Single molecule studies of intrinsically fluorescent macromolecules, such as photosynthetic light harvesting complexes (8) or the green fluorescent protein (9), have exhibited interesting quantum effects. Protein molecules with several chromophores, for example, were shown to behave as a single quantum system (10). The close packing of chromophores in such proteins leads to strong interchromophore interaction that gives rise to cooperative effects such as intermittent fluorescence (11, 12) and photon-antibunching (10). Single molecule fluorescence studies of green fluorescent protein revealed similar intermittent fluorescence (9).Most remarkably, intermittent fluorescence together with the observation of discrete emission levels was also reported for single molecules of conjugated polymers estimated to consist of over 1000 chromophores (13). Another surprising result is the recent observation of efficient photoluminescence quenching of polymer chains by extremely low amounts of cationic electron acceptors (14), which provides a unique bio-probe scheme with high ...

How Chromophore Shape Determines the Spectroscopy of Phenylene−Vinylenes: Origin of Spectral Broadening in the Absence of Aggregation

“…This blue shift is generally attributed to a reduction in effective conjugation length due to oxidation. 6,7 Our data suggest an additional important effect: the bent polymer chromophores emit in the red and are less stable to environmental influences; consequently, they will oxidize first. With the lower energy units removed from the intramolecular energy transfer cascade, 1,6 excitation energy becomes trapped on the less-deformed units of higher energy, with the overall molecule thus emitting in the blue.…”

“…[6][7][8][9][10][11][12] Energy transfer between these chromophores can mask the true photophysics of the individual spectroscopic unit. Although polarization-resolved spectroscopy has yielded detailed insight into the conformation of the polymer chain, 7,8,11,[13][14][15] very little is known about the shape of the indiVidual chromophore. Because a physical bend can potentially interrupt the π-electron conjugation, the chromophore is generally thought to be linearly extended in space leading to linearly polarized absorption.…”

“…Oligomers ensure that emission and absorption occur from the same chromophore without intramolecular interchromophoric energy transfer contributing to depolarization. 7,8,11,[13][14][15] Figure 1 summarizes quantum chemical molecular dynamics calculations of a DEH-PV heptamer, which serves us as a model of an MEH-PPV chromophore. The calculations follow the previously reported excited-state molecular dynamics procedure.…”

“…Samples were prepared by dissolving the compounds in toluene at concentrations of 10 -11 M, adding Zeonex as a host polymer matrix, and spin-coating the solutions on top of SiO 2 -covered silicon wafers to yield films 30 nm thick. The samples were prepared in a dry nitrogen glovebox to minimize any potential influence of oxygen or moisture on the molecular conformation [7][8]11 and subsequently mounted in a helium cold finger microscope cryostat under a vacuum of 10 -6 mbar and studied using an epifluorescence microscope described elsewhere. 9 Luminescence was excited by an Ar + laser (488 nm, up to 200 W/cm 2 intensity) and detected in 10 s integration windows.…”

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

“…Th e conformation in solution is retained even after depositing individual polymer chains onto a solid substrate. Th is has been elegantly demonstrated for single chains of MEH-PPV cast from toluene (a poor solvent) and chloroform (a good solvent) [28], as shown in Figure 4(a). In MEH-PPV/toluene samples, all chains show step-wise intensity changes, whereas MEH-PPV/chloroform chains decay continuously as the conjugated segments bleach one by one and there is little interaction between the segments in an extended chain.…”