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

Abstract

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support π-electron conjugation across ∼80°curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.Molecular-level engineering in plastic electronics requires a precise understanding of how a particular physical or chemical structure impacts on the physical properties of the material. Disorder effects on the ensemble level can often mask the subtle interplay between function and structure. Large macromolecules such as conjugated polymers are particularly prone to energetic disorder, which gives rise to substantial spectral broadening and is generally attributed to a "particle in a box"-like picture of varying chromophore lengths. 1 Disorder effects have commonly been investigated in matrix isolated materials, such as polyenes, where subtle interplays between molecular shape and electronic structure have been identified. 2-5 However, matrix isolation on its own is not sufficient to overcome disorder but merely helps to screen intermolecular effects. The intrinsic molecular properties themselves are accessible with single-molecule spectroscopy. Although this technique helps to overcome the ensemble limitations, a single polymer chain can still contain many chromophores. [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...

Polarization Resolved Measurements of Individual DNA‐Stabilized Silver Clusters

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

“…Another important recent development is the fi nding that even short isolated PPV oligomers can show large diff erences in fl uorescence spectral shapes and peak positions -bent oligomers of the same length have spectra that are red-shifted from those of the straight versions by as much as 20 nm (at low temperatures) [52].…”

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

“…Previous extensive work on conjugated polymer conformation and spectral properties 5,9 resulted only in information on an average conformation of an ensemble of individual chains and did not allow study of a one-on-one correspondence between the conformation and emission spectrum of the same chain. There has been a continuing interest in the spectral properties and their origin in the PPV family of conjugated polymers, [9][10][11][12][13][14][15][16][17][18][19] both in the bulk and on a single-chain level. Bimodal distribution of spectral maxima of single chains has been reported before 15 and ascribed to aggregation between different parts of the chain, similar to that proposed to explain spectral properties in solution.…”