Conformational Dynamics of Photoexcited Conjugated Molecules

Tretiak, Sergei; Saxena, Avadh; Martin, Richard L.; Bishop, A. R.

doi:10.1103/physrevlett.89.097402

Cited by 362 publications

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“…8 We find that the emission of the single oligomers is generally linearly polarized (34 single heptamers gave an average polarization anisotropy 〈M〉 ) 0.93), suggesting the involvement of one single linear transition dipole arising from the self-trapped exciton. 24 In contrast, the excitation, which is probed by rotating the laser polarization by a λ/2 plate and monitoring the PL intensity, indicates a departure from a linear absorbing unit for many single oligomer chains, in agreement with recent reports that even single para-phenylene dimers can undergo substantial bending and twisting. 22 This difference between absorption and emission anisotropy arises because absorption probes a larger region of the molecule than emission, where self-trapping of the exciton due to structural relaxation reduces the number of repeat units involved in the electronic excitation.…”

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confidence: 88%

“…The distortions only have a minimal effect on conjugation of the system, as determined by the oscillator strength of the transition and the spatial delocalization of the respective transition density (not shown). 24 Similar distortions are possible for chain bending within the plane (panel b). Because the oscillator strength is not affected significantly, we expect to observe equally bright molecules with different degrees of polarization anisotropy in excitation.…”

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confidence: 63%

“…22 This difference between absorption and emission anisotropy arises because absorption probes a larger region of the molecule than emission, where self-trapping of the exciton due to structural relaxation reduces the number of repeat units involved in the electronic excitation. 24 Following from Figure 1, a spatial distortion of the chain will induce a low polarization anisotropy in absorption without affecting the oscillator strength of the transition. Emission will occur due to a transition involving fewer monomer units following exciton self-trapping 24 and will therefore tend to display a higher degree of polarization.…”

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confidence: 99%

“…24 Following from Figure 1, a spatial distortion of the chain will induce a low polarization anisotropy in absorption without affecting the oscillator strength of the transition. Emission will occur due to a transition involving fewer monomer units following exciton self-trapping 24 and will therefore tend to display a higher degree of polarization.…”

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confidence: 99%

“…The calculations follow the previously reported excited-state molecular dynamics procedure. 24 To assess a particular bending angle θ, we enforce a molecular distortion by fixing the positions of the outermost atoms, optimize this constrained geometry, and compute the according heat of formation at optimal constrained geometry. This approach enables us to link theory to experiment: the molecules in the experiment are inevitably dispersed in a matrix and experience external forces that are not accounted for in a structure geometry optimized in Vacuo.…”

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

et al. 2008

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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...

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confidence: 88%

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confidence: 63%

mentioning

confidence: 99%

mentioning

confidence: 99%

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confidence: 99%

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