Simultaneous Raman and Fluorescence Spectroscopy of Single Conjugated Polymer Chains

Walter, Manfred J.; Lupton, John M.; Becker, K. W.; Feldmann, Jochen; Gaefke, Gerald; Höger, Sigurd

doi:10.1103/physrevlett.98.137401

Cited by 41 publications

(73 citation statements)

References 24 publications

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“…36 Once thermally activated intramolecular excitation energy transfer occurs, 37 emission will arise from the lower energy broad emitting bent chromophores. In this context it is also rewarding to note that we recently succeeded in mapping the progression of narrow PL transition lines of a poly(para-phenylene) directly to the single-chain Raman scattering spectra, 38 demonstrating directly that it is indeed the narrowest transitions that correspond to the chemically purest species.…”

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confidence: 96%

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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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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“…One of the foremost exploitations of this effect is surface-enhanced Raman scattering (SERS) where the interactions between light and the vibrational modes of matter within the hot spot can be magnified, enabling Raman scattering measurements of a single molecule. Such a feat necessitates overall enhancement factors approaching 12 orders of magnitude78910. Beyond SERS, plasmonic systems have also been employed in a wide range of nanophotonic applications such as near-field imaging11 and transmission spectroscopy12 using highly localized nonlinear light sources; nanoscale lasers13; and ultra-sensitive analysis of dielectric environments14.…”

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Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

Borys

Shafran

Lupton

2013

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The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.

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“…We used infrared spectroscopy to study microscopic changes induced by doping, as this technique is known to be sensitive to the presence of charged species in conjugated polymers through molecular vibrations [27][28][29] . An advantage of looking at vibrations is that their absorption resonances are less affected by inhomogeneous broadening, an effect which is instead markedly impacting the amount of information possibly extracted from electronic transitions in conjugated polymers 30,31 . In contrast to previous studies looking at the vibrational absorption bands of the dopant 21,32 , our work is concentrating on the modes of the conjugated polymer backbone.…”

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How intermolecular geometrical disorder affects the molecular doping of donor–acceptor copolymers

et al. 2015

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Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/ polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

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Simultaneous Raman and Fluorescence Spectroscopy of Single Conjugated Polymer Chains

Cited by 41 publications

References 24 publications

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

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

Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

How intermolecular geometrical disorder affects the molecular doping of donor–acceptor copolymers

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