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Florian Schindler

Publications and source records attributed to Florian Schindler.

5 recordsLinked to original sources
PubMed2006

How single conjugated polymer molecules respond to electric fields.

Conjugated polymers find applications in a range of devices such as light-emitting diodes, field-effect transistors and solar cells. The elementary electronic response of these semiconductors to electric fields is understood in terms of nanoscale perturbations of charge density. We demonstrate a general breaking of spatial charge symmetry by considering the linear Stark effect in the emission of single chromophores on individual chains. Spectral shifts of several nanometres occur due to effective dipoles exceeding 10 D. Although the electric field does not ionize the exciton, some molecules exhibit field-induced intensity modulations. This quenching illustrates the equivalence of charge symmetry breaking and polaron-pair or charge-transfer-state formation, and provides a microscopic picture of permanent charging, which leads to doping and exciton dissociation in actual devices. In addition to using this tuneable emission in single-photon electro-optic modulators, hysteresis in the Stark shift suggests a route to designing nanoscale memory elements such as molecular switches.

Journal Article↗
PubMed2005

Single chromophore spectroscopy of MEH-PPV: homing-in on the elementary emissive species in conjugated polymers.

Low-temperature, single-molecule spectroscopy can provide unparalleled access to the primary emissive species of conjugated polymers. We demonstrate this with the example of one of the most commonly studied polymers, poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), MEH-PPV, which is shown to exhibit sharp fluorescence signatures over one hundred times narrower than the ensemble. These unprecedented narrow emission features can be assigned to single chromophores on the polymer chain, which are selectively addressed by the narrow band excitation. As with organic dye systems, the emission from single chromophores is not static with time, but shows a substantial spectral fluctuation. We find that, for single chromophores, this spectral fluctuation always follows a universal Gaussian statistical distribution. High-resolution spectroscopy provides unique insight into low-energy vibrational modes in the polymer emission, which are generally inaccessible with conventional spectroscopic methods such as site-selective fluorescence or Raman spectroscopy. Interchromophoric coupling can also occur owing to the flexible nature of the polymer backbone. This leads to substantial spectral broadening and a loss of resolution in the vibronic progression. We observe reversible switching within one single molecule between narrow and broad emission, which directly correlates with a discrete switching in emission intensity. We conclude that one and the same single molecule can support aggregated and nonaggregated emission, that is, emission from isolated and aggregated chromophores in one single molecule, rather than the tendency for aggregate emission being intrinsic to the molecule.

Journal Article↗
PubMed2004

A universal picture of chromophores in pi-conjugated polymers derived from single-molecule spectroscopy.

Single-molecule spectroscopy can provide insight into the fundamental photophysics of large macromolecules containing tens of thousands of carbon atoms by circumventing disorder broadening. We apply this technique to comparatively ordered ladder-type poly(para-phenylene) and highly disordered poly(phenylenevinylene) (PPV), both of which are materials of substantial technological interest. Identical spectroscopic features are observed on the single-chromophore level, independent of the chemical structure or the chain morphology. Both materials exhibit narrow fluorescence lines down to 0.5 nm wide, which we attribute to the single-chromophore zero-phonon line, accompanied by a discrete vibronic progression providing a signature of the chemical structure. The chromophore units display spectral diffusion, giving rise to dynamic disorder on the scale of the linewidth. Although the energetic range of spectral diffusion is small, it can influence intramolecular excitation energy transfer and thus the overall molecular emission. The spectral diffusion dynamics of single chromophores are identical in both material systems and follow a universal Gaussian distribution. In the case of emission from multiple chromophores situated on the molecule, which we observe for PPV, spectral diffusion follows Lorentzian-like statistics. The fundamental difference between the two materials is the possibility of coherent interchromophoric coupling in PPV, resulting in strong spectral broadening caused by aggregation or superradiance. Such behavior is absent in the ladder-type polymers, where the linewidth of the emissive species is identical for all molecules. Our results demonstrate that structure-property correlations in conjugated polymers derive mainly from chain morphology rather than chromophoric properties and should be considered extrinsic in nature.

Journal Article↗
PubMed2004

Central poststroke pain and reduced opioid receptor binding within pain processing circuitries: a [11C]diprenorphine PET study.

Based on concepts that endogenous opioids participate in neural transmission of pain, the present study in central poststroke pain (CPSP) patients investigated changes in opioid receptor (OR) binding in neural structures centrally involved in the processing of pain. Five patients with central pain after lesions in the brain stem, thalamus or parietal cortex and twelve healthy volunteers underwent a [11C]diprenorphine positron emission tomography study. Binding potentials were calculated using a reference region model in all subjects. Statistical parametric mapping was applied for t-statistical analysis on voxel-basis. Binding potential values for each individual were extracted from a volume of interest at each identified significant peak. Spectral analysis was applied for quantification of global values. Significant regional reduced 11C-diprenorphine binding (corrected for multiple tests) was detected in contralateral thalamus, parietal, secondary somatosensory, insular and lateral prefrontal cortices, and along the midline in anterior cingulate, posterior cingulate and midbrain gray matter. Individual extracted binding values disclosed a reduced binding in these regions in all patients independent from the particular lesion site. The poststroke pain syndrome is associated with a characteristic pattern of reduced OR binding within the neural circuitry processing pain. It is suggested that an imbalance of excitatory-inhibitory mechanisms in certain brain structures, as evidenced in decreased [11C]diprenorphine binding, is one of the causes or the consequences of poststroke pain.

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