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Spectral Diffusion and Stokes’ Shift in Conjugated Polymers: From Hopping to Twist

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In conjugated polymers, important organic substances for photo-electronic devices, spectral diffusion is usually described in the framework of excitation energy transfer (EET), i.e. Förster-type hopping transfer, among so-called spectroscopic units. These spectroscopic units consist of several connected monomers of the polymer chain with common optical transition frequency and dipole. The transfer rate between two spectroscopic units can be calculated as dipole-dipole interaction between the respective transition (point-dipole approximation). We will present more sophisticated concepts of the excitonic interaction e.g. the line-dipole approximation, which result in a qualitative and quantitative better agreement with the energy transfer dynamics measured by e.g. anisotropy decay. Beside the EET , which dynamics of are on a ps-timescale, we found that there may exist other mechanisms, which result in the observed spectral diffusion and Stokes’ shift as well. These are directly connected with the nuclear relaxation of the polymer structure in the excited states. On timescales just a little bit longer than those characteristic for EET , torsions around the single-bonds connecting the monomeric units of the polymer become crucial. On the other hand stretching modes may be able to explain sub-ps kinetics, which have been found in anisotropy decay measurements. We will illustrate these effects by means of quantum-chemical calculations (ZINDO-S and TD-DFT) we have performed on oligo-thiophenes.

This talk is part of the Optoelectronics Group series.

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