Exciton dispersion in para-quaterphenyl: Significant molecular interactions beyond Coulomb coupling

Lukas Graf,Martin Knupfer,Bernd Büchner,Yulia Krupskaya

doi:10.1063/5.0058657

Lukas Graf, Martin Knupfer + Show 2 more

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https://doi.org/10.1063/5.0058657

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Abstract

We have experimentally determined the momentum dependence of the electronic excitation spectra of para-quaterphenyl single crystals. The parallel arrangement of para-quaterphenyl molecules results in a strong Coulomb coupling of the molecular excitons. Such crystals have been considered to be a very good realization of the Frenkel exciton model, including the formation of H-type aggregates. Our data reveal an unexpected exciton dispersion of the upper Davydov component, which cannot be rationalized in terms of inter-molecular Coulomb coupling of the excitons. A significant reduction of the nearest neighbor coupling due to additional charge-transfer processes is able to provide an explanation of the data. Furthermore, the spectral onset of the excitation spectrum, which represents a heavy exciton resulting from exciton–phonon coupling, also shows a clear dispersion, which had been unknown so far. Finally, an optically forbidden excitation about 1 eV above the excitation onset is observed.

Highlights

Organic semiconductor research and applications have developed in previous years, and applications such as light-emitting diodes and organic photovoltaic cells have entered the market already.[1,2,3,4,5,6] The properties of such devices strongly depend on the photophysical behavior of the used organic semiconducting materials
The impact of molecular packing on basic optical properties, such as absorption and photoluminescence, was originally worked out for arrangements dominated by Coulombic intermolecular interactions,[7,8] eventually leading to the classification of J- and H-aggregates and the description of the exciton band structure based on these interactions
We present an experimental determination of the exciton dispersion in para-quaterphenyl single crystals measured using electron energy-loss spectroscopy (EELS). para-quaterphenyl is a representative of conjugated oligomer herringbone aggregates, which are characterized by quite strong exciton coupling via the Coulomb interaction of the excitation dipole moment of adjacent molecules

Summary

Introduction

Organic semiconductor research and applications have developed in previous years, and applications such as light-emitting diodes and organic photovoltaic cells have entered the market already.[1,2,3,4,5,6] The properties of such devices strongly depend on the photophysical behavior of the used organic semiconducting materials. Para-quaterphenyl is a representative of conjugated oligomer herringbone aggregates, which are characterized by quite strong exciton coupling via the Coulomb interaction of the excitation dipole moment of adjacent molecules.

Results

Conclusion