Directional dispersion of the optical resonance ofπ−π*transitions ofα-sexithiophene single crystals

Abstract

The reflectivity of the (100) face of thin single crystals of $\alpha$-sexithiophene has been measured up to 6 eV to derive the optical properties of the strong Davydov component of the dominant intramolecular $\pi \to {\pi }^{*}$ transition, polarized along the long molecule axis. A Kramers-Kronig analysis of the data reveals numerous transitions of moderate strength above the main optical resonance near 3.4 eV, which is shifted by 0.8 eV to higher energy with respect to the corresponding absorption peak observed for the forbidden polarization of light. The remarkable shift results from coupling of strong excitons to the propagating electromagnetic wave. The experimental spectra provide input to model the dielectric tensor, and to calculate the optical resonance as a solution of the wave equation. This polariton approach reproduces the large shift of the dominant optical transition, and confirms that polariton effects are crucial to understand the optical properties of strongly correlated $\pi$ electrons in a crystalline solid.