Exciton interactions in oligophenyl nanoaggregates and single crystals

In crystals built of chainlike molecules such as oligophenyls or oligothiophenes, the value of the Davydov splitting, which is a measure of the strength of excited state interactions and as such of fundamental importance for the understanding of the optoelectronic properties, remains a matter of debate. To resolve the controversy on the subject we have performed a combined spectroscopic and theoretical study of the effect of intermolecular interactions on the electronic structure of conjugated oligomers in the solid state using a four ring oligophenyl as model compound. Strong excited state intermolecular interactions of oligophenyls in the crystalline environment lead to the formation of quasicontinuous exciton bands. Band structure calculations in terms of classical dipole theory indicate that oligophenyl crystals behave as two-dimensional semiconductors. Due to the long-range polarization field the splitting between the Davydov components depends on the propagation direction of the wave vector. For normal incidence on the (100) crystal face we find a splitting of 0.81 eV. The classical dipole theory accounts well for the experimental findings of the exciton resonance energy in single crystals as well as nanoaggregates of oligophenyls. (C) 2002 American Institute of Physics.