Electronic structure of aromatic 1,3,4-oxadiazoles studied by ultraviolet photoelectron spectroscopy

Abstract

The family of substituted 1,3,4-oxadiazole can be used in organic light-emitting diodes (OLEDs) acting both as hole blocking and as improved emissive layers. Thin films of aromatic 1,3,4-oxadiazoles, prepared in situ by evaporation from pinhole sources onto fresh deposited gold films on flat silicon wafers, were studied by means of ultraviolet photoelectron spectroscopy (UPS) in order to get detailed information about valence electronic band structure. The UPS measurements were performed at the beam line TGM-2 at BESSY (Berlin) in the energy range between 8 and 80 eV. Furthermore, we determined the vacuum level shift at the organic/metal interface. We used semi-empirical quantum-chemical calculations to evaluate the valence electronic structure of the isolated molecules. This was also the starting point used as input to simulate the UPS spectra obtained from the films. The good agreement between experimental and theoretical curves allows us to assign the different peaks to different molecular eigenstates and it also provides a more quantitative description of the experimental findings.