Electronic structure of ordered Langmuir-Blodgett films of an amphiphilic derivative of 2,5-diphenyl-l,3,4-oxadiazole

Abstract

Electronic structure of ordered films obtained from an amphiphilic substituted 2,5-diphenyl- 1,3,4-oxadiazole (NADPO-11) by Langmuir-Blodgett (LB) technique were studied, for the first time, by means of ultraviolet photoelectron spectroscopy (UPS). The complete one- dimensional valence band structure has been determined from the UPS spectra. The spectral features that correspond to the top of the valence band and that dominate the electronic properties of this material were assigned to the different molecular eigenstates by comparison with simulated spectra from quantum chemical calculations. Good agreement between simulated and experimental curves allowed to assign UPS peaks to certain groups of molecular orbitals (MOs) and to understand the electronic bonding state (σ- or π- type) of the involved MOs. Furthermore, we performed UPS measurements on in-situ evaporated amorphous films of the same substance, comparing their valence band features to the previous ones. The correlation between ionisation potential and electron affinity, determined by combined ultraviolet photoelectron spectroscopy/optical spectroscopy, and carrier-transport and light emitting properties of these material are also discussed.