Triarylamine on nanocrystalline TiO2 studied in its reduced and oxidized state by photoelectron spectroscopy

The electronic and molecular structures of an electrochemically reduced and oxidized triarylamine layer (((3-ethyl(p-N,N-dimethylamino)phenyl)amino)pr opyl-1-phosphonic acid) adsorbed on a nanocrystalline TiO2 film have been i nvestigated. Photoelectron spectroscopy (PES) measurements on electrochemic ally prepared surfaces and density functional theory (DFT) calculations for the free triarylamine molecule were performed. The major electronic differ ence between the reduced and oxidized state of the triarylamine molecules a ppears on the nitrogen atom. In the PES measurement a shift of the N 1s sig nal of 1.5 eV toward higher binding energies was measured between triarylam ine in its reduced and oxidized states. The molecular structure of the surf ace layer was found to depend on the electrolyte used in the electrochemica l preparation. The oxidized triarylamine molecule is positively charged and must therefore be accompanied by a negative counterion. The identity of th e counterion was found to influence electron transfer between adjacent mole cules, as explained by geometrical differences of the surface layer. During the measurements the PES N Is signal was affected by the continuous X-ray illumination. Interestingly, a reversible photoreduction of the oxidized tr iarylamine was observed. The effect is accounted for by X-ray induced elect ron hole-pair generation in the TiO2 and subsequent electron transfer from the TiO2 conduction band to the singly occupied molecular orbital of the ad sorbed molecule.