Adsorption geometry of carboxylic acid functionalized porphyrin molecules on TiO2(110)

Abstract

Controlling the adsorption geometry of porphyrin molecules on titania surfaces is an important step in the rational design of molecular devices such as dye-sensitized solar cells. We employed X-ray Photoelectron Spectroscopy (XPS) and Near-Edge X-Ray-Absorption Fine Structure (NEXAFS) spectroscopy to determine the binding mode, the electronic structure and the adsorption geometry of carboxylic acid functionalized tetraphenylporphyrin molecules. Molecules with one (mono), two (cis and trans) and four (tetra) carboxylic acid anchoring groups were adsorbed on rutile TiO2(110). XPS shows that the iminic nitrogen atoms at the macrocycle center are partially protonated after adsorption, and that the degree of protonation increases with the number of COOH functional groups in the molecule. NEXAFS measurements show that molecules with either one or two groups in cis configuration adsorb with the macrocycle tilted away from the surface. In contrast, molecules with two carboxylic-acid groups in trans configuration adsorb with what is probably a flat-lying, but distorted macrocycle. Finally, molecules with four carboxylic-acid groups show no linear dichroism, indicating an intermediate adsorption angle. Our results show how the number and position of the COOH functional groups determine the molecular adsorption geometry.