Adsorption geometry, molecular interaction, and charge transfer of triphenylamine-based dye on rutile TiO2(110).

The fast development of new organic sensitizers leads to the need for a better understanding of the complexity and significance of their adsorption processes on TiO(2) surfaces. We have investigated a prototype of the triphenylamine-cyanoacrylic acid (donor-acceptor) on rutile TiO(2) (110) surface with special attention on the monolayer region. This molecule belongs to the type of dye, some of which so far has delivered the record efficiency of 10%-10.3% for pure organic sensitizers [W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C. Pan, and P. Wang, Chem. Mater. 22, 1915 (2010)]. The molecular configuration of this dye on the TiO(2) surface was found to vary with coverage and adopt gradually an upright geometry, as determined from near edge x-ray absorption fine structure spectroscopy. Due to the molecular interaction within the increasingly dense packed layer, the molecular electronic structure changes systematically: all energy levels shift to higher binding energies, as shown by photoelectron spectroscopy. Furthermore, the investigation of charge delocalization within the molecule was carried out by means of resonant photoelectron spectroscopy. A fast delocalization (∼1.8 fs) occurs at the donor part while a competing process between delocalization and localization takes place at the acceptor part. This depicts the "push-pull" concept in donor-acceptor molecular system in time scale.