Direct Evidence of Chelated Geometry of Catechol on TiO₂ by a Combined Solid-State NMR and DFT Study

Catechol on TiO₂ is a model system for a class of molecules that bind and interact very strongly with metal oxides. This interaction gives rise to a marked charge-transfer absorption band that can be used to sensitize the complex to visible light. In solar cells, these are called type II sensitizers in contrast with type I sensitizers where an excitation of the molecule with subsequent charge injection is the main mechanism for placing an electron in the conduction band of the semiconductor. The adsorption geometry of these molecules is critical in their functioning. Nuclear magnetic resonance (NMR) spectroscopic methods can be used to elucidate structural information about the local geometry at the substrate-molecule interface. NMR methods coupled with density functional theory (DFT) allow for the detailed characterization of molecular binding modes. In the present work, we report a solid-state NMR and DFT study of catechol on TiO₂. DFT-GIPAW chemical shift predictions for the ¹³C CP-MAS experiments unambiguously indicate the presence of a chelated geometry. ¹H → ¹³C cross-polarization build-up kinetics were used to determine the protonation state of additional geometries and point toward the presence of molecular species. The most stable adsorption modes on regular slab models were found to be bidentate, and it is only in the presence of defective surfaces where the chelated mode is stabilized in the presence of undercoordinated titanium surface sites. The combined NMR and DFT approach thus allows characterization of the binding geometry, which is a stepping stone in the design of more complex light-harvesting architectures. This work constitutes, to the best of our knowledge, the first detailed instance of combined solid-state NMR and DFT studies on this class of materials.