Optimization of Surface Loading of the Silatrane Anchoring Group on TiO₂

Anchoring groups are usually needed for the attachment of small molecules to metal oxide surfaces such as in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs). Here, we optimize the surface loading onto titanium dioxide surfaces of the silatrane anchoring group, a triethanolamine-protected trialkoxysilane. This anchoring group is not yet widely used because prior protocols afforded low surface coverage, but it has the advantage of high stability over a wide pH range and at both oxidizing and reducing potentials when bound. A new and improved method for estimating surface coverage is described here and used to determine that loading using previously reported binding protocols is very low. However, we were able to uncover several factors contributing to this low loading, which has allowed us to develop methods to greatly improve surface coverage for a variety of silatranes. Most notably, we were able to increase the loading of a model arylsilatrane by 145% through use of a benzoic acid additive. This is not general acid catalysis because alkylsilatranes are not similarly affected and 4-t-butylbenzoic acid, having a similar pKa to benzoic acid, is not effective. Because the bulky t-butyl group of the latter additive is not expected to pi-stack with our arylsilatrane, we have tentatively assigned this enhancement to aromatic stacking between the aromatic additive and the arylsilatrane.