Orientation of 1,1′-Bi-2-naphthol Grafted onto TiO₂

Organic ligands grafted to oxide surfaces are used for sensing, chromatography media, catalysis, and area-selective atomic layer deposition. Beyond simply occupying space, the structure of the grafted ligands provides an opportunity to encode information onto surfaces. However, as the ligands become more complex than simple alkanes, it is not always clear how much of their structure is retained upon grafting. Here, chiral 1,1'-bi-2-naphthol (BINOL) and related BINOLates are supported onto TiO2 and Al2O3. The molecular structure and orientation of BINOL was probed in detail utilizing surface enhanced Raman spectroscopy. BINOL on TiO2 possesses significant in-plane (1493 cm(-1)), out of plane (785 cm(-1)), and torsional modes (427 cm(-1)), indicating naphthyl rings tilted off the perpendicular from the surface and with a dihedral angle between the two naphthyl rings of 65-80 degrees, broadly analogous to the structure of molecular BINOL complexes. In contrast, BINOL on Al2O3 has relatively fewer in-plane modes (1220 cm(-1)), in proportion to out-of-plane modes (552 cm(-1)) and torsional modes (382 cm(-1)), indicating that the naphthyl rings lie more flat with respect to the surface. BINOL on TiO2 was further characterized by a combination of infrared, ultraviolet-visible, and circular dichroism spectroscopies to confirm that BINOL is covalently bound to TiO(2)through the phenol oxygen and that BINOL retains its chirality upon immobilization onto TiO2. These spectroscopies also indicate that the BINOL orientation is preserved upon atomic layer deposition of Al2O3 on BINOL-TiO2. These results suggest strategies for the development of new functional hybrid and oxide materials.