Nanocylindrical confinement imparts highest structural order in molecular self-assembly of organophosphonates on aluminum oxide

We report the impact of geometrical constraint on intramolecular interactions in self-assembled monolayers (SAMs) of alkylphosphonates grown on anodically oxidized aluminum (AAO). Molecular order in these films was determined by sum frequency generation (SFG) spectroscopy, a more sensitive measure of order than infrared absorption spectroscopy. Using SFG we show that films grown on AAO are, within detection limits, nearly perfectly ordered in an all-trans alkyl chain configuration. In marked contrast, films formed on planar, plasma-oxidized aluminum oxide or α-Al₂O₃ (0001) are replete with gauche defects. We attribute these differences to the nanocylindrical structure of AAO, which enforces molecular confinement.