Metal oxide hybrid materials: on-surface modulation of aggregation-induced fluorescence

Abstract

Covalent grafting of molecular compounds to extended oxide surfaces provides variable access to hybrid materials. While in many cases the properties of both components are additive, some favorable combinations can lead to the emergence of new properties. In this work, we report on the aggregation-induced fluorescence of an N₂O₂ (proto-) ligand H₂L1 of the JäGER-type on oxide surfaces. The ligand is non-fluorescent in solution but shows strong greenish-blue fluorescence in the bulk and, importantly, once covalently anchored to oxidic surfaces via an appended carboxylic acid moiety. Intermolecular π−π stacking interactions dominate the packing in single crystals both of H₂L1 and a congener with methyl blocked acid function, H₂L2. As revealed by electronic spectroscopy, the high surface loadings of H₂L1 on Al₂O₃ and TiO₂ allow stacking in a dense surface layer also on the solid supports. While non-fluorescent dilute solutions of H₂L2 and H₂L1 resonate only in the UV range below 380 nm, the consistent shift of the spectral onset to > 430 nm for bulk ligand and Al₂O₃-grafted material lends additional support to a supramolecular origin of fluorescence. Sharply reduced (but not vanishing) ligand fluorescence on TiO₂ indicates electron-injection to the conduction band to be operative, as further supported by the reduced emission lifetimes recorded via time-correlated single-photon counting. On-surface synthesis of the respective zinc(II) complexes likewise gives strongly fluorescent materials, showing again a reduced emission on TiO₂.