Studies of tripodal pyrene chromophores on metal oxide surfaces.

Four tripodal linkers with large footprints for metal oxide nanoparticle sensitization, substituted with pyrene as the chromophore, and three COOR binding groups in the meta or para positions, were synthesized to study the effect of the anchoring group position and footprint size on the aggregation and sensitization processes. Two tripods based on tetraphenylsilane with a footprint size of ∼ 2.2 nm2, (1-pyrenyl-4-ethynyl-phenyl)-tris-(4-carbomethoxyphenyl-4-ethynyl-phenyl)silane, and (1-pyrenyl-4-ethynyl-phenyl)-tris(4-(4-methoxybenzyloxycarbonyl)phenyl-4-ethynyl-phenyl) silane, decomposed during hydrolysis due to limited chemical stability of silicon center. Attempts to bind the silicon (Si) centered tripods as esters to TiO2 and ZrO2 surfaces did not succeed. The instability of Si-tripods proved to be a major limitation to the photophysical studies of these sensitized films which convinced us to modify the tetrahedral core from silicon to the chemically stable adamantane (Ad) center. Ad-tripods, 1-(1-pyrenyl-4-ethynyl-phenyl)-3,5,7-(3-carbomethoxyphenyl)-adamantane and yl-4-ethynyl-phen1-(1-pyrenyl-4-ethynly-phenyl)-3,5,7-yl)-adamantane and (4-carbomethoxyphenyl-4-ethynyl-phenyl)-adamantane which were derivatives of tetraphenyladamantane, were chemically stable and were readily converted into the corresponding acids (footprint size ∼ 1.9 nm2 (meta) and ∼ 2.5 nm2 (para)) and bound to metal oxide surfaces. The UV-vis absorption and fluorescence emission spectra of all four tripods in solution were red-shifted with respect to pyrene due to the extended pi-conjugation. Higher extinction coefficients and near unity quantum yields were also observed. Binding of Ad-tripods to base pre-treated anatase TiO2 film and to diluted colloidal solutions resulted in quenching of the fluorescence indicating efficient electron injection into the semiconductor. The FT-IR-ATR spectra of Ad-tripods bound to TiO2 films indicated a bidentate carboxylate binding mode. Tripods with large footprints, by design, should isolate the pyrene molecule from the nearest neighbors and avoid the formation of excimers on ZrO2 (insulator) surface. However, excimer emission was observed on ZrO2 films and in colloidal solutions under various conditions, suggesting contacts between pyrene chromophores from adjacent nanoparticles and necking regions. Rapid multi-exponential kinetics of charge injection (k < 1015 s-1) was observed for Ad-tripods on TiO2. Recombination kinetics was described by a second-order kinetic model and was found to be dependent on the position of the carboxylate anchoring group.