Rigid-rod sensitizers for functionalization of titanium dioxide nanoparticles

Rigid-rod linkers varying in length were synthesized and used to bind sensitizing chromophores to the surface of TiO2 (anatase) and of ZrO2 nanoparticle thin films. The linkers were made of p-phenyleneethynylene (EPh)n bridges carrying two COOR anchoring groups at the end, and were caped with either Ru(II) polypyridyl complexes or organic chromophores, such as pyrene, anthracene, and azulene.; The Ru(II)-based and pyrene-based rigid-rod dye molecules did bind strongly (Kad ∼ 105 M-1) with high surface coverages (Gamma ∼ 10-8 mol/cm2 ) on the nanostructured ZrO2 and anatase TiO2 mesoporous thin films. The binding modes were found dependent on the surface treatment: on acid (pH = 1) pretreated films, an ester-type linkage was formed between the COOR binding groups and metal oxide surface; on non-pretreated and base (pH = 11) pretreated films, bidentate binding modes were dominant.; The photophysical properties of the Ru(II) rigid-rods were influenced by the length of the fully conjugated rigid linker (from 5.4 A to 19.1 A). The nanosecond transient absorption measurements indicated long-lived metal-to-ligand charge-transfer MLCT excited states (∼2 mus) with evidence for delocalization onto the rigid-rod linker. The interfacial electron transfer behavior on TiO2 was found to be dependent on the Bronsted acidity or basicity of the surface. On base (pH = 11) pretreated TiO 2, the excited state electron injection yields were low and could be increased by addition of LiClO4 to an external CH3CN solution. Under these conditions, a fraction of the injection process could be time-resolved on a 10 ns time scale. On acidic (pH = 1 pretreated) TiO 2, ultrafast excited state electron injection rate was observed ( kinj ∼ 1012 s-1) and decreased with increasing linker length. However, introduction of an insulating unit (bicyclooctane) into the rigid linker did not lower the injection rate. Recombination was found to be second-order with average rate constant, kobs ∼ 107 s-1, and it was independent of the rigid-rod length.; The rigid-rod linkers increased the extinction coefficient and shifted to the red the long wavelength absorbance of the pyrene chromophore. The appearance of a pyrene excimer-like emission on ZrO2 nanoparticles suggested that the rigid-rods did not spatially isolate the chromophores effectively. The emission on TiO2 was completely quenched and consistent with quantitative electron injection into the semiconductor. Nanosecond transient absorption measurements indicated rapid excited state electron injection ( kinj > 108 s-1), and second-order recombination with observed average rate constants independent of which rigid-rod was excited. Preliminary photoelectrochemical studies in regenerative solar cells with 0.5 M LiI/0.05 M I2 showed quantitative conversion of the absorbed photons into an electrical current.