Heavy atom effects on anthracene-rigid-rod excited states anchored to metal oxide nanoparticles

Two new organic rigid-rod linker compounds, dimethyl 5-(1-anthracenylethynyl)isophthalate and dimethyl 5-(4-(1-anthracenynyl)phenylethynyl)-isophthalate, were used to couple anthracene to the surface of TiO2 (anatase) and ZrO2 nanoparticle thin films. These anthracene-rigid-rod molecules have high extinction coefficients and absorbance spectra that are red-shifted relative to 9-anthracenecarboxylic acid (9-AC). The rigid-rod linkers afford high surface coverages, ∼10−8 mol/cm2, on the nanostructured films in acetonitrile. Excimer-like emission on ZrO2 nanoparticles suggests that the rigid-rods do not spatially isolate the anthracene chromophores effectively. On TiO2 fluorescence was observed for both anthracene-rigid-rods while it was not detected for 9-AC consistent with quantitative electron injection into the semiconductor. Organoiodides and thallium cations (Tl+) were found to be heavy atom quenchers of the anthracene fluorescence in acetonitrile with a concentration dependence that followed the Stern-Volmer model. Fluorescence on ZrO2 was quenched less efficiently by the organoiodides while Tl+ cations exhibited enhanced quenching that followed the Stern-Volmer model at low Tl+ concentrations and saturated at higher concentrations, behavior attributed to surface adsorption.