Organic Rigid-Rod Linkers for Coupling Chromophores to Metal Oxide Nanoparticles

Two new organic rigid-rod linkers, dimethyl 5-(1-pyrenylethynyl)isophthalate and dimethyl 5-(4-(1-pyrenylethynyl)-phenylethynyl)isophthalate, were used to couple pyrene to the surface of TiO2 (anatase) and ZrO2 nanoparticle thin films. The rigid-rods increase the extinction coefficient and shift to the red the long-wavelength absorbance of the pyrene chromophore. The rigid-rod linkers afford high surface coverages, ∼10-8 mol/cm2, and high surface stabilities on the nanostructured metal oxide films in acetonitrile. The appearance of a pyrene excimer-like emission on ZrO2 nanoparticles indicates that the rigid-rods do not spatially isolate the chromophores effectively. The emission on TiO2 is completely quenched, consistent with quantitative electron injection into the semiconductor. Nanosecond transient absorption measurements (λexc = 417 nm, 8−10 ns fwhm, 3.1 mJ/cm2) indicate rapid excited-state electron injection, k inj > 108 s-1, and second-order recombination with an observed average rate constant of k obs = 2.5 ± 0.3 × 107 s-1, independent of which rigid-rod was excited. Preliminary photoelectrochemical studies in regenerative solar cells with 0.5 M LiI/0.05 M I2 indicate a quantitative conversion of absorbed photons into an electrical current.