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)lsophthalate, 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, similar to10(-8) mol/cm(2), 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 (lambda(exc) = 417 nm, 8-10 ns fwhm, 3.1 mJ/cm(2)) indicate rapid excited-state electron injection, k(inj) > 10(8) s(-1), and second-order recombination with an observed average rate constant of k(obs) = 2.5 +/- 0.3 X 107 s(-1), independent of which rigid-rod was excited. Preliminary photoelectrochemical studies in regenerative solar cells with 0.5 M Lil/0.05 M I-2 indicate a quantitative conversion of absorbed photons into an electrical current.