Pyrene-Terminated Phenylenethynylene Rigid Linkers Anchored to Metal Oxide Nanoparticles

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Abstract
Phenylenethynylene (PE) rigid linkers (para and meta) were used to anchor pyrene to the surface of TiO2 (anatase) and ZrO2 nanoparticle thin films through the two COOH groups of an isophthalic acid (Ipa) unit. Four chromophore-linker models were studied in solution and bound. Two are novel meta-pyrene−PE linker systems: dimethyl 5-(3-(1-pyrenylethynyl)phenylethynyl)-isophthalate, carrying one pyrene, and dimethyl 5-(bis-3,5-(1-pyrenylethynyl)phenylethynyl)-isophthalate, carrying two. These were compared with para rigid-rods dimethyl 5-(1-pyrenylethynyl)isophthalate and dimethyl 5-(4-(1-pyrenylethynyl)phenylethynyl)-isophthalate, each carrying one pyrene but varying in length. The length of the PE linkers and the para or meta substitution influence the photophysical properties of the compounds. The extinction coefficient increased, and the long wavelength absorbance of the pyrene chromophore was shifted to the red with increasing conjugation. Compared to unsubstituted pyrene, the pyrene-linker systems were characterized by short fluorescence lifetimes (τ ∼ 2 ns in tetrahydrofuran solutions), but quantum yields were close to unity. ZINDO/S CI calculations attribute this effect to a switching in the order of the two lowest-lying singlet states of pyrene. High surface coverages, ∼10-8 mol/cm2, and carboxylate binding modes on nanostructured TiO2 films were obtained in all cases. The appearance of a pyrene excimer emission on ZrO2, an insulator, indicates that the pyrene-linker system is closely packed (Py−Py < 4 Å) on the surface. The fluorescence emission on TiO2 was completely quenched, consistent with quantitative and rapid electron injection into the semiconductor indicating that the pyrene excimer acts as a sensitizer. Photoelectrochemical studies in regenerative solar cells with I3-/I- as the redox mediator indicated near-quantitative conversion of absorbed photons into an electrical current.