Molecular devices for energy conversion: TiO2 compact films in DSPECs and Ir(III) complexes

The main focus of this thesis is: application of nanostructured TiO2 compact layers in dye-sensitized photoelectrosynthesis cells (DSPECs) and syntheses and characterization of Ir(III) polypyridyl complexes towards molecular devices. The first investigation focused on the preparation of nanostructured TiO2 sols using a sol-gel method. The deposition of these nanoparticles through the Layer-by-Layer method led to a thin film growth, which were applied at the photoanode of DSPECs between the conducting glass and the mesoporous network to evaluate the performance of these devices. The obtained thin films are highly compact and the photoelectrochemical characterization of DSPECs showed photocurrent enhancements of 53%, compared with non-modified devices. Voltammetry analyses depicted that the compact layers promote shifts to more negative potentials, which may lead to higher oxygen evolution efficiencies. Incident photon-to-current efficiencies (IPCE) corroborate the role of compact layers in these devices. Potential decay/electron lifetime measurements − pioneered in DSPECs − demonstrated a successful accomplishment of such an innovative approach. The second part of this work focused on the obtention of Ir(III) complexes with potential application in luminescent systems. Three new polypyridyl heteroleptic Ir(III) complexes [Ir(Fppy)2(CF3pic)], [Ir(ppy)2(CF3pic)] and [Ir(Meppy)2(CF3pic)], where Fppy = 2-(2,4-difluorophenyl)pyridine, ppy = 2-phenylpyridine, Meppy = 2-(p-tolyl)pyridine and CF3pic = 3-(trifluoromethyl)pyridine-2-carboxylate, were obtained to evaluate their photophysical properties in different media. Among the investigated complexes, [Ir(Fppy)2(CF3pic)] showed a distinct profile and its excited state in fluid medium, acetonitrile, is typically 3MLCT (metal-to-ligand charge transfer) in character with an emission quantum yield of 13%. In an intermediate-rigidity medium, as poly(methyl metacrilate) (PMMA), an inversion of the lowest-lying state is observed, with a 3IL (intra-ligand) character emission, whereas for other complexes in PMMA remain 3MLCT. In glassy medium, highly rigid, all compounds depict characteristic 3IL bands, well structured and blue-shifted. This rigidochromic effect was rationalized through energy diagrams. Chromaticity diagrams were also obtained and show theCIE (Comission Internationale dEclairage) coordinates for the emitted color of each complex. This work features an appealing study of complex photophysics, where potential application of these compounds are pointed out. (AU)