Nessa tese, duas frentes de trabalho foram abordadas: a aplicação de camadas compactas de TiO2 nanoestruturado em células fotoeletrossintéticas sensibilizadas por corante (DSPECs) e a síntese e caracterização de uma série de complexos polipiridínicos de Ir(III), visando à aplicação em dispositivos moleculares. A primeira investigação focou na obtenção de sóis de TiO2 nanoestruturado utilizando um método sol-gel. As deposições dessas nanopartículas pelo método Layer-by-Layer levaram ao crescimento de filmes finos, que foram aplicados no fotoanodo das DSPECs, entre o substrato condutor e o óxido mesoporoso, com o objetivo de avaliar o desempenho desses dispositivos. Os filmes obtidos são altamente compactos e a caracterização fotoeletroquímica das DSPECs apontou que eles propiciaram o aumento da fotocorrente em até 53%, em comparação aos dispositivos não modificados. A voltametria mostrou que as camadas compactas atuam também no deslocamento a potenciais mais negativos, o que pode levar a um aumento na eficiência da geração de oxigênio. As eficiências de conversão de fótons incidentes em corrente (IPCE) corroboraram o papel dessas camadas nos fotoanodos dos dispositivos. As medidas de decaimento de potencial/tempo de vida do elétron no eletrodo − realizados pioneiramente em DSPECs − demonstraram o sucesso na implementação dessa proposta inovadora. A segunda parte deste trabalho focou na obtenção de complexos de Ir(III) com potenciais aplicações em sistemas luminescentes. Três novos complexos polipiridínicos heterolépticos de Ir(III) [Ir(Fppy)2(CF3pic)], [Ir(ppy)2(CF3pic)] e [Ir(Meppy)2(CF3pic)], em que Fppy = 2-(2,4-difluorofenil)piridina, ppy = 2-fenilpiridina, Meppy = 2-(p-toluil)piridina e CF3pic = 3-(trifluorometil)piridina-2-carboxilato, foram obtidos para avaliar suas propriedades fotofísicas em diferentes meios. Dentre os compostos investigados, o [Ir(Fppy)2(CF3pic)] apresentou um perfil bastante diferenciado. Seu estado excitado em meio fluido, acetonitrila, é característico de 3MLCT (transferência de carga metal-ligante) com 13% de rendimento quântico de emissão. Em meio de rigidez intermediária, poli(metil metacrilato) (PMMA), observase uma inversão do estado excitado de mais baixa energia, com um decaimento radiativo típico de 3IL (transição eletrônica intra-ligante), ao passo que para os demais complexos investigados, a emissão em PMMA ainda é do tipo 3MLCT. Em meio vítreo, altamente rígido, a emissão de todos os compostos apresenta bandas características de 3IL, bastante estruturadas e deslocadas para o azul. O efeito rigidocrômico foi discutido e sumarizado em termos de diagramas de energia. Diagramas de cromaticidade também foram obtidos, mostrando as coordenadas CIE (Comission Internationale dEclairage) referentes às cores de emissão de cada complexo. Esse trabalho apresenta a importância da fotofísica desses compostos para potenciais aplicações em dispositivos moleculares. / 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.

Titania has been the focus of attention for several decades owing to its chemical stability, non-toxicity, inexpensiveness and robust surface chemistry. Its technological applications include use in diverse areas such as photocatalytic reactors, antibacterial coatings, dye sensitive solar cells (DSSC) and more recently the perovskite solar cells to name a few. All of these applications are based on the ability to inject or generate electronhole pairs in titania and transport them to a suitable interface at which they are ejected to either engender a reaction as in photocatalysis or drive a load as in photovoltaics. From a technological perspective it is also important that such science be achieved and controlled in supported titania structures. The research reported in this thesis, thus, started with the development of a process for obtaining adherent titania films by oxidation of sputtered Ti films on stainless steel, a very commonly used substrate. Challenges that had to be overcome included the need to oxidize titanium to obtain the right phase mixture while preventing film cracking or delamination due to compressive stresses generated during anodic oxidation of Ti. During this process of obtaining nanostructured TiO2 through anodization, it was serendipitously discovered that planar TiO2 films obtained by oxidation of sputtered Ti films did significantly better than anodized nanoporous titania in bactericidal studies. This was then replicated in organic dye degradation studies. Analysis of the material showed that this improved performance was due to the unintentional contamination during sputtering by Cu, Zn, Mo possibly due to arcing across brass contacts. This quaternary system was then systematically explored and it was shown that an optimal metastable composition in the Ti- Cu-Mo oxide ternary system performs the best. DFT studies showed that this was due to introduction of shallow and deep states in the band gap that, depending on the level of dopants, either enhances carrier lifetimes or leads to recombination. In continuation of this work on supported titania structures by oxidation of Ti, a novel photoanode for use in dye sensitized photovoltaics was developed by oxidation of Ti foam. This results in an interconnected 3-D network of TiO2 that possess at its core a network of Ti. Such architecture was designed to provide a large surface area for anchoring the sensitizer while simultaneously reducing the distance that charge carriers have to travel before reaching the ohmic contacts to prevent recombination losses. The thesis discusses the preparation of such anodes, the properties of the 3-D oxide and cells, with up to 4% efficiency, developed using such anodes. Reasons for such behaviour and avenues for further exploration to improve cell efficiency will also be discussed.