Photophysicochemistry of porphyrins and ruthenium complexes adsorbed on mesoporous TiO2 thin films

Abstract

For many years, the search of new organic, organometallic and inorganic molecules has been attracting considerable interest in science due their potential applications in technology, medicine and fundamental science. Among these new materials, porphryins and ruthenium complexes have called special attention to the scientific community due to its versatile structural manipulation through molecular engineering, and their unique photophysical and photochemical properties. To estimate the perspectives of such classes of compounds to potential applications, it is imperative to systematically carry out characterizations of their molecular properties, especially those involving its excited state formation and possible interactions with the surrounding quantities. A topic of extensive work during the last ten years is the application of dye molecules, such as porphyrin and ruthenium compounds, as sensitizers in dye sensitized solar cells (DSSCs). DSSCs represent an attractive technology to addressing the world s demand for renewable energy. Such devices are composed of mesoporous thin films of nanometer sized metal oxide spheres, most commonly titanium dioxide (TiO2), and a molecular sensitizer anchored to the surface of the metal oxide nanoparticles. The most recent works reported the efficiency of a porphyrin based DSSC to be 13%.1 Although still lower in efficiency when compared with the mostly used photovoltaic systems based on silicon, DSSC represents a promising alternative for solar energy conversion devices of low cost and ease of manufacturing. In the first part of this dissertation, PART I, the conventional spectroscopic characterization of two classes of free base porphyrins is presented as a start up investigation. Chapter 1 investigates the UV-vis absorption spectrum of a free base cationic porphyrin derivative [5,10,15,20-tetrakis (1,3-dimethylimidazolium-2-yl) porphyrin tetraiodide], abbreviated to H2TDMImP, in polar solvents. Our results showed that, for diluted solutions (< 14 μM), the outlying cationic groups linked to the porphyrin ring mesocarbons triggers strong interactions with the polar solvent molecules, leading to significant changes in the spectral features of the porphyrin moiety. When the porphyrin concentration is increased, spectral changes suggested that H-type porphyrin aggregates were formed, preventing the outlying cationic groups of the porphyrin to interact with solvent molecules. Chapter 2 is focused on the comparison of steady state and time-resolved photophysical properties of the [5,10,15,20- tetra(pyridyl)-21H,23H-porphyrin], or H2TPyP, when an ruthenium complex, [Ru(terpy)(PPh3)Cl], is bounded to the peripheral pyridine groups of the H2TPyP. The systematic spectral study showed that the porphyrin ring and the peripheral ruthenium complexes have weak electronic interaction due to the absence of favorable driving force for intermolecular electron transfer between the porphyrin and the ruthenium moieties. Also, time-resolved photoluminescence and transient absorption studies showed that the radiative singlet relaxation was quenched after complexation with external ruthenium groups, which is thought to arise from new faster non-radiative decay channels from the S1 state. The PART II of this dissertation composed by Chapter 3, tries to create a coherent bridge between what was done prior to the experience at JHU, and what ended up being the convergence to a possible new research line adopted by Prof. Newton group the observation of photodriven charge and energy transfer in organic and inorganic system. In PART II is introduced the concepts and the working mechanism of the DSSCs, as a preparation to following Chapters 3, 4 and 5 of this dissertation. Chapter 3 is focused on a new free base tetrapyridyl porphryin meso-substituted with a ruthenium complex, Ru(dmb)2NO2. The said molecule, although absent of functional binding groups, was suitable for sensitization of TiO2 through what is believed to be electrostatic interactions with the nanocrystallite surface. The limited number of experiments performed in this chapter couldn\'t precisely inform the possibility of photoinduced electron injection. Although not achieved the desired success in theses experiments, Chapter 3 led to substantial contributions to the understanding of how DSSC should operate with porphyrin sensitizers. In PART III, it is proposed a work focused on ruthenium complexes used as sensitizers to DSSC. More specifically, it offers a discussion on a particular effect that arises after photoinduced electron injection, the Stark effect. This effect is the response of the ground state molecules and the other chemical components to the surface electric fields originated from injected electrons into the TiO2 and/or adsorbed cations on the TiO2 surface. In Chapter 4, it is proposed the analysis of a heteroleptic ruthenium complex adsorbed on TiO2 thin films that have its absorption spectrum perturbed by electric fields from either electrochemically accumulation of electrons or injected electrons right after pulsed laser excitation. The Stark effect is systematically studied as a function of four different types of cations adsorbed on the TiO2 surface: Na+, Li+, Mg2+ and Ca2+. The nature of each cation critically defines the strength of the electric field at the TiO2 dye molecule interface based on screening mechanisms. Chapter 5 gives an approach in which a ruthenium compound composed by a mono-pyridine ligand is absorbed to the TiO2 surface. Critical insights, which concerns about charge motion and screening the high TiO2 permittivity, were satisfactorily obtained. Finally, Chapter 5 proposes a new model to report the time evolution of the Stark effect, under restrict conditions.