Optical properties of Iridium(III) cyclometalates:excited state interaction with small molecules and dynamics of light-harvesting materials.

Abstract

The research presented in this thesis concerns the use and understanding of luminescent Ir(III) cyclometalates. Areas of research involve the design, synthesis, and characterization of novel luminescent Ir(III) cyclometalates, including photophysical investigation of their phosphorescent excited states using steady-state and time resolved absorption/luminescence spectroscopies. This broad research description may be further separated into two subareas: study of excited state interaction with small molecules and excited-state dynamics of metal-organic light harvesting dyads. The first chapter of this thesis examines the interaction of Ir(III) cyclometalates with the small molecule carbon dioxide (CO2). It has been the goal of investigators to develop methods for direct optical detection of CO2. This has been difficult as CO2 is considered chemically inert and there are few luminescent probes directly sensitive to CO2. Most optical detection schemes previously developed for CO2 use indirect detection methods, which rely upon measuring changes in pH brought about by hydrolysis of CO2. Research efforts to design a reliable method for the direct optical detection of CO2 were accomplished through development of a system where hydrazine, a simple amino ligand, when coupled into the coordination sphere of an Ir(III) cyclometalate reacts with CO2. The result of this reaction provides a significant shift in the luminescence λmax of the phosphorescent probe, a previously unobserved optical response for the direct detection of CO2. The second chapter investigates phosphorescent excited states and their ability to function as triplet sensitizers for the generation of singlet oxygen (1O2) and luminescent probes for molecular oxygen (O2) concentration. Interaction of phosphorescent excited states with O2 results in energy transfer from the luminescent probe to O2, quenching the phosphorescent excited state. Energy transfer also generates the reactive oxygen species (ROS) 1O2. We have used this duality to develop an analytical methodology to follow the serendipitously discovered photoreactivity of 1O2 with common organic solvent dimethyl sulfoxide (DMSO) using the luminescence profile of Ir(III) and Ru(II) phosphors. Reaction of the triplet sensitized 1O2 with a photooxygenation substrate results in the consumption of O2 from the system and an increase in the observed luminescence intensity. Detailed kinetic investigations of the luminescence recovery and O2 depletion were preformed on air-saturated closed cell systems. Determinations of the quantum efficiencies for the photooxygenation system were performed and differences in choice of triplet sensitizer discussed. Study of 1O2 reactivity with substrates of biological and environmental relevance using this methodology should provide an additional tool to understand better oxidative damage induced by 1O2 within these systems. In chapter three a detailed study involving the design, synthesis, and characterization of the electrochemical and phototophysical properties of Ir(III) cyclometalates with pendant terthiophenes as secondary organic chromophores is presented. The interplay of the excited states between each chromophore represents an interesting photoredox active system for energy-to-light or light-to-energy devices. Greater knowledge of the primary photophysical events within these complexes will provide a better understanding of how energy moves in these hybrid systems after light absorption, leading to increased device efficiency.