Photophysical and Photosensitizing Properties of Dimetal Quadruply Bonded Paddlewheel Complexes Probed Through Ultrafast Spectroscopy

Over the past few decades, research concerning the discovery and study of new materials capable of harnessing sunlight and transforming it into a more useful form of energy has increased dramatically. With the large volume of work being performed in this area, innovations are constantly being made towards higher solar cell efficiencies or better photocatalytic systems. This growth has also coincided with technological advancements such as the discovery of femtosecond lasers and the numerous spectroscopic techniques that utilize them. Now more than ever researchers are able to monitor the essential electron transfer processes and chemical transformations as they occur in real time. By knowing and understanding how a molecule or material behaves after it absorbs light, better materials may be rationally designed with improved performance for specific applications.;The chemical and photophysical properties of dimetal quadruply bonded complexes have long been of interest. Tetracarboxylate (or an analogous ligand) paddlewheel compounds are known to exhibit low energy metal-to-ligand charge transfer (MLCT) transitions with high molar absorptivities that can easily be tuned through out the visible region by modifying the organic ligand or changing the metal center from molybdenum to tungsten. The advent of ultrafast spectroscopy also allowed the singlet excited states of these compounds to be investigated and they have shown many unique and potentially useful properties. Unlike other common transition metal complexes, the M2 compounds have long 1MLCT lifetimes (1 -- 20 ps) and in many cases the singlet state is delocalized over both ligands through the metal center, the distance of which can be upwards of 20A.;This dissertation describes work which has primarily focused on building upon these known properties by making perturbations to the system and observing the changes that occur in the ground and excited states. In Chapter 3 this involves varying the heteroatom of a thiophene-type ligand and studying how much of a role the heavy atom effect plays in the 1MLCT lifetimes. Chapters 4 and 5 investigate another form of ligand modification that incorporates a secondary transition metal center into the ligands to produce molecular triad complexes. The secondary metals contain carbonyl ligands that act as IR reporters so that changes in the electron density around the ligands can be monitored by time-resolved infrared (TRIR) spectroscopy. Most compounds examined previously have been of the linear bis-bis or dimer-of-dimers type, however in Chapter 6 compounds with a cis ligand geometry are studied with steady state and time-resolved spectroscopic methods. Their excited state properties are found to be in many ways similar to their trans counterparts. One important advantage is that this ligand orientation affords the opportunity to attach the dimetal complexes to surfaces of materials. This aspect is explored in Chapter 7 where the compounds are used as sensitizers for titanium dioxide nanoparticles. Their ability to undergo photoinduced electron transfer to the semiconducting metal oxide is evaluated with TRIR.;These compounds have highly favorable properties for use in solar energy conversion and there is a need for further investigation of their abilities in this area.