Dr. Watts received his Ph.D. from the University of Colorado in 1969. He did postdoctoral research at Washington State University and came to Santa Barbara in 1972.
My research interests are in spectroscopic characterization and photochemistry of electronic excited states of metal complexes. The immediate goals are to design, prepare and characterize new complexes with possible applications to photoconversion - the utilization of solar energy for production of chemical fuels. Since this requires materials which can absorb sunlight and then initiate chemical transformations leading to energy rich products, electronic spectroscopy and photochemical studies are the primary disciplines utilized. Examples of reactions which might be used for chemical photoconversion include splitting of H2O into hydrogen and oxygen as well as a variety of reactions in which CO2 is reduced to products such as methane, methanol, formaldehyde or formic acid. These CO2 reduction reactions are distant relatives of reactions which occur in natural photosynthesis in plants. As is the case in plants, a photochemical process leading to these reactions must utilize a molecule other than CO2 or H2O to initially absorb sunlight. The key roles of metal complexes as sensitizers and photocatalysts in such photochemical processes is the focus of this research.
A variety of transition metal complexes have been partially successful in yielding photoconversion via both H2O splitting and CO2 reduction. In particular, complexes of second and third row transition metals such as Ru(II), Re(I), Rh(III) and Ir(III) have been used as sensitizers, electron transfer agents, and catalysts for photoconversion. Excitation of these complexes by visible light leads to a variety of electronic excited states, some of which are characterized by a large displacement of charge density, and are denoted as charge-transfer excited states.
Much of the initial research on photoconversion utilized excited states involving charge transfer from the metal to an organic ligand. These metal-to-ligand charge-transfer excited states have proven to be useful as photooxidants and photoreductants. Recently, my research has come to focus upon a class of charge-transfer excited state where excitation results in movement of electron density from a metal-ligand bond into an organic ligand. This results in weakening and cleavage of the bond. With ligands bonded to the metal through several binding sites, one can use visible excitation to produce highly reactive transient intermediates that may be useful in the photoconversion processes cited above. This reactivity arises from a highly reactive reduced ligand, which is extremely electron rich, as well as a metal which is electron deficient due to cleavage of a metal-ligand bond. Return of the displaced charge to the original distribution is inhibited by movement of the ligand away from the bonding distance as a result of bond cleavage. Thus, the reactive intermediate lasts substantially longer than an excited state, and the metal complex may function as both a sensitizer and catalyst in promoting photoconversion.
Although complexes with sigma-bond-to-ligand charge-transfer excited states are not common, several are known. My research has emphasized metal-carbon and metal-silicon bonded species. The spectroscopy and photochemistry of several of these is presently being studied in various organic solvents as we seek a system that may be useful in carrying out photoreduction of CO2 with visible light as a driving force.
Selected Research Publications
Widely Different Lumiscence Lifetimes of the ÆRRR, ÆSSS and the ÆRRS, ÆSSR Diastereometers of fac-Tris [(8-quinolyl) phenylmethylsilyl]iridium(III): Exciplex formation with Solvents by Distinct s-Donor and ¹-Acceptor Binding Mechanisms. P.I., Djurovich, W.Cook, R. Joshi and R.J. Watts, J. Phys. Chem., (1994).
Time-Resolved Transient Difference Absorption Spectra of Iridium-Silicon Bonded Complexes: Evidence for a Solvated Intermediate Formed Via Iridium-Silicon Bond Cleavage, P.I. Djurovich and R.J. Watts, J. Phys. Chem. (1994).
Luminescence Characterizations of Cyclometallated Rhenium (I) Carbonyl Complexes, P. Spellane, R.J. Watts and A. Vogler, Inorg. Chem., 32, 4681-4682 (1993).
Synthesis and Spectroscopic Properties of Ortho-Metalated Iridium(III) Solvento Complexes. B. Schmid and R.J. Watts, Inorg. Chem., 33, 9-14 (1994).
Electrochemistry and Absorption and Emission Spectroscopy of New Ortho-metalated Complexes of Rh(III) and IR(III) with the Ligands 1,4,5,8-Tetraazaphenanthrene and 1,4,5,8,12-Hexaazatriphenylene. P. Didier, A. Kirsch-de Mesmaeker and R.J. Watts, Inorg. Chem., 32, 5239-5245 (1993).