Chemists can play a significant role in the search for new sources of renewable energy to help meet future world energy needs. Of all the prospective renewable energy sources, solar offers the greatest potential return (as much as ten times the world energy needs expected at the end of this century). One area of solar energy related research for chemists is the study of photochemical reactions of molecules that absorb visible light. One significant goal is to develop photochemical systems that can be applied to the effective conversion of light (solar) energy into other forms of energy, most often chemical and electrical energy.
Our recent efforts have focused on preparing clusters of visible light absorbing chromophores that may be used to "harvest" light. Such light harvesting arrays can be used to channel energy into electrochemical cells designed to convert light to electrical energy (photoelectrochemical cells). The types of chromophores we have examined include porphyrins, phthalocyanines and a variety of transition metal complexes. Through a combination of synthetic and laser spectroscopic investigations, our group studies the factors that influence the reactivity and stability of these classes of visible chromophores. In addition we are attempting to develop new systems optimized for application in solar photochemical devices (most notably dye sensitized photoelectrochemical cells).
Another area of research in our group, related to solar energy conversion to chemical energy, is the study of what is referred to as one photon-two electron reactions. Using transition metal complexes that undergo two electron redox processes (i.e. Pt(II) to Pt(IV)), we are attempting to develop photochemical systems that can undergo redox cycling (M(II) to M(IV) to M(II)) to serve as photocatalysts for the production of useful reagents. For instance, a group of Pt(II) complexes have been shown to act as photocatalysts for the conversion of dihydropyridines to pyridines and hydrogen, useful for fuel cells and hydrogen internal combustion engines.
Many of the N-heterocyclic ligands we prepare for light harvesting systems are strongly luminescent and are related to materials used for the development of organic light emitting diodes (OLEDs). The ligands have emission that can be tuned to span the visible spectrum and, upon complexation of metals such as Zn(II), exhibit strong emission at lower energy. We are currently exploring the luminescence behavior of these complexes in solution and on surfaces, where aggregation of the chromophores often leads to luminescence quenching and/or spectral shifts. By gaining control of surface morphology via various surface deposition techniques, we hope to prepare highly luminescence surface bound chromophores that can be used in OLEDs.
Our research includes considerable organic and inorganic synthesis; most of the synthetic efforts are directed toward the preparation of N-heterocyclic ligands and coordination complexes of the ligands with Ru(II), Os(II) and Re(I). In addition, a significant part of our research involves characterization of the photophysical properties of molecules we prepare. This work includes measurement of luminescence lifetimes, quantum yields, emission spectra and excited state absorption spectra and provides students with the opportunity to gain experience with pulsed lasers. In the surface modification studies deposition is via either self assembly techniques or transfer of Langmuir-Blodgett films to solid supports and characterization is by absorption, luminescence and atomic force microscopy.
