Design, Synthesis and Study of Multi-Component and Integrated Systems for Light-Driven Hydrogen Generation (Final Technical Report 2006-2009)
The research focused on fundamental problems in the conversion of light to stored chemical energy. Specifically, work was completed on the design, synthesis and study of multi-component super- and supramolecular systems for photoinduced charge separation, one of the key steps in artificial photosynthesis, and on the use of these and related systems for the photochemical generation of H 2 from water. At the center of these systems are chromophores comprised of square planar coordinated Pt(II) ions with arylacetylide and either diimine or terpyridyl ligands. Previous work had shown that the chromophores are photoluminescent in fluid solution with long-lived metal-to-ligand charge transfer ( 3 MLCT) excited states that are necessarily directional. An advance which set the stage for a number of proposed studies was the light-driven production of hydrogen from water using a Pt(terpyridyl)(arylacetylide)+ chromophore and a sacrificial electron donor. The reaction is catalytic and appears to rival previously reported ruthenium bipyridyl systems in terms of H 2 production. Variation of system components and mechanistic studies were conducted to understand better the individual steps in the overall process and how to improve its efficiency. Success with light driven H 2 generation was employed as a key probe as new systems were constructed consisting of triads for photoinduced charge separation placed in close proximity to the H 2 generating catalyst - a Pt colloid - through direct linkage or supramolecular interactions with the polymer used to stabilize the colloid. In order to prepare new donor-chromophore-acceptor (D-C-A) triads and associated D-C and C-A dyads, new ligands were synthesized having functional groups for different coupling reactions such as simple amide formation and Pd-catalyzed coupling. In these systems, the donor was attached to the arylacetylide ligands and the acceptor was linked to the diimine or terpyridyl chelate. Research under the contract proved successful in the development of synthetic methodologies to make multi-component systems designed so as to maintain electronic communication between components held in a defined spatial arrangement. Systems effective for light driven H 2 generation were examined by photophysical methods including transient absorption spectroscopy to observe charge-separated states and chart their dynamics. Quantum yields for hydrogen production were also measured. Additional studies examined the effectiveness of these systems for H 2 generation and involved the development of new catalysts and systems based thereon. From these studies, a better understanding of initial steps in the light driven generation of hydrogen were obtained.