Synthesis and investigation of ruthenium polypyridyl based donor-bridge-acceptor complexes toward control of electron transfer photochemistry

Developing efficient methods for converting solar photons to renewable fuels is a goal that is inspiring researchers in many areas of science. One specific area of related research involves the design and synthesis of Donor-Bridge-Acceptor (DBA) systems as platforms to mimic, explore, and ultimately improve upon the electron transfer (ET) photochemistry that drives many light-to-energy conversion schemes.In this work, a series of novel DBA complexes are designed, synthesized and characterized. Each is comprised of a ruthenium polypyridyl donor complex and a methyl viologen acceptor covalently bound through an aromatic bridge. We have hypothesized that by exploiting large amplitude ligand-based motions we will exert control over forward and reverse ET rate constants.Novel electroactive asymmetric ligands were synthesized and then complexed to generate a series of ruthenium compounds containing various ancillary ligands. Complementary donor model complexes were synthesized to help determine driving forces for ET in corresponding DBA systems through the use of electrochemical and photophysical techniques. ET rate constants were measured using picosecond time-resolved absorption spectroscopy. The proposed control mechanism is also tested with a detailed study of structure and energetics using density functional theory (DFT) calculations.Two series of complexes were studied. The first had the same asymmetric bridging ligand while the ancillary ligands on the donor were varied to change the driving force for ET. An ET photoproduct was observable and the rate constant for its formation was measured to be larger than that for its loss (back ET). The rate constant for forward ET was found to be nearly identical for these complexes, and was insensitive to changes in driving force.The second series held the donor moiety static while changing substitution on the bridging moiety. These complexes all have nearly the same driving force. However, when rate constants for ET were collected the rate constant for charge recombination was highly varied suggesting that other variables are being manipulated with these synthetic modifications. Temperature dependent studies suggest that as sterics are increased on the bridge, the amount of electronic coupling between reactant and product in the back direction decreases. This confirms our hypothesis as well as predictions from the DFT results.