Design, Synthesis a nd Characterization of Ruthenium Polypyridyl Complexes Towards Structural and Conformational Control of Intramolecular Electron Transfer

摘要

The demand for carbon neutral sources of energy has led researchers to turn toward strategies involving conversion of solar photons to produce electricity and fuels. Solar conversion is initiated by photoinduced electron transfer (ET) processes. Donor-Bridge-Acceptor (DBA) complexes are a common platform for investigating photoinduced intramolecular electron transfer processes.In this work, two series of DBA complexes have been synthesized and characterized to investigate the role of structural elements on rates of photoinduced intramolecular electron transfer. The DBA complexes are comprised of a ruthenium polypyridyl donor covalently linked to a 4,4u27-bipyridinium acceptor by a bridging aryl subunit. We hypothesize that the addition of steric bulk on the donor chromophore and the bridging subunit will alter the lifetimes for forward and back electron transfer processes.New electroactive asymmetric ligands were synthesized in high yields and complexed to produce seven new DBA complexes which were characterized by absorption and emission spectroscopies, cyclic voltammetry and spectroelectrochemistry, electronic structure calculations and picosecond transient absorption techniques.The first series of DBA complexes was prepared to investigate how lifetimes of intramolecular ET are affected by increased methylation on ancillary ligands which is expected to alter driving forces for ET. The electron lifetimes were measured for three complexes that share an electroactive ligand but differ in ancillary ligand. It was determined that the lifetime for forward ET was not significantly altered but the lifetimes for back ET differed by a factor of two.For the second DBA series, methyl groups were systematically introduced onto various positions of the bridging subunit to determine if steric bulk is effective at disrupting electron communication within the complex thereby altering ET lifetimes. Reductive spectroelectrochemistry and transient absorption spectra revealed that steric bulk was effective at localizing electron density onto the acceptor moiety. The measured ET lifetimes showed that the number of methyl substituents on the bridging subunit was more of a determining factor of ET lifetimes than the methyl substituent position.