Tuning the Photo-Sensitive Intramolecular Electron Transfers in a Rhenium Supramolecular Coordination Compound

Abstract

To investigate photo-sensitive intramolecular electron transfers and spatially large charge-separated states in a supramolecular coordination compound, a covalently linked donor bridge acceptor complex was synthesized with the general formula: [dmbRe(CO)₃-μ-L-Pt(NH₃)₂-μ-L-Re(CO)₃deb]4+, where L=4,4-bpy. The donor substituent is a rhenium dimethyl polypyridyl linked through a square planar Pt(II) moiety to an acceptor rhenium diester polypyridyl. Emission of a photon is observed at 612 nm on the acceptor polypyridyl when the compound is irradiated at 338 nm, the wavelength of excitation from the Re(I) metal to the donor 4,4’-dmbpy ligand. It is hypothesized that this excitation of the metal-to-ligand charge transfer (MLCT) band from the donor Re leads to a Re(II) excited state with the electron in a π* molecular orbital of the donor 4,4’dmbpy ligand. Through a Förster resonance type mechanism due to the length of the molecule, the electron then undergoes a series of energetically downhill intramolecular electron transfers to lead to a spatially large charge-separated state with the electron lying on the diester polypyridyl ligand, the acceptor complex. We theorize that communication occurs from the conjugation in the 4,4’-bpy bridging ligands on either side of the Pt(II) moiety, which allows electrons to transfer from the donor complex to the acceptor complex. To tune these photo-sensitive intramolecular electron transfers, we are adapting the bridging ligand to have 3,3’-dimethyl-4,4’-bipyridine ligands on either side of the Pt(II) moiety. Since the dihedral angle between the two pyridine rings is 81.1°, this non coplanar conformation should slow the electron transfer or inhibit the transfer all together. We are currently working on a proper synthesis for this ligand, which will then allow us to analyze the absorption and fluorescence properties of the molecule