Photophysics and Redox Behavior of Chiral Transition Metal Polymers

The absorption and emission spectra, excited-state lifetimes, quantum yields, and electrochemical measurements have been obtained for a new series of chiral complexes based on three different chiral 2,2ʹ:6ʹ,2-terpyridine ligands, (-)-ctpy, (-)-[ctpy-x-ctpy], and (-)-[ctpy-b-ctpy], with one, two, or multiple Ru metal centers. The roomtemperature absorption and emission maxima of {[((-)-ctpy)Ru]-(-)-[ctpy-b-ctpy]-[Ru((-)-ctpy)]}(PF6)4 and ((-)[ctpy-b-ctpy])-{[Ru((-)-[ctpy-b-ctpy])](PF6)2} were shifted to lower energies and also exhibited significantly longer luminescence lifetimes when compared to [Ru((-)-ctpy)2](PF6)2, {[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]}(PF6)4, and ((-)-[ctpy-x-ctpy])-{[Ru((-)-[ctpy-x-ctpy3)](PF6)2}n. In terms of their electrochemical behavior, all of the complexes studied exhibited one Ru-centered and two ligand-centered redox waves and the {[((-)-ctpy)Ru}-(-)[ctpy-x-ctpy]-[Ru((-)-ctpy)]}(PF6)4, ((-)-[ctpy-x-ctpy])-{[Ru((-)-[ctpy-x-ctpy])](PF6)2}n, and ((-)-[ctpy-b-ctpy])-{[Ru((-)-[ctpy-b-ctpy])](PF6)2}n complexes were found to electrodeposit upon ligand-based reduction. The difference between the formal potentials of the Ru-centered and the first ligand-centered (least negative) waves corresponded linearly with the changes in the observed emission energies. The shifts in energy are discussed using a particlein-a-box model, and the luminescence lifetimes are discussed in terms of the structure of the excited-state manifold.