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 2,2':6',2"-terpyridine ligands, (-)-ctpy, (-)-[ctpy-x-ctpy], and (-)-[ctpy-b-ctpy], with one, two, or multiple Ru metal centers. The room-temperature absorption and emission maxima of {[((-)-ctpy)Ru]-(-)-[ctpy-b-ctpy]-[Ru((-)-ctpy)]}(PF6)(4) and ((-)-[ctpy-b-ctpy])-{ [Ru((-)-[ctpy-b-ctpy])](PF6)(2)}(n) 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-ctpy])](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 particle-in-a-box model, and the luminescence lifetimes are discussed in terms of the structure of the excited-state manifold.