Ground-State Properties and Excited-State Reactivity of 8-Quinolate Complexes of Ruthenium(II)

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Abstract
In an effort to explore new systems with highly reducing excited states, we prepared a series of Ru(II) complexes of the type Ru(L)2quo+ (L = bpy (2,2‘-bipyridine), phen (1,10-phenanthroline), dmphen (4,7-dimethyl-1,10-phenanthroline), tmphen (3,4,7,8-tetramethyl-1,10-phenanthroline); quo- = 8-quinolate) and investigated their photophysical and redox properties. The absorption and emission spectra of the Ru(L)2quo+ are significantly red-shifted relative to those of the parent complexes Ru(L)32+, with emission maxima in the 757−783 nm range in water. The Ru(L)2quo+ systems are easily oxidized with E1/2(RuIII/II) values ranging from +0.62 to +0.70 V vs NHE, making the emissive Ru → phen MLCT (metal-to-ligand charge transfer) excited states (E00 ∼ 1.95 eV in CH3CN) of the Ru(L)2quo+ complexes significantly better reducing agents than the MLCT states of the parent Ru(L)32+ complexes. Emission lifetimes of 17.0 and 32.2 ns were measured for Ru(phen)2quo+ in water and acetonitrile, respectively, and 11.4 ns for Ru(bpy)2quo+ in water. Transient absorption results are consistent with the formation of reduced methyl viologen upon Ru(phen)2quo+ excitation with visible light in water. The possibility of observing the Marcus inverted region in the forward bimolecular electron transfer reaction from the highly reducing *Ru(phen)2quo+ excited state was explored with neutral electron acceptors with reduction potentials ranging from +0.25 to −1.15 V vs NHE.

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