New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex

The new Ru(II)–anthraquinone complex [Ru(bpy) 2 (qdpq)](PF 6 ) 2 ( Ru-qdpq; bpy = 2,2′-bipyridine; qdpq = 2,3-di(2-pyridyl)naphtho[2,3- f ]quinoxaline-7,12-quinone) possesses a strong 1 MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru(bpy) 2 (qdppz)](PF 6 ) 2 ( Ru-qdppz; qdppz = naphtho[2,3- a ]dipyrido[3,2-h:2′,3′-f]phenazine-5,18-dione), with λ max = 450 nm. Ru-qdppz possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of Ru-qdpq results in the population of a relatively long-lived (τ = 19 ns) Ru(dπ) → qdpq(π*) 3 MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. These observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy.