posted on 2014-06-02, 00:00authored byStephanie Bellinger-Buckley, Tse-Cing Chang, Seema Bag, David Schweinfurth, Weihong Zhou, Bela Torok, Biprajit Sarkar, Ming-Kang Tsai, Jonathan Rochford
A series of ruthenium polypyridyl
complexes are presented incorporating π-extended electron rich
derivatives of the 8-oxyquinolate (OQN) ligand. The π-donating
property of the OQN ligand introduces covalent character to the Ru(dπ)–OQN(π)
bonding scheme enhancing its light harvesting properties and diversifying
its redox properties, relative to the classic ruthenium(II) trisbipyridyl
complex [Ru(bpy)3]2+. Synthesis and characterization
is presented for the complexes [Ru(bpy)2(R-OQN)](PF6), where bpy = 2,2′-bipyridine and R = 5-phenyl, 5,7-diphenyl,
2,4-diphenyl, 5,7-bis(4-methoxyphenyl), 5,7-bis(4-(diphenylamino)phenyl).
A comprehensive bonding analysis is presented for the [Ru(bpy)2(OQN)]+ system illustrating the origin of its unique
spectroscopic and redox properties relative to [Ru(bpy)3]2+. This model is then extended to enable a consistent
interpretation of spectra and redox properties for the π-extended
[Ru(bpy)2(R-OQN)](PF6) series. Electronic structures
have been probed experimentally by a combination of electrochemical
and spectroscopic techniques (UV–vis–NIR absorption,
emission, EPR spectroscopy) where (metal–ligand)-to-ligand
(MLLCT) charge-transfer properties are described by time dependent-density
functional theory (TD-DFT) analysis, at the B3LYP/6-31g(d,p) level
of approximation. Substantial mixing, due to bonding and antibonding
combinations of Ru(dπ) and OQN(π) orbitals, is observed
at the HOMO and HOMO – 3 levels for the ruthenium–oxyanion
bond in [Ru(bpy)2(OQN)]+, which is responsible
for the low-energy MLLCT based electronic transition and destabilization
of the HOMO level viz. cyclic voltammetry. This noninnocent π-bonding
phenomenon is consistent throughout the series which allows for controlled
tuning of complex redox potentials while maintaining panchromatic
absorption properties across the visible spectrum. Extensive charge
delocalization is observed for the one-electron oxidized species using
a combination of UV–vis–NIR, EPR spectroelectrochemistry,
and Mulliken spin-density analysis, giving strong evidence for hole-delocalization
across the delocalized Ru(dπ)–OQN(π) system, in
particular for the electron rich 5,7-bis(4-methoxyphenyl) and 5,7-bis(4-(diphenylamino)phenyl)
systems.