A Tridentate-Bridged Ruthenium-Rhodium Complex as a Stereochemically Defined Light-Absorber-Electron-Acceptor Dyad

The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) (tpy = 2,2',6',2"-terpyridine and tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) has been prepared and its spectroscopic, electrochemical, and photophysical properties investigated. This complex couples a ruthenium light absorber to a rhodium electron acceptor to create the first tpp-bridged light-absorber-electron-acceptor dyad. This study illustrates the applicability of this (tpy)Ru(II)(&mgr;-tpp) chromophore in the construction of photochemical molecular devices. This system is of interest since the tpp ligand has been shown to provide stereochemically defined polymetallic complexes with reasonably long-lived metal to ligand charge transfer excited states. The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) displays a Ru-->tpp CT transition centered at 516 nm that is the lowest lying electronic transition. The electrochemistry of [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) shows a Ru(II/III) couple at 1.60 V vs Ag/AgCl, an irreversible Rh(III/I) reduction at -0.23 V and, a tpp(0/)(-) couple at -0.60 V. This illustrates that although this complex has a lowest lying spin-allowed spectroscopic transition that is Ru-->tpp CT in nature, the lowest occupied molecular orbital is Rh based. Thus, following excitation of this [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) complex into the Ru-->tpp CT state, electron transfer to the rhodium is thermodyamically favorable. This electron transfer leads to a quenching of the emission normally observed for this Ru-->tpp CT excited state. Emission quenching for [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) via electron transfer is 80% efficient with a k(et) of 4 x 10(7) s(-)(1). Details of these studies are presented herein.