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

The complex [(tpy)Ru(tpp)RhCl3](PF6)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)RuII(μ-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)RhCl3](PF6)2 displays a Ru→tpp CT transition centered at 516 nm that is the lowest lying electronic transition. The electrochemistry of [(tpy)Ru(tpp)RhCl3](PF6)2 shows a RuII/III couple at 1.60 V vs Ag/AgCl, an irreversible RhIII/I reduction at −0.23 V and, a tpp0/- 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)RhCl3](PF6)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)RhCl3](PF6)2 via electron transfer is 80% efficient with a k et of 4 × 107 s-1. Details of these studies are presented herein.