A new class of light absorber–electron acceptor dyad

Photoinduced electron-transfer within osmium(II) and ruthenium(II) bis-terpyridine donor acceptor dyads

The synthesis and photophysical characterization of two different series of electron donor-acceptor dyads containing Ru(II) and Os(II) bis-terpyridines (M(tpy)(2)(2+)) were prepared and studied in order to compare the oft-studied Ru(tpy)(2)(2+) chromophore with the less studied Os(tpy)(2)(2+) chromophore. The first series of dyads incorporates a benzoquinone (BQ) group as the electron acceptor, whereas the second contains a substituted pyromellitimide (PI) group as the electron acceptor. Steady-state emission experiments indicated efficient quenching of the 3MLCT emission of the electronically excited Os(II)-BQ complexes (7-8) compared to both model complexes (3-4) and the Os(II)-PI complex 10. Femtosecond pump-probe absorption experiments on 7-8 yielded ultrafast electron transfer rate constants (kET) of approximately 2.0 x 10(11) s(-1) (7) and 1.3 x 10(10) s(-1) (8) that were in good agreement with the low emission quantum yield results. Charge-recombination (kCR) in these complexes was also quite rapid, with rate constants of approximately 6.7 x 10(10) s(-1) (7) and 1.2 x 10(10) s(-1) (8). The analogous Ru(II) complexes underwent charge separation with rate constants of 7.6 x 10(10) s(-1) (5) and approximately 2.3 x 10(10) s(-1) (6), while charge recombination in these complexes occurred with rate constants of approximately 2.1 x 10(10) s(-1) (5) and approximately 5.3 x 10(10) s(-1) (6). Electron transfer in the pyromellitimide-containing complexes occurred only for Os(II)-PI (10), which exhibited significantly slower electron transfer (approximately 4.3 x 10(6) s(-1)) and charge recombination (approximately 7.7 x 10(6) s(-1)) rate constants. The nearly thermoneutral free energy of electron transfer and short excited state lifetime in the case of Ru(II)-PI (9) presumably prevents electron transfer in this compound.

Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.

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.