Electrochemical, Spectroscopic, and Photophysical Properties of Structurally Diverse Polyazine-Bridged Ru(II),Pt(II) and Os(II),Ru(II),Pt(II) Supramolecular Motifs

Five new tetrametallic supramolecules of the motif [{(TL)2M(dpp)}2Ru(BL)PtCl2]6+ and three new trimetallic light absorbers [{(TL)2M(dpp)}2Ru(BL)]6+ (TL = bpy = 2,2′-bipyridine or phen = 1,10-phenanthroline; M = Ru(II) or Os(II); BL = dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline, or bpm = 2,2′-bipyrimidine) were synthesized and their redox, spectroscopic, and photophysical properties investigated. The tetrametallic complexes couple a Pt(II)-based reactive metal center to Ru and/or Os light absorbers through two different polyazine BL to provide structural diversity and interesting resultant properties. The redox potential of the MII/III couple is modulated by M variation, with the terminal RuII/III occurring at 1.58–1.61 V and terminal OsII/III couples at 1.07–1.18 V versus Ag/AgCl. [{(TL)2M(dpp)}2Ru(BL)](PF6)6 display terminal M(dπ)-based highest occupied molecular orbitals (HOMOs) with the dpp(π*)-based lowest unoccupied molecular orbital (LUMO) energy relatively unaffected by the nature of BL. The coupling of Pt to the BL results in orbital inversion with localization of the LUMO on the remote BL in the tetrametallic complexes, providing a lowest energy charge separated (CS) state with an oxidized terminal Ru or Os and spatially separated reduced BL. The complexes [{(TL)2M(dpp)}2Ru(BL)]6+ and [{(TL)2M(dpp)}2Ru(BL)PtCl2]6+ efficiently absorb light throughout the UV and visible regions with intense metal-to-ligand charge transfer (MLCT) transitions in the visible at about 540 nm (M = Ru) and 560 nm (M = Os) (ε ≈ 33,000–42,000 M–1 cm–1) and direct excitation to the spin-forbidden 3MLCT excited state in the Os complexes about 720 nm. All the trimetallic and tetrametallic Ru-based supramolecular systems emit from the terminal Ru(dπ)→dpp(π*) 3MLCT state, λmax em ≈ 750 nm. The tetrametallic systems display complex excited state dynamics with quenching of the 3MLCT emission at room temperature to populate the lowest-lying 3CS state population of the emissive 3MLCT state.