Synthesis, electrochemistry and photophysics of rigid norbornylogous-bridged complexes of ruthenium and osmium

A series of rigid phenanthroline-annulated ligands incorporating fused B-bond norbornane bicyclo[2.2.0]hexane bridges have been synthesized. The resulting dipyridoquinoxaline (dpq) ligands with covalently bound 1,4-naphthoquinone (dpq-6-NQ) or 1,4-benzoquinone (dpq-6-BQ) moieties were treated with cis-[Ru(bipy)(2)Cl-2]. 2H(2)O to yield the [Ru(dpq-6-NQ}(2+) and [Ru(dpq-6-BQ)](2+) [Ru = Ru(bipy)(2)] complexes as PF6- salts. The symmetric bis-annulated dpq-6-dpq ligand was similarly metallated to yield the dinuclear [Ru(dpq-6-dpq)M](4+) [Ru = Ru(bipy)(2), M = Ru(bipy)(2) or Os(bipy)(2)] species, whose electrochemistry and UV-Vis absorption spectra are consistent with class I or weakly coupled class II (localised) mixed-valence character and weak through-bond coupling across the norbornylogous bridge. Preliminary photophysical measurements using steady-state emission spectroscopy revealed quenching of the Ru-II-based (MLCT)-M-3 emissive state in the [Ru(dpq-6-NQ)](2+) and [Ru(dpq-6-BQ)](2+) complexes, which is assumed to arise from intramolecular electron transfer between Ru-II* and the electron-accepting quinone groups. In contrast, the efficient quenching of the (MLCT)-M-3 emissive state observed in [Ru(dpq-6-dpq)Os](4+) is consistent with intramolecular electronic energy transfer (k(EET) = 1.5 x 10(7) s(-1)) between the Ru-II* and Os-II centres.