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

Abstract

A series of rigid phenanthroline-annulated ligands incorporating fused 6-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)₂Cl₂]·2H₂O 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 ³MLCT 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 ³MLCT emissive state observed in [Ru(dpq-6-dpq)Os]⁴⁺ is consistent with intramolecular electronic energy transfer (k_EET = 1.5 × 10⁷ s^–1) between the Ru^II* and Os^II centres.