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 RuII-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 RuII* 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 (kEET = 1.5 × 107 s–1) between the RuII* and OsII centres.