Trinuclear ruthenium dioxolene complexes based on the bridging ligand hexahydroxytriphenylene: electrochemistry, spectroscopy, and near-infrared electrochromic behaviour associated with a reversible seven-membered redox chain

The trinuclear complexes [{(R₂bipy)₂Ru}₃(µ³-HHTP)](PF₆)₃ [1(PF₆)₃, R = H; 2(PF₆)₃, R = 4-^tBu] contain three {Ru(R₂bipy)₂}²⁺ fragments connected to the triangular tris-chelating ligand hexahydroxytriphenylene (H₆HHTP). This bridging ligand contains three dioxolene-type binding sites, each of which can reversibly convert between dianionic catecholate (cat), monoanionic semiquinone (sq) or neutral quinone (q) redox states. The bridging ligand as a whole can therefore exist in seven different redox states from fully reduced [cat,cat,cat]⁶⁻ through to fully oxidised, neutral [q,q,q]. Cyclic voltammetry of 1(PF₆)₃ in MeCN reveals six redox processes of which the three at more positive potentials (the sq/q couples) are reversible but the three at more negative potentials (the sq/cat couples) are irreversible with distorted wave shapes due to the insolubility of the reduced forms of the complex. In contrast, the more soluble complex 2(PF₆)₃ displays six reversible one-electron redox processes making all components of a seven-membered redox chain accessible. UV/Vis/NIR spectro-electrochemical studies reveal rich spectroscopic behaviour, with—in particular—very intense transitions in the near-IR region in many of the oxidation states associated with Ru(II)→(dioxolene) MLCT and bridging ligand centred π–π* transitions. TDDFT calculations were used to analyse the electronic spectra in all seven oxidation states; the calculated spectra generally show very good agreement with experiment, which has allowed a fairly complete assignment of the low-energy transitions. The strong electrochromism of the complexes in the near-IR region has formed the basis of an ‘optical window’ in which a thin film of 1(PF₆)₃ or 2(PF₆)₃ on a conductive glass surface can be reversibly and rapidly switched between redox states that alternate between strongly absorbing or near-transparent at 1100 nm, with—for 2(PF₆)₃—the switching being stable and reversible in water over thousands of cycles.