The coordination potential of indigo, anthraquinone and related redox-active dyes

Abstract

The chromophores of several classes of well-established dyes, e.g., indigo, 9,10-anthraquinones, formazanes, diketopyrrolopyrroles (DPPs), or certain azo compounds, are not only responsible for intense low-energy absorptions but are also suited for dye ligand-based electron transfer and for (multiple) metal chelate coordination at N and O donor centers, after due deprotonation. Coupling with the redox activity of bonded transition metals such as ruthenium can lead to intricate electronic structures with variable oxidation state combinations, especially when the non-innocent dye-derived ligands are bridging two or more metal centers. Employing an array of experimental methods such as crystal structure determination, voltammetry, EPR, IR and UV–vis-NIR spectroelectrochemistry, and using supporting DFT calculations, it has been possible to analyze the geometrical and electronic situations in compounds such as [LxRun(µ-BL)RumLx]k, involving radical formation of the bridging chromophore and/or metal-metal mixed valency (BL: bridging ligand derived from dye). Among the prominent suitable examples for BL components are deprotonated indigo with variable ligand oxidation states and coordination patterns (N,O; N,N′; O,O′; N,O:N′,O′; N,N′:O,O′), the non-innocent nindigo ligands with indigo-O replaced by NR, the widely employed 9,10-anthraquinones with additional O or NR donor substituents at various chelate-forming positions, bidirectionally non-innocent formazanato ligands, the DPP(diketopyrrolopyrrole) π system used for technical pigments, heterocyclic quinones, and selected bis-chelating azo compounds with strong and potentially useful NIR absorbance. The deprotonated bridging ligands can adopt several oxidation states such as. BL0, BL−, BL2−, BL3− and BL4− for the indigo and anthraquinone examples. With the increasing application of classical dye chromophores in information technology systems based on optical and electronic properties, their hitherto neglected potential for metal coordination, including metal-metal bridging and coordination-enhanced electron transfer reactivity, awaits to be exploited – the scope of such capability is illustrated in this article.