Spectral tuning and emission enhancement through lanthanide coordination in a dual Vis-NIR emissive cyanide-bridged heterometallic Ru(II)-Er(III) complex

Owing to their unique luminescent properties and photosensitizing capability, cyanoruthenium(II) complexes with diimine ligands are the subject of intense research striving for routes for tuning their electronic properties and improving their emission quantum yield. In this work, we describe a heterometallic d-f cyanide-bridged Ru(II)–Er(III) assembly obtained by the direct reaction of trivalent erbium salt with the neutral [Ru(bipy)2(CN)2] (bipy = 2,2′-bipyridine) metalloligand. This strategy allows for accommodating inorganic negatively charged anions such as nitrate and oxalate in the coordination sphere of the lanthanide ion. As a result, a dimeric tetranuclear discrete molecular architecture is obtained, where the two constituting monomeric Ru(II)-CN-Er(III) units are bridged by an oxalate anion coordinating two Er(III) ions in a bis-bidentate fashion. Strikingly, this heterometallic compound shows intense dual emission in the visible and near-infrared spectral ranges under single-wavelength excitation in both solution and the crystalline state. The effect of Er(III) coordination through a cyanide bridge is thoroughly discussed, also with the support of DFT calculations, to highlight the factors that induce the observed spectral hypsochromism and, more importantly, the remarkable 10-fold-increased emission quantum yield of the [Ru(bipy)2(CN)2] moiety in the visible range. We show that the described coordination mode induces an energy raise of the emissive triplet metal-to-ligand charge transfer (3MLCT) state and even a more pronounced lifting of the nonemissive Ru(II) triplet metal-centered (3MC) states, suppressing thermal deactivation channels. Furthermore, owing to the reduced number of water molecules and quenching groups surrounding the lanthanide ion in the molecular architecture, relatively intense erbium emission at 1.5 μm telecom wavelength is detected through sensitization from the Ru(II) metalloligand. We suggest that this compound can find applications as an efficient solid-state dual emitter.