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 lig-ands 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 the first example of a heterometallic d-f cyanide-bridged Ru(II)-Er(III) assembly ob-tained by the direct reaction of trivalent erbium salt with the neutral [Ru(bipy)2(CN)2] 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 heter-ometallic compound shows intense dual emission in the visible and near-infrared spectral ranges under single-wavelength exci-tation both in solution and in the crystalline state. The effect of Er(III) coordination through a cyanide bridge is thoroughly dis-cussed, also with the support of DFT calculations, to highlight the factors that induce the observed spectral hypsochromism and, more importantly, the remarkable tenfold-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 3MLCT state and even a more pronounced lifting of the non-emissive Ru(II) 3MC states, suppressing thermal deactivation channels. Furthermore, thanks to the reduced number of water molecules and quenching groups surrounding the lanthanide ion in the molecular architecture, rel-atively intense erbium emission at 1.5 μm telecom wavelength is detected through the sensitization from the Ru(II) metallolig-and. We suggest that this compound can find applications as efficient solid-state dual emitter and luminescent chemical sensor.