2-Pyridine cyclic triimidazole as a chelating and bridging ligand in mono- and hexa-nuclear Re(I) complexes with emissive properties in solution and in the solid state

Two luminescent Re(I) complexes, namely the mononuclear fac-[ReCl(CO)3(TT-Py)] (1) and the hexanuclear [{Re(μ- Cl)(CO)3(μ-TT-Py)}{Re2(μ-Cl)2(CO)6}]2 (2), have been isolated in high yields by one-pot reaction between [ReCl(CO)5] and different amounts of nitrogen rich 2-pyridine cyclic triimidazole (TT-Py), which acts as chelating ligand in both derivatives. The metallo-supramolecular cyclic structure of 2, which can be described as a metallacycle in which two mononuclear units of 1 act as bridging ligands towards two “Re2(μ-Cl)2(CO)6” fragments, has been confirmed by single crystal X-ray diffraction analysis. Both complexes have been fully characterized in solution by 1D and 2D multinuclear NMR spectroscopy. 1 and 2 exhibit photoluminescence at room temperature in solution, with a broad unstructured emission in the 530–570 nm range. The nature of the excited states involved in the electronic absorption spectrum of 1 has been ascertained by a combined density functional and time-dependent density functional (TD-DFT) study. Both compounds exhibit strong aggregation induced emission enhancement (AIEE), which is more pronounced for 2 leading to a solid state photoluminescence quantum yield equal to 2.5% that is one of the highest value ever reported for Re(I) polynuclear metallacycles so far. The versatility of TT-Py to adopt variegated coordination modes, previously observed for Cu(I) and Cu(II) derivatives, is confirmed supporting further use of this ligand to obtain new and peculiar emissive mono-, polynuclear and extended coordination systems.