Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(i) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement

Since intramolecular pi-pi stacking interactions can modify the geometry, crystal packing mode, or even the electronic properties of transition metal complexes, they are also likely to influence the solid-state luminescence properties. Following this concept, a new tricarbonylrhenium(I) complex (Re-BPTA) was designed, based on a simple symmetrical 5,5 "-dimethyl-4,4 "-diphenyl-3,3 "-bi-(1,2,4-triazole) organic ligand. The complex was prepared in good yield using a three-step procedure. The crystallographic study revealed that both phenyl rings are located on the same side of the molecule, and twisted by 71 degrees and 62 degrees, respectively, with respect to the bi-(1,2,4-triazole) unit. They overlap significantly, although they are slipped parallel to each other to minimize the intramolecular interaction energy. The pi-pi stacking interaction was also revealed by H-1 NMR spectroscopy, in good agreement with the results of theoretical calculations. In organic solutions, a peculiar electrochemical signature was observed compared to closely related pyridyl-triazole (pyta)-based complexes. With regard to the optical properties, the stiffness of the Re-BPTA complex led to the stabilization of the (MLCT)-M-3 state, and thus to an enhancement of the red phosphorescence emission compared to the more flexible pyta complexes. However, an increased sensitivity to quenching by oxygen appeared. In the microcrystalline phase, the Re-BPTA complex showed strong photoluminescence (PL) emission in the green-yellow wavelength range (lambda(PL) = 548 nm, Phi(PL) = 0.52, <tau(PL)> = 713 ns), and thus a dramatic solid-state luminescence enhancement (SLE) effect. These attractive emission properties can be attributed to the fact that the molecule undergoes little distortion between the ground state and the triplet excited state, as well as to a favorable intermolecular arrangement that minimizes detrimental interactions in the crystal lattice. The aggregation-induced phosphorescence emission (AIPE) effect was clear, with a 7-fold increase in emission intensity at 546 nm, although the aggregates formed in aqueous medium were much less emissive than the native microcrystalline powder. In this work, the rigidity of the Re-BPTA complex is reinforced by the intramolecular pi-pi stacking interaction of the phenyl rings. This original concept provides a rhenium tricarbonyl compound with very good SLE properties, and could be used more widely to successfully develop this area of research.