Tuning the photoluminescence properties of SLE- and MRL-active tricarbonylrhenium(I) complexes through minor structural changes of the organic ligand

Solid-state luminescence properties depend on numerous parameters, including the molecular geometry and the intermolecular interactions that take place in the solid. To clarify the role played by these parameters on the photoluminescence (PL) properties of tricarbonylrhenium(I) complexes, four molecules incorporating a 3-(2-pyridyl)-1,2,4-triazole (pyta) ligand with appended phenylbenzoxazole (PBO) unit were compared. One or two methyl groups were inserted at different places of the organic ligand of the parent compound RePBO, resulting in three new complexes RePBO-Me1, RePBO-Me2 and RePBO-Me3. As shown by NMR, the presence of the methyl group(s) induced some changes in molecular flexibility, but the electronic effects were relatively weak. As a result, the electrochemical and optical properties were little impacted, and the four complexes behaved almost similarly in organic solution, in agreement with theoretical calculations. In contrast, marked differences appeared between the complexes when considering the aggregation-induced emission (AIE) effect, mainly due to the formation of tiny microcrystals in aqueous medium. In the same way, the three methylated complexes in the form of microcrystalline powders showed clear crystallization-induced emission enhancement (CIEE) with respect to solutions, but with distinct characteristics. They emitted less intensely and at longer wavelengths than the unsubstituted complex. Most likely, the methyl groups strongly affect the geometry and the packing mode of the molecules in the crystals, which influence the PL properties in the solid state. After grinding the powders, the emission spectra of the three methylated complexes were shifted to the red, although this shift was weaker than that previously observed for RePBO. This effect was almost reversible after THF fuming. It was attributed to transitions between the crystalline and amorphous phases. Remarkably, in the amorphous phase where molecules regain their mobility, the emission differences between the four complexes almost disappeared. It was then concluded that the amplitude of the mechanoresponsive luminescence (MRL) effect strongly depends on the geometry of the molecules in the pristine powder. This study is one more step toward the rational design of photoluminescent tricarbonylrhenium(I) complexes. More generally, it is also a good example of how very small structural modifications can drastically govern the PL and MRL properties.