Luminescent Mechanochromic Dinuclear Cu(I) Complexes with Macrocyclic Diamine-Tetracarbene Ligands

Dinuclear Cu­(I) complexes bearing hexadentate, macrocyclic N-heterocyclic carbene (NHC) ligands, [Cu2­(L1)­(CH3CN)]­[PF6]2 (1) and [Cu2­(L2)­(CH3CN)]2­[Cu2­(L2)­(CH3CN)2]­[PF6]6 (2), have been synthesized by the reactions of [H4L]­[PF6]4 (L = L1, L2) with excess Cu2O in acetonitrile. Crystallizations of the heat-treated samples of 1 and 2 from acetone/​methanol/ether or CH3NO2/​ether result in [Cu2­(L1)]­[PF6]2 (3) and [Cu2­(L2)]­[PF6]2 (4). Complexes 1–4 are emissive with luminescent maxima at 464, 472, 540, and 488 nm in the solid state, respectively. The origin of the red shift of the emission maximum of 3 relative to the other three complexes has been studied by theoretical calculations, showing the cuprophilic interactions in the excited state of 3. The mechanochromic luminescent properties of 1–4 have been studied. After grinding in a mortar, a significant emission color change is found with a red shift of 98 nm for 1, 82 nm for 2, 20 nm for 3, and 64 nm for 4, respectively. These mechanochromic transformations are found to be a crystalline-to-amorphous conversion, which can be reverted by adding drops of the organic solvent or recrystallization. The possible correlations between the luminescent properties and structural modifications such as Cu···Cu distances are discussed