Luminescent Mechanochromism in a Gold(I)-Copper(I) N-Heterocyclic Carbene Complex

The silver(I) species [Ag(benzim(CH2py)2)2]PF6 (1) was prepared by refluxing the ligand precursor [H(benzim(CH2py)2)2]PF6 with Ag2O and aqueous sodium hydroxide in dichloromethane. Simple transmetalation of 1 with tetrahydrothiophenegold(I) chloride forms the gold(I) analogue [Au(benzim(CH2py)2)2]PF6 (2). The addition of 2 equiv of [Cu(NCCH3)4]PF6 to 2 in acetonitrile produces a blue-luminescent, trimetallic complex, [AuCu2(benzim(CH2py)2)2(NCCH3)4](PF6)3·2CH3CN (3·2CH3CN). When blue-luminescent 3·2CH3CN is exposed to air, the complex loses four acetonitrile molecules, and the emission of the desolvated complex (4) appears aquamarine. Crystallization of 4 from different solvents produces the complexes [AuCu2(benzim(CH2py)2)2](PF6)3 (5) and [AuCu2(benzim(CH2py)2)2(NCCH2CH3)2](PF6)3 (6). Upon grinding, both 3·2CH3CN and 4 exhibit mechanochromic transformations to a yellow-luminescent powder (ground-4). The reversible mechanochromic transformation of 3·2CH3CN to ground-4 is a crystalline-to-amorphous conversion accompanied by partial desolvation. The luminescent mechanochromism of 4 to ground-4 is an "amorphous-to-amorphous" process and does not require solvent loss. In addition to their mechanochromic properties, both 3·2CH3CN and 4 exhibit luminescent thermochromism through desolvation to form a weak luminescent powder (7).