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).