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

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