Modulation of Metallophilic Bonds: Solvent-Induced Isomerization and Luminescence Vapochromism of a Polymorphic Au–Cu Cluster

We report a homoleptic Au–Cu alkynyl cluster that represents an unexplored class of luminescent materials with stimuli-responsive photophysical properties. The bimetallic complex formulated as [Au2Cu2(C2OHC5H8)4]n efficiently self-assembles from Au­(SC4H8)­Cl, Cu­(NCMe)4PF6, and 1-ethynylcyclopentanol in the presence of NEt3. This compound shows remarkably diverse polymorphism arising from the modulation of metallophilic interactions by organic solvents. Four crystalline forms, obtained from methanol (1a); ethanol, acetone, or choloroform (1b); toluene (1c); and diethyl ether or ethyl acetate (1d), demonstrate different photoluminescent characteristics. The solid-state quantum yields of phosphorescence (Φ) vary from 0.1% (1a) to 25% (1d), depending on the character of intermetallic bonding. The structures of 1b–d were determined by single-crystal X-ray diffraction. The ethanol (1b, Φ = 2%) and toluene (1c, Φ = 10%) solvates of [Au2Cu2(C2OHC5H8)4]n adopt octanuclear isomeric structures (n = 2), while 1d (Φ = 25%) is a solvent-free chain polymer built from two types of Au4Cu4 units. Electronic structure calculations show that the dramatic enhancement of the emission intensity is correlated with the increasing role of metal–metal bonding. The latter makes the emission progressively more metal-centered in the order 1b 1c 1d. The metallophilic contacts in 1a–d show high sensitivity to the vapors of certain solvents, which effectively induce unusual solid-state isomerization and switching of the absorption and luminescence properties via non-covalent interactions. The reported polymorphic material is the first example of a gold­(I) alkynyl compound demonstrating vapochromic behavior.