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)] efficiently self-assembles from Au(SC4H8)Cl, Cu(NCMe)(4)PF6, 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 (Phi) 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, Phi = 2%) and toluene (1c, Phi = 10%) solvates of [Au2Cu2(C2OHC5H8)(4)](n) adopt octanuclear isomeric structures (n = 2), while 1d (Phi = 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.