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.