Modulation of Metallophilic
Bonds: Solvent-Induced
Isomerization and Luminescence Vapochromism of a Polymorphic Au–Cu
Cluster
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Abstract
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 [Au<sub>2</sub>Cu<sub>2</sub>(C<sub>2</sub>OHC<sub>5</sub>H<sub>8</sub>)<sub>4</sub>]<sub><i>n</i></sub> efficiently self-assembles
from Au(SC<sub>4</sub>H<sub>8</sub>)Cl, Cu(NCMe)<sub>4</sub>PF<sub>6</sub>, and 1-ethynylcyclopentanol in the presence of NEt<sub>3</sub>. This compound shows remarkably diverse polymorphism arising from
the modulation of metallophilic interactions by organic solvents.
Four crystalline forms, obtained from methanol (<b>1a</b>);
ethanol, acetone, or choloroform (<b>1b</b>); toluene (<b>1c</b>); and diethyl ether or ethyl acetate (<b>1d</b>),
demonstrate different photoluminescent characteristics. The solid-state
quantum yields of phosphorescence (Φ) vary from 0.1% (<b>1a</b>) to 25% (<b>1d</b>), depending on the character of
intermetallic bonding. The structures of <b>1b</b>–<b>d</b> were determined by single-crystal X-ray diffraction. The
ethanol (<b>1b</b>, Φ = 2%) and toluene (<b>1c</b>, Φ = 10%) solvates of [Au<sub>2</sub>Cu<sub>2</sub>(C<sub>2</sub>OHC<sub>5</sub>H<sub>8</sub>)<sub>4</sub>]<sub><i>n</i></sub> adopt octanuclear isomeric structures (<i>n</i> =
2), while <b>1d</b> (Φ = 25%) is a solvent-free chain
polymer built from two types of Au<sub>4</sub>Cu<sub>4</sub> 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 <b>1b</b> < <b>1c</b> < <b>1d</b>. The metallophilic contacts in <b>1a</b>–<b>d</b> 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
