NOVELTY - A vapochromic gold-copper complex (C1) or its solvate is new.    USE - In chemical sensor device for detecting an analyte (preferably water, alcohol, nitrile, ketone, aldehyde, carboxylic ester, carbonic ester, thioether, amine, imine, phosphonic ester, phosphoric ester, phosphorus ester, methanol, ethanol, acetone, methyl ethyl ketone, tetrahydrothiophene, dimethylsulfide, acetonitrile, or phosphorus-containing compound) (all claimed). Also used in the chemical, energy and environmental sectors; for personal and badge monitors in chemical laboratories (e.g. industrial chemical or pharmaceutical research laboratories, paint and coatings manufacturing, cosmetics manufacturing) for hazardous vapor detection; in portable or stationary threshold monitors for chemical vapors in laboratory environments or chemical storage facilities for hazardous vapor detection or regulated emission requirements; in environmental sensor for volatile organic compounds (VOCs) or gases ("electronic noses") for use at environmental remediation sites, landfills, and air-quality monitoring; for the area of biofuels, where fermentation would produce methanol as a liquid fuel for transportation or fuel cells; and for responsive coatings, art supplies, color-changing paint and other related applications where a color-changing material is desired.    ADVANTAGE - The vapochromic gold-copper complex (C1) exhibits reversible luminescent vapochromic behavior through ligand exchange reactions allowing the sensing of organic compounds (e.g. MeCN, H2O, MeOH, Me2CO, and the like vapors), which permit use in luminescence vapochromic sensors. The vapochromic gold-copper complex (C1) is suitable for use in chemical sensor devices, such as volatile organic compounds (VOC) sensors.    DETAILED DESCRIPTION - A vapochromic gold-copper complex (C1) of formula (Au(L')2(Cu(Y')n)2)+3X3 or its solvate is new.    L'=N-heterocyclic carbene;    Y'=heteroatom-containing ligand;    X=anion;    n=1 or 2.    INDEPENDENT CLAIMS are included for the following:    (1) detecting (M1) an analyte involving a) exposing a first complex to the analyte, where the first complex is the vapochromic gold-copper complex (C1), and the first complex is reversibly transformed to a second complex by replacement of at least one of the heteroatom-containing ligands by the analyte upon exposing the first complex to the analyte; and b) observing a photoluminescent or infrared absorbance change in the second complex relative to the first complex resulting from exposure to the analyte; and    (2) making (M2) the complex (C1).