NOVELTY - Neutral mononuclear copper(I) complex (I), for emitting light, is new. The copper(I) complex exhibits: a singlet-triplet splitting delta E(S1-T1) value between the lowest excited singlet state (S1) and the underlying triplet state (T1), of less than 2500 cm-1; an emission lifetime of not greater than 20 mu s; an emission quantum yield of greater than 40%; and a solubility in organic solvents of at least 1 g/liter.    USE - (I) is useful for emitting light, preferably in an emitter layer of an optoelectronic device, preferably OLEDs, light-emitting electrochemical cells, OLED sensors, preferably non-hermetically and outwardly shielded gas- and vapor sensors, optical temperature sensors, organic solar cells, organic field effect transistors, organic lasers, organic diodes, organic photodiodes and down conversion systems (all claimed).    ADVANTAGE - (I): exhibits high emission quantum yields, preferably greater than 40%, and good solubility in various solvents; and does not undergo unwanted changes in geometry. The optoelectronic device comprising (I) exhibits longer service life.    DETAILED DESCRIPTION - Neutral mononuclear copper(I) complex of formula (I), for emitting light, is new. The copper(I) complex exhibits: a singlet-triplet splitting delta E(S1-T1) value between the lowest excited singlet state (S1) and the underlying triplet state (T1), of less than 2500 cm-1; an emission lifetime of not greater than 20 mu s; an emission quantum yield of greater than 40%; and a solubility in organic solvents of at least 1 g/liter.    M = Cu(I);    L-L = single, negatively charged, bidentate ligand;    A-A = diimine ligand (substituted by R and FG), preferably substituted 2,2'-bipyridine derivative or 1,10-phenanthroline derivative;    R = at least one sterically hindered substituent for preventing the change in the geometry of copper(I) complex in the direction of planarization, in the excited state; and    FG = functional group, preferably at least one second substituent for electron conduction and for increasing solubility in organic solvents, or at least one second substituent for hole conduction, and for increasing solubility in organic solvents, where the functional group is bonded to the diimine ligand directly or through a bridge.    INDEPENDENT CLAIMS are also included for:    (1) producing an optoelectronic device, comprising using (I);    (2) an optoelectronic device comprising (I); and    (3) selecting copper(I) complex, whose singlet-triplet splitting delta E(S1-T1) value is less than 1500 cm-1, preferably less than 500 cm-1, comprising determining the singlet-triplet splitting delta E(S1-T1) value using an ab-initio molecular calculation, by measuring the temperature function of fluorescence- and phosphorescence intensity, or by measuring the temperature function of the emission lifetime, and determining the singlet-triplet splitting delta E(S1-T1) value of organic molecules, whose singlet-triplet splitting delta E(S1-T1) value is less than 2500 cm-1, preferably less than 500 cm-1.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the OLED. Drawing includes non-English language text.