NOVELTY - Neutral mononuclear copper(I) complexes (Q) are new.    USE - (Q) is useful for emitting light, preferably in an emitter layer of an optoelectronic device, which is organic LED (OLED), light-emitting electrochemical cells, light-emitting cells, OLED-sensors, preferably non- hermetically shielded outward gas and vapor sensors, optical temperature sensor, organic solar cells, organic field-effect transistors, organic lasers, organic diodes, organic photodiodes and down conversion systems (all claimed).    ADVANTAGE - (Q) can be prepared without using costly precious and nontoxic metals, hence economical and ecofriendly. (Q) exhibit high luminous efficiency, short emission lifetime, high emission quantum yield and good solubility in organic solvents. (Q) is free of unwanted geometrical changes.    DETAILED DESCRIPTION - Neutral mononuclear copper(I) complexes of formula (Q) are new.    ring A = nitrogen containing group of formula (a);    ring B1 = aryl group of formula (b)-(f), Z4=N-R, X-N=C(R)2 or P(R)2;    M = Cu(I);    L-B2-L = a neutral bidentate ligand, preferably a phosphine or arsanyl group of formula (E(R1)(R2));    E = P or As;    R1, R2 = branched or cyclic alkyl, aryl, heteroaryl, alkenyl, alkynyl (all boned over oxygen (-OR), nitrogen (-N(R)2) or silicon (-Si(R)3) and optionally substituted by halo), or aryl, heteroaryl, alkenyl (substituted by halo or D), further known donor and acceptor groups e.g. amines, carboxylates or esters, or CF3 (bonded to L over B2 to form a bidentate ligand) or H;    Z1-Z7 = N or CR;    R = organic group comprising branched or cyclic alkyl, aryl, heteroaryl, alkenyl, alkynyl (all boned over oxygen (-OR), nitrogen (-N(R)2), silicon (-Si(R)3) or phosphorus atom (P(R)2) and optionally substituted by halo), or aryl, heteroaryl, alkenyl (substituted by halo or D), further known donor and acceptor groups e.g. amines, carboxylates or esters, or CF3, or H;    X = CR2 or NR;    Y1 = O, S or NR;    Z8 = CR1a;    R1a = OR, NR2 or PR2, where the biding to Cu-atom takes place via these groups;    b1 = an atom binding to the complex;    a1 = an atom binding with two chemical units;    R2a = a bulky substituent, preferably in the ortho position to the coordination site and has a geometry change in the direction to prevent the planarization of the complex in the excited state, preferably branched alkyl group (-(CH2)n-CH3), 6-20C-aryl (e.g. phenyl), alkoxy group (-O-(CH2)n-CH3), aryloxy (e.g. -O-phenyl) or a silane group (e.g. Si(CH3)3) (where the alkyl and aryl are optionally substituted by e.g. halo, alkoxy or silane and optional lead to fused ring systems); and    n = 0-20.    Where: (Q) comprises no, 1 or 2 R2a; (Q) optionally have a functional group as a further substituent, which is attached either directly or through appropriate bridges at NarcL substituent, where the functional group is electron conductor, hole conductor and groups, which change the solubility of (Q), preferably increase the solubility of (Q) in organic solvents; and (Q) optionally exhibits delta E of less than 2500, preferably 500 cm-, which is the difference between the lowest excited singlet and the underlying triplet state, an emission lifetime of maximum than 20, preferably 3 mu s, and an emission quantum yield of greater than 40%, preferably 70% and/or solubility in organic solvent of at least 1, preferably 10 g/l. INDEPENDENT CLAIMS are included for:    (1) the preparation of an optoelectronic device using (Q); and    (2) selecting (Q) comprising determination of delta E by ab-initio molecular calculation, measuring the temperature dependence of the fluorescence and the phosphorescence intensity or measuring the temperature dependence of the emission lifetime, and determination of the organic molecules, whose delta E value is less than 2500, preferably 500 cm-1.