Four neutral mononuclear Cu(I) complexes, [Cu(pyin)(PPh₃)₂] (1a), [Cu(pyin)(DPEphos)] (1b), [Cu(quin)(PPh₃)₂] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin = 2-(2-quinolyl)indole and DPEphos = bis(2-(diphenylphosphino)phenyl)ether) have been synthesized. X-Ray crystal structure analysis revealed that the central Cu(I) ion in all complexes is in a distorted tetrahedral coordination environment. All four complexes display the typical metal-to-ligand charge transfer (MLCT) absorption band at 371, 363, 413 and 402 nm, respectively. No emission was observed from any complexes in the solid state due to triplet–triplet annihilation. However, the complexes show unusual dual-emission originating from intraligand charge-transfer (ILCT) and MLCT transitions, when dispersed in a rigid matrix (e.g. PMMA) or in frozen CH₂Cl₂. The oxidation potential of Cu(I)/Cu(II) in these neutral complexes, ∼0.5 V (vs.Ag/AgCl), is lower than those of cationic Cu(I) complexes. Films containing 10 wt% of these complexes in PMMA shows ratiometric fluorescent oxygen gas sensing property with a response ratio of 0.3–3.2 and response time of 3–4 s. Complex 2b acts as a ratiometric oxygen gas sensor with good reversibility through energy and electron transfer mechanisms under the loss of a counteranion.