NOVELTY - A heteroleptic complex comprises a metal bound to first and second ligands. Each ligand has a pair of substituents. The substituents on the first ligand is different from the second ligands. One of the ligands is sized and shaped to prevent forming of homoleptic complex with the metal. The ligands do not interfere sterically with each other.    USE - The heteroleptic complex is useful as optical oxygen sensors. It is also used to detect and quantify the concentration of oxygen, ammonia, nitric oxide, and nitrogen dioxide molecules in liquids and gases; in dye sensitization for photovoltaic devices/solar cells; and in hydrogen production from the photochemical splitting of water.    ADVANTAGE - The heteroleptic complex exhibits photoluminescence property. It is obtained without the difficult and unproductive, chemical separation processes. It has a longer solid-state lifetime and demonstrates inexpensive metal-based heteroleptic complexes, which are better than costly ruthenium-based homoleptic complexes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a method of preparing heteroleptic complex;    (2) a method of detecting molecules in a gas;    (3) a method for the visible photosensitization of titanium dioxide comprising contacting the titanium dioxide with a complex;    (4) a method of producing hydrogen comprising photochemically splitting water by contact of water with the complex; and    (5) a method of sensitizing dyes for photovoltaic devices/solar cells comprising contacting dyes with the complex.    The heteroleptic complex is prepared by reacting the metal and each of the ligands in a solution of equal molar amount so that formation of heteroleptic complex proceeds to the exclusion formation of heteroleptic complexes and ligand.    The molecules in gas are detected by contacting the gas with the heteroleptic complex which is attached to a solid support and quantifying quenching photoluminescence of complex by the gas.    The degree of quantification of the quenching is indicated by the concentration of molecules in the gas.