The efficient parallel emissions (photoluminescence = PL and electroluminescence = EL) from bi-molecular (B-M) and mono-molecular (M-M) excited states are observed and their nature described in Pt L 26 Cl complex (for neat films in the figure) and employed to fabricate varying-color, high-efficacy OLEDs. Display omitted ► How various bi-molecular states are involved in determining spectral properties of Pt organic complexes. ► How bi-molecular excited states can be used and manipulated in fabrication of highly-efficient OLEDs within a broad spectral span covering its visible and near IR region. ► New significant aspects of chemical physics in the important group of advanced organic compounds to be widely used in organic optoelectronics. The nature of excited states in Pt(II) organic complexes used as efficient phosphorescent emitters in organic light-emitting diodes (OLEDs) is examined. These compounds were chosen because their square planar structures allow them to facially aggregate through attractive intermolecular interactions of Pt–Pt or ligand–ligand or combination of these, thereby facilitating formation of bi-molecular (B-M) states such as ground-state dimers and/or excited states – excimers and electromers. The efficient parallel emissions from B-M and mono-molecular (M-M) excited states allow to tune color and efficiency of OLEDs. Based on various spectroscopic characteristics of selected Pt L xCl complexes with different ligands L x, we find that both excited triplet dimers and excimers are formed in fluid and solid solutions in addition to monomer excitonic states, though their relative populations are strongly dependent on the sample composition and particularly on its polarity. Of two possible routes of the formation of the excimer states, the one based on molecular triplet interaction with ground state molecules is found to dominate in both optically (PL) and electrically (EL) pumped film emissions. The contribution of the direct Coulombic interaction of complex cations and anions (omitting the formation of molecular triplets) to the formation of excimers is insignificant. These findings have a direct impact on the design and general understanding of OLEDs and ultimate development of stable, high-efficiency Pt(II) organic phosphor-based LEDs. However, they apply as well to any organic material containing planar molecules or their planar parts. Thus, we would expect their importance in phthalocyanine dyes and derivitized bucky-balls that are often used in organic photovoltaics and in pentacene films that are incorporated into organic transistors.