Bi-molecular emissive excited states in platinum (II) complexes for high-performance organic light-emitting diodes

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 x Cl 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.