Enhanced blue phosphorescence in platinum acetylide complexes via a secondary heavy metal and anion-controlled aggregation

Abstract

Organoplatinum compounds represent a promising class of blue-phosphorescent molecules for electroluminescent color displays. Much recent work has focused on decreasing the nonradiative rate constant (k_nr) to improve the photoluminescence quantum yield (Φ_PL) of these compounds, but in most cases small radiative rate constants (k_r) lead to long excited-state lifetimes (τ) poorly suited for electroluminescence applications. In this work, we present an approach to increase k_r and Φ_PL in blue-phosphorescent platinum acetylide complexes with the general formula cis-[Pt(CN–R)₂(CC–2-py)₂] (CN–R is an alkyl isocyanide and CC–2-py is 2-pyridylacetylide). This method incorporates secondary heavy metals, Cu(I) or Ag(I), bound by the pyridyl moieties. We observe the formation of dimer complexes in the solid state due to noncovalent interactions between the secondary metal and the acetylide ligands, especially strong in the case of Cu(I). Incorporation of Cu(I) also erodes the desired blue-phosphorescence by introducing a low-lying metal-to-ligand charge transfer (³MLCT) state that dominates the observed phosphorescence. In the complexes bound to Ag(I), we find that phosphorescence profile is strongly dependent on the counteranion, which we propose is caused by different degrees of aggregation. With this insight, we show that coordination of AgBAr^F₄ (BAr^F₄⁻ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), with a large noncoordinating counteranion, inhibits aggregation and results in a 4–8× increase in k_r and a 5–10× increase in Φ_PL while preserving a pure blue phosphorescence profile.