Rational Design of Strongly Blue-Emitting Cuprous Complexes with Thermally Activated Delayed Fluorescence and Application in Solution-Processed OLEDs

A new series of strongly greenish-blue to blue emitting Cu­(NN)­(POP)⁺ (POP = bis­[2-(dipenylphosphino)­phenyl]­ether) complexes containing N-linked 2-pyridyl pyrazolate diimine ligands [Cu­(pypz)­(POP)]­BF₄ (<b>1</b>), [Cu­(pympz)­(POP)]­BF₄ (<b>2</b>), and [Cu­(pytfmpz)­(POP)]­BF₄ (<b>3</b>) (pypz = 1-(2-pyridyl)­pyrazole, pympz = 3-methyl-1-(2-pyridyl)­pyrazole, and pytfmpz = 3-trifluoromethyl-1-(2-pyridyl)­pyrazole) have been designed and synthesized. Their structural, electrochemical, and photophysical properties have been characterized. The complexes <b>1</b>–<b>3</b> exhibit high photoluminescence quantum yields (PLQYs) at room temperature both in nitrogen-saturated CH₂Cl₂ (up to 45%) and in neat solid (up to 87%), which are comparable to the reported highest values for the cuprous complexes. The temperature dependence of spectroscopic properties and emission decay behaviors reveal the presence of two thermally equilibrated emitting states. At temperatures below 150 K, the lowest triplet state (T₁) is the predominant emitting state resulting in the typical phosphorescence with the emission decay times in the order of hundreds of microseconds. However, at ambient temperature, the lowest singlet state (S₁), which lies only about 0.17–0.18 eV above the T₁ state, is populated thermally and in turn generates efficient thermally activated delayed fluorescence (TADF), and the emission decay times are reduced dramatically to, e.g., 12.2 μs for <b>2</b>. Solution-processed OLEDs containing <b>1</b>–<b>3</b> in the emissive layer demonstrated excellent device performances by taking advantage of the singlet harvesting mechanism, among which the electroluminescent device using <b>3</b> shows a peak external quantum efficiency (EQE) of 8.47%, a peak current efficiency (CE) of 23.68 cd/A, and a maximum brightness of 2033 cd/m<sup>2