Extended ligand conjugation and dinuclearity as a route to efficient platinum-based near-infrared (NIR) triplet emitters and solution-processed NIR-OLEDs

Abstract

Near infrared (NIR) emission from molecular materials is typically targeted by using more extended conjugated systems compared to visible-emitting materials. But efficiencies usually fall off due to the combined effects of increasing non-radiative and lower oscillator strengths as the energy of emissive excited states decreases. Efficient NIR-emitting organic light emitting diodes (OLEDs) are rare compared to the huge progress that has been made for visible-light devices. For organometallic emitters that contain a heavy metal ion to promote phosphorescence through the effect of enhanced spin–orbit coupling (SOC), the problem is typically exacerbated by decreased metal character in the S_n and T₁ excited states as the conjugation in a bound ligand increases. Here we show how the use of a dinuclear metal complex with an extended conjugated ligand allows such effects to be mitigated compared to analogous structures with just one metal centre. The complex Pt₂(bis-dthpym)(dpm)₂ (complex 5) is readily prepared by a double N^C cyclometallation of 4,6-bis(dithienyl)-pyrimidine (H₂bis-dthpym), with the coordination sphere of each Pt centre being completed by O^O-coordinating dipivaloylmethane (dpm). This new complex displays intense NIR emission in solution, λ_max = 725 nm, with essentially no “contamination” by visible light <700 nm. The photoluminescence quantum yield of 0.17 in toluene at 300 K is vastly superior to that of the analogous mononuclear complex, where reduced SOC leads primarily to ligand-based fluorescence and only very weak phosphorescence. Computational results indicate that a key reason for the superior performance of the dinuclear system is a doubling of the number of higher-lying excited singlet states with which the T₁ state may couple, to promote the formally forbidden phosphorescence process. Complex 5 has been evaluated as an NIR emitter in solution-processed OLEDs. An external quantum efficiency (EQE) of 3.6% is attained using 5 doped into TBP:PBD at 5% w/w, with a turn-on voltage of 5.6 V (at 0.01 mW cm⁻²). The maximum radiosity of 2.7 mW cm⁻² for this device is particularly high compared to most reported NIR-emitting phosphorescent OLEDs.