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 Sn 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, λmax = 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.