Upper limit to the ultimate achievable emission wavelength in near-IR emitting cyclometalated iridium complexes

Iridium complexes bearing cyclometalated (C^N) ligands are the current emitters of choice for efficient phosphorescent organic light emitting diodes (OLEDs). Homoleptic iridium complexes Ir(C^N)₃ and the analogous heteroleptic ones carrying a β-diketonate ancillary ligand (C^N)₂Ir(O^O) often exhibit similar photophysical properties and device performances; the choice among them usually depends both on the yield/ease of the respective synthetic preparations as well as on the device fabrication methods (i.e. vacuum-deposition or solution-process). In our recent study we found a significant spectral red shift on going from the homoleptic to the β-diketonate Ir(iii) derivatives. The NIR emitting complex Ir(iqbt)₂dpm (λ_max = 710 nm) has almost 20 nm red shifted emission compared to the homologue Ir(iqbt)₃ making only the former a real NIR emitter. For comparison, we studied the Pt(iqbt)dpm complex as the suitable example to investigate metal ligand interactions. Noteworthily the Pt(iqbt)dpm emission perfectly overlaps that of the Ir(iqbt)₂dpm. In this paper we provide an in-depth investigation of these systems by electrochemical and spectroscopic analyses and corroborate the results with DFT and TDDFT calculations to investigate whether the Pt(ii) complex can be used as a model system to predict how far the emission can be pushed in a Ir(iii) heteroleptic derivative bearing the same C^N ligand.