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 carring [small beta]-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). During our recent study we found a significant spectral red shift on going from the homoleptic to the [small beta]-diketonate Ir(III) derivatives. NIR emitting complex Ir(iqbt)2dpm ([small lambda]max = 710 nm) has almost 20 nm redshifted emission compared to the homologues Ir(iqbt)3 making only the former a real NIR emitter. For comparison we studied Pt(iqbt)dpm complex as resemble to investigate metal ligand interactions. Noteworthy 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 spectroscopical analysis and corroborate the results with DFT and TDDFT calculations to investigate whether Pt(II) complex can be used as model system to predict how far can be pushed the corresponding emission in Ir(III) heteroleptic derivative bearing the same C^N ligand.