Featuring long-lifetime deep-red emitting iridiumIII complexes with high colour purity: insights into the excited state dynamics from spectroscopic and theoretical perspectives

The significant attention drawn to IrIII-complexes in recent years has boosted the development of new compounds with advantageous photophysical features. However, obtaining IrIII deep-red-emitting complexes with long lived excited state, high colour purity and high quantum yield (Φ) remains a challenging task. To address this issue, this study reports the synthesis and photophysical investigation of three novel zwitterionic complexes, [Ir(C^N)2bqdc] (C^N = ppy, phq, and bzq), with ppy = 2-phenylpyridine (Ir-pb), phq = 2-phenylquinoline (Ir-qb), bzq = benzo[h]quinoline (Ir-bb), and bqdc = potassium 2,2′-biquinoline-4,4′-dicarboxylate. These complexes exhibit high quantum yields and long emission lifetimes with high colour purity in the deep-red region. The structural characterisation carried out by usual spectroscopic measurements supports the proposed structures, while the photophysical study unveiled the high contribution of the 3MLCT state to the hybrid emitter state, as endorsed by theoretical investigations. The desired correspondence between the calculations and the experimental data set affirmed the accuracy of the theoretical analysis, which enabled us to establish a relationship between the ground-to-excited-state geometry distortion and the lifetime through the nonradiative decay (knr). Furthermore, these newly synthesized complexes exhibit quenching in the presence of molecular oxygen. In deoxygenated DMSO solution the knr values halve, increasing the quantum yields (34.0, 10.6, and 26.6%) and the lifetimes (1.13, 1.11, and 1.72 μs), while leading to quite pure deep-red emission - CIE coordinates: (0.67, 0.33), (0.60, 0.40;), (0.65, 0.35) for Ir-pb, Ir-qb, and Ir-bb, respectively. These complexes display considerable potential for a wide range of applications, such as photodynamic therapy, due to their attractive photophysical properties, and are among the deep-red-emitting complexes reported in the literature with longer lifetimes and higher Φ.