Substituents engineered deep-red to near-infrared phosphorescence from tris-heteroleptic iridium(iii) complexes for solution processable red-NIR organic light-emitting diodes

Research on near-infrared- (NIR-) emitting materials and devices has been propelled by fundamental and practical application demands surrounding information-secured devices and night-vision displays to phototherapy and civilian medical diagnostics. However, the development of stable, highly efficient, low-cost NIR-emitting luminophores is still a formidable challenge owing to the vulnerability of the small emissive bandgap toward several nonradiative decay pathways, including the overlapping of ground- and excited-state vibrational energies and high-frequency oscillators. Suitable structural designs are mandatory for producing an intense NIR emission. Herein, we developed a series of deep-red to NIR emissive iridium(iii) complexes (Ir1-Ir4) to explore the effects of electron-donating and electron-withdrawing substituents anchored on the quinoline moiety of (benzo[b]thiophen-2-yl)quinoline cyclometalating ligands. These substituents help engineer the emission bandgap systematically from the deep-red to the NIR region while altering the emission efficiencies drastically. Single-crystal X-ray structures authenticated the exact coordination geometry and intermolecular interactions in these new compounds. We also performed an in-depth and comparative photophysical study in the solution, neat powder, doped polymer film, and freeze matrix at 77 K states to investigate the effects of substitution on the excited-state properties. These studies were conducted in conjunction with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Most importantly, the -CH3 substituted Ir1, unsubstituted Ir2, and -CF3 substituted complex (Ir4) were promising novel compounds with bright phosphorescence quantum efficiency in doped polymer films. Using these novel molecules, deep-red to NIR emissive organic light-emitting diodes (OLEDs) were fabricated using a solution-processable method. The unoptimized device exhibited maximum external quantum efficiency (EQE) values of 2.05% and 2.11% for Ir1 and Ir2, respectively.