Advancing Near-Infrared Phosphorescence with Heteroleptic Iridium Complexes Bearing a Single Emitting Ligand: Properties and Organic Light-Emitting Diode Applications
Marta Penconi, Aravind B. Kajjam, Moon Chul Jung, Marco Cazzaniga, Clara Baldoli, Davide Ceresoli, Mark E. Thompson, Alberto Bossi
Abstract
Cyclometalated iridium complexes, emitters of choice in organic light-emitting diodes (OLEDs), hold great potential for near-infrared (NIR) applications. Upon increasing the conjugation size and chemical complexity of the ligands, as required to push the emission toward the NIR), overall high-molecular-weight complexes (both homoleptic and heteroleptic β-diketonate ones) are obtained, posing related issues in OLED processing. One, so far barely explored, question arises: “Why endow Ir(III) with three or two emissive ligands when one might work just as well?” Herein, as proof of concept for OLED technology, we disclose three novel deep-red to NIR emitters of formula Ir(C^N)2(iqbt), with a single iqbt (1-(benzo[b]thiophen-2-yl)-isoquinolinate) ligand responsible for the emission in the NIR range. (C^N) are cyclometalated ligands with higher triplet energy than that of iqbt. We demonstrate that the presence of a single iqbt ligand is sufficient to enable efficient phosphorescence matching that of homoleptic Ir(iqbt)3; moreover, the Ir(C^N)2(iqbt) based OLEDs display efficiency exceeding the one of Ir(iqbt)3. These compounds offer several important benefits: (i) advantageous synthetic protocols limiting to the last step of the use of novel and synthetic costly NIR ligands (implying a lower amount of ligand required), (ii) commercially available (C^N) to prepare the starting Ir chloro-dimers, and (iii) lower molecular weight of the complexes compared to that of the homoleptic parent ones fruitful for easier vacuum thermal processing of the emitters.