Structural isomerism and π-extension effects in iridium(III) complexes: From green to deep-red emission tuning

Abstract

In this study, Ir1 was synthesized using 2-(dibenzo[b,d]thiophen-2-yl)pyridine as the cyclometalating ligands (C^N) and 2,2,6,6-tetramethylheptane-3,5-dione (tmd) as the ancillary ligand. To achieve deep-red and near-infrared emission, the π-conjugation of the pyridine ring was systematically extended, leading to the design of four Ir(III) complexes (Ir1-Ir4) with emission wavelengths spanning from green to deep red (523–650 nm). Notably, π-extension in the N-containing moiety induced a significant red shift, while dibenzothiophene annulation at the C-moiety showed negligible effects. Furthermore, Ir2 and Ir3, as structural isomers, differ in the connection positions of dibenzothiophene and isoquinoline (C-1 vs. C-3). This variation in connectivity significantly influences the intramolecular electron distribution and the extent of π-conjugation. Differences in electron density and steric hindrance at the C-1 and C-3 positions likely alter the intramolecular charge transfer (ICT) and excited-state properties, resulting in a pronounced red shift of 55 nm in the phosphorescence emission peak of Ir3 compared to Ir2. DFT calculations revealed that pyridine ring π-conjugation effectively lowers LUMO energy and narrows the HOMO-LUMO gap, establishing a ligand design-photophysics correlation for targeted Ir(III) complex development.