Cationic Iridium Complexes with Intramolecular π–π Interaction and Enhanced Steric Hindrance for Solid-State Light-Emitting Electrochemical Cells

Cationic iridium complexes incorporated 4,5-diaza-9,9′-spirobifluorene as N^∧N ancillary ligands, in which one (2) or two (3) phenyl groups were introduced onto 4,5-diazafluorene to afford intraligand π–π interactions. The X-ray crystal structures of complexes 2 and 3 show that the pendant phenyl ring forms strong intramolecular face-to-face π-stacking with the difluorophenyl ring of the cyclometalated ligand with distances of 3.38 Å for complex 2 and 3.40 and 3.46 Å for complex 3. This π–π stacking interaction minimizes the expansion of the metal–ligand bonds in the excited state, resulting in a longer device lifetime in the light-emitting electrochemical cell (LEC) devices.