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.