Do the intramolecular π interactions improve the stability of ionic, pyridine-carbene-based iridium(III) complexes?

Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π-π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devices with diimine-based complexes. To interpret this phenomenon clearly, the features of a family of pyridine-carbene-based iridium(III) complexes, in which phenyl groups are sequentially attached to the carbene and the pyridine rings of the ancillary ligand, have been thoroughly studied by using a theoretical approach. Our most valuable findings shed light onto the lack of significant improvement regarding device stability when the pyridine-carbene-based iridium(III) complexes are used. Quite likely, the easy population of the metal-centered ³MC excited states, in which the pyridine ring of the carbene-based ligand is totally decoordinated to the iridium(III) center, is the most plausible explanation for the device behavior. The theoretical study performed in complexes, in which the phenyl substituent is attached to the pyridine ring as well as those with two π-π interactions, clearly confirms that the use of π interactions is not successful in providing a cage conformation in both ground and excited states. This limitation is tentatively attributed to an intrinsic feature when pyridine-carbene-based ligands are utilized.