Photophysical Properties of Substituted Homoleptic and Heteroleptic Phenylimidazolinato Ir(III) Complexes as a Blue Phosphorescent Material

Iridium
complexes are one of the most important materials for fabrication
of organic light emitting diodes (OLEDs). There are difficulties in
the preparation of blue phosphorescent complexes with respect to chromaticity,
emission efficiency, and stability of the material, compared with
green and red phosphorescent complexes. Control of the frontier orbital
energy level (HOMO–LUMO) is the sole method to achieve better
blue phosphorescent iridium complexes by appropriate ligand selection
and the introduction of adequate substituents. Homoleptic and heteroleptic
iridium­(III) tris­(phenylimidazolinate) complexes were synthesized,
and the effect of the substituents on their nature in the excited
state was examined. Density functional theory calculation showed that
the imidazolinato complexes have the HOMO localized at the iridium
d- and phenyl π-orbitals. The LUMO is also localized on the
phenyl moiety with a much higher population than HOMO. This LUMO is
quite different from other complexes, such as iridium­(III) tris­(phenylpyridinate)
and tris­(phenylpyrazolinate) complexes. Therefore, substitution with
π-electron donating groups and electron withdrawing groups induces
blue and red spectral shifts, respectively, which is the reverse shift
exhibited by other complexes. The ancillary ligand (acetylacetone)
acts as a path for nonradiative deactivation in the blue phosphorescent
complexes.