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 pi-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 pi-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.