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

Abstract

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.

Materials for Organic Light Emitting Diode (OLED)

“…Similar behavior has been observed in fac-Ir(ppz) 3 [67]. On the other hand, detailed analysis of the MOs of fac-Ir(CF 3 pim) 3 , fac-Ir(OCF 3 pim) 3 , and fac-Ir(Fpim) 3 revealed the HOMO contribution of the phenyl moiety was 30.7-32.8 % [16], which is smaller than that of fac-Ir(ppy) 3 (38.9 %). Accordingly, the contribution of the imidazole ring part (13.3-15.0 %) is higher than that of the pyridine part (8.2 %) in fac-Ir(ppy) 3 .…”

“…These results strongly indicate a 3 ; however, much larger localization of the LUMO at the phenyl moiety. Therefore, substitution of the phenyl group by EWGs affects not only the HOMO, but also the LUMO [16]. This type of HOMO-LUMO relation has also been reported for iridium phenyltriazolinate complexes [56].…”

“…The inefficiencies of other diketonated complexes at 298 K are due to thermal activation to the upper excited state responsible for nonradiative deactivation. Therefore, these thermal activations are prohibited at 77 K. The smallest Φ PL of Ir(OCH 3 pim) 2 acac and Ir(pim) 3 acac among the diketonate complexes at 77 K is explained by the LUMO being localized on acetylacetone [16]. In contrast, the homoleptic complexes showed efficient emission, not only at 77 K, but also at 298 K (Φ PL ¼ 0.40-0.60).…”

“…Therefore, in fac-Ir(CF 3 pim) 3 , fac-Ir (OCF 3 pim) 3 , and fac-Ir(Fpim) 3 , the substitution of EWGs on the phenyl ring is less effective to stabilize the HOMO than in the case of fac-Ir(ppy) 3 (Table 11.5) [16].…”

“…In addition, comparing with blue, green, and red phosphorescent complexes, blue phosphorescent complexes have difficulties for their emission color purities and materials stabilities. Therefore, one section is spent to explain recent development of blue phosphorescent iridium complexes [15,16]. Iridium complexes have been used as dopant in an emitting layer, therefore, host materials are also important.…”