580
S. Yagi
emitters, introduction of another electron-withdrawing group to dfppy has so far
been reported. Yagi and coworkers reported 5
-banzoylated analogues of FIrpic
and Ir(dfppy) 3 , i.e., Ir(Bz-dfppy) 2 (pic) and Ir(Bz-dfppy) 3 , respectively, to achieve
3–4 nm blue shifts of λ PL (Okamura et al. 2016). The Φ PL s of Ir(Bz-dfppy) 2 (pic)
and Ir(Bz-dfppy) 3 are still high (0.82 and 0.90, respectively, in dichloromethane).
A similar type of FIrpic analogue, so-called FCNIrpic, exhibits blue emission,
and the FCNIrpic-based OLED afforded deep blue EL with the CIE coordinate
of (0.14, 0.17) (Jeon et al. 2011). Deep blue phosphorescent bis-cyclometalated
iridium(III) complexes consisting of trifluoromethylcarbonyl-substituted 2-(4
,6
-
difluororphenyl)-3-methylpyridinate (TFAc-dfppy) cyclometalated ligands were
also reported by a Korean group; Ir(TFAc-dfppy) 2 (pic) and Ir(TFAc-dfppy) 2 (fptz)
(fptz; 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridinate), exhibiting deep
blue emission at 453 nm (Φ PL ; 0.74) and 447 nm (Φ PL ; 0.63), respectively
(Lee et al. 2013). Deep blue EL with the CIE coordinate of (0.147, 0.116) was
obtained from the OLED containing Ir(TFAc-dfppy) 2 (fptz) as an emitting dopant.
To develop deep blue phosphorescent organoiridium(III) emitters, employment
of 2,3
-bipyridine as a preligand is also a good strategy. Kang and coworkers
reported the deep blue phosphorescent tris-cyclometalated complex, fac-Ir(dfpypy) 3
(dfpypy; 2
,6
-difluoro-2,3
-bipyridinate), which shows the emission spectrum with
comparable peaks at 438 and 463 nm in dichloromethane (Lee et al. 2009). The
color purity of the emission, CIE (0.14, 0.12), is excellent for a blue emitter, and
Φ PL of 0.71 is also fairly high. However, the low-lying HOMO of Ir(dfpypy) 3 (ca.
–6.5 eV) prevents the hole injection from a host material, when it is used as an
emitting dopant for OLED. Then, Kang and coworkers developed the deep blue
phosphorescent bis-cyclometalated complex, Ir(dfpypy) 2 (pic), which shows the
emission spectrum with a peak at 445 nm (Kang et al. 2013). This complex is highly
emissive both in solution and polymer film: Φ PL s in dichloromethane solution
and poly(methyl methacrylate) film are 0.90 and 0.95, respectively. The HOMO
level of Ir(dfpypy) 2 (pic) lies at –6.14 eV, destabilized in comparison with that of
Ir(dfpypy) 3 . As other examples of blue phosphorescent organoiridium(III) materials,
the complexes with carbene-based cyclometalated ligands, such as Ir(dfbmb) 2 (fptz)
(dfbmb; 1-(2,4-difluorobenzyl)-3-methylbenzimidazole) (Chang et al. 2008) and
mer-Ir(dbfmi) 3 (dbfmi; 1-(dibenzofuran-4-yl)-3-methylimidazole) (Sasabe et al.
2010), have so far been reported. The OLED containing the latter complex as an
emitter afforded highly efficient blue EL with CIE (0.15, 0.19), where high device
performance (η j max of 28.6 cd A
–1 , η p max of 35.9 lm W
–1 , and η ext max of 18.6%)
was obtained (Sasabe et al. 2010).
Generally, deep blue phosphorescent emitters possess high triplet energies and
deep HOMO, and thus, the confinement of the excitons generated at EML is very
difficult. This is one of the main reasons why the development of excellent blue
phosphorescent emitters for OLED is still a challenging theme.
S. Yagi
emitters, introduction of another electron-withdrawing group to dfppy has so far
been reported. Yagi and coworkers reported 5
-banzoylated analogues of FIrpic
and Ir(dfppy) 3 , i.e., Ir(Bz-dfppy) 2 (pic) and Ir(Bz-dfppy) 3 , respectively, to achieve
3–4 nm blue shifts of λ PL (Okamura et al. 2016). The Φ PL s of Ir(Bz-dfppy) 2 (pic)
and Ir(Bz-dfppy) 3 are still high (0.82 and 0.90, respectively, in dichloromethane).
A similar type of FIrpic analogue, so-called FCNIrpic, exhibits blue emission,
and the FCNIrpic-based OLED afforded deep blue EL with the CIE coordinate
of (0.14, 0.17) (Jeon et al. 2011). Deep blue phosphorescent bis-cyclometalated
iridium(III) complexes consisting of trifluoromethylcarbonyl-substituted 2-(4
,6
-
difluororphenyl)-3-methylpyridinate (TFAc-dfppy) cyclometalated ligands were
also reported by a Korean group; Ir(TFAc-dfppy) 2 (pic) and Ir(TFAc-dfppy) 2 (fptz)
(fptz; 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridinate), exhibiting deep
blue emission at 453 nm (Φ PL ; 0.74) and 447 nm (Φ PL ; 0.63), respectively
(Lee et al. 2013). Deep blue EL with the CIE coordinate of (0.147, 0.116) was
obtained from the OLED containing Ir(TFAc-dfppy) 2 (fptz) as an emitting dopant.
To develop deep blue phosphorescent organoiridium(III) emitters, employment
of 2,3
-bipyridine as a preligand is also a good strategy. Kang and coworkers
reported the deep blue phosphorescent tris-cyclometalated complex, fac-Ir(dfpypy) 3
(dfpypy; 2
,6
-difluoro-2,3
-bipyridinate), which shows the emission spectrum with
comparable peaks at 438 and 463 nm in dichloromethane (Lee et al. 2009). The
color purity of the emission, CIE (0.14, 0.12), is excellent for a blue emitter, and
Φ PL of 0.71 is also fairly high. However, the low-lying HOMO of Ir(dfpypy) 3 (ca.
–6.5 eV) prevents the hole injection from a host material, when it is used as an
emitting dopant for OLED. Then, Kang and coworkers developed the deep blue
phosphorescent bis-cyclometalated complex, Ir(dfpypy) 2 (pic), which shows the
emission spectrum with a peak at 445 nm (Kang et al. 2013). This complex is highly
emissive both in solution and polymer film: Φ PL s in dichloromethane solution
and poly(methyl methacrylate) film are 0.90 and 0.95, respectively. The HOMO
level of Ir(dfpypy) 2 (pic) lies at –6.14 eV, destabilized in comparison with that of
Ir(dfpypy) 3 . As other examples of blue phosphorescent organoiridium(III) materials,
the complexes with carbene-based cyclometalated ligands, such as Ir(dfbmb) 2 (fptz)
(dfbmb; 1-(2,4-difluorobenzyl)-3-methylbenzimidazole) (Chang et al. 2008) and
mer-Ir(dbfmi) 3 (dbfmi; 1-(dibenzofuran-4-yl)-3-methylimidazole) (Sasabe et al.
2010), have so far been reported. The OLED containing the latter complex as an
emitter afforded highly efficient blue EL with CIE (0.15, 0.19), where high device
performance (η j max of 28.6 cd A
–1 , η p max of 35.9 lm W
–1 , and η ext max of 18.6%)
was obtained (Sasabe et al. 2010).
Generally, deep blue phosphorescent emitters possess high triplet energies and
deep HOMO, and thus, the confinement of the excitons generated at EML is very
difficult. This is one of the main reasons why the development of excellent blue
phosphorescent emitters for OLED is still a challenging theme.