Recent Advances in Multicolor Emission and Color Tuning of Heteroleptic Iridium Complexes

We review recent advances in the spectroscopic properties of heteroleptic Ir(N^C)2(LX)‐type iridium complexes, which are known as color‐tuning materials. Most Ir(N^C)2(LX)‐type Ir complexes give single emission, in accordance with Kasha’s rule. Dual emission, however, has been observed from a single Ir(N^C)2(LX) complex, depending on the choice of the N^C moiety and LX ligands. For example, Ir(dfppy)2(pq), Ir(ppy)2(dpq‐3F), Ir(ppy)2(pq), and Ir(pq)2(tpy) (dfppy=2‐(2,4‐difluorophenyl)pyridine, pq=2‐phenylquinoline, ppy=2‐phenylpyridine, dpq‐3F=2‐(3‐fluorophenyl)‐4‐phenylquinoline, tpy=2‐p‐tolylpyridine). Recently, triple emission was observed from Ir(ppy)2(BTZ)‐type iridium complexes with two ppy ligands as (N^C)2 and one 2‐(2‐hydroxyphenyl)benzothiazole (BTZ) ligand, while quadruple emission from Ir(ppy)2Q‐type iridium complexes with two ppy ligands as (N^C)2 and one quinolinolato (Q) ligand. These multiple emissions cover a spectral range from blue to red, leading to white emission. Of the four emission bands from Ir(ppy)2Q, the UV and violet emissions are attributed to the emission from the singlet states of IrQ and Ir(ppy), respectively, while the green and red emissions are attributed to emission from the triplet states of Ir(ppy) and IrQ. The appearance of the emission from each of the Ir(ppy) and IrQ (or Ir(BTZ)) components is understood by reduced Förster energy transfer between IrQ (or Ir(BTZ)) and Ir(ppy) due to an orientation factor of nearly zero, that is, due to orthogonality between the two ligand planes, while the appearance of both the fluorescence and phosphorescence bands from each of the ligands is understood by inefficient intersystem crossing from the upper singlet state to the lower triplet state.