Theoretical study on the influence of different N∧N ligands on the electronic structures and optoelectronic properties of heteroleptic Iridium(III) complexes

DFT/TDDFT calculations were carried out to investigate the electronic structures, absorption and phosphorescence properties of a series of heteroleptic Ir(III) complexes consisting of two N-heterocyclic carbene ligands and a conjugated bicyclic N,N'-heteroaromatic (N<^>N) ligand. On the basis of the results reported herein, we attempt to explain the experimental observations according to which complex (mpmi)(2)Ir(pybi) (1) [Hmpmi = 1-(4-tolyl)-3-methyl-imidazole; Hpybi = 2-(pyridin-2-yl)-1H-benzo[d]imidazole] emits green light with an extremely high-quantum phosphorescence efficiency (Phi(PL)) of 79.3%, while a relatively lower Phi(PL) (only 11%) was measured for (fpmi)(2)Ir(tfpypz) (2) [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C, C-2'; tfpypz = 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato] emitting blue light by tuning the N<^>N ligands. Besides, we also designed (fpmi)(2)Ir(pyN3) (3) [pyN3H = 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine] and (fpmi)(2)Ir(pyN4) (4) [pyN4H = 2-(1H-tetrazol-5-yl)pyridine] to explore the influence of electron-withdrawing substituents on N<^>N ligands on the electronic and optical properties of these Ir(III) complexes. The results revealed that electron-withdrawing substituents can stabilise both HOMOs and LUMOs and induce HOMO-LUMO energy gap change. Moreover, the emission properties can be significantly tuned by introducing different N<^>N ligands. While new insights were gained on structural and electronic properties, the extremely high Phi(PL) of 1 was found to be not inherent to spin-orbital coupling effects, but determined by its large transition dipole moment (mu(S1)) upon S-0-S-1 transition compared with that of 2. On the basis of these results, the designed complexes 3 and 4 are considered to be the promising candidates for blue-emitting phosphorescence materials with higher Phi(PL) than the complex 2.