Heteroleptic Cyclometalated Iridium(III) Complexes Displaying Blue Phosphorescence in Solution and Solid State at Room Temperature

A series of heteroleptic Ir(III) metal complexes 1−3 bearing two N-phenyl-substituted pyrazoles and one 2-pyridyl
pyrazole (or triazole) ligands were synthesized and characterized to attain highly efficient, room-temperature blue
phosphorescence. The N-phenylpyrazole ligands, dfpzH = 1-(2,4-difluorophenyl)pyrazole, fpzH = 1-(4-fluorophenyl)pyrazole, dfmpzH = 1-(2,4-difluorophenyl)-3,5-dimethylpyrazole, and fmpzH = 1-(4-fluorophenyl)-3,5-dimethylpyrazole,
show a similar reaction pattern with respect to the typical cyclometalated (C∧N) chelate, which utilizes its ortho-substituted phenyl segment to link with the central Ir(III) atom, while the second 2-pyridylpyrazole (or triazole)
ligand, namely, fppzH = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, fptzH = 3-(trifluoromethyl)-5-(2-pyridyl)triazole,
and hptzH = 3-(heptafluoropropyl)-5-(2-pyridyl)triazole, undergoes typical anionic (N∧N) chelation to complete the
octahedral framework. X-ray structural analyses on complexes [(dfpz)2Ir(fppz)] (1a) and [(fmpz)2Ir(hptz)] (3d) were
established to confirm their molecular structures. Increases of the ππ* energy gaps of the Ir(III) metal complexes
were systematically achieved with two tuning strategies. One involves the substitution for one or two fluorine atoms
at the N-phenyl segment or the introduction of two electron-releasing methyl substituents at the pyrazole segment
of the H(C∧N) ligands. Alternatively, we have applied the more electron-accepting triazolate in place of the pyrazolate
segment for the third (N∧N)H ligand. Our results, on the basis of steady-state, relaxation dynamics, and theoretical
approaches, lead to a conclusion that, for complexes 1−3, the weakening of iridium metal−ligand bonding strength
in the T1 state plays a crucial role for the fast radiationless deactivation. For the case of [(fmpz)2Ir(hptz)] (3d), a
thermal deactivation barrier of 4.8 kcal/mol was further deduced via temperature-dependent studies. The results
provide a theoretical basis for future design and synthesis of the corresponding analogues suited to blue
phosphorescent emitters.