Theoretical Studies of Blue Phosphorescent Iridium(III) Complexes with Phenylpyrazole and Phosphines

Abstract

New blue emitting mixed ligand iridium(III) complexes comprising one cyclometalating, two phosphines trans to each other such as Ir(dppz)(PPhMe2)2(H)(C1) and Ir(dppz)(PPhMe2)2(H)(CN), [dppz = 3,5-Diphenylpyrazole] were studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To achieve deep blue emission and increase the emission efficiency, the following must occur: (1) substitution of phenyl group on the 3-position of the pyrazole ring that lower the triplet energy enough that the quenching channel is not thermally accessible, (2) changing ancillary ligands coordinated to iridium atom to phosphine and cyano groups known as very strong field ligands. Using the density functional theory (DFT) and time-dependent DFT method calculations on the ground and excited states of the complexes, we have studied how the ancillary ligand influences the emission peak as well as MLCT transition efficiency. It is showed that the strong-field ancillary ligand such as CN, PPhMe2 alters the energy gap mainly by changing the highest occupied molecular orbitals (HOMO) energy level. Their inclusion in the coordination sphere can increase the energy gap to achieve the hypsochromic shift in emission color and also lower the triplet energy level to avoid the thermal quenching.