Theoretical study on the absorption spectra of fac-Ir(ppy)3 in the amorphous phase of organic electro-luminescent devices

Absorption spectra in the amorphous phase consisting of Ir(ppy)₃ and 4,4-bis(carbazol-9-yl)-2,2-biphenyl (CBP) molecules were theoretically investigated. The equilibrium structures in amorphous phase were simulated by QM/MM MD calculations. The results of calculation suggest that eleven CBP molecules exist as the closest neighbors of Ir(ppy)₃. Time-dependent density functional theory (TD-DFT) calculations successfully reproduced the experimental absorption spectra. For Ir(ppy)₃ in the gas phase, the most important spectral peak at the wavelength of 400 nm can be assigned principally to the one-electron excitation from HOMO-1 to LUMO+2, where the main component of HOMO-1 is the d orbital of the Ir atom and that of LUMO+2 is the π* orbital of the ligands. When Ir(ppy)₃ strongly interacts with a CBP molecule, the π* orbital of the ligand is delocalized into the CBP molecule. This is the reason why the spectral peak at the wavelength of 400 nm almost disappears in the amorphous phase.