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 pi* orbital of the ligands. When Ir(ppy)(3) strongly interacts with a CBP molecule, the pi* 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.