Are Re(I) phenanthroline complexes suitable candidates for OLEDs? Answers from DFT and TD-DFT investigations

The electronic structure and spectroscopic properties of seven recently reported rhenium(I) phenanthroline complexes were investigated theoretically by density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. All the seven complexes are shown here to be better electron transport materials with high quantum efficiency in OLED devices due to their high electron transport mobility and low lambda(electron) values. Particularly, among these seven chosen complexes the difference between lambda(hole) and lambda(electron) for tricarbonyl Re(I) complexes is smaller, suggesting that these complexes have a better hole- and electron-transport balance in OLED devices. The absorption is associated with (MLCT)-M-1/(LLCT)-L-1 transitions while the emission transition has (MLCT)-M-3/(LLCT)-L-3/(ILCT)-I-3 character as revealed by natural transition orbital (NTO) analysis. The calculated results show that the absorption and emission transitions and device efficiency can be changed by varying the nature and position of the axial ligands as revealed by the structural and bonding features of the complexes through natural bond orbital (NBO) and quantum theory of atoms in molecule (QTAIM) analysis.