Enhancing the blue phosphorescence of iridium complexes with a dicyclometalated phosphite ligand via aza-substitution: a density functional theory investigation

Abstract

Electronic structure methods based on judiciously chosen DFT and TD-DFT approaches were applied to explore the influence of aza-substitution on the geometrical, electronic and photophysical properties of iridium complexes of dicyclometalated phosphite recently reported as good blue phosphorescents (Angew. Chem. Int. Ed., 2011, 123, 3240). With respect to the pristine molecule, the frontier molecular orbitals (HOMO & LUMO) were found to be stabilized upon aza-substitution, with the magnitude of the decrease in orbital energy depending on the position of the substitution. The results also revealed a slightly enhanced LUMO–HOMO energy gap and consequently a deep blue shifted phosphorescence spectrum. In addition, the electron affinity, ionization potential, and charge mobility, which are some other key factors in the design of optoelectronic materials, were found to be in general, improved by the aza-substitution considered in this study. When compared to the parent molecule, the aza-substituted counterparts show a destabilized metal centered (³MC d–d*) triplet and larger energy separation from the low-lying (³MLCT π–π*) triplet, which is likely to enhance the phosphorescence efficiency reported. These features make aza-substitution an efficient strategy to increase the ³MLCT π–π*–³MC d–d* energy splitting without altering the blue emission of the type of complexes studied.