Synthesis and photophysical characterization of phosphorescent cyclometalated iridium(III) complexes and their use in organic light emitting devices

Organic light emitting devices (OLEDs) are a new type of display technology based on organic thin films. The materials that comprise these films must be able to meet certain criteria in order to be considered for these devices. The work presented here describes the development of novel phosphorescent materials along with their photophysical characterization and applications in OLEDs. Chapter 1 illustrates how these devices work, the materials used in these devices, and how the properties of these materials affect device performance. Chapter 2 describes the synthesis and characterization of high energy phosphorescent materials from Ir(III) complexes with cyclometalated pyrazolyl-based and N-heterocyclic carbene (NHC)-based ligands. Chapter 3 portrays the synthesis and characterization of heteroleptic Ir(III) complexes consisting of two chromophoric cyclometalating (C.;N) ligands and a singlehigh energy ancillary ligand (L.;X). The incorporation of high energy ancillaryligands such as pmi on bis-cyclometalated Ir(ppz)2 does not lead to emission at room temperature. However, the replacement of the ppz chromophoric ligands with carbazolyl, diphenylamino, or fluorenylpyrazolyl-based chromophoric ligands leads to emission at room temperature. In Chapter 3, more reducible flz-based Ir(III) complexes have also been synthesized by incorporation of a high triplet energy, more reducible ancillary ligand. Their electrochemical, spectroscopic, and electroluminescent properties are discussed. Chapter 4 discusses the synthesis and photophysics of triscyclometalated Ir(III) benzoquinoline complexes, Ir(bzq)3. White phosphorescent OLEDs with mer -Ir(bzq)3 as the broad band emitter have been fabricated with a maximum external quantum efficiency of ∼12%. In Chapter 5, we utilize extensive temperature dependent lifetime studies to estimate the zero-field splitting (ZSF) and ligand-field (LF) state energies for blue and near-UV phosphorescent cyclometalated Ir(III) complexes. This is the first time we can identify where the LF state is for blue cyclometalated Ir(III) complexes. The thermal population of the LF state is most likely one of the deactivation processes that blue cyclometalated Ir(III) complexes exhibit. From the temperature dependent study, we learn that the activation energies (Ea) needed to thermally populate LF state and the nonradiative decay rate constant (k nr(T)) are important factors that affect the quantum efficiencies of high energy phosphorescent cyclometalated Ir(III) complexes.