Tuning the physical and optoelectronic properties of phosphorescent iridium(III) complexes : applications to organic semiconductor devices
2017
This thesis explores the design, synthesis and characterisation of iridium(III) complexes for
optoelectronic applications; in particular, cationic [Ir(CˆN)₂(NˆN)]⁺-type emitters (where
CˆN is an anionic bidentate cyclometalating ligand such as 2-phenylpyridinato, ppy, and
NˆN is a neutral bidentate ligand such as 2,2’-bipyridine, bpy) for use in light-emitting
electrochemical cells (LEECs). Design strategies aim to achieve high photoluminescence
quantum yields (Φ[sub](PL)) for these complexes.
Chapter 1 provides an overview of the fundamental photophysics of luminescent transition metal complexes, before reviewing state of the art iridium complexes employed in
LEEC devices.
Chapter 2 employs a combination of the electron-deficient 2,4-difluorophenylpyridine
(dFppy) CˆN ligand and various functionalised biimidazole (biim) NˆN ligands. Within
the family of different biim ligands the emission energy does not vary significantly, but the
excited state kinetics di.er depending on the rigidity of the biim ligand. Combining the
lead biim ligand with a sterically bulkier CˆN ligand gives an iridium complex that emits
deep blue light with 90% Φ[sub](PL) in MeCN.
Chapter 3 describes an approach to replacing the electrochemically unstable aryl carbon-fluorine bonds in dFppy, while maintaining the deep blue emission colour observed for the
complexes in Chapter 2.
Chapter 4 expands on the concept of rigid biim ligands to bibenzimidazoles (bibenz).
Combining conjugated bibenz NˆN ligands with more conjugated CˆN ligands allows for the
emission colour of these complexes to be tuned to the orange/red. The Φ[sub](PL) necessarily falls
due to the energy gap law, but is nevertheless higher than values measured for reference complexes.
Chapter 5 explores the use of an arylazoimidazole ligand with donor-acceptor intraligand
charge transfer characteristics in order to red-shift the emission further. The resultant
complex is poorly emissive, but shows a panchromic absorption profile and high molar
absorptivity, which is unusual for iridium(III) complexes. The absorption profile can be
tuned as a function of the protonation state of the imidazole.
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