Synthesis and Photophysical Properties of New Di- and Mononuclear Phosphorescent Iridium(III) Complexes

Cyclometallated Ir(III) complexes have attracted significant attention as luminescent materials due to a range of favourable properties, such as their high photoluminescence quantum yields, microsecond phosphorescence lifetimes, thermal stability, robust electrochemistry and synthetic versatility, which has enabled their emission to be tuned across the visible spectrum from the near-UV to the near-IR. They have been applied to various applications, such as in bioimaging, sensing, photocatalysis, and as sensitisers for singlet oxygen and as emitters in organic light emitting devices (OLEDs). While mononuclear Ir(III) complexes have been extensively studied, dinuclear derivatives have received less attention. This is likely related to their high molecular weights, which limits their application to solution-processed rather than vacuum-processed OLEDs. Historically, diiridium complexes have also been noted to exhibit poorer luminescence efficiency than their mononuclear analogues. Nevertheless, a number recent of studies have indicated that correctly designed dimers can indeed be highly emissive and interest in dinuclear Ir(III) complexes has increased. They are particularly interesting as they introduce a bridging ligand, which can be used to modify the electronic communication between the Ir centers as well as the various photophysical and physical properties of the complex. In this thesis a range of new diiridium complexes bridged by hydrazide (N^O) chelates will be discussed. In Chapter 2 further structural variation of the peripheral cyclometallating and bridging ligands of the prototypical complex 34 is explored through complexes 35–38. Results indicated that functionalisation of either the bridging or peripheral ligands can facilitate colour tuning, and the matrix dependent photophysical properties of 37 and 38 were explained. Intramolecular π–π interactions were also observed between the peripheral and bridging ligands of the complexes. In Chapter 3, such interactions were enhanced through fluorination of aryl moieties on the bridging ligands, and were utilised to modify the photophysical properties of diiridium complexes (complexes 62–66). The first examples of sky-blue emitting diiridium complexes are also presented (complexes 68–70). In Chapter 4 the series was extended through the application of topical bulky 1,2-diarylimidazole cyclometallating ligands, leading to the first example of sky-blue aggregation-induced phosphorescent emission (AIPE) from a diiridium complex. Inspired by the findings in Chapter 3, in Chapter 5 mononuclear Ir(III) diasteromers featuring chiral oxazoline ancillary ligands were investigated as a platform for a deeper fundamental study into the effect of intramolecular π–π interactions on the photophysical properties of Ir(III) phosphors

Homoconjugation enhances the photophysical and electrochemical properties of a new 3D intramolecular charge transfer iptycene displaying deep blue emission

A new structural class of 3D molecule capable of intramolecular charge transfer (ICT) is introduced, based upon an electron poor ring-fused triptycene core. Its photophysical and electrochemical properties are evaluated in comparison with an analogous molecule, representative of a single fin of the iptycene. Homoconjugation through the delocalised LUMO of the iptycene facilitates a great increase in transition probability both to and from the ICT state, while the deep blue photoluminescence of the single fin is retained. The peripherally distributed HOMO of the iptycene also permits reversible access to a tricationic state in a single step and at an oxidation potential lower than that of the single fin. This first example demonstrates great potential for this 3D design concept in producing new optoelectronic molecular materials

Intramolecular Pi-Pi Interactions with a Chiral Auxiliary Ligand Control Diastereoselectivity in a Cyclometalated Ir(III) Complex

The application of a chiral auxiliary ligand to control the diastereoselectivity in the synthesis of a cyclometalated iridium(III) complex is presented. The diastereomeric iridium(III) complexes 1a and 1b are reported, in which a phenoxyoxazoline auxiliary ligand incorporates a chiral center functionalized with a pendant pentafluorophenyl group. The diastereomers were readily separated, and their structural, electrochemical and photophysical properties are discussed. Solution-state NMR data and X-ray crystal structures establish that the pentafluorophenyl group engages in intramolecular π–π interactions. The X-ray analysis reveals that the two diastereomers display very different modes of intramolecular stacking. The variable-temperature 19F NMR data indicate that rotation of the pendant pentafluorophenyl rings in 1b and 1a is a temperature-dependent process and that there is a smaller energy barrier to rotation in 1b in comparison to 1a. This correlates with variable-temperature photoluminescence data, which show that upon heating the integrated emission intensity is reduced substantially more for 1b than for 1a, which is ascribed to the enhanced rotation in 1b, providing a more easily populated nonradiative pathway in comparison to 1a. These experimental data are supported by computational calculations. Phosphorescent organic light-emitting devices (PhOLEDs) using 1a as the dopant complex give blue-green emission with a high maximum external quantum efficiency (EQEmax) of 25.8% (at ca. 270 cd m–2) and with a low efficiency roll-off to 24.9% at 1000 cd m–2. Our results extend the scope of ligand design for cyclometalated iridium complexes which possess interesting structural and emission properties

3,4-Phenylenedioxythiophenes (PheDOTs) functionalized with electron-withdrawing groupsand their analogs for organic electronics. Remarkably efficient tuning the energy levels in flatconjugated polymers

A novel, facile and efficient one-pot, microwave-assisted method of synthesis allowing an access to a new series of 3,4-phenylenedioxythiophene derivatives with electron-withdrawing groups at the benzene ring (EWG-PheDOT) and their analogs (with an expanded side π-system or with heteroaromatic rings, ArDOT) by the reaction of 2,5-dialkoxycarbonyl-3,4-dihydroxythiophenes with electrophilic aromatic/heteroaromatic compounds in dipolar aprotic solvents has been described. Its applicability over a wide range of novel functionalized ArDOTs as promising building blocks for organic electronic materials has been demonstrated. The structures of selected ArDOTs have been determined by single-crystal X-ray diffraction. The electronic structure of conjugated polymers p[ArDOTs] based on synthesized novel thiophene monomers has been studied theoretically by the DFT PBC/B3LYP/6-31G(d) method. The performed calculations reveal that while the side functional groups are formally not in conjugation with the polymer main chain, they have an unprecedentedly strong effect on the HOMO/LUMO energy levels of conjugated polymers, allowing their efficient tuning by over the range of 1.6 eV. In contrast to that, the energy gaps of the polymers are almost unaffected by such functionalizations and vary within a range of only ≤0.05 eV. Computational predictions have been successfully confirmed in experiments: cyclic voltammetry shows a strong anodic shift of p-doping for the electron-withdrawing CF3 group functionalized polymer p[4CF3-PheDOT] relative to the unsubstituted p[PheDOT] polymer (by 0.55 V; DFT predicted the decrease of the HOMO by 0.58 eV), while very similar Vis-NIR absorption spectra for both polymers in the undoped state indicate that their optical energy gaps nearly coincide (ΔEg < 0.04 eV). © 2018 The Royal Society of Chemistry

A Simple Molecular Design Strategy for Delayed Fluorescence toward 1000 nm.

Harnessing the near-infrared (NIR) region of the electromagnetic spectrum is exceedingly important for photovoltaics, telecommunications, and the biomedical sciences. While thermally activated delayed fluorescent (TADF) materials have attracted much interest due to their intense luminescence and narrow exchange energies (ΔEST), they are still greatly inferior to conventional fluorescent dyes in the NIR, which precludes their application. This is because securing a sufficiently strong donor-acceptor (D-A) interaction for NIR emission alongside the narrow ΔEST required for TADF is highly challenging. Here, we demonstrate that by abandoning the common polydonor model in favor of a D-A dyad structure, a sufficiently strong D-A interaction can be obtained to realize a TADF emitter capable of photoluminescence (PL) close to 1000 nm. Electroluminescence (EL) at a peak wavelength of 904 nm is also reported. This strategy is both conceptually and synthetically simple and offers a new approach to the development of future NIR TADF materials

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence.

Funder: Royal Society; Id: http://dx.doi.org/10.13039/501100000288Funder: Winton Programme for the Physics of SustainabilityConjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π-interchain stacking distance. This suggests that decreasing π-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching

Highly luminescent 2-phenylpyridine-free diiridium complexes with bulky 1,2-diarylimidazole cyclometalating ligands

While a number of highly emissive dinuclear Ir(III) complexes have been reported, they have generally been restricted to structures based on 2-phenylpyridine (Hppy) cyclometalates. We now present a series of new hydrazide-bridged diiridium complexes (5–8) which incorporate bulky 1,2-diarylimidazole cyclometalating ligands in the place of Hppy. Complexes 6–8 are strongly emissive when doped into poly(methyl methacrylate) (PMMA), displaying the highest PLQYs yet reported for ppy-free diiridium emitters (ΦPL = 47–55 ± 10%). Notably, complex 8 has an emission peak at 452 nm and CIExy colour coordinates in the sky-blue region (0.18, 0.27), which is competitive with state-of-the-art monoiridium analogues. X-ray crystallography and solution-state 19F NMR spectra reveal the presence of rigidifying intramolecular π–π interactions for complexes 6–8, which explains their improved photophysical performance compared to 5 which does not have these interactions. Structure–property relationships are further rationalised through density functional theory (DFT) and cyclic voltammetry (CV) data. All the complexes studied in this work display aggregation induced phosphorescent emission (AIPE). This series of compounds increases the structural diversity of highly luminescent dinuclear Ir(III) complexes to include luminophoric ligands that are not restricted to Hppy-type fragments. The colour range accessible to AIPE-active diiridum complexes has also been substantially broadened