Study of Luminescent Copper(I), Iridium(III) and Rhenium(I) Complexes with Different Types of Isocyanide Ligands – Design, Synthesis and Photophysics

含不同異氰配體的發光銅(I),銥(III)及錸(I)配合物 - 設計、合成與光物理研究

Student thesis: Doctoral Thesis

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Award date21 May 2018


Although a large number of luminescent transition metal complexes have already been developed, many of these complexes are not suitable for OLED applications because of their poor processability and incompatibility with vacuum sublimation. One of the major reasons can be attributed to the ionic character of these metal complexes, which resulted in high sublimation temperatures. The incorporation of anionic isocyanoborate ligands to convert cationic MLCT phosphorescent rhenium(I) and osmium(II) complexes into charge-neutral phosphorescent complexes with similar or improved emission properties have recently been demonstrated by our research group. Moreover, the electroluminescent devices based on those charge-neutral isocyanoborato rhenium(I) complexes have also been successfully fabricated.

It is anticipated that the same strategy can also be applied to develop charge-neutral luminescent iridium(III) emitters from cationic iridium(III) cyclometalates. In Chapter 2, the preparation, characterization and study of a series of isocyanoborato cyclometalated iridium(III) and iridate(III) complexes have been reported. These complexes display phosphorescence from a predominant 3LC excited state with energy covering nearly the full visible region from blue to orange-red depending on the types of the cyclometalated ligands. The incorporation of isocyanoborate ligands have found to increase the emission energy when compared to related cyano complex precursors. Importantly, their emission quantum yields have also been improved compared to the cyano complexes. Further fine tuning of photophysics of these complexes can also be achieved by the substituents on the isocyanoborate ligand.

Although luminescent cyclometalated iridium(III) complexes are useful for developing emissive materials for OLED applications, these materials are usually considered as non-sustainable because of their cost and low natural abundance. Therefore, earth-abundant copper(I) diimine complexes with thermally-activated delayed fluorescence (TADF) property is considered to be one of the most important and promising candidates for the development of third generation emissive materials for OLEDs. On the basis of the results with rhenium(I), osmium(II) and iridium(III) emitters, it is anticipated that the incorporation of isocyanoborate ligands could also be applied to develop emissive charge-neutral copper(I) systems with improved emission properties. In this context, a series of isocyanoborato copper(I) diimine complexes have been synthesized and characterized. The synthesis, characterization and properties of these complexes have been summarized in Chapter 3. These complexes display yellow emission originated from 1MLCT excited state in the solid state at room temperature. The emission energies of these complexes are sensitive to both steric and electronic properties on the isocyanide ligand. In 77 K glassy medium, the complexes display both 3MLCT and 3LC phosphorescence with different lifetimes, which can be resolved by the time-resolved emission spectroscopy.

On the other hand, despite the well-reported MLCT [dπ(M) → π*(CNR)] transitions in the isocyano transition metal complexes, emissive complexes with phosphorescence derived from MLCT [dπ(M) → π*(CNR)] less explored. To develop the phosphorescent rhenium(I) complexes with emissive 3MLCT [dπ(Re) → π*(CNR)] excited state, a series of pentaisocyano rhenium(I) complexes have been designed and reported in Chapter 4. By judicious ligand design on the isocyanide ligands, phosphorescent pentaisocyano rhenium(I) complexes have been developed. In contrast to most reported penta- or hexaisocyano rhenium(I) complexes with unsubstituted or alkyl- or monohalo- substituted phenylisocyanide ligands, which only exhibit emission in 77 K glassy medium, the solutions of all these complexes were found to show photoluminescence at room temperature. A detailed study on their emission properties revealed that they are derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state. Moreover, it has been shown that the strong electron-withdrawing substituents on the isocyanide ligands can lower the MLCT [dπ(Re) → π*(CNR)] and raise the LF deactivating state. This is a crucial criteria for developing the phosphorescent pentaisocyano rhenium(I) complexes. On the other hand, the emission properties in terms of energy, lifetime and quantum yields can also be enhanced by the anionic ancillary ligand.

In the synthesis of the isocyanide ligands for luminescent isocyano complexes, the formation of emissive indoloquinoline derivatives from isocyanide was discovered. The indoloquinoline derivatives are well-known for their pharmaceutical activities, which are generally synthesized by tedious synthetic procedures. In Chapter 5, a new synthetic methodology for the preparation of indoloquinoline derivatives from isocyanides has been developed and reported. With the aid of structural characterization of byproduct, the mechanism for this synthetic route was proposed. It is anticipated this should open up an alternative method for the preparation of widely diverse indoloquinolines for the discovery of new drug candidates from readily available isocyanide derivatives. Moreover, in view of the highly emissive nature of the indoloquinoline derivatives, the photophysical properties of the newly synthesized compounds have also been investigated.