Multi-stimuli Responsive Luminescent Cyclometalated Iridium(III) and Ruthenium(II) Complexes with Bidentate Acyclic Carbene Ligands - Design, Photophysics and Application Study

多重刺激響應發光環金屬銥(III)和釕(II)雙齒非環卡賓配合物 - 設計、光物理與應用研究

Student thesis: Doctoral Thesis

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Award date9 Nov 2021

Abstract

The developments of luminescent transition-metal complexes and their photofunctional materials have attracted tremendous interest because they are well-reported to show rich photophysical properties, which can be easily tuned through the modification of their ligands. Among these complexes, cyclometalated Ir(III) and polypyridyl Ru(II) complexes are very popular and promising luminescent systems because of their high thermal stability, photostability, and excellent phosphorescent properties. With conformational flexibility of acyclic carbene ligands, it is anticipated that highly environmental sensitive and stimuli-responsive luminescent transition metal complexes can be obtained through the introduction of π-accepting bidentate acyclic carbene ligands into the cyclometalated Ir(III) and polypyridyl Ru(II) complex systems. To improve the physical as well as excited state properties of the targeted complexes, bidentate acyclic carbene ligands with wide varieties of electronic properties have been incorporated. Compared with transition-metal complexes bearing N-heterocyclic carbene (NHC) ligands, acyclic carbene complexes have been rarely studied. Through the study of the cyclometalated Ir(III) and polypyridyl Ru(II) complexes with bidentate acyclic carbene ligands, the structure-property relationship can be established. 

To systematically modify photophysical properties of the cyclometalated Ir(III) complexes with pyridyl acyclic carbene ligands, different substituents on the diaminocarbene ligand has been introduced. Some of the Ir(III) complexes have been structurally characterized by X-ray crystallography. All the complexes display phosphorescence derived from a predominant 3MLCT [dπ(Ir) → π*(N^Ccarbene)] state mixed with 3LC (R-ppy) character. Besides, the solid states of the resulting complexes show strong luminescent mechanochromism, which is ascribed to the mechanoresponsive switching of two forms having distinguishable luminescent properties. Detailed study with time-resolved spectroscopy, Raman spectroscopy and DFT calculation revealed that the mechanoresponsive behavior is derived from the from trans- to cis- isomerization through the rotation of the N-phenyl ring on the carbene ligands. The potential application of these complexes for ink-free rewritable paper based on this novel property has been studied.

Based on the study of cyclometalated Ir(III) complexes with pyridyl acyclic carbene ligands, it is envisioned that the efficient modification through the changes of their ligands is not confined to photophysical properties; the relative stability between two isomeric forms can also be amended. By incorporating various electron‐withdrawing and -conjugating substituents on pyridyl acyclic carbene ligands of cyclometalated Ir(III) complexes, it not only enhances the mechanochromic shift of the complexes but also render complexes with readily interconverting trans‐ and cis- conformations. For complex with two carboxyl ester [–COOEt] substituents on N-phenyl moiety of the carbene ligand, single crystal in cis- form and the polycrystal in trans- form have been obtained from recrystallization. With two readily interconverting trans‐ and cis- forms having highly distinguishable emission properties, a multiple stimuli‐responsive dual emissive system has been obtained. The investigation of the thin film applications of this series of complexes showed that the drop-casted neat films with different self-assembled morphologies displaying different emission colors can be fabricated by simply altering solvent/non-solvent ratio of film-forming solvents. As the result, the emissive properties of the films of this complex can be modified. The different emission colors of these films of the same complexes can be used to design luminescent quick-responsive (QR) code, readable from the camera of a mobile device. 

Apart from Ir(III) complexes, a new series of bis(bipyridyl) Ru(II) complexes with structurally-related bidentate acyclic diaminocarbene ligands have been prepared. These complexes exhibit multiple stimuli-responsive phosphorescent behaviour derived from environmentally sensitive H-bonding dimerization. Two of the dimerized complexes have been confirmed by X-ray crystallography. The two strong intermolecular H-bonds are characterized by short N···N distances (2.97 – 2.99 Å) in the crystal structures. Based on dimerization equilibrium analysis and time-resolved spectroscopy, the equilibrium, absorption, and emission properties of the monomeric and dimeric forms have been elucidated. Importantly, the monomeric and dimeric forms show remarkable differences in their UV-vis and emission properties. Using different coating/deposition techniques and conditions, thin films/depositions of the monomeric and dimeric forms can be fabricated. Hence, their potential applications as sensors for amine vapor, humidity, and chemical-induced security data storage systems have also been described.