Study of Luminescent Rhenium(I) Carbene and Cyclometalated Iridium(III) Complexes - Photophysics, Photocatalysis and Anion-Binding Properties

發光錸(I)卡賓及環金屬銥(III)配合物的研究: 光物理、光催化及陰離子結合特性

Student thesis: Doctoral Thesis

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  • Chi On NG


Awarding Institution
Award date21 Jul 2016


Tricarbonyl Re(I) diimine complexes have been well-known to show intense and long-lived phosphorescence. This property has led to their various applications in different areas such as the development of phosphorescent materials, chemosensors, photosensitizers and photocatalysts. However, when the diimine or ancillary ligand is functionalized for these applications, it is challenging to further modify and fine-tune their physical and excited state properties. To improve the ease for the modification, isocyano Re(I) diimine complexes have been designed. These complexes also exhibit rich photophysical and excited state properties, which can be readily modified or fine-tuned by varying both the isocyanide and diimine ligands. However, the photocatalytic applications of these isocyano Re(I) diimine complexes are limited by the instability of the coordinated isocyanide ligands, which are subjected to nucleophilic attack in the presence of nucleophiles particular upon light excitation.
To improve the stability, a series of luminescent Re(I) diimine complexes with various types of N-heterocyclic carbene (NHC) ligands has been synthesized. These NHC complexes are formed by the reaction of the isocyano Re(I) diimine complexes with suitable nucleophiles. These Re(I) NHC complexes were characterized by 1H, 13C NMR and IR spectroscopies, mass spectrometry and elemental analysis. One of the precursor complexes fac-{Re(CO)3[CN(H)C6H4-2-O]2Br} and five of the NHC Re(I) diimine complexes were also structurally characterized by X-ray crystallography. During the preparation of these Re(I) NHC complexes, the reactivity of the coordinated isocyanide ligands has been studied. All these NHCRe(I) diimine complexes displayed 3MLCT [dπ(Re)→ π*(N-N)] phosphorescence in degassed CH2Cl2 and CH3CN solutions at room temperature. Through the study of the photophysical and electrochemical properties of these NHC Re(I) complexes, the electronic properties of different types of NHC ligands were investigated.
Apart from the design of luminescent Re(I) with carbene ancillary ligands, we have also designed luminescent Re(I) complexes with their emissive excited-states directly involving the orbital of carbene ligand. A series of luminescent isocyano Re(I) complexes with various bidentate acyclic diaminocarbene have been synthesized, characterized and their photophysical and electrochemical properties studied. Some of these complexes were also structurally characterized by X-ray crystallography. Detailed photophysical, electrochemical and computational studies have been carried out. In view of the close resemblance of the NH moieties of these carbene ligands with urea-based anion receptor, the anion binding properties of the selected Re(I) complexes have also been studied. These complexes show different UV-vis and emission spectral changes upon addition of different types of anions. Their reactivity towards these anions was also investigated by UV-vis and emission titration study. On the other hand, the CO2 capture properties of these complexes have also been investigated.
Furthermore, in search of methods for replacing or converting sacrificial electron donor (amine) to useful organic products in the study of the photocatalytic CO2 reduction of the luminescent NHC Re(I) complexes, we discovered the visible-light induced photocatalytic reactions for the preparation of perfluorinated amides from perfluoroalkyl halides and amines using MLCT emitters as photocatalysts under a mild aerobic condition. Detailed studies showed that [Ir(ppy)3] gave the best performance amongst the studies MLCT emitters. This photocatalytic reaction is applicable to primary aliphatic, secondary aliphatic and aromatic amines as well as perfluoroalkyl iodides and bromides. The emission quenching studies revealed that the excited state of [Ir(ppy)3] is firstly quenched by perfluoroalkyl halides by oxidative quenching to generate perfluoroalkyl radicals. The mechanism for these photocatalytic amide formation reactions has also been proposed.