Chi-On Ng

Photochemical Synthesis of Intensely Luminescent Isocyano Rhenium(I) Complexes with Readily Tunable Structural Features

A new class of readily tunable isocyano rhenium(I) diimine luminophores, cis,cis-[Re(CO)(2)(CNR)(2)(N-N)](+) (R=2,4,6-Cl(3)C(6)H(2), 4-ClC(6)H(4), 4-Br-2,6-(CH(3))(2)C(6)H(2), 2,6-(CH(3))(2)C(6)H(3), 4-[(CH(3))(2)N]C(6)H(4), 4-(C(6)H(5))C(6)H(4), 4-nBuC(6)H(4), tBu), has been synthesized in high yield by a highly selective photochemical substitution reaction. These complexes were characterized by (1)H NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The X-ray crystal structures of one of the complexes and one of the precursor complexes for the photosubstitution reaction were also determined. As the isocyanide ligands are readily tunable, complexes with excellent solubility in benzene or other nonpolar solvents could be designed through slight modification of the isocyanide ligands with a short nBu substituent. With the characteristic strong infrared absorptions of the carbonyl (C≡O) and isocyanide (C≡N) stretches as well as the high solubility of the reactant and product in benzene, which is the solvent for the photoreaction, the photosubstitution reaction of [Re(CO)(3)(nBuC(6)H(4)NC)(2)Br] with 4,4-di-tert-butyl-2,2-bipyridine was also studied by in situ IR spectroscopy. The photophysical and electrochemical properties of these complexes were also investigated. Except for the complex with [(CH(3))(2)N]C(6)H(4)NC ligands, all complexes displayed intense luminescence with quantum yields of up to 0.37 in degassed CH(2)Cl(2) solution at room temperature. These emissions were assigned as the phosphorescence derived from the metal-to-ligand charge transfer [dπ(Re)→π*(N-N)] excited state. The emissive excited states of these complexes have also been characterized by transient absorption spectroscopic studies. The capability of tuning the emissive excited-state energy through the modification of the isocyanide ligands could be reflected by the significant shifting of the phosphorescence from 530 to 620 nm with the same phenanthroline ligand.

A New Class of Isocyanide-Containing Rhenium(I) Bipyridyl Luminophore with Readily Tunable and Highly Environmentally Sensitive Excited-State Properties

A new class of readily tunable and highly environmentally sensitive luminescent rhenium(I) tetra(isocyano)bipyridyl complexes has been synthesized and characterized, and their luminescent properties have been investigated. Preliminary studies showed that the metal-to-ligand charge-transfer [dpi(Re) --> pi*(bpy)] absorption and emission are extremely sensitive to the nature of the solvent and the rigidity of the environment.

Luminescent rhenium(I) complexes with acetylamino- and trifluoroacetylamino-containing phenanthroline ligands: anion-sensing study.

A series of rhenium complexes with acetylamino- and trifluoroacetylamino-containing 1,10-phenanthroline ligands have been synthesized, characterized and their photophysical and electrochemical properties studied. These complexes were found to show significant UV-vis and emission changes on addition of CN(-), F(-) and AcO(-) anions. Their reactivity towards CN(-), F(-) and AcO(-) anions, was also investigated by UV-vis, emission and (1)H NMR spectroscopy. The reaction product between the trifluoroacetylamino-containing 1,10-phenanthroline ligand and the CN(-) anion has also been structurally characterized by X-ray crystallography.

Synthesis, characterization, and photophysical study of luminescent rhenium(I) diimine complexes with various types of N-heterocyclic carbene ligands.

A series of luminescent Re(I) diimine complexes with various types of N-heterocyclic carbene (NHC) ligands has been synthesized through the reaction between isocyano Re(I) diimine complexes with different nucleophiles. These Re(I) NHC complexes were characterized by (1)H and (13)C NMR and IR spectroscopy, mass spectrometry, and elemental analysis. One of the precursor complexes fac-{Re(CO)3[CN(H)C6H4-2-O]2Br} and five of the Re(I) diimine complexes with different types of NHC ligands were also structurally characterized by X-ray crystallography. In the preparation of these Re(I) NHC complexes, it is found that the reactivity of the isocyanide ligands in the synthetic complex precursors is significantly affected by the electronic nature of the trans ligand. All these complexes displayed (3)MLCT [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 Re(I) NHC complexes, the electronic properties of different types of NHC ligands were investigated.

Synthesis, characterisation and photophysical studies of leucotriarylmethanes-containing ligands and their rhenium(I) tricarbonyl diimine complexes

A series of pyridine- and bipyridine-containing leucotriarylmethane ligands has been successfully synthesised and incorporated into tricarbonyl rhenium(I) diimine complexes. The X-ray crystal structures of two of the complexes have also been determined. The photoreactivity, photophysical and electrochemical properties of these ligands and their rhenium complexes were investigated. The photo-ionisation of the leucotriarylmethanes in the free ligands and their metal complexes and the subsequent change in absorption properties were also studied. Additionally, the electrochemistry of these ligands and complexes were investigated.

Luminescent Rhenium(I) Pyridyldiaminocarbene Complexes: Photophysics, Anion-Binding, and CO2-Capturing Properties.

A series of luminescent isocyanorhenium(I) complexes with chelating acyclic diaminocarbene ligands (N^C) has been synthesized and characterized. Two of these carbene complexes have also been structurally characterized by X-ray crystallography. These complexes show blue-to-red phosphorescence, with the emission maxima not only considerably varied with a change in the number of ancillary isocyanide ligands but also extremely sensitive to the electronic and steric nature of the substituents on the acyclic diaminocarbene ligand. A detailed study with the support of density functional theory calculations revealed that the lowest-energy absorption and phosphorescence of these complexes in a degassed CH2Cl2 solution are derived from the predominantly metal-to-ligand charge-transfer [dπ(Re) → π*(N^C)] excited state. The unprecedented anion-binding and CO2-capturing properties of the acyclic diaminocarbene have also been described.

Design of Luminescent Isocyano Rhenium(I) Complexes: Photophysics and Effects of the Ancillary Ligands

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)] were not extensively studied. To provide insights into the design strategy of phosphorescent rhenium(I) complexes with an emissive 3MLCT [dπ(Re) → π*(CNR)] excited state, a series of pentaisocyano rhenium(I) complexes have been synthesized. In contrast to most of the reported penta- or hexaisocyano rhenium(I) complexes with unsubstituted or alkyl- or monohalo-substituted phenylisocyanide ligands, which only exhibit photoluminescence in 77 K glassy medium, the solutions of all of these complexes were found to show phosphorescence at room temperature. Detailed study on their emission properties revealed that they are derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state mixed with LLCT character. It has been shown that the strong electron-withdrawing substituents on the isocyanide ligands can lower the energy of the MLCT [dπ(Re) → π*(CNR)] state and raise the deactivating ligand-field state. These effects are the crucial criteria to render the pentaisocyano rhenium(I) complexes emissive. Moreover, the emission properties in terms of energy, lifetime, and quantum yields can also be enhanced by the ancillary ligand.

Self-assembled nanostructures of linear arylacetylenes and their aza-substituted analogues

A series of linear phenylene ethynylene molecules have been synthesized, and aza-substitution has been used as a strategy to fine-tune the properties of the molecules. All the compounds exhibited pure blue emission both in solution and solid state, and fluorescence quantum yields as high as 0.66, 0.63 and 0.82 were found in chloroform solutions. The well-defined nanostructures such as quasi-cubes, cubes and rods were fabricated by self-assembly method from these compounds. The photophysical properties and aggregation behavior of self-assembled structures were analyzed in detail. The morphology as well as photophysical properties of these nanostructures have been tuned with selective requirements.