Kaori Fujisawa

Reversible thermal-mode control of luminescence from liquid-crystalline gold(I) complexes

The synthesis and characterisation of liquid-crystalline (LC) gold complexes designed to have a rod-like structure in a dimeric form are described, and the relationship between their photophysical properties and aggregated structure is discussed. The luminescence intensities of the complexes were enhanced in the condensed phase, meaning that the complexes showed aggregation-induced emission. Observed photoluminescence in the condensed phase could be assigned to a monomer emission; however, luminescence properties were strongly affected by the aggregated structures of the complexes. A reversible “on–off” switching of the luminescence induced by the phase transition between LC and isotropic phases is demonstrated. Moreover, complex 1b showed thermochromic photoluminescence controlled by the aggregated structure; the colour of luminescence could be reversibly controlled by the phase transition between crystalline and LC phases. These LC gold complexes show potential application as materials for novel light-emitting devices.

Tuning the photoluminescence of condensed-phase cyclic trinuclear Au(I) complexes through control of their aggregated structures by external stimuli

A series of new cyclic trinuclear Au(I) complexes with alkoxy side chains of various lengths were synthesized as photoluminescence materials. None of the complexes emitted luminescence in solution; however, some showed photoluminescence in the crystalline phase. Single crystal X-ray structural analyses revealed that an intermolecular interaction between two Au atoms (aurophilic interaction) existed only in the emissive complexes, which formed molecular aggregates in the crystal. Because isolated molecules show no luminescence in the present system, we conclude that only molecules aggregated via aurophilic interactions can luminesce. We demonstrated that luminescence properties, such as colour and intensity, were very sensitive to the aggregated structure of the molecules. We also found that such luminescence properties can be controlled by a change in the aggregated structure induced by external stimuli, such as heat, solvent, and mechanical stress.

Photoluminescent properties of liquid crystalline gold(I) isocyanide complexes with a rod-like molecular structure

Novel liquid crystalline (LC) gold(I) complexes were synthesised and their LC behaviour and photophysical properties were investigated. Simple ligands were utilised in order to prevent steric hindrance and facilitate a strong intermolecular aurophilic interaction, resulting in highly efficient photoluminescence. It was found that the complexes easily formed dimers in the crystalline, LC, and isotropic phases due to strong interaction between the gold atoms of neighbouring molecules, and that these dimers acted as unit mesogens in the LC phase. The complexes exhibited intense blue photoluminescence with quantum yields of 8–50% in the condensed phases, but no photoluminescence in dilute solutions. It was therefore concluded that the luminescence was due to the dimers, and that the formation of these was facilitated by the simplicity of the ligands surrounding the gold atoms, resulting in a low level of steric hindrance and strong aurophilic interaction.

Liquid-Crystalline Behavior and Photoluminescence Properties of Gold(I) Complex: Relationship between Aggregation Structure and Properties

Liquid-crystalline (LC) behavior and photoluminescent properties of a gold(I) complex, 2 were investigated. It was confirmed that the complex showed enantiotropic LC phases. In addition, we found that the complex showed strong photoluminescence in the condensed phases, but no luminescence was observed in a dilute solution. The single-crystal X-ray structure analysis suggested that the complex formed dimer in the condensed phases and the dimer formation plays crucial role in both LC behavior and photoluminescent properties.

Effects of Molecular Structure and Aggregated Structure on Photoluminescence Properties of Liquid-crystalline Gold(I) Complexes with Various Aromatic Rings

Rod-like gold(I) complexes with phenyl, biphenyl, or naphthyl rings in a mesogenic core were synthesized by complexation of ethynyl-substituted aromatic derivatives with (tetrahydrothiophene)AuCl, followed by ligand exchange with alkyl isocyanide, to investigate the effects of molecular and molecular-aggregated structures on their liquid-crystalline (LC) behavior and photoluminescence properties. The gold complexes exhibited enantiotropic liquid crystallinity. Introduction of multi-ring systems into the mesogenic core effectively expanded the LC temperature range. A comparison of the photoluminescence properties in the crystalline phase showed that the molecular-aggregated structure plays a crucial role in the luminescence of these gold complexes.

Photoluminescent Gold(I) Complex with Biphenyl Acetylene Ligand Showing Stable Nematic Liquid-Crystalline Phase

A novel liquid-crystalline (LC) gold(I) complex (Biph6) having biphenyl acetylene ligand was designed and synthesized, and its LC behavior and photoluminescence properties were compared with those of a previously reported gold complex (Ph6) with phenyl acetylene ligand. Biph6 exhibited an enantiotropic nematic phase in a temperature range wider than that of Ph6. Ph6 and Biph6 showed intense photoluminescence with different colors in the crystalline phase: blue and yellowish green luminescence, respectively. We demonstrated that the color of luminescence of the gold complexes could be changed by changing the structure of the molecules and molecular aggregates.

Self-Organization of Gold Nanoparticles Coated with a Monolayer of Azobenzene Liquid Crystals

A gold nanoparticle having a monolayer of azobenzene liquid crystal (LC) was synthesized. The size of the gold nanoparticle obtained was 4.7 nm in average diameter, and it was soluble in common non-polar organic solvents such as toluene and dichloromethane. We confirmed that the reversible photochemical and thermal isomerization of the azobenzene could take place on the surface of the gold nanoparticles in analogy with free azobenzenes in solutions. Furthermore, a 1-dimensional self-organization of the nanoparticle was observed by transmission electron microscopy. However, a gold nanoparticle without LC molecules never showed such self-organization. Therefore, we conclude that the self-organization of the gold nanoparticle is based on the liquid crystallinity of the organic ligands attached on their surface.

Photoluminescence behavior of liquid-crystalline gold(I) complexes with a siloxane group controlled by molecular aggregate structures in condensed phases

AbstractLiquid-crystalline (LC) Au complexes with siloxane groups at the termini of flexible chains were synthesized. The effects of the molecular and molecular aggregate structures on the luminescence behavior of the complexes were investigated. All complexes used in this study showed LC phases. No direct effect of the siloxane group on the luminescence behavior of the complexes in solution was observed. However, in condensed phases, different luminescence colors were observed depending on the aggregate structure due to the effect of intermolecular interactions. Thus, the luminescence color of the Au complexes can be controlled by the intermolecular interactions based on the structure of the molecular aggregates. The complex with the siloxane group developed in this study showed two different luminescence colors, which can be controlled by changing the aggregate structures induced by the phase transition.Liquid-crystalline Au complexes with siloxane groups at the termini of flexible chains were synthesized. The effects of the molecular and molecular aggregate structures on the luminescence behavior of the complexes were investigated. In condensed phases, different luminescence colors were observed depending on the aggregate structure due to the effect of intermolecular interactions; thus, the luminescence color of the Au complexes can be controlled by the intermolecular interactions based on the structure of the molecular aggregates.

Effects of aromatic core and flexible terminal chain structures on the properties of luminous liquid-crystalline gold(i) complexes for functional materials

ABSTRACT Luminescent liquid-crystalline (LC) Au complexes with various aromatic cores and flexible terminal chains were synthesized. The aromatic core aspect ratio crucially affected the LC temperature range and melting point of the complexes. Moreover, branched flexible chains reduced the melting point, thus expanding the LC temperature range. In the crystalline and LC phases of the complexes, fluorescence was quenched by the aggregation-caused quenching effect, but phosphorescence was enhanced by the crystallization-induced phosphorescence effect. However, no phosphorescence was observed in the isotropic phase. Thus, not only aggregation but also regularity of molecular aggregates is necessary for the observed crystallization-induced phosphorescence effect.

Morphological Control of Gold Nanoparticle Aggregates via Photoisomerization of Azobenzene Liquid Crystals

Gold nanoparticles having a monolayer of azobenzene liquid crystals (LCs) were synthesized and their aggregation structures on solid surfaces were observed. The gold nanoparticles formed a single-particle layer on bare mica and mica with a polyimide layer. We demonstrated that the morphology of the gold nanoparticle aggregates could be controlled by UV light irradiation at 366 nm; the mass transfer of gold nanoparticles from the exposed area to the nonexposed area was induced by the isomerization of azobenzene LC molecules.