Soon Wook Cha

Dimesogenic compounds consisting of a cholesteryl moiety and an aromatic mesogen interconnected through a central pentamethylene spacer

New non-symmetric dimesogens consisting of a cholesteryl moiety and an aromatic mesogenic unit interconnected through a pentamethylene spacer have been prepared. The aromatic units consist of two phenyl rings linked by carboxy, oxycarbonyl, ethylene, ethynylene, azo or Schiffs base groups. The mesomorphic and transitional properties have been investigated by using optical microscopy, differential scanning calorimetry, helical pitch measurements, and electric field and miscibility studies. The structures of the mesophases have been studied using X-ray diffraction. Several kinds of smectic packings are observed depending on the nature of the linking group and an attempt to understand the origin of the different phase structures at a molecular level is made.

Anomalies of periodicity in TGB structures in new liquid crystal dimers

Non-symmetric liquid crystal dimers consist of two different mesogenic units linked through a polymethylene flexible spacer. Our previous studies have shown that dimers containing a cholesteryl moiety as one of the mesogenic groups and a Schiffs base unit as the second, exhibit a rich polymorphism and that several types of smectic packing are obtained depending on the molecular parameters: specifically, a smectic periodicity similar to the molecular length and an intercalated structure with a smectic parameter lower than half the molecular length can be obtained. The competition between these two incommensurate lengths can induce two-dimensional phases and/or an incommensurate smectic phase in which the two smectic periodicities coexist over a long range. Small modifications of the molecular structure can significantly influence the phase sequence. Here we have replaced the Schiffs base by a tolan unit and the terminal alkyl chains by alkoxy chains. As a result, anomalies of periodicity are also observed in this new dimeric series, but they occur mainly in TGB structures.

Synthesis and luminescence properties of four-armed conjugated structures containing 1,3,4-oxadiazole moieties

New 9,10-bis(phenylethynyl)anthracene cored, four-armed conjugated structures in which the 1,3,4-oxadiazole ring is utilized to link the surface alkyl tail groups, were synthesized and the luminescence properties of thin films prepared therefrom were studied. It was found that the four-armed compound (ANTH-OXA6t-OC12) with the dodecyloxy surface group is a high glass transition temperature (Tg: 211 °C) material and exhibits good solubility in common organic solvents and, surprisingly, results in good quality thin films by spin-coating from solutions. In contrast, the compound with tert-butyl surface groups shows poor solubility and gives films of inferior quality. When the films were exposed to UV light of 335 nm, they emitted mainly strong red light (500–750 nm) with much weaker emission at 375–450 nm. The (phenylethynyl)anthracene core is the only red light emitting chromophore in the structure, and the diphenyl oxadiazole moieties are the shorter wavelength light emitters. In contrast, an electroluminescence (EL) device having the configuration of ITO/PEDOT-PSS/ANTH-OXA6t-OC12/Li ∶ Al emits only red light with an external quantum efficiency of 0.02% indicating that the outer diphenyl oxadiazole moieties act only as electron transporters.

Photoluminescence and electroluminescence properties of poly(9-vinylcarbarzole) doped with anthracence derivatives containing bis(ethynylphenyl oxadiazole) or bis(vinylphenyl oxadiazole) substituents

The photoluminescence (PL) and electroluminescence (EL) properties of two novel light-emitting anthracence derivatives containing bis(ethynylphenyl oxadiazole) or bis(vinylphenyl oxadiazole) substituents (ANOs) blended with poly(9-vinylcarbazole) (PVK) are characterized. The energy transfer process in the blends is discussed. By employing 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) and tris(8-hydroxyquinoline) aluminum (Alq3) as electron transporting layers (ETLs) in the EL devices, remarkable improvements in EL efficiency were observed. The effect of the transporting layer on the EL intensity of the devices is also discussed.

A field-dependent organic LED consisting of two new high Tg blue light emitting organic layers: a possibility of attainment of a white light source

Two new blue light emitting trimeric compounds of the Y-shape type having high glass transition temperatures were synthesized and EL behavior of LED devices consisting of bilayers of the two compounds was studied. One of the compounds is of hole-transporting type containing carbazole moieties, whereas the other is of electron-transporting type bearing phenyloxadiazole moieties. The bilayer LED devices exhibit a strong field-dependence and emit white light (simultaneous light-emittance in blue, green and red regions), at high applied electric fields. Increased interfacial formation of exciplexes at stronger external fields appears to be responsible for this field-dependence.

Reversible polarization gratings on thin films of polyoxetanes bearing 4-(N,N-diphenyl)amino-4′-nitroazobenzene chromophores

Polarization gratings were fabricated by the two-beam coupling method on the surface of a series of polyoxetanes containing 4-(N,N-diphenyl)amino-4′-nitroazobenzene pendants with the azobenzene chromophores either side-on attached or end-on attached through short or long spacers to the main chain. The dynamics of formation of the gratings or writings was studied in relation to the diffraction efficiency. The erasing behavior of the gratings was also examined, irradiating a linearly polarized single beam. The temporal stability of the diffraction efficiency was further studied after removing the coupled excitation beam. Both the mode of attachment of the chromophore to the main chain and the length of the side-chain spacer greatly influenced the dynamic properties of the diffraction gratings. The presence of a long side-chain spacer and side-on attachment of the chromophore resulted in faster writing and erasing behavior. A preliminary study by solid-state 13C NMR of the relaxational behavior of the main-chain CH2 groups and the pendant chromophores provided a reasonable picture to help explain the photo-induced reorientation and relaxation of the azo chromophores at the molecular level.