Azodendrimers as a Functional Material

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Azodendrimers as a Functional Material

Dendrimers can behave as active functional materials, if they are substituted at their tail ends by azo groups. The dendrimers we developed spontaneously attach to interfaces or surfaces, providing us with a variety of functions. After describing the synthesis of dendrimers and azodendrimers, we first review functions for static uses; alignment surface for liquid crystals (LCs). Then, a major part of this review is devoted to the introduction for dynamic uses. Because of photo-induced trans-cis isomerization, the azodendrimers act as a command surface, which enables us to control LC orientation. Azodendrimer layers were formed at glass substrates, LC droplets in polymers, and surfaces of microparticles in LCs. Photo-controlled trans- and cis-forms, respectively, provide homeotropic and planar orientations of LCs. The photo-irradiation induces dewetting of dendrimer layers as well. Photo-induced orientation changes of LCs provide us with various applications and novel phenomena; photo-controlled macroscopic physical properties such as thermal transport, defect structure changes in LC colloids and LC systems with microinclusions, and even dynamics of inclusions in LCs

Effect of Photonic Structures in Organic Light-Emitting Diodes – Light Extraction and Polarization Characteristics

Edited by Jai Sing

Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals

We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters

Sub-second dynamic phototuning of alignment in azodendrimer-doped nematic liquid crystal shells

The alignment of nematic liquid crystal 5CB in micron-thick shells, suspended in and containing aqueous liquid phases, can be rapidly switched between radial (homeotropic) and tangential (planar) director field by doping them with a photoresponsive dendrimer with multiple azobenzene moieties in the branches. The dendrimer spontaneously segregates to the inner as well as outer shell interfaces, folding into an amphiphilic conformation irrespective of the sign of interface curvature. The branches are directed into the liquid crystal, inducing a homeotropic ground state. Upon UV irradiation, the trans-cis isomerization of azobenzene triggers immediate switching to planar alignment. The very fast realignment and the simultaneous response throughout the shell leads to an initially random planar director field, with many topological defects of both positive and negative signs becoming visible within a second of irradiation. All but two +1 defects quickly annihilate, and the remaining defect pair moves up towards the thinnest part of the shell to form the planar steady state. By illuminating with visible light the homeotropic alignment is quickly recovered. By exchanging the solvent used for assisting the dendrimer dissolution, also dynamic phase separation phenomena can be studied in the shells, revealing that the dendrimer solubility in 5CB is greater in the UV-induced cis state than in the trans ground state

Optical cavity with a double-layered cholesteric liquid crystal mirror and its prospective application to solid state laser

The authors have fabricated an optical cavity with silver (Ag) and double-layered cholesteric liquid crystal (CLC) mirrors facing each other. This CLC mirror consists of left-handed CLC and right-handed CLC films for high light reflection irrespective of polarization states. A single-mode lasing was observed in dye-doped CLC sandwiched between Ag and double-layered CLC mirrors. The authors also fabricated a flexible solid state device with a spin-coated dye molecular film sandwiched between Ag and double-layered CLC mirrors. Amplified spontaneous emission was observed from the solid state device, suggesting a possible structure for a flexible and tunable solid state laser.open

Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.open2

Analysis of the intralayer molecular orientation in the B1 phase of a bent-core liquid crystal molecule using X-ray microbeam(New Frontiers in Colloidal Physics : A Bridge between Micro- and Macroscopic Concepts in Soft Matter)

この論文は国立情報学研究所の電子図書館事業により電子化されました。アキラル屈曲型液晶のB1相構造に関して、X線マイクロビームでその配向解析を行った。結果、これまで提案されていた2つの構造のうち、Col(pm2_1m)の対称性を持つ屈曲面に垂直な面内に2次元周期構造を有することがわかった。Achiral bent-core liquid crystals are very interesting from a viewpoint of "chiralily" which appears in an "achiral" molecular system, and have been investigated experimentally and theoretically by many researchers. The bent-core molecules exhibit specific mesogenic phases, B1～B8, different from those in rod-like molecules. The determination of the structure in each phase is crucial but difficult, because of difficulties in obtaining large uniform domains. The B1 phase was found in P-6-O-PIMB (Fig. 1(a)). A two-dimensional (2D) modulated structure (Col(p2mg)) was proposed for the phase, as shown in Fig. 1(b), based on macroscopic x-ray diffraction. On the other hand, another 2D structure (Col(pm2, m)) shown in Fig. 1(c) has been very recently proposed in the other compound