Solute and Temperature Induced Pitch Changes and Pretransitional Effects in Cholesteric Liquid Crystals

Voss, Jonathan P.; Sackmann, E.

doi:10.1515/zna-1973-0918

Cited by 20 publications

(8 citation statements)

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“…The molecules in the smectic clusters align parallel to each other without twisting, so the cholesteric pitch is elongated as the number and size of the cluster increased. This effect has been reported for explaining the cholesteric pitch change upon temperature change for the mixtures of cholesteryl derivatives or an achiral biphenylcyclopexane derivative containing chiral additives. − …”

Section: Resultsmentioning

confidence: 66%

Photochemical Phase Transition and Molecular Realignment of Glass-Forming Liquid Crystals Containing Cholesterol/Azobenzene Dimesogenic Compounds

et al. 2003

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We synthesized dimesogenic liquid crystals with both cholesterol and azobenzene groups by esterification of 10,12-docosadiynedioic acid or docosanedioic acid with 4-alkyl-4′hydroxyazobenzene (or 4-hydroxyazobenzene) and cholesterol and studied the thermal, optical, and glass-forming properties. Especially, we focused on the effect of the length of the alkyl chain substituted on the azobenzene unit and the photochemical isomerization of the azobenzene unit on the properties of the mesophases of the compounds or a host cholesteric liquid crystal forming stable glass when using them as a dopant. Two compounds without an alkyl chain on the azobenzene unit show a cholesteric phase, while five compounds with an alkyl chain from butyl to hexadecyl show only a smectic phase. The cholesteric reflection bands of the compounds without an alkyl chain on the azobenzene unit shifted toward shorter wavelengths upon photoisomerization of the azobenzene unit from the trans to cis form. Further photoirradiation induced an isotropic phase in the compounds. The compounds with an alkyl chain showing a smectic phase directly changed to an isotropic phase after the photochemical formation of a certain amount of the cis form. All compounds attained a glassy state by maintaining a molecular order if the compounds between the substrates were rapidly cooled from their liquid-crystalline temperature to 0 °C. When the compounds were doped with 2-10 wt % of a dicholesteryl ester forming a stable cholesteric glass, the cholesteric band of the mixtures shifted toward shorter wavelengths following the photoisomerization from trans to cis of the azobenzene unit on the dopant. The extent of the shift depends on the irradiation energy, temperature, and length of the alkyl chain on the azobenzene unit. The mixture went into the glassy state, keeping the shifted cholesteric reflection band after rapid cooling from the liquid-crystalline temperature to 0 °C. The cholesteric glassy state was stable up to 80 °C. Since the molecular arrangement of the liquid-crystalline compounds and the conformation of the azobenzene unit completely returned to the initial state upon reheating to its isotropic temperature, the photoresponsibility and glass-forming property of the compounds or mixtures are applicable to rewritable recording materials.

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Section: Resultsmentioning

confidence: 66%

Photochemical Phase Transition and Molecular Realignment of Glass-Forming Liquid Crystals Containing Cholesterol/Azobenzene Dimesogenic Compounds

et al. 2003

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“…This phenomenon can be rationalized by taking into account that the microscopic molecular disordering induced by bent Z -isomer of an azobenzene should give analogous impact to the system (which appears as pitch shifting in this case) with thermally induced disordering of molecular arrangement. However, in some cholesteric mixtures having pretransition to smectic states, an anomalous temperature behavior close to the smectic−cholesteric transition temperature was observed, due to the formation of smectic clusters . Our cases deal only with normal cholesteric states.…”

Section: Resultsmentioning

confidence: 75%

Conformational Effect on Macroscopic Chirality Modification of Cholesteric Mesophases by Photochromic Azobenzene Dopants

and

Ichimura

2000

J. Phys. Chem. B

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Azobenzenes having different positional substituents were dissolved in cholesteric liquid crystals to make cholesteric pitches tunable by photoisomerization. E-to-Z photoisomerization of 3,3‘-disubstituted or 2,2‘-dimethyl-3,3‘-disubstituted azobenzenes resulted in a moderate change or even no change in the maximum wavelengths of reflectivity of the cholesteric phase, whereas azobenzene or 4,4‘-disubstituted azobenzenes showed much bigger changes. The difference in the conformations between the E- and Z-isomers of the azobenzenes is discussed to explain the observed characteristics. The photoinduced modification of helical pitches by photoisomerization of achiral azobenzenes was identical between a pair of enantiomeric cholesteric solvents, suggesting no preferred intrinsic handedness of the achiral azobenzenes. Experimental results with the corresponding chiral azobenzene suggested that the contribution of the conformation of the azobenzene to the effective helical twisting power is more pronounced than that of the molecular chirality arising from the asymmetric carbon at the terminal alkyl substituents. The photoinduced shortening or lengthening of the helical pitch seems to be determined crucially by the thermal characteristics of the cholesteric liquid crystals.

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“…The width of a small angled peak is inversely proportional to the width of smectic-like clusters. 3,5,18 A plot of half-width against the reflected wavelength in these dimesogens is shown in Fig. 4.…”

Section: Effect Of Temperature On the Cholesteric Pitchmentioning

confidence: 99%

“…[2][3][4][5][6][7][8][9][10][11][12] A change in the pitch length of a cholesteric helix with variation in temperature can be attributed to the associated change in the orientational parameter of the molecule or to smectic cybotactic groups (smectic fluctuations). [5][6][7][13][14][15][16][17][18] It is however difficult to relate the molecular structure to the physical behavior of the pitch and the mechanism of helical disordering is still uncertain.…”

Section: Introductionmentioning

confidence: 99%

Photoresponsive vitrifiable chiral dimesogens: photo-thermal modulation of microscopic disordering in helical superstructure and glass-forming properties

Mallia

Tamaoki

2002

J. Mater. Chem.

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Thermal, photooptical as well as glass-forming properties of a series of photoresponsive chiral dimesogenic compounds containing cholesterol-azobenzene moieties linked to varying alkyl chain lengths (6-14) are described. All compounds except the dimesogen containing 7 methylene spacers show a cholesteric phase. Selective reflections observed in the cholesteric liquid crystals were found to be blue shifted upon increasing the temperature due to variations in smectic fluctuations. X-Ray diffraction studies of these compounds in the cholesteric phase show a low intensity reflection at small angles, the half widths of which decrease on decreasing the temperature, confirming the presence of smectic cybotactic groups. These materials also exhibit a strong odd-even effect in their reflection wavelength as a function of spacer length. Compounds with even numbers of methylene units show a drastic change in their reflection wavelength (102-225 nm) on temperature change compared to those with odd numbers of methylene units (9-14 nm). UV photolysis, using light of 366 nm wavelength, in the cholesteric super-cooled state exhibits a hypsochromic shift in the reflection wavelength. This can be explained by the formation of the cis isomer destroying the smectic clusters in the helical superstructure due to its bent geometry, which in turn causes a decrease in the reflected wavelength. A maximum wavelength shift of 215 nm (703-488 nm) has been observed on photolysis at 80 uC (cooling cycle) for the dimesogen having 12-methylene spacers. By rapid cooling of the compounds from the cholesteric temperature to 0 uC glassy liquid crystalline films maintaining helical molecular order were obtained. The optical properties of these dimesogens, such as cholesteric reflections, controlled by light and temperature combined with their glass forming properties, can be utilized for the construction of information storage materials.

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Solute and Temperature Induced Pitch Changes and Pretransitional Effects in Cholesteric Liquid Crystals

Cited by 20 publications

References 1 publication

Photochemical Phase Transition and Molecular Realignment of Glass-Forming Liquid Crystals Containing Cholesterol/Azobenzene Dimesogenic Compounds

Photochemical Phase Transition and Molecular Realignment of Glass-Forming Liquid Crystals Containing Cholesterol/Azobenzene Dimesogenic Compounds

Conformational Effect on Macroscopic Chirality Modification of Cholesteric Mesophases by Photochromic Azobenzene Dopants

Photoresponsive vitrifiable chiral dimesogens: photo-thermal modulation of microscopic disordering in helical superstructure and glass-forming properties

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