/pdf/chiroptical-molecular-switches-1mgstibwk7.pdf

Chiroptical Molecular Switches

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast films and thermotropic mesomorphic state in some copolypeptides are also discussed.

Polypeptide liquid crystals

Chiralitatsuntersuchungen mit Flussigkristallen basieren darauf, das nematische Flussigkristalle von chiralen Verbindungen in sehr niedrigen Konzentrationen in cholesterische Flussigkristalle umgewandelt werden. Im vorliegenden Beitrag wird zunachst ein Uberblick uber Struktur und Eigenschaften von Flussigkristallen sowie uber neuere Methoden zur Bestimmung der Ganghohe und des Schraubungssinns von cholesterischen Flussigkristallen gegeben. Bei der Bildung induziert-cholesterischer Phasen interessieren speziell die Verdrillungsstarken optisch aktiver Verbindungen und die strukturellen Beziehungen, die zwischen der induzierenden Verbindung und dem Losungsmittel – der nematischen Phase – bestehen. Flussigkristalle lassen sich unter anderem zum Nachweis kleiner optischer Aktivitaten und zur Bestimmung von Racemisierungsbarrieren und absoluten Konfigurationen anwenden.

Flüssigkristalle: ein Werkzeug für Chiralitätsuntersuchungen

„Sergeants‐and‐Soldiers”︁‐Prinzip in chiralen, säulenförmigen Stapeln scheibenförmiger Moleküle mit C3‐Symmetrie

The effects of small amounts of achiral photochromic dopants,
4,4′-di-n-alkylazobenzenes, with various lengths of substituted
alkyl chain (CnH2n + 1 ∶ n = 4,
7, 11, 12, 13, 14, 15, 16) on the helical pitch of a cholesteric liquid
crystal (CLC) with ca. 1000 molecular weight were investigated
in order to utilise the cholesteric reflected colour of these combinations
for a rewritable colour image recording. The (E)-azobenzenes (~2 wt%)
in the host CLC induced pitch changes dependent on temperature, the concentration
of the azobenzenes and the length of substituted alkyl chain on the azobenzenes.
In the low region of the cholesteric temperature (below 100 °C),
some (E)-azobenzenes remarkably lengthened the pitch. Contrary to
this, upon irradiation, E–Z photoisomerization of all the azobenzenes
in the CLC phase induced consecutive pitch shortenings in response to the
formation of the Z isomers. The changed pitch (reflected colour)
could be fixed in cholesteric glassy solids. In the glassy solid state, the
photo and thermal isomerization of the azobenzenes could not influence the
cholesteric pitch any longer, therefore, the colour was stably stored. However,
since the molecular arrangement of the CLC and the conformation of the doped
azobenzenes completely returned to the initial state upon re-heating to isotropic
temperatures, the stored colour could be easily erased. Based on these fundamental
investigations, it is suggested that the mixed systems of the medium-molecular-weight
CLCs and the 4,4′-dialkylazobenzenes will be effective materials for
a rewritable full-colour image recording in a photon mode.

Effects of doped dialkylazobenzenes on helical pitch of cholesteric liquid crystal with medium molecular weight: utilisation for full-colour image recording

The helical twisting power in nematic ($\mathrm{methoxybenzylidene} p\ensuremath{-}\mathrm{aminophenyl}\mathrm{acetate}$)-cholesteric (cholesteryl propionate) mixtures as a function of cholesteric material concentration is studied by measuring the circular dichroism, the wavelength of selective reflection, and the Grandjean fringes. It is found that as the cholesteric material concentration increases, the twisting power of the mixtures increases linearly up to a certain concentration, beyond which, however, it increases more slowly, taking a maximum value at some specific concentration which normally falls in the high-concentration region.

K. K. Kobayashi

Papers

Helical Twisting Power in Mixtures of Nematic and Cholesteric Liquid Crystals