Photoswitching of Ferroelectric Liquid Crystals Using Unsymmetrical Chiral Thioindigo Dopants: Photoinduced Inversion of the Sign of Spontaneous Polarization

Abstract

A new unsymmetrical chiral thioindigo dopant 6‐[(R,R)‐2,3‐difluorooct‐1‐yloxy]‐5′‐nitro‐6′‐[(R)‐2‐octyloxy]thioindigo (4) designed to photoinvert the sign of spontaneous polarization (PS) in a ferroelectric chiral smectic C (SmC*) liquid crystal was prepared using a synthetic approach previously developed in our laboratory. In this new “ambidextrous” design, the (R)‐2‐octyloxy side‐chain is sterically coupled to the thioindigo core and induces a positive PS, whereas the (R,R)‐2,3‐difluorooctyloxy side‐chain is decoupled from the core and induces a larger negative PS. In the trans form, this dopant induces a negative polarization in the SmC host (+)‐4‐(4‐methylhexyloxy)phenyl 4‐decyloxybenzoate (PhB). Irradiation of a 1 mol‐% mixture of 4 in PhB at λ = 510 nm caused a sign inversion of PS, from –0.88 to +0.42 nC cm–2 at T – TC = –5 °C, which is consistent with an increase in the polarization power of the coupled 2‐octyloxy/thioindigo unit over that of the 2,3‐difluorooctyloxy unit, due to the increase in transverse dipole moment of the thioindigo core upon trans–cis photoisomerization. The PS sign inversion was confirmed by a surface‐stabilized ferroelectric liquid crystal photoswitching experiment. Spectroscopic measurements on films of the doped liquid crystal mixtures showed that trans–cis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation.

Novel Synthesis of Thioaurones by the Regioselective Cyclization of 1‐(2‐Benzylthio)phenyl‐3‐phenyl‐2‐propyn‐1‐ones Derived from Thiosalicylic Acid

“…Thioaurones, 2‐arylidenebenzo[ b ]thiophen‐3(2 H )‐ones, are the sulfur analogues of aurones and are used as thioindigo dyes and photoswitchable materials, but also have been reported to be cytotoxic to cancer cells and to be useful for measuring liphophilicity . However, their biological properties have not been well investigated.…”

Photoswitching of ferroelectric liquid crystals using photochromic dopants

“…More recently, we extended the design of unsymmetrical thioindigos such as 10 to produce a new chiral thioindigo dopant 12 with competing stereo-polar units of opposite handedness, i.e., an ''ambidextrous'' dopant similar conceptually to a switch hitter in baseball. 37,38 A similar concept has been used by Green et al in polyisocyanates with different chiral units competing to control the helical sense of the polymer chain. 39 In the ambidextrous dopant 12, the (R)-2-octyloxy side-chain and the thioindigo core are strongly coupled via the nitro substituent and form one stereo-polar unit that induces a positive polarization.…”

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications

“…FLC shows bistable switching (i.e., upon reversal of the electric field the direction of spontaneous polarization inverts). Recently, photoresponsive FLCs have been fabricated and studied. − Ikeda et al have demonstrated photochemical switching of the spontaneous polarization of FLC by doping a FLC with a photochromic compound capable of undergoing trans–cis photoisomerization . It was noticed that isomerization of the photochromic compound changes the switching potential of the FLC host.…”

“…An ambidextrous chiral thioindigo dopant 206 was designed, and its photomodulation behavior was investigated. ,, It was found that this compound brings about photoinduced polarization inversion of a FLC due to the presence of competing chiral side chains that promote spontaneous polarization of the opposite sign as shown in Figure …”

“…An ambidextrous chiral thioindigo dopant 206 was designed, and its photomodulation behavior was investigated. 576,578,579 It was found that this compound brings about photoinduced polarization inversion of a FLC due to the presence of competing chiral side chains that promote spontaneous polarization of the opposite sign as shown in Figure 62 Subsequently, the unsymmetrical chiral thioindigo dopant was found to cause photoinduced sign inversion of the spontaneous polarization of FLCs. The sign inversion was ascertained experimentally by photoswitching surface-stabilized FLCs.…”