Gelation of smectic liquid crystal phases with photosensitive gel forming agents

Abstract

This paper studies the potential of 3 newly synthesized low molecular weight gelling agents (2 of them with azo groups) with the semicarbazide group as a H-bonding motif to gel the smectic phase of a commercial chiral smectic C * material. To detect gelation two methods were used: the stabilization of the director pattern, which measures the interaction of the physical gel network with the LC-director, and the suppression of the onset of electrohydrodynamic instabilities, which measures the increase of viscosity. Special emphasis was put on the influence of the photochemical trans-cis isomerization of the gelling agents on gelation.From the stabilization of the director pattern an elastic component can be seen by the memory effect (two different switching times). Photochemical trans-cis isomerization destroys this memory effect. The increase of the threshold field for the onset of electrohydrodynamic instabilities in free-standing films measures the increase of viscosity and the supression of flow. Both methods detect the same melting temperature of the gel and the same reduction of the gelling ability upon trans-cis isomerization. This proves that the gelling abilities of the new azo-containing gelling agents can be reversibly switched on and off by photoisomerization.

Photochromic and fluorescent LC gels based on a bent-shaped azobenzene-containing gelator

“…[11][12][13][14][15][16][17][18][19][20][21][22] In the paper by Kato et al [13] photoinduced gel-sol transition in photochromic LC-gels with hydrogen-bonded azobenzene-containing gelators was described. Authors have demonstrated…”

Anomalously large bend elastic constant and faster electro-optic response in anisotropic gels formed by a dipeptide

“…On the other hand, the splay elastic constant shows relatively a small increase. The importance [8][9][10][11][12][13][14][15] of the LC gels arises from the attractive combination of mesomorphism, which makes them amenable to orientation by external fields, and the elastic properties that gives a built-in robustness. Attractive features of the electro-optic switching when the sol transforms to the gel state are the vanishing of the undesirable backflow effect, and nearly an order of magnitude decrease in the switching speed.…”

From LC‐polymers to Nanomedicines: Different Aspects of Polymer Science from a Materials Viewpoint

“…After my return back to Mainz in 2000, again some new topics were started and other topics like helical polymers were finished. A smaller topic—in this context—were low molar mass gel forming agents and their photoisomerizable variants, which were used to gel liquid crystals reversible (or—for photoisomerizable systems—on demand). In addition, they could be used to form nanostructures like platelets or tubes by self‐assembly …”