Cholesteric GLC

Fig. 6.5 Cholesteric GLCs consisting of a and b hydrogen-bonded, and c chemically bonded...

Esterification of a tricarboxylic acid with nematic and chiral precursors, Nnu-OH Oij-OH at a 2 1 molar ratio as annotated in Reaction Scheme 6.1, was relatively straightforward. Subsequent isolation and purification produced a nematic and a cholesteric GLC, (19) and (20), at a 20 and 31 % yield, respectively. The elaborate work-up procedures involved dissolution-precipitation cycles, repeated liquid column chromatograpy complemented by recrystallization as needed [33]. 

Monohydrolysis of trimethyl esters is the key to two alternative deterministic approaches as described in Reaction Scheme 6.3 [35]. About 50 % overall yield of cholesteric GLCs, (23) and (24), using pig liver enzyme as the first step in Approach (a), and a 23 % overall yield of (25) involved base catalysis in Approach (b), the former being superior to statistical synthesis with much less laborious work-up procedures. 

Scheme 6.3 Deterministic synthesis of cholesteric GLCs starting with monohydrolysis of a 1,3,5-cyclohexanetricarboxylic acid trimethyl ester with pig liver enzyme, PLE and b 1,3,5-benzenetricarboxylic acid trimethyl ester...

In comparison to the synthesis of separate chiral and nematic precursors, the idea of a hybrid chiral-nematic precursor, exemplified as Ch2 in Reaction Scheme 6.4, is quite appealing. Moreover, the synthesis of a cholesteric GLC can be completed in a single step at a 66 % yield [36], which compares favorably with the preceding statistical and deterministic approaches. Of particular significance is that (26) with (5 )-3-bromo-2-methylpropanol incorporated as the chiral linkage produced a left-handed cholesteric GLC film with Xr located at 413 nm. 

Scheme 6.4 Deterministic synthesis of a cholesteric GLCs consisting of hybrid chiral-nematic pendant groups...

As prescribed in Sect. 6.4, a constant-pitch cholesteric LC film typically gives rise to a selective reflection bandwidth around 100 nm in the visible region as determined by the An value. The bandwidth can be broadened with a gradient pitch through film thickness, according to the Hajdo-Erigen theory [40]. Photoracemizable (/ )-di-naphtho[2, ID l, 2 -/] [1,3] dioxepin was mixed with a nematic and cholesteric GLC at a molar ratio of (27) (28) (29) = 1.00 0.23 0.13 as depicted in Fig. 6.7a. With a and Tc at 65 and 156 °C, respectively, the ternary mixture was prepared into a... 

Two sets of chiral-nematic hybrid are chemically bonded to the same dithienylethene core as for the photoswitchable nematic GLC (30), resulting in a smectic C -cholesteric GLC as depicted in Fig. 6.10. A sample of (31) was placed between two alignment-coated fused silica substrates for heating to 230 °C followed by cooling to 207 °C, where shearing was applied to induce alignment. The... 

Fig. 6.10 Molecular structure of a smectic C -cholesteric GLC (31) for photoswitchable absorptive coloration via ring closure and opening on top of reflective coloration in a 5-pm-thick film. Used with permission [46]...

Cholesteric GLC Film as Robust Solid-State Laser... 

Fig. 6.15 Molecular structures of a rerf-emitting dopant (41), nematic (42) and cholesteric (43) GLCs mixed to serve as the host. Temporal evolution of lasing output with specified pump fluences at 10 Hz from a a cholesteric GLC film prepared with (42) (43) at a 76.0 24.0 mass ratio, and b a cholesteric fluid film, CB-15 ZLI-2244-000 at a 35.6 64.4 mass ratio, both films containing 2.0 wt% (41). Used with permission [53]...

Spatially Resolved Lasing from a Cholesteric GLC Film... 

In view of the unique roles of cholesteric GLCs in material synthesis and device application, the commonly practiced statistical synthesis and the continuing effort in deterministic synthesis are highlighted. All factors considered, a hybrid chiral-nematic precursor is much preferred over separate chiral and nematic precursors in terms of synthesis and subsequent product separation and purification. 

---