Asymmetric synthesis of liquid crystalline materials and chiral smart moelcules
The synthesis and investigation of a series of bichromophoric molecules, based upon 7-methylene-cis-bicyclo[3.3.0]octan-3-ones, which exhibit optical dissymmetry, have been investigated for use as liquid-crystal-based chiroptical switches. Doping of these optically active materials into nematic liquid crystals (LC's) induced a cholesteric phase. The handedness of the dopant controls the direction and magnitude of the helical twist in the cholesteric phase. Irradiation of these matrices with circularly polarised light leads to reversible photoswitching processes which, induce an enantiomeric excess of one stereoisomer, and either a left or right handed helix is generated. These mirror image isomers represent the 1 and O states of binary logic. Such systems have potential applications in three-dimensional memory storage devices. Selective excitation of these bichromophoric molecules at 305nm leads to an intra-molecular energy transfer from the donor keto chromophore to the exocyclic double bond at the opposite end of the rigid bicyclo[3.3.0]octane spacer. We have investigated the extent of the achievable photoresolution and observed the magnitude of the helical pitch that can be induced in the nematogen PCB -7. Seven novel trigger groups were synthesised and their properties investigated; (i) Hanessian non-ester moieties, (ii) Mosher esters, (iii) aliphatic esters, (iv) propargyl ester, (v) biphenyl ester, (vi) alkyl, biphenol and polyfluoro esters, and (vii) PCH-7 ester. Their synthesis involved Wittig type chemistry to add the exocyclic alkene-groups. Enantiomeric excesses were determined by capillary electrophoresis. Kuhn dissymmetry factors (gA.) were determined, using UV and CD spectroscopy, and helical twisting powers (ßm), by the droplet method. Group (i) triggers were initially investigated but our model studies suggested that the synthesis of a library of molecules based on this synthetic route would be problematic. Group (iii) esters displayed lower dissymmetry then their parent acid and required to be doped in too high a concentration, compromising the integrity of the LC host. Group (vi) showed excessive overlap of the n-n* band destroying dissymmetry but displayed a potential for LC formation. In contrast groups (ii), (iv) and (vii) possessed useful molecular properties making them useful as molecular switches.