PHOTOPOLYMERIZATION OF CARBOHYDRATE-BASED DISCOTIC MESOGENS - SYNTHESES AND PHASE PROPERTIES OF O-(TRANS-3,4-DIALKOXYCINNAMOYL)-(D)-GLUCOPYRANOSES AND THEIR OLIGOMERS

The neat phase properties of the alpha and beta anomers of 1,2,3,4,6-p enta-O-(trans-3,4-dialkoxycinnamoyl) (D)-glucopyranose (n alpha GP and n beta GP; n, the number of carbon atoms in the alkyl chains, is 6-8, 10, and 14) have been investigated by optical microscopy, differentia l scanning calorimetry, and X-ray powder diffraction. Each of the GP f orms an enantiotropic and thermotropic discotic columnar liquid-crysta lline phase. Irradiation of the 14GP in dilute solutions leads to two intramolecular [2 + 2] cycloaddition reactions between cinnamoyl doubl e bonds. In the solid I (obtained by recrystallization from solvent an d in which 14GP molecules are paired in each columnar stack) and ''iso tropic'' melt phases, only short (<10 monomer unit), probably branched oligomers form upon irradiation due to the continued preference for i ntramolecular cycloadditions and the ability of each monomer to intera ct with more than two neighbors. Irradiation of the solid II phase (ob tained by cooling the liquid-crystalline phase) provides unbranched ol igomers with <20 monomer units. However; more than 30 14 alpha GP mono mer units have been linked in unbranched chains upon irradiation of it s liquid-crystalline phase. This is the greatest degree of columnar po lymerization reported to date from a discotic mesophase. Qualitatively similar results are obtained upon irradiation of other GP in their va rious phases. Oligomerization of GP liquid-crystalline and solid I pha ses is intracolumnar, and the columnar packing arrangement of-the mono mers is retained to ca. 100 degrees C above their clearing temperature s; each GP produces nanoscale cylinders of one diameter but varying le ngths. The oligomers unwind and do not retake their original form when they are dissolved and reprecipitated or heated to above their cleari ng temperatures and cooled. The nature of the oligomerization processe s is discussed, especially with regard to inter-/intramolecular and in ter-/intracolumnar competitions for [2 + 2] cycloaddition reactions an d the possibility of an anomeric effect on oligomer length.