Surface chromonics

Ruslim C, Hashimoto M, Matsunaga D, Tamaki T, Ichimura K. 2004. Optical and surface morphological properties of polarizing films fabricated from a chromonic dye by the photoalignment technique. Langmuir 20 95 100. 

Mechanistic insights as proposed by Kumar and coworkers supported the very design of the branching cascade strategy because the successful nucleophiles have utilized the dispersed electrophilic sites over the surface of common precursors 27 beautifully to provide different products via different cascade reactions (Scheme 27.4). In many cases, the reaction sequence apparently begins with the addition of nucleophiles to aldehyde or ketoester moiety and followed by further cyclization on to the chromones moiety (cascades I, III, IV, VI) before the second nucleophilic addition or rearrangement leading to diverse scaffolds. 

The Aggregation of Chromonic Molecules in Dilute Solution and on Substrate Surfaces... 

Figure 3. The contrast between the pattern of aggregation of amphiphiles and chromonic molecules For amphiphiles, the energetically unfavorable contact between the hydrophobic alkyl chains and water molecules is reduced to more or less zero when a closed micelle is formed. In contrast, for chromonic systems, the fraction of the total molecular surface exposed to the solvent decreases as the columns lengthen, but there is no optimum aggregate size directly comparable to a micelle.

There is then a sudden drop in the free energy as virtually all of the amphiphile molecules are incorporated into micelles, enabling their hydrophobic alkyl chains to be more or less completely shielded from the aqueous part of the phase. This leads to the familiar abrupt change in physical properties at the cmc. In chromonic systems there is also the aggregation of molecules in dilute solution before mesophase formation, but the pattern of association is different. The hydrophobic surfaces of the molecules cause them to aggregate in stacks like packs of cards. As these stacks grow, the fraction of the total hydrophobic surface area exposed to the aqueous part of the phase steadily falls, but there is no minimum free energy state, no cmc, and there is no structure directly comparable to the micelle [38]. 

Chromonic phases can in general be oriented by surface alignment or magnetic fields, but when samples are introduced into untreated glass tubes for an X-ray study, there is usually sufficient spontaneous alignment for axial and equatorial reflections to be easily distinguished. (In rare cases a sample in the M phase will form a large scale zig-zag texture like that observed by Bernal and Fankuchen for TMV solutions [49].)... 

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