“Crystal Engineering” Based on Two-Dimensional Molecular Assemblies. Relation between Chemical Structure and Molecular Orientation in Cast Bilayer Films

Optically transparent films of single chain ammonium amphiphiles having an azobenzene chromophore in the hydrophobic chain (CnAzoCmN+) weee prepared by simple casting of their water or ethanol solutions. The UV—visible absorption spectrum of the azobenzene chromophore was strongly affected by the chemical structure of the amphiphile, especially on the chain length of the alkyl tail (n = 3-14) and the spacer group (m = 4—12), and is classified into six groups: absorption maximum of group I, 300—305 nm; group II, around 320 nm; groups III and IV, 345-355 nm; group V, 360-370 nm; group VI, 375 nm. Structural requirement of the group I is found to be m-n ≥ 2. The amphiphiles with a long alkyl chain (n + m > 18) and structural relation of m-n ≤ 1 are comprised into the group II. Amphiphiles with m = 8 and 7, except for the amphiphiles in groups I and II, are classified in group III. Groups IV and V consist of the amphiphiles with m = 4 and 6, respectively. Group VI consists of the series of amphiphiles with m = 5, except n = 3 and 4. X-ray diffraction experiments indicate that the cast films are composed of multiple stacked bilayers. Intensity distribution in the X-ray diffraction pattern of group V was very similar to group VI but different from groups I and II. Supposing a tilt molecular packing with a head-to-tail chromophore orientation, the molecular axes inclines by 36° and 26° to the bilayer surface in groups V and VI, respectively. The molecules in groups I and II are supposed to be packed laterally in an antiparallel and mutually interdigitated fashion. Spectral difference of I and II is ascribed to full and partial overlapping of the azobenzene chromophores, respectively. A peculiar small void space under the bromide counterion is expected to be formed in the group I films if the spacer chain is longer than the alkyl tail (m-n> 2). Thermally induced structural transformation to V or VI was observed when the films of groups I and II weee annealed above their phase transition temperatures.