Periodic change in absorption maxima due to different chain packing between the bilayers of amphiphiles possessing even and odd numbers of carbons in the hydrophobic chain

Abstract

N-{4-[4-(10-Trimethylammoniodecyloxy)phenylazo]benzoyl}-L-alanine alkyl ester bromides with n-C₆ to -C₁₃ alkyl chains have been newly synthesized. The bilayer aggregates absorbed at 355 nm when the number of carbons was even and at 320 nm when odd. Regardless of whether the number was even or odd, X-ray diffraction patterns suggested a chain penetration structure, where the tail chains in one molecular layer penetrated into the opposite molecular layer, and hence every other azobenzene group was distributed into the opposite layers. However, a remarkable splitting of the CH₂ scissoring band into 1473 and 1463 cm^–1 indicated perpendicular alignments of the trans-zigzag planes of the chains containing an even number of carbons, whereas the singlet CH₂ scissoring band at 1465 cm^–1 indicated parallel alignments of the planes of the chains containing an odd number of carbons. In accord with the chain packing, when the amphiphiles possess an even number of carbons, the adjacent neighbouring azobenzenes in each layer were arranged perpendicular to each other. The perpendicular location causes a loss of interchromophore interaction, giving λ_max at 355 nm. In contrast, the azobenzenes in the bilayers of the amphiphiles containing an odd number of carbons were all parallel to each other. Because the molecules were tilted above about 60°, a H aggregate should be predominantly formed, which absorbed at 320 nm. In addition, the amphiphiles containing an even number of carbons formed stronger H bonding than those containing an odd number of carbons.