Orientation and Spectral Characteristics of the Azobenzene Chromophore in the Ammonium Bilayer Assembly

Abstract

AbstractAzobenzene‐containing, single‐chain ammonium amphiphiles in which the alkyl chain length in the tail and spacer portions was systematically varied were prepared, and their aggregation behavior in water was studied by electron microscopy, differential scanning calorimetry and absorption spectroscopy. These amphiphiles exist as molecular dispersions, hydrated microcrystals, micelles, or bilayers. The formation of stable bilayers was deduced on the basis of three criteria which include the spectral agreement between aqueous dispersions and the corresponding cast films. Stable bilayers were obtainable when the sum of tail and spacer is not smaller than C14 to C16, and the tail is at least C8. Double‐chain ammonium amphiphiles which possess an octadecyl chain in addition to the azobenzene‐containing chain were prepared and shown to form stable bilayers. The absorption spectrum reflected the aggregate structure. In particular, the bilayer spectra were classified into four types: type A (λmax ca. 300 nm) for the parallel chromophore orientation (H aggregate), type B (330 – 340 nm) for the dimeric chromophore, type C (ca. 355 nm) for the isolated chromophore, and type D (360–390 nm) for the head‐to‐tail chromophore orientation (J‐like aggregate). Liquid‐crystalline bilayers gave type B spectra, while crystalline bilayers gave either of types A, B, and D, depending on the alkyl chain length. Finally, various examples of spectral control were discussed in relation to the present results.