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

Abstract

Absorptionsspektren, sichtbar und ultraviolett / Biophysikalische Chemie / Doppelschicht-Membranen / Phasenum wandlungen / Zwischenmolekulare Wechselwirkungen Azobenzene-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 C,4 to C16, and the tail is at least C,. 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 (,Imax 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 Dresent results.

Monolayers of rod-shaped and disc-shaped liquid crystalline compounds at the air-water interface

“…This corresponds to a tight packing of the hydrocarbon chains as found for fatty acids and alcohols [22]. Compared to the cyanoazobenzene 3 [13], the nitrile 4 is characterized by a short aliphatic chain. 4 shows the formation of an expanded phase at room temperature: The reduction of the length of the hydrophobic chain causes the decrease of the phase transition temperature [22], in agreement with the spreading behaviour of classical amphiphiles.…”

“…Recently, the combination of amphiphilic and mesogenic properties of thermotropic LCs has been realized, and spreading experiments at the air-water interface were performed [3][4][5][6][7]. In addition, the functionalization of amphiphilic compounds led to the incorporation of rod-shaped mesogenic units in amphiphiles, such as biphenyl [6,8], stilbene [9,10], azobenzene [11][12][13][14], benzylidenaniline [15] or bipyridine [16,17]. In these compounds the rod-shaped units function as chromophores, fluorescence markers or highly polarizable units.…”

Features of Photomechanical Response in Monolayers Composed of a Charged Amphiphilic Azobenzene Polymer

“…17 The side-by-side aggregation of Az is characterized by the hypsochromic shift of this band. Therefore, the magnitude of the spectral shift can be an indicator of the lateral packing state of the side chain array.…”

Challenges and breakthroughs in recent research on self-assembly

“…Examples of artificially designed amphiphiles are shown in figure 14B. A dialkyl structure is often used for the tail, but trialkyl structures, tetraalkyl structures and azobenzene-type rigid structures are also available [130][131][132].…”