Monolayer azobenzene-containing

Weener et al. prepared photo-responsive monolayers from azobenzene modified polypropylene imine) dendrimers which also hold promise in the area of optical data storage [74], A fifth generation polypropylene imine) dendrimer was functionalized with equal amounts of palmitoyl and azobenzene containing alkyl chains, resulting in the formation of an amphiphilic copolymer with a random shell structure (Figure 16.5). 

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

In monolayers of azobenzene containing amphiphiles, the characteristics of photoisomerization are dependent on the chain length. ... 

Kimizuka, N., and Kunitake, T. Molecular Orientation and Domain Formation in Surface Monolayers of Azobenzene Containing Amphiphiles and Their Polyion Complexes. Chem. Lett. 827 (1988). 

Malcolm, B. R., and O. Pieroni, O. The Photoresponse of an Azobenzene-Containing Poly(L-Lysine) in the Monolayer State. Biopolymers 29, 1121 (1990). 

FIG. 15.1 Photocontrol of various soft materials by azobenzene-containing monolayers. 

Seki, T., Tanaka, K., and Ichimura, K. Phototropic discrimination of the polymerization behavior of diacetylene Langmuir-Blodgett films by an azobenzene-containing monolayers. Adv. Mater., 9, 563 (1997). 

Seki, T., Fukuda, R., Yokoi, M., Tamaki, T., and Ichimura, K. Photomechanical response of azobenzene containing monolayers on water surface. Bull. Chem. Soc. fpn., 69, 2375... 

Seki, T., Sekizawa, H., Morino, S., and Ichimura, K. Inherent and coopoerativc photomechanical motions in monolayers of an azobenzene containing polymer at the air-water interface. /. Phys. Chem. B., 102, 5313 (1998). 

Xing, L., and Mattice, W. L. Atomistic Simulations of Self-Assembled Monolayers Thar Contain Azobenzene. Langmuir 12,3024 (1996),... 

Higuchi M, Minoura N, Kinoshita T. 1995b. Photo responsive behavior of a monolayer composed of an azobenzene containing polypeptide in the main chain. Colloid Pol Sci 273(11) 1022 1027. 

Malcolm BR, Pieroni 0.1990. The photoresponse of an azobenzene containing poly(L lysine) in the monolayer state. Biopolymers 29(6 7) 1121 1123. 

There are a large number of publications on photoinduced anisotropy in supramolecular-ordered azobenzene-containing systems, such as liquid crystals (Kreuzer et al., 2000 Szabados et al., 1998), liquid crystalline polymers (Rosen-hauer et al., 2001 Han et al., 2000), LbL (Geue et al., 1997 Stumpe et al., 1996), self-assembled monolayers (Sekkat et al., 1995), and adsorbed azobenezene molecules (West et al., 2001). But mass transport resulting in SRG was only reported till recently in glassy systems. 

Kawashima Y, Nakagawa M, Ichimura K, Seki T. 2004. Photo orientation of mesoporous sUica materials via transfer from azobenzene containing polymer monolayer. J Mater Chem 14 328 335. 

Photomechanical Responses of Azobenzene-Containing Materials. Ji et al. (2004) reported that a self-assembled monolayer (SAM) of azobenzene molecules on a silicon microcantilever created a photon-driven switch. Commercial silicon microcantilevers were electron-beam coated on one side with 3nm of primer layer (chromium), followed by 20 nm of gold. An azobenzene derivative with a decanthiol substituent on one ring was used to functionalize the gold surface. The thiol (-SH) end attached to the gold surface and the other azobenzene end disposed outward. 

Hamelmamm, R Heinzmann, U. Siemling, U. Bretthauer, R der Bruggen, J. V. Light-stimulated switching of azobenzene-containing self-assembled monolayers. Appl. Surf. Sci. 2004, 222,1-5. 

Delorme, N. Bardeau, J. R Bulou, A. Poncin-EpaiUard, F. Azobenzene-containing monolayer with photoswitchable wettabflity. Langmuir 2005, 21, 12278-12282. 

The examples of photo-induced trans-to-cis isomerization of azobenzene dyes described above are very rare and valuable in supramolecular assembled systems, because the difficulty of the photoisomerization of the azobenzene moiety has been recognized in micellar and monolayer supramolecular systems [86,87]. In addition, other azobenzene-containing organogels, in which the trans-azobenzene moieties are tightly packed in the organogel supramolecular structme, did not show any photo-induced trans-to-cis iso-merizations [59]. Presiunably, in 22b and 44 there is sufficient room for the isomerization aroimd the azobenzene moiety. Thus, photo-induced isomerization is very sensitive to the organogel structure. 

The photochromic behaviour of azobenzene-containing poly(L-lysine) has also been reported in the monolayer state [19]. When the polypeptide monolayer is kept at constant area, alternate irradiation with visible and ultraviolet light produces reversible changes ( 25%) of the surface pressure of the monolayer. 

We have approached the goal of functionalised monolayer fabrication by synthesis of a group of azobenzene-containing amphipathic molecules. Such molecules can be deposited by either chemisorption or physisorption and should allow hybrid films to be fabricated. The monolayers are reversibly photochromic (which might prove useful in sensor fabrication) and are capable of chemical activation for surface attachment of macromolecules, etc. In addition, the activated surface can be photochemi-cally modified so that photolithography could be used to fabricate surface structures such as danains of particular ccxnposition. With particular regard to conductive polyers, polyanilines could be locally formed as discussed later. 

An interesting example of a system whose components become mechanically bound upon surface immobilization is rotaxane trans-21 (Fig. 13.24), consisting of a ferrocene-functionalized /3-cyclodextrin (fi-CD) macrocycle threaded on a molecule containing a photoisomerizable azobenzene unity and a long alkyl chain.33 A monolayer of trans-21 was self-assembled on a gold electrode. Therefore, the ring... 

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