Cyclodextrin azobenzene

Another type of photoactive capped cyclodextrin, azobenzene-capped cyclodextrin, is also reported, but regioisomer distribution is not mentioned and their treatment does not seem quite satisfactory. See Yoshimura, H., Saka, H., and Osa, T., J. Am. Chem. Soc. 101, 2779 (1979) Chem. Lett. 841, 1007 (1979) Chem. Lett. p. 29 (1980). 

Callari F, Petraia S, Sortino S. 2006. Efighly photoresponsive monolayer protected gold clusters by self assembly of a cyclodextrin azobenzene derived supramolecular complex. Chem Commun 1009 1011. 

Fujimoto, T, Nakamura, A., Inoue, Y., Sakata, Y., and Kaneda, T, Photoswitching of the association of a permethylated a-cyclodextrin-azobenzene dyad forming a janus[2]pseudorotaxane, Tetrahedron Lett., 42, 7987, 2000. 

Yabe A, Kawabata Y, Niino H, Tanaka M, Ouchi A, Takahashi H, Tamura S, Tagaki W, Nakahara H, Fukuda K. Cis-trans isomerization of azobenzenes included as guests in Langmuir-Blodgett films of amphiphilic beta-cyclodextrin. Chem Lett 1988 l-4. 

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... 

A similar template-directed approach was employed (Figure 31) by Anderson et al. [48] to self-assemble azo-dye [2]rotaxanes. The azobenzene diazonium salt 98 is bound by a-cyclodextrin 99 and also by / -cyclodextrin 100 in H20 with pseudorotaxane geometries. Reaction of the terminal diazonium functions with / -naphthol 101 affords the purple [2]rotaxanes 102 and 103 in yields of 12 and 15%, respectively. These two [2]rotaxanes were characterized by electrospray mass spectrometry which revealed peaks corresponding to the loss of all four sodium cations in both cases. By irradiating some of the protons of the dumbbellshaped components of these [2]rotaxanes, enhancement of the resonances of some of the cyclodextrin protons could be observed as a result of an intercomponent... 

Nakashima et al. [49] have self-assembled (Figure 32) a Tight-driven molecular shuttle by relying upon the threading of one a-cyclodextrin (99) on to an acyclic guest, followed by the covalent attachment of two stoppers at both its ends. The azobenzene-based guest 104 is bound by a-cyclodextrin 99 with a pseudorotaxane... 

J and Yabe et al. [155J have formed LB films of amphiphilic derivatives of jS-cyclodextrin and have incorporated azobenzene derivatives into them in such a way as to form a host-guest complex. Many workers have used the cis to trans change of structure referred to above and brought about by ultraviolet irradiation to change some measurable physical parameter of LB films formed from azobenzene derivatives [156-62J. 

The hetero-dimerization behavior of dye-modified -cyclodextrins with native CDs was investigated by means of absorption and induced circular dichroism spectroscopy in aqueous solution [43], Three types of azo dye-modified /i-CDs show different association behavior, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl Red-modified fi-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4 position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its / -CD cavity (Figure 13). 1 also associates with native a-CD by inserting the dye residue into the a-CD cavity. The association constants for such hetero-dimerization are 198 M"1 at pH 1.00 and 305 M 1 at pH 6.59, which are larger than the association constants of 1 for / -CD (43 M 1 at pH 1.00). 

Inclusion properties of molecular nanotubes composed of crosslinked a-cyclodextrin was investigated [47], Induced circular dichroism was used to probe the formation and dissociation of complexes between the nanotubes and azobenzene modified polyethylene glycol), either with or without a hydrophobic alkyl chain. The inclusion complex between the nanotubes and polymers formed at room temperature, and the polymers dissociated from the nanotubes with increasing temperature. 

Ueno et al. [55,56] showed that /3-cyclodextrin arched with azobenzene-4,4 dicarbonyl- and azobenzene-4,4 -disulfonyl- residues (Figure 19) have enlarged hydrophobic cavity, particularly in the case of the cis isomers. 

Sueishi, Y. Kasahara, M. Inoue, M. and Matsueda, K. (2003) Effect of substituent and solvent on inclusion complexation of //-cyclodextrins with azobenzene derivatives, J. Incl. Phenom. 46, 71-75. 

Yamaguchi, I. Osakada, K. and Yamamoto, T. (2000) Pseudopolyrotaxane composed of an azobenzene polymer and y-cyclodextrin. Reversible and irreversible photoisomerization of the azobenzene groups in the polymer chain, Chem. Commun. 1335-1336. 

Ueno, A. Shimizu, T. Mihara, H. Hamasaki, K. and Pitchumani, K. (2002) Supramolecular Chemistry of Cyclodextrin-Peptide Hybrids Azobenzene-Tagged Peptides,/. Incl. Phenom. 44, 49-52. 

Kikuchi, T. Narita, M. and Hamada, F. (2002) Synthesis of bis dansyl-modified /f-cyclodextrin dimer linked with azobenzene and its fluorescent molecular recognition, J. Incl. Phenom. 44, 329-333. 

Ueno, A. Fukushima, M., and Osa, T. (1990) Inclusion Complexes and Z-E Photoisomerization of P-Cyclodextrin Bearing an Azobenzene Pendant, Perkin Trans. 2 1067-1072. 

Fukushima, M., Osa, T., and Ueno, A. (1990) Photocontrol of Molecular Association Attained by Azobenzene-modified Cyclodextrin, Chem. Commun. 15-17. 

Yoshida, K. Nakagama, T. Uchiyama, K. and Hobo, T. (1998) Photo responsive chromatographic behavior of dansyl amino acid enantiomers using azobenzene-modified cyclodextrin stationary phases in... 

Figure 5.23 Schematic illustration of the organization of the molecular train monolayer assembly on a gold electrode and its photoinduced translocation. Reprinted from /. Electroanal. Cheat., 497, I. Willner, V. Pardo-Yissar, E. Katz and K. T. Ranjit, A photoactive "molecular train" for optoelectronic applications light-simulated translocation of a /3-cyclodextrin receptor within a stoppered azobenzene-alkyl chain supramolecular monolayer assembly on an Au-electrode, 172-177, Copyright (2001), with permission of Elsevier Science...

Figure 5.41 Optoelectronic functions of the cyclodextrin-monolayer-modified electrode and bipyridinium-azobenzene diad, as reported by Willner and co-workers [73]. Photoisomerization of the diad alters its ability to include in the cyclodextrin cavity and consequently write to the electrode...

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

Scheme 12 Photochemically driven shuttling movement of an a-cyclodextrin in an azobenzene-containing thread through reversible E/Z photoisomerization [61]...

Switching devices that are reversible and work on the molecular level are essential features of nanomachinery. Control of the access to capsules, the transport of molecules in and out of the cavities, is desirable and we examined a well-established system that uses light as a switching device the cis-trans photoisomerization of azobenzenes [58, 59]. The azobenzenes have been applied in the supramolecular chemistry of crown ethers [60-62], cyclodextrins [63,64], and even proteins [65, 66]. The photoisomerization changes the shape in a predictable way and we used azobenzene photoisomerization in an indirect sense to control reversible encapsulation. 

Threading of two cyclodextrins onto a symmetrical dumbbell can occur head-to-head, head-to-tail, or tail-to-tail, defining a new class of diastereoisomeric [3]-rotaxanes, as shown schematically in Figure 2.12d. Anderson and co-workers have shown that end-capping of 4,4,-bis(diazonio)azobenzene chloride 43 with 2,6-dimethylphenol 44 in the presence of a-CDX produced the [3]-rotaxane 45 in 12% yield, in addition to the [2]-rotaxane 46 in 9% yield, and free dumbbell 47 in trace amounts (Figure 2.18).42 The stereochemistry of the [3]-rotaxane species is remarkable because the two cyclodextrin beads have their smallest rims facing to each other. Therefore the threading reaction was stereoselective. The reasons for the exclusive formation of the tail-to-tail stereoisomer are not clearly established. 

Tanaka, M., Ishizuka, U., Matsumoto, M., Nakamura, T., Yabe, A., Nakanishi, H., Kawabata, Y.,Takahashi, H.,Tamura, S.,Tagaki, W., Nakahara, H., and Fukuda, K. (1987), Host-guest complexes of an amphiphilic P-cyclodextrin and azobenzene derivatives in Langmuir-Blodgett films, Chem. Lett., 1307-1310. 

Two studies have been reported on azobenzenes 2, 2 5 4 jp case of the sterically hindered 0,0,0substituted azobenzines the effect of exciting lower M n, tt ) and higher (ir, ir ) states has been studied25. in cyclodextrin inclusion complexes of azobenzene there is partial blockage of rotational motion about the N=N bond25. ... 

Rearrangements.- E-Photoisomerization occurs readily in imines and in azo compounds. The syn-isomer (1), for example, is the major product of irradiation of nitrofurazone (2) in solution and is formed together with the corresponding azine on exposure to laboratory illumination. The photoisomerization of azobenzene derivatives in solution, in membranes, in host-guest complexes of cyclodextrins, and in polymers continues to attract attention. The reversibility of E-photoisomerization of azobenzene in cyclo-hexane solution has been established, and the E/ -ratios generated by irradiation of various azobenzene derivatives adsorbed on... 

Confinement of azobenzene in defined structures changes the quantum yields. In the cavity of fi-cyclodextrine, the < )e z are reduced and become nearly wavelength-independent, whereas the practically unaffected... 

Besides solving the quantum yield enigma, this concept also rationalizes some other results. If rotation is inhibited by, say, structural design as, for instance, in azobenzenophanes or constraint from outside as, for instance, in restricted spaces as in iS-cyclodextrin " or zeolites or in solid matrices or low temperature down to 4 K, then the internal conversion from the (7i,7t ) to the (n,7t ) state provides a virtually barrierless path of isomerization. The fact that the stilbenophane analogue of Tamaoki s azobenzeno-phane shows isomerization does not invalidate this reasoning—the azobenzenes choose the easiest isomerization path. 

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