Crown-ether, azobenzene

Photochemical butterfly-like E — Z photoisomerization of a bis(crown ether) azobenzene derivative 354 was found to be thermally reversible and the stereoisomers exhibit unique contrasting behaviour in the presence of metal ions.1108 The concentration of the Z-isomer in the photostationary state was noticeably enhanced by the addition of K+, Rb + or Cs +, because the corresponding Z-complex achieved a stable sandwich geometry (Scheme 6.162). As a result, the cations could be selectively extracted by the Z-derivative from an aqueous phase to an organic solvent (o-dichlorobenzene), whereas no complexation (i.e. no transfer) took place in the case of the E-isomer. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

Photoresponsive systems incorporating an azobenzene moiety. The capped crown ether (196), shown as the (E) isomer, was synthesized initially by a high-dilution condensation between diaza-18-crown-6 and 3,3 -bis(chlorocarbonyl)azobenzene (Shinkai et al., 1980). Extraction patterns for the alkali metals differed between the (E) and (Z) isomers giving a clear example of photochemical control of the complexation behaviour. Subsequently, the analogue (197) was synthesized in which 2,2 -azopyridine was used for the cap (Shinkai Manabe, 1984). Photo-... 

Three azobenzeneophane-type crown ethers in which the 4,4 positions of azobenzene are joined by a polyoxyethylene chain have been synthesized (Shinkai, Minami, Kusano Manabe, 1983). On irradiation with UV light, the ( ) (or trans) form (198) is isomerized to the (Z) (or cis) isomer (199). The ( ) isomer may be regenerated by heating, or by irradiation with visible light the interconversion is completely reversible. 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

Compound 1 is an early example of a photoresponsive crown ether [2,3], 1 has a photofunctional azobenzene cap on an N2O4 crown ring, so that one can... 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

Photoresponsive host-guest systems based on azobenzene-substituted crown ethers have been shown to be particularly effective in the control of molecular recognition by light, due to their large geometrical changes upon E-Z isomerization. 55 A num-... 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

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. 

Photoisomerization of an azobenzene function located in a complex molecule is often accompanied by conformational changes. This approach has again been employed in the construction of photoresponsive crown ethers, a topic which has been the subject of a recent review. Cylindrical ionophores in which two diaza-crown ethers are linked by two photoresponsive azobenzene groups change their ability to bind polymethylene-diammonium salts on irradi-... 

There are, however, azobenzenes that have wavelength-independent isomerization quantum yields and thus obey Kasha s rule. The structure of these molecules inhibits rotation. Rau and Liiddecke investigated azobenzeno-phane 9 and Rau the azobenzene capped crown ether 14, and these researchers found identical E,E —> E,Z, and E —> Z quantum yields respectively, regardless of which state was populated. The photoisomerization of azobenzenophanes and 13 could not be evaluated in the same way because the photoisomerization is intensity-dependent. A series of azobenzenes substituted in all ortho positions to the azo group has equal quantum yields for n —> n and k —> k excitation if the substituents are ethyl, isopropyl, tert.butyl, or phenyl. This provides clues for the elucidation of the isomerization mechanism (Section 1.6). 

In addition to crown-ethers, calix[4]resorcinarenes 19 have also been used as head groups to obtain azobenzene amphipbiles with sufficient photo-isomerization in LBK films. For this purpose, azobenzene moieties have been tethered to the lower rim of the crown conformer of the calixarene. O-octacarboxymethoxylated calix[4]resorcinarenes 19 (X = CH2-COOH) display efficient trans to cis photoisomerizability in densely packed mono-layers on a water surface, in LBK films, and in surface-adsorbed monolayers, whereas the noncarboxymetylated 19 (X = H) derivative gives films that are too densely packed. However, the aggregation is already suppressed efficiently compared to the azobenzene derivative without calixaren. ... 

Membranes containing azobenzene-modified crown ether and crown ether linked spirobenzopyran also showed changes in the photostimulated membrane potential [55]. 2,3-Diphenylindenone oxide was also effective in changing the membrane potential of poly(vinyl chloride) by photoirradiation [56]. 

New photoresponsive crown ethers incorporating an azobenzene moiety have been prepared. The binding properties of the crown ether (3) containing an intraannular 4-methoxyphenylazo substituent... 

Fig. 8. A light-driven movement of an pendant arm which bears an ammonium group and is covalently linked to a crown ether. UV irradiation induces the trans-to-cis rearrangement of the azobenzene fragment. The self-complexing process is favored by the establishing of hydrogen bonding interactions between the ammonium group and the oxygen atoms of the crown. Decomplexation (cis-to-trans re-isomerization) can take place either via irradiation with visible light or thermally...

Fig. 8A-C. Molecular switches based on the binding ability of crown ethers. A A photo-switchable binding event based on conformational change upon azobenzene isomerization. B A photoswitchable binding event based on both conformational and electrostatic changes. C A redox-switchable calix[4]arene-crown ether...

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

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