Azobenzene photoswitches

Imaging/Labeling Applications Amino acids " analytes azobenzene photoswitching in vivo, leucine lysines " small molecules metal ions transition metal complexes " nucleic acids peptide analog proteins " " silane coupling agent " sUica particles eads " uronium salts ... 

Beharry, A. A. Wong, L. Tropepe, V. Woolley, G. Andrew fluorescence imaging of azobenzene photoswitching in vivo. Angew. Chem., Int. Ed. 2011, 50,1325-1327. 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

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... 

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

McGrath and Junge [36] reported a photoresponsive poly(aryl ether) dendrimer with azobenzene as the dendrimer core. These dendrimers exhibited reversible trans to cis photoisomerization by irradiation at 350 nm. The authors proposed the use of this type of dendrimer as novel photoswitchable transport vectors. This is based on the expected ability of dendrimers to encapsulate or eject small molecules reversibly upon light perturbation. 

Ingerman, L. A. Waters, M. L. Photoswitchable dynamic combinatorial libraries Coupling azobenzene photoisomerization with hydrazone exchange. J. Org. Chem. 2009, 74, 111-117. 

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. 

Functionalisation of PO PAM-dendrimers with the push/pull azobenzene derivative methyl orange (cf. Fig. 5.21) [27] offers further photoswitching opportunities. The change in colour of this pH indicator from red (pH 3.1) to yellow (pH 4.4) can be explained by protonahon of the azo function to form the me-somerically stabilised azonium ion (Fig. 5.22). 

POPAM dendrimers bearing up to 32 photoswitchable azobenzene groups at the periphery were used as host compounds for eosin Y (2, 4, 5, 7 -tetrabromo-fluorescein dianion) (see also Section 5.1.2). One reason for the choice of this dye was that it shows strong fluorescence which should be influenced by being enveloped by the dendrimer. The other reason was that the energy of its lowest triplet state is higher than that of the lowest triplet state of azobenzene, leading... 

Starting from l,3,5-tris(bromomefhyl)benzene as core unit, Vogtle et al. constructed the first dendrimer with intramolecularly accumulated azobenzene entities in the periphery by convergent synthesis (Fig. 8.25) it expectedly exhibits complex photoswitchable properties owing to the possibility of intramolecular E/Z isomerisation [79, 80]. 

As a different type of photoswitchable rotaxane, Murakami et al. reported on the rotaxane, in which a-CD is threaded by an azobenzene-containing chain with two methylenes as the spacer between azobenzene and viologen units (109) [97],... 

The azobenzene-appending 7-CD 116 exhibits different circular dichroism bands for trans and cis forms, and 116 was used as a photoswitchable multiresponse sensor on the basis of the guest-induced intensity variations [106],... 

Fukushima, M. Osa, T. and Ueno, A. (1991) Photoswitchable Multi-response Sensor of Azobenzene-modified y-Cyclodextrin for Detecting Organic Compounds, Chem. Lett. 709-712. 

Scheme 24 Chiral photoswitchable polyisocyanates A) schematic representation of the shift in equilibrium between P and M helices upon irradiation. B) illustration ofPto M helix transition in polyisocyanates upon photoisomerization ofthe azobenzene unit (adapted from references 75-78).

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 . 

Figure 3.47 Photoswitchable azobenzene-cydooctapeptide in (a) cis and (b) trans conformations. The N atoms of azobenzene and the S atoms of the azobenzene-peptide bridges are labelled. In (b) the atoms treated at the QM level in the simulations are highlighted. The water molecules are not shown.

Figure 5.42 Cyclic amperometric responses of a ft-CD functionalized monolayer on photoswitching of the bipyridinium-azobenzene diad currents are measured at a constant potential of —0.6 V. From M. Lahav, K. T. Ranjit, E. Katz and I. Willner, Photosimulated interactions of bipyridinium-azobenzene with a /3-aminocyclodextrin monolayer-functionalized electrode An optoelectronic assembly for the amperometric transduction of recorded optical signals, Isr.. Chem., 37, 185-195 (1997). Reproduced by permission of Laser Pages...

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