Azo group(s) in selected macrocyclic compounds

Journal of Inclusion Phenomena and Macrocyclic Chemistry, Jan 2018

Azobenzene derivatives due to their photo- and electroactive properties are an important group of compounds finding applications in diverse fields. Due to the possibility of controlling the trans–cis isomerization, azo-bearing structures are ideal building blocks for development of e.g. nanomaterials, smart polymers, molecular containers, photoswitches, and sensors. Important role play also macrocyclic compounds well known for their interesting binding properties. In this article selected macrocyclic compounds bearing azo group(s) are comprehensively described. Here, the relationship between compounds’ structure and their properties (as e.g. ability to guest complexation, supramolecular structure formation, switching and motion) is reviewed.

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Azo group(s) in selected macrocyclic compounds

Journal of Inclusion Phenomena and Macrocyclic Chemistry https://doi.org/10.1007/s10847 Azo group(s) in selected macrocyclic compounds Ewa WagnerW‑ysiecka 0 Natalia Łukasik 0 Jan F. Biernat 0 Elżbieta Luboch 0 0 Department of Chemistry and Technology of Functional Materials, Faculty of Chemistry, Gdańsk University of Technology , Narutowicza Street 11/12, 80-233 Gdańsk , Poland 1 Elżbieta Luboch Azobenzene derivatives due to their photo- and electroactive properties are an important group of compounds finding applications in diverse fields. Due to the possibility of controlling the trans-cis isomerization, azo-bearing structures are ideal building blocks for development of e.g. nanomaterials, smart polymers, molecular containers, photoswitches, and sensors. Important role play also macrocyclic compounds well known for their interesting binding properties. In this article selected macrocyclic compounds bearing azo group(s) are comprehensively described. Here, the relationship between compounds' structure and their properties (as e.g. ability to guest complexation, supramolecular structure formation, switching and motion) is reviewed. Ewa Wagner-Wysiecka, Natalia Łukasik and Elżbieta Luboch dedicate this article to Professor Jan F. Biernat on occasion of His 80th birthday. Macrocyclic compounds; Azo group; Trans-cis isomerization; Host-guest interactions; Molecular switches Introduction The year 2017 appears to be a very special for supramolecular chemistry. 50 years ago Charles Pedersen [ 1 ] published papers describing the syntheses and completely untypical and unknown until that time intriguing complexing properties of macrocyclic polyethers, i.e. crown ethers [ 2, 3 ]. The discovery turned out to be a milestone in chemistry that changed the whole chemical world, gave new fascination and opened up new perespectives for science and technology. Crown ethers are excellent example of unexpected discovery that gained worldwide fame. Since discovery of crown ethers, many their applications have been developed, for example in chromatography [ 4, 5 ], sample preparations [ 6 ], catalysis [ 7–9 ], and chemical sensing [ 10 ]. Macrocyclic compounds had entered the laboratories all over the world, in particular after the discovery of macrocycles containing oxygen and nitrogen electron donors, being the base for three dimensional cryptands, synthesized and studied by Lehn [ 11, 12 ] and spherands, obtained and investigated by Cram [ 13–16 ]. All these discoveries initiated host–guest [ 17, 18 ] and supramolecular chemistry [ 19–22 ]. For their achievements Pedersen [ 23 ], Cram [ 18 ] and Lehn [ 20 ] were honored in 1987 with a Nobel Prize. The Nobel Prize in Chemistry 2016 was awarded to: Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Ben L. Feringa “for the design and synthesis of molecular machines” [ 24–26 ], which have close relationship with the above mentioned branches of chemistry. A year after Pedersen’s publication on crown ethers and their unique metal cation binding abilities, Park and Simmons published work on macrobicyclic amines i.e. catapinands, the first anion receptors [ 27–29 ]. Since that time supramolecular chemistry of anions for many years seemed to be almost forgotten, but last two decades were a renaissance of anion recognition studies [ 30–33 ]. Within the last 50-years a lot of macrocyclic compounds of sophisticated structures have been synthesized and investigated [ 34 ]. The skeleton of the vast majority of macrocyles can be more or less easily modified by introducing functional groups, which bring about additional chemical or physical features in comparison to the parent compounds as well as to the respective supramolecular species. Functionalized supramolecular systems can be applied in many branches of science and life [ 35 ], including e.g. the development of new analytical [ 36–41 ] and therapeutic [ 42–44 ] systems, modern, intelligent (nano)materials [ 45–50 ] and molecular devices and machines [ 51–57 ]. One of the most convenient and useful functionalization of macrocyclic compounds is the introduction of azo group(s): incorported in the ring or on its periphery. Azo moiety due to its ability to alter the geometry upon photochemical or thermal trans–cis isomerization can be utilized as a light triggered switch in vast variety of functional materials such as for example molecular containers, polymers, supramolecular protein channels, and sensors. As upon photoisomerization process of azo bearing molecules electromagnetic radiation is converted to mechanical work, those compounds can be used in light-driven molecular machines. Here, we present an extensive review of selected azomacrocyclic compounds with the special focus on supramolecular interactions (host–guest complex formation, self-assembly) and trans–cis isomerization of azo group. Azobenzene and its derivatives The properties and functions of the supramolecular systems can be controlled by external stimuli such as changing of pH, temperature, irradiation with the selected wavelength, action with electric or magnetic field. For specific goals, moieties sensitive to one or more of the above factors must be present or introduced to macrocycle structure upon its functionalization. The synthetic routes leading to macrocyclic compounds are often laborious, hence additional functionalization preferably needs relatively simple procedures. A nice example of relatively easy-to-implement functional unit with photo- and redox active properties is the azo group – N̄=N̄ –, which is also pH sensitive. Azo compounds are one of the oldest synthesized organic compounds, being produced till now on a large scale in dye industry [ 58 ]. The main synthetic approach is based on diazotization reaction discovered by Peter Griess in Fig. 1 Schematically: the main methods for the synthesis of azo compounds nineteenth century. The most common methods of azo group incorporation are schematically shown in Fig. 1. Nowadays, diverse modifications of the orginal process of diazocoupling are available; also new, synthetic procedures are proposed for preparation of azo compounds for varied purposes [ 59–66 ], including methods identified as environmentally friendly [ 67–69 ]. Colored azobenzenes and their more sophisticated derivatives, among others, can undergo light-driven reversible trans–cis (E⇄Z) isomerization. The reversible E⇄Z photoisomerization of azobenzene presents well-understood process widely used for construction of light-driven functional molecules for energy storage or conversion of light energy into mechanical motion, exemplified by molecular devices and machines [ 70 ]. Cis isomer of azobenzene was discovered in 1937 by Hartley [ 71 ]. Trans (E) and cis (Z) azobenzene isomers are shown in Fig. 2 that also illustrates the reversible isomerization. Trans isomer of azobenzene is thermodynamically more stable than the cis isomer. In most cases trans→cis isomerization occurs upon irradiation with UV light (Fig.  2a). However, azobenzene derivatives undergoing reversible trans⇄cis isomerization upon visible light illumination have been also reported [ 72–74 ]. Such molecular switches are more applicable and safer for biological uses where harmful ultraviolet light should be avoided. The cis⇄trans isomerization may occur by spontaneous thermal back reaction or reverse photoisomerization cycle. The light-driven reversible E⇄Z isomerization of azobenzene is associated with substantial changes of structure, size, geometry and physical properties. Structural changes of azobenzene moiety inbuilt into a larger or more complicated compound affect also the behavior and properties of the azofunctionalized molecular systems like it is for example in photoswitches. Dipole moment of trans isomer of azobenzene is near zero. Cis isomer of azobenzene has dipole moment 3.1 D, what determines hydrophobic/hydrophilic character of isomers. Trans (E) azobenzene is almost planar, opposite to cis (Z) isomer. In solid state in cis azobenzene the parallel (a) 4 0.9 nm phenyl rings are twisted 56° out of the plane of the azo group (Fig. 2). The different geometry of trans and cis isomers of azobenzene affects their UV–Vis spectra. The spectra (Fig. 2c) of trans and cis isomers are overlapping, but differ significantly. Band at ~440 nm originating from n→π* transition is more distinct for cis isomer. Strong absorption band at ~ 320 nm for trans isomer can be attributed to π→π* transition. In a spectrum of cis-azobenzene less intensive π→π* transition bands are observed at lower wavelength. The spectral differences cause different colors of both isomers, what makes the observation of isomerization process possible also in non-instrumental manner (by naked eye). Spectral properties of azobenzene derivatives are strongly dependent on the substituents in phenyl rings. Azobenzene can also act as an important functional unit if incorporated into electrochromic materials (ECMs), which properties can be stimulated by applied potential. Such substances are outstanding candidates for materials used for production of electronic paper [ 75–78 ] or dual-stimuliresponsive systems [ 79, 80 ]. The electrochemistry of azobenzene and its derivatives in different solvents was studied exhaustively in details for both trans and cis isomers [ 81–85 ]. It was found that the electrochemical reduction of azobenzene is strongly dependent on conditions, such as type of the solvent, pH or reagent concentrations. However, in general it can be summarized that the reduction of azobenzene occurs in a single two electrons, two protons process with a final formation of hydrazobenzene. The simplified way of the electrochemical reduction of azobenzene is shown in Scheme 1. The properties of self-assembled monolayers of azobenzene derivatives—also macrocyclic—on different surfaces [ 86–92 ] showed, that such materials are promising candidates for molecular devices for energy storage and conversion. hydrazobenzene benzo[c]cinnoline 2 N N N N cinnoline 1 reduction 2H+, 2e-2H+, -2eoxidation N H H N Scheme 1 The electrochemical reduction–oxidation of azobenzene NO2 NO2 reduction electrochemical or chemical N N 2,2'-dinitrodiphenyl Scheme 2 Cinnoline and reductive cyclization of 2,2′-dinitrobiphenyl as preparation method of benzo[c]cinnoline Cyclic and macrocyclic derivatives of azobenzene(s) Small rings Derivatives of cinnoline 1, e.g. benzo[c]cinnoline 2 (Scheme 2) can be considered as structural, cyclic analogs of azobenzene. These compounds are used in manufacturing of dyes, electrochromic polymers, coloured polyamide fibers and have microbial and herbicidal activities [ 93, 94 ]. Cinnolines were also studied as potential anticancer agents [ 95, 96 ]. The reduction of 2,2′-dinitrobiphenyl to 3,4-benzocinnoline (benzo[c]cinnoline) 2 (Scheme 2) was first described by Wohlfart [ 97 ] and later by other groups [ 98–107 ]. The crystal structure of benzo[c]cinnoline complex with ytterbium Yb(BC)3(thf)2 (BC = benzo[c]cinnoline) was described [ 108 ]. Fe2(BC)(CO)6 complex was examined as a candidate for a new structural and functional model for [FeFe]-hydrogenases [ 109, 110 ]. Modified with benzo[c]cinnoline or its derivatives surfaces of e.g. glassy carbon [ 111, 112 ], gold [113] or platinum [ 114, 115 ] are often used in organic, inorganic, and biochemical catalytic transformations. Öztürk et  al. [ 116 ] reported an amperometric lactate biosensor based on a carbon paste electrode modified with benzo[c]cinnoline and multiwalled carbon nanotubes. Its characteristics showed, that it can be used for determination of lactate in human serum. Incorporation of benzo[c]cinnoline moieties into poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV) indicated that p-type semiconductors based on the above polymer can be transformed into n-type materials [ 117 ]. Larger analog of cinnoline,  (5,6-dihydrodibenzo[c,g] [ 1,2 ]diazocine) (Fig. 3, compound 3) comprising azobenzene moiety joined by ethylene bridge at 2,2′-positions was identified as a molecular switch with interesting photochemical characteristics [ 118–120 ]. Interestingly, in this case cis isomer is thermodynamically more stable than trans isomer. Remarkably, both isomers of 3 have well pronounced n→π* bands in UV–Vis absorption spectra. Reversible trans to cis photoisomerization occurs with efficiency close to 100% under the illumination with visible light of 480–550 nm. The back process of rapid kinetics can be achieved at near ultraviolet at 380–400 nm. Cyclic oligomers of azobenzene (oligoazobenzenophanes) Oligoazobenzenophanes are compounds consisting of at least two or more azobenzene units forming macrocycles. Azobenzene moieties can be joined in para-, meta- or ortho- positions by sp3 hybridized spacers with or without heteroatoms forming relatively flexible, non-conjugated Fig. 3 a 5,6-dihydrodibenzo [c,g][ 1,2 ]diazocine (3): equilibrium structures of 3Z and 3E in the electronic ground states from quantum chemical calculations at the B3LYP/def2TZVP level of theory using the TURBOMOLE program and b colors of 3Z before irradiation and color 3E upon irradiation in n-hexane. Adapted with permission from [119]. Copyright 2009 American Chemical Society azobenzophanes (Fig. 4a). More rigid, conjugated azobenzophanes are obtained by joining azobenzene units without sp3 tether. As an example of conjugated azobenzophane the simplest cyclotrisazobenzene is shown in Fig. 4b. Synthetic procedures leading to azobenzophanes involve also approaches typical for macrocyclization, e.g. high dilution technique or template synthesis. The formation of azo group can be the final step of ring closure or can be achieved from substrate(s) bearing functional group(s) by substitution or condensation reactions. An exhaustive review on synthetic protocols was published by Reuter and Wegner [ 121 ] that shows preparation of vast varieties of azobenzophane skeletons by cyclizations based on nucleophilic reactions, Schiff bases condensations, reductive or oxidative azocouplings, palladium catalyzed N-arylations, and electrophilic aromatic substitutions of diazonium salts. The utility of azobenzophanes lies in reversible photoisomerization. Opposite to azobenzene for which only two possible states Z or E can be achieved by photoisomerization, macrocyclic azobenzophanes offer multiple molecular states, depending on the number of azo units. For example, for azobenzophane composed of two azobenzene fragments three states can be considered: E,E, E,Z and Z,Z (Fig. 4c) with the ratio of the isomers depending on e.g. the structure of macrocycle and photoisomerization conditions. The simplest conjugated azobenzophane cyclotrisazobenzene 4 (Fig. 4b) exits only in all-E form and has no tendency to be converted into Z form under illumination [ 121 ]. Unusual behavior of cyclotrisazobenzene was exhaustively investigated by Dreuw and Wachtveitl [ 122 ]. According to experimental and theoretical studies on ultrafast dynamics of this macrocycle the authors stated that the structural constrains prevent isomerization of azo units. The azo bonds respond elastically to the motion along the isomerization coordinates leading to complete and ultrafast dissipation of the UV excitation as heat. It was proposed that the molecules of this type can be used as UV absorbers e.g. in sunscreens. Non-conjugated azobenzophanes (a) Azobenzophanes of various structures are studied inter alia as metal cation complexing reagents. Tamaoki and coworkers [ 123 ] have obtained a series of azobenzophanes 6–8 (Fig. 5) by reductive macrocyclization of bis(3-nitrophenyl) methane under high dilution conditions. Macrocycles constructed of two, three or four azobenzene units with methylene linkers were isolated as all-E isomers. For comparative studies trans-3,3′-dimethylazobenzene 5 was prepared (Fig. 5). The position of UV–Vis absorption maxima for compounds 6–8 and 5 registered in benzene are ring size dependent. The shift of the main band (π→π*) towards longer wavelength can be ordered as follows: 8 > 7 > 6 and reverse order for n→π* (cf. Fig.  5b) and can be associated with steric distortion of the azobenzene moieties. The position of the main absorption band of the largest compound 8 is comparable to a spectrum of open chain analog trans-3,3′-dimethylazobenzene. (a) CH3 H3C H3C N N 5 N N N N N N N N 5' CH3 6. n = 1 7. n = 2 8. n = 3 n The UV–Vis spectra of all-trans 6 macrocycle and alltrans isomer of acyclic dimer 5′ (Fig. 5a) in acetonitrile are compared in Fig. 6 [ 124 ]. The same Figure shows also changes upon irradiation with 313 nm wavelength light. Photoisomerization of macrocycles 6–8 (Fig. 5) and acyclic compound 5′ from all-trans(E) to all-cis(Z) isomers proceeds gradually via respective trans(E)/cis(Z) isomers (depending on the number of azo groups). Comparison of UV–Vis spectra of macrocycle 6 trans/trans, and its acyclic analog 5′ is shown in Fig. 6 (left). Photoisomerization studies of all-trans isomers of compounds 6–8 showed that the ratio of all-cis isomers is irradiation wavelength dependent. The increase in quantity of cis azobenzene units upon irradiation at 366 nm and decrease at 436 nm was observed for 7 and 8 in chloroform. Similar behaviour was found for photoisomerization of 6 in acetonitrile. The ratios of isomers at the photostationary state for compounds 6–8 are schematically shown in Fig. 7a–c [ 123 ]. It was found that macrocyclic compounds 6–8 form complexes with alkali metal cations in methanol (determined by mass spectrometry, MS ESI). For all-trans isomers the highest peak in mass spectra was observed for cesium complexes; peak intensities for particular macrocycles can be ordered as: 6 > 7 > 8. The observed trend was disturbed upon irradiation when cis isomers also participate in complex formation. It was explained by the softer character of trans isomers. However, the clear relationship: the intensity of peaks versus ion diameter in correlation with the size of macrocyle ring was not defined. It was concluded that other factors than only host–guest geometrical complementarity affect the binding strength of metal cations by azobenzophanes 6–8 [ 123 ]. Norikane et al. [ 125 ] obtained azobenzophanes 9 and 10 (Fig. 8), having structures similar to 6–8 (Fig. 5). The modification of the macrocyclic skeleton by attaching long alkoxyl chains resulted in photoresponsive liquid crystallinity. The effect of different bulky substituents on the properties of azobenzophanes having the same macroring size was investigated by Mayor and  co-workers [ 126 ]. Four m-terphenyl compounds 11–14 (Fig. 9) comprising different peripheral substituents were synthesized by multistep reactions, and different strategies, with the final step of reductive macrocyclization (LiAlH4, THF, r.t.) of the respective nitro compounds. Compounds 11–13 are symmetric opposite to an HPLC system. Spectra are normalized at the isosbestic points (269 and 272  nm for 6 and 5′, respectively). Numbers for compounds in reproduced material correspond to following numbers of compounds in this work: 1 = 6, 2 = 5′. Reprinted from [ 124 ]. Copyright 2006 with permission from John Wiley and Sons 366 nm 436 nm 366 nm 436 nm (a) rse1.0 om0.9 ifs0.8 tno0.7 tne0.6 rco0.5 loa0.4 em0.3 itv0.2 a l re0.1 the0.0 (b) derivative 14 with two different substituents at peripheral positions. The structures of obtained macrocycles were confirmed spectroscopically and the molecular weights of oligomers were determined by vapor pressure osmometry. UV–Vis spectra registered for macrocycles 11–14 in THF are shown in Fig. 10 (left) [ 126 ]. Similar shape of spectra, i.e. π→π* ~350 nm and n→π* ~450  nm was found for all compounds, although below 300 nm in UV–Vis spectra of macrocycles 11–14 the differences are more pronounced. The blue shift of absorption bands for compounds 12 and 13 can be attributed to the effect of substituents on the central phenyl rings. Fig. 10 Left: absorption spectra of azobenzophanes 11–14: 11-solid black line, 12-dashed line, 13-solid grey line, 14-dotted line, in THF. Right: changes in absorption spectra of macrocycle 11 in THF upon irradiation at 313  nm. Inset: absorbance change versus irradiation time. Reprinted from [ 126 ]. Copyright 2009 with permission from John Wiley and Sons UV–Vis spectra of 11–14 undergo changes upon illumination (313 nm, in THF). For all compounds comparable changes were observed, what is exemplified for 11 in Fig. 10 (right). The photostationary state was reached within 8 min. Photoisomerization is observable in UV–Vis spectra by the decrease of the π→π* and the increase in the n→π* absorption bands upon irradiation over time. These changes are associated with the formation of Z isomer. Photoisomerization was monitored by 1H NMR measurements along with UV–Vis experiments (Fig. 11). By integration of the corresponding 1H NMR signals the amounts of E isomer at the photostationary state was determined to be 15%. In all cases no intermediate E,Z isomer was observed as it was in the case of similar systems studied by Tamaoki [ 123 ]. This property can be attributed to the extremely rigid structure of macrocycles 11–14. The back Z→E isomerization proceeds upon illumination or thermally. The Z isomers of the macrocycles 11–14 are stable pointing to very slow thermal backreaction (the rate constant 1.15 × 10−6 s−1). The reversibility of the photoisomerization was investigated under illumination (450 nm). Contrary to the thermal-back process, under which macrocyles were fully converted back to E isomers, upon light stimuli ~ 15% remain in their Z form. However, this process seems to be reversible what was confirmed by experiments performed in several cycles. The effect of the strain in azobenzophanes on the photoisomerization of azobenzene unit is well seen in cyclotriazobenzenes, a special class of azobenzenophanes, in which Fig. 11 a UV-Vis spectra of macrocycle 11 showing the corresponding E/Z ratio (black: thermally stable state; dark grey: 50% isomerized; light grey: photostationary state). b Corresponding 1H NMR spectra (markers indicate the peaks corresponding to the Z isomer). Reprinted from [ 126 ]. Copyright 2009 with permission from John Wiley and Sons all azobenzene units are conjugated. The simplest compound of this class has been already shown in Fig. 4b. Wegner and co-workers [ 127 ] prepared bromo- and t-butyl derivatives of the simplest cyclotrisazobenzene 4 using o-phenylenediamine as a substrate (Scheme 3). Irradiation of 4, 15 and 16 (Scheme 3) showed no isomerization under various conditions. The unfavorable change of geometry upon possible photoisomerization should result in extreme strain in the macrocyclic skeleton, thus 4, 15 and 16 exist only as all-E isomers [cf.122] Light controlled sol–gel transition of azobenzene bismacrocycle 17 (Fig. 12) was described by Reuter and Wegner [ 128 ]. Due to significant π–π-stacking interactions macrocycle 17 forms 3D networks. Its gelation was observed in aromatic solvents, attributable to the incorporation of the solvent molecule inside the 3D π-stacking network. After UV irradiation at 365 nm the gel in o-xylene slowly liquefies as a result of dissociation of 3D network. The gel–liquid conversion of 17 upon irradiation till now is the first example of switchable 3D system which was controlled by two factors: incorporation of azobenzene units and non-covalent interaction, namely π-stacking of the azobenzene macrocyles. The proposed system can be potentially used in process where small molecules are released from the 3D network upon light stimulation. In photoswitchable cyclic azobenzenes several factors such as ring strain and the number of azo units are crucial for photochemical properties. These features depend also on rigidity and the position of linkers connecting the azobenzene units, the symmetry of the macrocycle and the degree of bonds conjugation. If at the beginning, i.e. before the illumination, a compound with several azo units is fully symmetrical in all-E configuration, the change of one of the azo groups into Z isomer affects the geometry of the macrocycle. The more azo units in macrocyle the more configuration variations (number of isomers) and geometrical changes can be expected. Wegner and co-workers [ 129 ] investigated the effect of symmetry changes on the photostationary state upon E→Z isomerization stimulated by both light and temperature. For this purpose they used macrocyle 18 with four H2N NH2 NH2 N N H2N R NO NHAc R H2N N N NHAc N N 17 C9H19O2C N N C9H19O2C N N O N N CO2C9H19 C9H19O2C N N N N N N N N O N N CO2C9H19 N N CO2C9H19 Fig. 12 Macrocycle 17 described by Reuter and Wegner [ 128 ] azo moieties shown in Fig. 13. The isomerization of 18 was monitored by UV–Vis measurements and 1H NMR spectroscopy with in situ light irradiation. 18 in THF exists in the form of all-E isomer. Upon irradiation of this solution (125 μM) at 424 nm for 73 min. a mixture of five among six possible isomers was detected: the starting all-E (21%), E,E,E,Z (49%), E,E,Z,Z (19%), E,Z,E,Z (7%), and E,Z,Z,Z (4%). Under elevated (50 °C) temperature, at photostationary state, much higher ratio of the all-E isomer (55%) was detected, but lower quantities of E,E,E,Z (32%), E,E,Z,Z (4%) and E,Z,Z,Z (1.7%) isomers and almost unchanged amount of E,Z,E,Z (6.8%) isomer. It was concluded that at photostationary state the E,E,Z,Z isomer is favored over the E,Z,E,Z isomer. Comparison of the rates of thermal back isomerization reveals that the E,Z,E,Z isomer has the highest and the E,E,Z,Z isomer the lowest thermal stability. This can be ascribed to the ring strain of the particular forms. Different states can be achieved by the arrangement of the azo groups in macroring reflecting the overall symmetry of the molecule without introduction of additional substituents or applying different wavelength of the light used for illumination. Scheme 3 The general synthetic route for preparation of cyclotrisazobenzenes 4, 15 and 16 reported by Wegner and co-workers [ 127 ] N N N N N N R no photoisomerization R: 4. H 51% 15. t-Bu 49% yield (last step) 16. Br 42% Interesting, well organized system utilizing highly ordered pyrolytic graphite (HOPG) based on the photosensitive macrocycle bearing four azobenzene units 19 (4NN-M, Fig. 14) immobilized in the TCDB network was obtained and investigated by Wang and co-workers [ 130 ]. Upon UV illumination of the prepared material E,E,E,Z and E,Z,E,Z isomers are present at photostationary state. The proposed methodology was found to be useful for fabrication of nanostructures and can be valuable for production of photosensitive nanodevices. A ternary switch utilizing chiral macrocycle was presented by Reuter  and  Wegner [ 131 ]. They obtained both enantiomers R and S and racemic form of chiral bismesitylcyclotrisazobiphenyl compound 20 (Fig. 15) in about 40% yield. Three different photostationary states were gained by irradiation of 20 with different UV (302 and 365 nm) and visible light. The photoisomerization was investigated by CD spectroscopy. A large increase in the optical rotation angle for (S)-20: [α]20D = 2128° and for (R)-20: [α]20D = − 2077° in comparison with acyclic 3,3′-diaminobismesityls comes from the helical shape of macrocyclic compounds. All E-isomer was obtained by heating samples of (S)-20 and (R)-20 at 45 °C overnight. CD spectra of two all-E enantiomers are mirror images with four different absorption maxima. Upon irradiation of all E-isomer of (S)-20 with three different wavelength the photostationary state was reached after ~ 15 min. For (S)-20 seven different isomers were detected by 1H NMR measurements: six species being different E/Z isomers (one (E,E,E), two (E,E,Z), two (E,Z,Z), and one (Z,Z,Z)). The seventh one was described as a stable conformer of the (E,E,Z)-isomer with azo bond next to the bimesityl unit in Z-form. The different ratio of these isomers at particular photostationary state is manifested in CD spectra that varied mostly in intensities, but with preserved similar overall shape. However, a difference can be observed at 275 nm, when irradiating sample with mentioned above three different wavelengths: at 302 nm—positive value, visible light—no dichroism, at 365 nm negative value what is promising for ternary switch with +, − and 0 output (Fig. 15, bottom). Crown ethers with azobenzene moiety(‑ies) Azobenzene unit incorporated into crown ethers skeleton was first reported by Takagi and co-workers almost 40 years ago [ 132, 133 ]. Azo bearing crowns 21–24 (Fig. 16a) were obtained by Williamson reaction from dihydroxyazobenzene and alkylating agents. The synthesis of this type of compounds (21–23, Fig. 16a) was also elaborated in details by Biernat and co-workers [ 134–140 ]. Reductive macrocyclization of dinitropodands allowed the preparation of vast number of macrocyclic compounds showing diverse properties. By this method azoxycompounds are formed next to Fig. 15 Top: (R)-20 and (S)-20 enantiomer of bismesitylcyclotrisbiphenyl macrocyle obtained by Reuter and Wegner [131]. Bottom: CD spectra at different photostationary states at 302 nm (solid line), visible light (dashed line), and 365 nm (dotted line) (5.9 × 10−5 M), with an enlarged graph for the region 270–280 nm (inset). Reprinted with the permission from [ 131 ]. Copyright 2011 American Chemical Society O O O O N N O O azocompounds. They were studied e.g. as ionophores in ionselective membrane electrodes and chromogenic agents for metal cation complexation. At first glance—these simple compounds bring a great potential in supramolecular chemistry not only as metal cation complexing properties, but also due to photosensitivity. There are also known crown ethers with azo group located at the periphery of the molecule with brilliant example of so called “butterfly crown ethers” obtained and investigated by Shinkai et al. [ 141, 142 ]. These photo-switchable compounds were used for light-driven transport of potassium and sodium. Figure 16b shows the scheme of light-driven transport of potassium cations across organic bulk membrane with the use of photoresponsive azobis(benzo-15-crown-5) 25. These early works on azo group bearing crown ethers inspired further development of synthetic methods, challenging functionalization, and studies (both experimental and theoretical) of properties and finally applications of macrocyclic polyethers. Crown ethers with inherent azobenzene group(s) Among the first synthesized crown ethers with azo unit incorporated into the macrocycle were so called “all or nothing” crown ethers exemplified by 26 (Fig.  17) obtained by Shinkai et al. [ 141, 143 ]. These photoswitchable compounds form complexes with metal cations with affinity that depends on the geometry of azo group. The cis isomer obtained by illumination binds cations, whereas in the dark the cation is released due to decreasing the cavity size being a consequence of isomerization to trans form. The spectral behavior of “all or nothing” crowns of different size of the macrocycle and their ability to form complexes with alkali metal cations was later studied theoretically using density functional theory (DFT) [144]. The results showed good agreement between experimental and computational attempts. Computational methods were also used by Wang and co-workers [ 145 ] to study trans-azobenzene embedded N-(11-pyrenyl methyl)aza-21-crown-7 27 (Fig.  18) as a fluorogenic receptor for alkaline-earth metal cations. According to density functional theory using B3LYP/631G(d) it was determined that the ether chain of trans isomer of the compound becomes almost a straight line forming a strip crown ring. Calculated structure of cis isomer shows cavity enables coordination of metal cation inside the macrocycle. The optimized structures of complexes of the host n O O N O N trans (E) isomer * hv1 hv2 O n O O N N cis (Z) isomer Fig. 18 Azobenzene embedded N-(11-pyrenyl methyl)aza-21crown-7, 27 studied by DFT by Wang and co-workers [ 145 ] molecule and alkaline earth metal cations (Mg2+, Ca2+, Sr2+, and Ba2+) indicate that the ligand binds calcium cations the strongest due to the best match of ion radius to the cavity size. These results showed, that proposed system can act as molecular device of double function. Tamaoki and  co-workers [ 146 ] studied the effect of trans–cis isomerization of [5.5](4,4′)azobenzeno(1,5)naphthalenophane 28 (Fig. 19) on silver(I) complexation. The resolved crystal structure of 1:1 complex showed that two silver cations are complexed to form dimeric structure with azobenzenonaphthalenophane in trans form (Fig. 19 left). 1H NMR studies showed that complexation of silver cation is controlled by reversible trans–cis isomerisation of azo moiety; photoisomerization of trans to cis isomer causes the cleavage of the π–cation interaction. The opposite change was found under reverse isomeriation (Fig. 19, right). Kirichenko  and co-workers [ 147 ] described synthesis and complexing properties of four crownophanes 29–32 (Fig. 20) containing 2,7-dioxyfluorenone and 4,4′-azobiphenoxy groups joined with di-, tri-, tetra-, and pentaethylene glycol moieties. Based on NMR, UV–Vis, and X-ray data it was concluded that all macrocycles exist in solution and in solid state in trans-configuration of azobenzene unit. The trans to cis isomerization of 30 can be achieved by UV-light (365 nm) irradiation. Macrocycles 30–32 bind 4,4′-dimethylbipyridinium (paraquat) bis(hexafluorophosphate), an electron-deficient model compound. The derivatives of this compound are used in synthetic procedures leading to interpenetrating complexes (pseudorotaxanes). Complex formation of paraquat with macrocyles is based on π–π interactions between π-donor aromatic moieties of cyclophanes and π-acceptor dipyridinium core of the guest. 1H NMR and MS measurements showed the formation of 1:1 inclusion complexes of pseudorotaxane type. The stability of complexes changes in the order: 31 > > 30 > 32. The smallest macrocycle 29 does not complex the guest due to lack of complementarity between size of the guest and cavity of the host. O O O n O Fig. 20 Crownophanes 29–32 bearing 2,7-dioxyfluorenone and 4,4′-azobiphenoxy groups synthesized and studied by Kirichenko and  co-workers [ 147 ] showing binding ability of dimethylbipyridinium (paraquat) bis(hexafluorophosphate) Described by Takagi’s and Biernat’s groups 13- and 16-membered crown ethers 22 and 23, as it was stated earlier, form complexes with metal cations. The X-ray structure of complexes of 13-membered crown with lithium bromide [ 148 ] and sodium iodide [ 149 ] were described. Metal cation complexes of larger, 16-membered crowns were also obtained. In solid state 16-membered crown forms sodium complex of 1:1 stoichiometry [ 150 ] while with potassium salt sandwich type 2:1 (crown:ion) complex [ 151 ] is created. In all cases the azo group is in trans configuration. It was also shown that the analysis of crystal structures of complexes of azobenzocrown ethers with alkali metal cations can be helpful in interpretation of the selectivity of ion-selective electrodes doped with particular macrocyle [ 152 ]. The X-ray structures of uncomplexed trans isomers of crown 22 and 23 were also investigated [ 153 ]. In the unit cells there are two independent molecules 22A and 22B or 23A and 23B (Fig. 21). The kinetics of the buildup and decay of photoinduced birefringence of crown ethers with inherent azo groups 21–24 (Fig. 16a) of different size of the macrocyle was investigated in poly(methyl methacrylate) matrix [ 154 ]. For all cases it was found that the kinetics of the buildup Fig. 19 Left: crystal structure of dimeric AgI complex E-28. Right: schematic illustration of photoresponsive cleavage/binding of cation-π bond. Numbering of compound in the reproduced material corresponds to number of compound in this work: 1 = 28. Adapted with permission form [ 146 ]. Copyright 2010 American Chemical Society. (Color figure online) Fig. 22 16-membered crown 33 and voltammograms for 22-monolayers LB films on ITO, based on this macrocyclic compound, electrode in solutions of KCl, NaCl and LiCl (0.1 M) registered with a sweeping rate of 50 mVs−1. Reprinted from [ 155 ]. Copyright 2009 with permission from Elsevier O O O O N N 33 azocrown ether with naphthalene moiety in polyether linkage of the birefringence was suitably described by a sum of two exponential functions, the time constants (being function of the pumping light characteristic) and sample thickness. The dark decays were described the best by the stretched exponential function, with the characteristic parameters (time constant and stretch coefficient) being practically independent of the type of crown ether. The time constants of the signal decay were orders of magnitude shorter than the respective constants of the dark isomerization of the azo crown ethers. Thus it indicates that the process controlling the decay was a relaxation of the polymer matrix and/or a rearrangement of the flexible parts of the crowns. The introduction of the azo group into compounds results not only in photoresponsive but also redox active properties. An example can serve 16-memebered crown 33 (Fig. 22) [ 155 ] with naphthalene joined by two oxyethylene chains. This macrocycle was used for the preparation of Langmuir–Blodgett (LB) film deposited onto solid ITO substrate. The complexation of metal cations on these electrodes can be successfully observed by cyclic voltammetry (CV). Figure 22 shows CV obtained for a 22-monolayers LB film on an ITO electrode in solutions of KCl, NaCl and LiCl (0.1 M). Bare ITO shows no redox peaks in the presence of K+, Na+ or Li+ ions. For the LB film based on crown 33 film, H17C8 O N N O O O O O O N N C8H17 Fig. 23 29-Membered diazocrown 34 showing electrochemical response towards potassium cations [ 156 ] an electrochemical response in the presence of metal salts was observed. The change of observed signal was attributed to the specific interactions between the film and the metal ions. The peaks in voltammograms can be ascribed to the electro-reduction of the azo moiety to the hydrazo group, which consumes two electrons and two protons according to the overall reaction. The strongest effect was observed in the presence of lithium cation, showing the possibilities of its electrochemical sensing. Similar experiments were performed for a number of macrocyclic compounds, e.g. larger 29-membered macrocyle 34 (Fig. 23), bearing two n-octyl substitutents in benzene rings and two azo groups as a part of macrocycle [ 156 ]. Langmuir–Blodgett (LB) and physical vapor deposition (PVD) films on ITO showed electrochemical response towards metal cations. Cyclovoltamperometric curves registered for LB films of 29-membered compound 34 point out that among alkali metal cations Li+, Na+ K+, potassium ion was preferentially complexed under applied conditions suggesting the best host and guest size complementarity. The selectivity of crown ethers and other host molecules towards metal cation can be controlled also by changing the type of donor atoms. 16- and 18-membered azo- and azoxythiacrown (forming next to azo compounds) ethers 35–40 (Fig. 24, right) were obtained in satisfactory yields by Kertmen and Szczygelska-Tao [ 157 ] using reductive macrocyclization procedure. Thiacrowns were tested as ionophores in ion-selective, graphite screen printed electrodes. Opposite to their oxygen analogs, sulfur containing compounds preferentially supposed to form complexes with softer metal cations. All electrodes doped both with azo- and azoxythiacrowns 35–40 (Fig. 24) showed high sensitivity towards heavy metal cations. The effect of softer sulfur donor atom in the skeleton of macrocycles on the response of ISE with membrane doped with 35–40 can be visualized by comparison of the order of potentiometric selectivity of thia-crown and its oxaanalog [ 137 ], shown in Fig. 24 (right, in a frame). Potassium selectivity of electrodes based on derivatives of 16-membered crown ether 23 was well-proved over years of working with ISEs. 13-membered azobenzocrowns, O S derivatives of compound 22 (Fig. 16a) are sodium ionophores [ 134, 136–138, 158–160 ]. To improve the characteristic of the sodium and potassium sensors, important for clinical analyses, new derivatives of both 13- and 16-membered crowns were prepared and at the same time new technical solutions, including miniaturization of the sensors, were applied. Recently, a series of bis-(azobenzocrown) s (compounds 41–48, Scheme 4) based on the skeleton of parent 13- and 16-membered crowns 22 and 23 (Fig. 16a) linked by α,ω-dioxaalkane chains between two macrocycles have been obtained [ 162 ]. Bis-crowns were synthesized from the respective hydroxyazobenzocrowns obtained in reaction analogous to Wallach rearrangement elaborated by Luboch [ 161 ]. The unique structure of intermolecular of 2:2 stoichiometry sandwich-type complex of bis-(azobenzocrown) 41 with sodium iodide was obtained [ 162 ]. It is presented in Fig. 25. Bis-(azobenzocrown)s 41–48 were used as ionophores both in classic and miniature, all-solid  state, screenprinted, graphite ion-selective electrodes. New sodium and potassium sensors feature by short response times, stable potential and high selectivity, in particular high K/ Na selectivity. Bis-(azobenzocrown)s 41–48 form complexes with metal cations also in acetonitrile. The increase of stability N O N n O Scheme 4 Synthetic route for preparation of bis-(azobenzocrown)s 41–48 from hydroxyazobenzocrowns as substrates [ 162 ] Fig. 25 Two projections of macrocyclic cation [Na2(trans-41)2]2+ in 41-NaI complex with a partial labeling scheme. Reprinted from [ 162 ]. Copyright 2012 with permission from Elsevier constant values comparing analogous monocrown bearing alkoxy substituent proves beneficial effect of the presence of two binding sites in one molecule. Another example of biscrowns are diester derivatives of dodecylmethylmalonic acid joining two 13-membered azobenzocrown moieties obtained in Luboch group [ 163 ] (compounds 49 and 50, Scheme  5). Biscrowns were obtained using bromoalkoxy derivatives of azobenzocrowns [ 164 ] and potassium salt of dodecylmethylmalonic acid in ~ 40% yield. For comparative studies monoester derivative 51 was synthesized. For biscrowns 49 and 50 three isomers trans–trans, trans–cis and cis–cis can be considered. From 1H NMR spectra registered in d-acetone it was found that in solutions of 49 and 50 trans–trans and trans–cis isomers dominate representing altogether ~ 90% of the total amount of compounds. The presence of cis–cis isomer of 49 was observed upon irradiation with UV light. For monoester derivative 51 the ratio of trans to cis isomer was evaluated as 6:4. Trans–trans and trans–cis isomers of 49 and especially of 50, differ significantly in TLC properties. This can be associated with different complexation properties of both isomers [ 166 ]. Trans isomers of azobenzocrowns show higher affinity towards metal cations than cis forms. Thus trans–trans isomer is probable able to form intramolecular sandwich type complexes (Fig. 26) with metal cations whereas for trans–cis isomer rather intermolecular complexes are expected. This hypothesis finds confirmation in previously published works of the above authors and in articles published by other groups [ 149, 166, 167 ]. O O O N O N 51 O O N O N O H3C O Scheme 5 Synthesis of bis-(azobenzocrown)s 49 and 50, diesters of dodecylmethylmalonic acid and monoazobenzocrown 51 [ 163 ] Formation of sodium complex by trans–trans isomer of 49 was confirmed also by 1H NMR measurements. Stability constant value of (1:1) complex of 49 in acetone was estimated as logK ~ 3.0 from UV–Vis titrations. Bis-crowns 49 and 50 based on 13-membered rings, were tested as sodium ionophores in classic and miniature, solid contact: screen-printed and particularly glassy carbon membrane ion-selective electrodes. Plasticizers 2-nitrophenyl octyl ether (o-NPOE) and more lipophilic di(2ethylhexyl) sebacate (DOS) can be successfully used for bis(azobenzocrown) containing membranes. It was proved that possible isomerization under usual conditions does not significantly affect the characteristics of the prepared electrodes. The influence of UV irradiation on the properties of glassy carbon electrode with ionophore 49 is shown in Fig. 27. After exposition to UV light (1 h, 365 nm), the electrode regains its properties practically after 2 h conditioning in NaCl solution. Fig. 26 Proposed organization of biscrown 49 sodium cation complex. Reprinted without changes from [ 163 ]. Copyright 2016 with permission from Springer Publishing Company (http://creativecommons.org/licenses/by/4.0/) (a) 0.00 -0.05 -0.10 -0.15 -0.20 X ,-0.25 N-2.0 a K log -2.5 -3.0 -3.5 -4.0 49 gc 49 gc(UV) shortly after exposure 49 gc(UV) after 1h 49 gc(UV) after 20h Electrodes with the tested biscrowns 49 and 50 were found to have better selectivity coefficients KNa/K than the electrodes with the monocrown 51. The best selectivity coefficient Na/K was achieved for the screen printed graphite electrode with the addition of carbon nanotubes into the membrane (50 as the ionophore, logKNa,K = − 2.6). No significant differences were also observed between the selectivities of the classic and solid contact electrodes. In the last case lower detection limits (LDL) may be obtained. The membrane doped with carbon nanotubes deposited onto graphite screen-printed electrodes results in the better potential stability, detection limit and selectivity of biscrown-based electrodes. The electro-conductive material was introduced directly into the membrane in a manner analogous to that proposed by Ivaska and co-workers [ 168 ]. For glassy carbon electrodes to improve the conductivity, between the membrane and glassy carbon the conductive PEDOT/PSS polymer blend was introduced by electropolymerization. Such electrodes have better (lower) LDL than plain glassy carbon electrode. Electrodes with ionophores 49 and 50 characterize with response times not longer than 10 s, illustrated in Fig. 28 for membrane electrode doped with 49. Electrodes based on 49–51 characterize by stable potential in a wide range of pH, depending on the type of the used plasticizer, e.g. electrodes with compound 49 and DOS show stable potential in the pH range 2–10 (0.1M NaCl). Proposed sodium sensor (based on 50) fulfills requirements for electrodes used in clinical analysis [ 169 ]. The response of electrodes based on 50 for sodium in the presence of interfering metal cations corresponding to their blood plasma levels are shown in Fig. 29. The electrodes were tested for sodium in blood plasma giving consistent results with independent measurements carried out in clinical analytical laboratory. (b) 60 50 40 30 20 10 0 -10 mV/dec log a 49 gc 49 gc(UV) shortly after exposure 49 gc(UV) after 1h 49 gc(UV) after 20h LDL (loga) S [mV/dec] Fig. 28 Response time of glassy carbon electrode with membrane with ionophore 49 (o-NPOE as plasticizer) A 0.9 mL of NaCl solution (0.1M) was injected to 100  mL of NaCl solution (10−4  M), B  0.9  mL of NaCl solution (1M) was injected to 100  mL of NaCl solution (10−3 M). Reprinted without changes from [ 163 ]. Copyright 2016 with permission from Springer Publishing Company (http://creativecommons.org/licenses/by/4.0/). (Color figure online) Crown ethers with peripheral azo group The interactions between photoswitchable azobis-(benzo18-crown-6) and alkaline earth metal cations were studied by DFT and reactive molecular dynamics (reactive MD) by Pang et al. [ 170 ]. Optimized structures of complexes revealed that in the case of Ba2+ complex the distance between two cations is the largest among tested complexes in their trans form, and the shortest among cis complexes. Macrocycles become face-to-face when complexing Ba2+ ions. Small energy difference between Ba2+ complex in its trans and cis form indicates facile cis to trans thermal conversion. Calculation the Ba2+ complex allows to conclude that it is a suitable candidate for photocontrolled catalysis. To mimick the structure and function of biological ion channels the light-regulated transmembrane system was proposed by using tris(macrocycle) system based on diaza-18crown-6 joined by azobenzene photoswitchable moieties 52 (hydraphile 1, Fig. 30) [ 171 ]. The liposome-based ion transport assays revealed that compound 52 displays an efficient transmembrane activity with Ymax around 0.7 at 40 μmol/L of 52 in DMSO. Due to the presence of azobenzene moieties the potassium ion transport by the molecule across bilayer membranes can be regulated by applaying of external source of light. The photoisomerization of azo groups induces changes of transmembrane length of the ion channel and this way regulating the efficiency of the ion transport. In many chemical and photochemical processes donor–acceptor complexes (D-A complexes) play an important role. Such systems are also investigated as organic conductors and photoconductors that find applications in nonlinear optics. D–A complexes of a series of bis(crown) stilbenes, and also of bis(crown)azobenzene with salts of alkylammonium viologen derivatives were studied in solution and in a solid state by Gromov and co-workers [ 172 ]. X-ray structure of complex of bis(18-crown-6)azobenzene 53 (Fig. 31) with viologen derivative 54 showed that the central parts of donor and acceptor molecules feature planar geometry. The proposed systems can be used for the design of optical sensors and molecular devices. Fig. 29 Response curves for Na+ obtained with ISEs based on ionophre 50 a graphite screen-printed electrode b glassy carbon electrode. Curve A indicates the response for Na+ without and curve B Na+ in the presence of interfering ions (4.2 mM K+, 1.1 mM Ca2+, 0.6 mM Mg2+). Reprinted without changes from [ 163 ]. Copyright 2016 with permission from Springer Publishing Company (http://creativecommons.org/licenses/by/4.0/) O O O O N N O O N N O O Colorimetric and spectrophotometric ion receptors Molecular recognition can be utilized in many branches of science and technique if the information about host–guest interaction could be converted into analytically useful signal, e.g. optical or electrochemical. Optical signaling in the visible range of the electromagnetic spectrum draws special attention because it enables non-instrumental sensing of various chemical species such as ions or neutral molecules, e.g. for monitoring of ions of biological or/and environmental importance. The receptor molecule besides binding site should be equipped with additional signaling unit, a functional group joined via linker or chromophoric/fluorophoric moiety forming an integral part of the molecule. Schematically, the idea of chromo- and fluorogenic molecular receptors is shown in Scheme 6. The mechanism of sensing depends on the nature of both the host and the guest. The binding mode, selectivity and sensitivity can be also influenced or controlled by the effect of the solvent and/or receptor immobilization on solid surfaces of various properties. Inter alia functionalized macrocyclic compounds bearing azo moiety belong to this relatively popular group of sensing materials. O O + N O O O O In the case of para- and ortho- hydroxyderivatives of azocompounds the color signaling mechanism may be associated with the change in the tautomeric equilibrium upon complexation. This is well illustrated by tautomeric switch based on functionalized azacrown ether 55 (Fig. 32) synthesized and investigated by Antonov and co-workers [ 173 ]. Uncomplexed ligand in acetonitrile exists in azophenol form stabilized by intramolecular hydrogen bond between phenolic OH group and nitrogen atom of crown ether residue. In the presence of alkali and alkaline earth metal cations— the color of the solution turns from yellow to orange–red, what is a result of bathochromic and hyperchromic effects in UV–Vis spectra. The complex formation is connected with the shift of the tautomeric equilibrium towards ketone (quinone-hydrazone) form. Metal cations are complexed by ether oxygen donor atoms and by carbonyl oxygen atom of ketone form. Lithium and sodium cations form complexes of 1:1 stoichiometry with azacrown 55 (Fig. 33). For magnesium and calcium initially 1:1 complex is formed. Under an excess of a metal salt 2:2 complex dominates. Direct 2:2 complex formation was found for barium perchlorate. Absorption spectra of azacrown registered in the presence of metal perchlorates are shown in Fig. 33a. In Fig. 33b the values of the stability Scheme 6 Schematic: the idea of chromo- and fluorogenic molecular receptors Fig. 32 The mechanism of color change of azacrown ether modified with 4-(phenyldiazenyl) naphthalen-1-ol 55 synthesized by Antonov et al. exemplified by sodium complexation [ 173 ] constants of 1:1 and 2:2 metal complexes with discussed azacrown 55 are presented. Aza-15-crown-5 56 (Fig.  34) skeleton is a hopeful building block for colorimetric sensors. Lincoln and Sumby [ 174 ] used this macrocyle to synthesize N-[4(phenyldiazo)benzenesulfonyl]-aza-15-crown-5 57 (Fig. 34). This chromogenic compound was obtained in 55% yield by treating commercially available 4-phenyldiazobenzenesulfonyl chloride with aza-15-crown-5 in DMF in the presence of triethylamine. The synthesized lariat ether was studied as metal cation reagent in ethanol–water (75:25 v/v, pH 6.66) mixture. The stability constant values of 1:1 complexes of sodium and potassium cations with 57 are higher than for the parent aza-15-crown-5 56 (Fig. 34) and its derivatives [ 175–178 ]. The solved X-ray structure of [Na(57)(H2O)]2(ClO4)2 complex showed that it is a dimer with the sulfonamide oxygen atom engaged in cation complexation. This indicates the cooperation of sulfonamide side arm and crown ether moiety in ion binding and explains the higher values of the stability constant compared with data for unsubstituted aza-15-crown-5. The selectivity of metal cation binding can be controlled by using macrocycles with softer, sulfur donor atoms. Lee and Lee [ 179 ] synthesized, under high dilution recognition of phosphoric moieties of ATP is provided by dipicolylamine—Cu2+ unit. These multipoint interactions are probably responsible for high selectivity of ATP recognition. Photoswitching properties of azobenzene make it an interesting candidate for controllable drug therapy. For example, azobenzene units were used in fabrication of a triple-layer nanocomposites tested in vitro anticancer therapy as a drug delivery system [ 251 ]. The single particle consists of gold nanobipyramids (the core), mesoporous silica nanoparticles (the middle layer), and hyaluronic acid functionalized with α-cyclodextrin and azobenzene. Inside the silica pores anticancer drug—doxorubicin is loaded. Experiments carried out for human squamous carcinoma cells (representative cancer cells) and human keratinocyte cell (representative normal cells) revealed that these nanocomposites are able to specifically accumulate around the tumor tissue due to noncovalent interactions between hyaluronic acid and CD44 receptor overexpressed in cancer cells. Localized irradiation N 185 O N O O O HN O Fig. 82 A gate synthetic ion channel based on cyclodextrin as a channel and azobenzene moiety as a gate. Reprinted with permission from [ 248 ]. Copyright 2008 American Chemical Society Fig. 83 Top: Synthetic procedure for 186, bottom: A UV–Vis spectra and B  photographs of 186 (0.03  mM) with amines (1000 equiv) in CHCl3: a 186, b 186 + n-octylamine, c 186 + di-n-butylamine, and d with near-infrared light (780 nm) converts cis-azobenzene to its trans isomer what leads to hydrogel formation due to noncovalent interactions between α-cyclodextrin and transazobenzene. Thanks to the presence of specific enzyme— hyaluronidase around the tumor cells the network in the hydrogel is degraded resulting in the anticancer drug release and its transport to the cancer cell nuclei. The association of artemisinin (ART) 188 with an azobenzene bridged bis(β-CD) derivative with an azobenzene 6–6′ linker 189 in aqueous solution was investigated by circular dichroism (CD) spectroscopy (Fig. 85) [ 252 ]. It was shown that bis(β-CD) with trans-azobenzene unit binds artemisinin (1:1 complex) and this process can be light controlled. Upon irradiation at 363 nm trans–cis isomerization causes loss of the binding ability of artemisinin. 186 + tri-n-butylamine. Reprinted with permission from [ 249 ]. Copyright 2006 American Chemical Society H2N N N H O N N N HO O Fig. 84 Complex of cyclodextrin derivative 187 with ATP [ 250 ] Polymers bearing macrocycle(s) and azo motif(s) Azo derivatives found a vast range of applications in polymer science. Polymers containing azobenzene moiety have been intensively studied due to their photoresponsive properties ensuring the obtainment of functional materials. An azo group can be a part of a supramolecular system in polymer matrix (non-covalent interactions) or can be covalently bound within a polymer chain. Polymers, responding to light irradiation are widely investigated systems due to reversible (or irreversible) changes of physical properties [ 253 ]. This can be utilized in many branches of science. The change of the polymer properties can be very often achieved by using molecules, which act as molecular containers. Elegant molecules of such properties are cyclodextrins, described ealier, which can be also used for design and synthesis of functional polymers. A system utilizing noncovalent interactions between synthesized in a click reaction AZO-β-CD (Fig. 86), which interacts as a “dimer” with azo bearing polyester obtained in reaction of ε-caprolactone with p-aminoazobenzene (AZOPCL) (Fig. 86a), was described by Ma et al. [ 254 ]. It was suggested that in aqueous solution micellar aggregates are formed due to host–guest interaction between (AZO-β-CD) and AZO-PCL (Fig. 86b). On the basis of 1H NMR spectra it was suggested that the guest molecule in its trans form is included shallowly into cyclodextrin cavity from its wider site. After UV-light irradiation the transparent opalescence solution becomes turbid, what is a result of decomplexation followed by disaggregation. The uniform vesicles are reformed upon exposure of the solution to visible light. Authors propose possible use of the system in the control or release of drugs. Photosensitive hydrogel based on α-CD, dodecyl-modified poly(acrylic acid), and a photoresponsive competitive guest [ 255 ] inspired further studies of self-assembling systems with polymer side chains. Poly(acrylic acid)s (pAA) with p3αCD and p6αCD functionalities and pAA carrying azobenzene moieties (pC12Azo), were used for the construction of the photoresponsive system based on polymer–polymer interactions (Fig. 87) [ 256 ]. The properties β–CD O N N 189 O β–CD H3C O O Fig. 85 Azobenzene bridged bis(β-CD) 189 and the formula of artemisinin 188 [ 252 ] of obtained systems were studied in details among others by steady-shear viscosity (η) measurements. The method was chosen because the interaction of the CD polymers with pC12Azo (formation of inclusion complexes of CD moieties in the CD polymers with side chains of guest polymers) may cause an increase of solution viscosity. The mixture of the p3αCD/pC12Azo and p6αCD/pC12Azo has shown contrast η changes upon photoirradiation: decrease in the case of the p3αCD/pC12Azo mixture, and increase of η for p6αCD/ pC12Azo mixture. Irradiation with visible light causes the reverse process in the above cases, i.e. η values became similar to those before the UV exposure. The differences in η values were explained by the fact that UV light causes dissociation of inclusion complexes for the p3αCD/pC12Azo mixture, and the formation of interlocked complexes for the p6αCD/pC12Azo mixture (Fig. 87, bottom). PEG-substituted CD with an azobenzene residue at the end of the PEG chain (6-Az-PEG600-HyCiO-β-CD) was obtained by Harada and co-workers (Fig. 88) [ 257 ]. The photochemically and thermally induced conformational changes in aqueous solutions were studied by 1D and 2D NMR analyses. It was found that at low concentration, 6-trans-Az-PEG600-HyCiO-β-CD forms different types of intermolecular, self-inclusion complexes or exists in an uncomplexed form depending on the temperature. An intermolecular complex is formed at high concentration. Regardless of the concentration, irradiation by UV light promotes complexation with the CD including the azobenzene part. The attaching of azobenzene groups to side-chains of liquid crystalline polymers results in light-controllable polymer materials. Such films and coatings can be applied for example as optical molecular devices. One group of such materials are derivatives of crown ethers bearing residues able to form liquid crystalline (LC) phases. Complex formation by crown ether moiety can lead to the appearance or disruption of supramolecular structures. A series of photochromic azobenzene-crown-containing compounds forming crystalline and nematic phases were described by Shinkai and coworkers [ 258 ]. Photochromic crown ether-containing LC homopolymers and copolymers based on azobenzenes were later described also by Bobrovsky and co-workers [ 259 ]. The complexation of metal ions by these compounds cause the decrease of clearing temperature and sometimes the transition into the amorphous state. The investigation of the relationship between molecular architecture of this type polymers and their photo-optical properties and phase behavior was the main scope of studies. Bobrovsky and co-workers [ 260 ] described among others the synthesis and properties of two types of polymers differing in the position of the crown ether in relation to the photoresponsive azobenzene residues (Fig. 89). Macrocyclic moiety was linked directly to chromogenic residue (Fig. 89, left) or via carboxymethylene spacer (Fig. 89, right). Phase behavior, spectral properties Fig. 87 Top: p3αCD and p6αCD and pAA carrying azobenzene moieties (pC12Azo) used for studies of self-assembly. Bottom: schematic representation for interactions of CD and azo moieties upon irradiation with UV and visible light for (a) p3αCD/pC12Azo, (b) p6αCD/ pC12Azo. Reprinted with permission from [ 256 ]. Copyright 2006 American Chemical Society and kinetics of photo-orientation processes inside thin films of polymers shown in Fig. 89 were found to be dependent on the location of crown ether with respect to the residue bearing azo group. In the case where a crown ether was introduced as separated non photochromic side group the decrease of the degree of photoinduced orientational order was found. Complex formation with potassium ions by compound shown in Fig. 89 (left) results in the decrease in degree of the photoinduced order. Possible application in the creation of new sensing materials was suggested. Zhu and co-workers [ 261 ] described linear 190 and 191 and cyclic 192 and 193 (Scheme 23) amphiphilic polymers containing azobenzene moieties. Macrocyclic polymers were obtained in Cu(I)-catalyzed azide-alkyne cycloaddition to achieve intramolecular macroring closure process, one of the most popular and powerful “click” synthetic reaction [ 262 ]. According to the obtained results, azomacrocycles exhibit increased glass transition temperatures, faster trans–cis–trans photoisomerization, and enhanced fluorescence intensity in comparison to their acyclic analogues. In water:THF mixture (1:1, v/v) both macrocylic and linear polymers self-assemble into spherical nanoparticles. The size of aggregates formed by cyclic compounds are significantly Fig. 88 Top: 6-Az-PEG600-HyCiO-β-CD, Bottom: proposed conformational changes of 6-Az-PEG600-HyCiO-β-CD in aqueous solutions by external stimuli. Reprinted with permission from [ 257 ]. Copyright 2007 American Chemical Society smaller than those of corresponding linear analogues due to more dense and compact packing of macrocyle-bearing particles. Alternating irradiation of nanospheres with ultraviolet (365 nm) and visible (435 nm) light causes isomerization of the azo group located in polymer main chain. This induces reversible shift of the hydrophilic-hydrophobic balance of macromolecules and leads to the dissociation and reaggregation of the particles. The photoresponsive behavior is slower for nanospheres containing cyclic polymers than in the case of particles with materials of linear structure. The skeleton of the photoresponsive polymers also can be enriched with other functionalities that for example are able to form complexes with metal cations. Wiktorowicz et al. [ 263 ] prepared polymers comprising dibenzo-18-crown-6 moieties joined by azo bridges 194, 195 using reductive coupling procedure (Fig. 90). Spectrophotometric measurements showed that the polymers are pH-sensitive and exhibit solvatochromic properties. Alternating irradiation of the polymers with UV and visible light induces reversible trans–cis–trans photoisomerization. Due to the presence of crown ether cavity, the described polymers interact with Ba2+ ions and also with low molar mass pyridinium type guests, leading to complex-induced phase separation in solvents of lower polarity. In alcohols the polymers reveal thermo-responsive behavior exhibiting the upper critical solution temperature type transitions. This effect depends on the polymer concentration and the degree of polymerization. Cozan and co-workers [ 264 ] described the preparation of copoly(ether sulfone)s with azocrown ether and fluorene fragments. The polymers 196–200 (Fig. 91) showed good solubility in solvents of different polarity. Thermogravimetric analysis showed the lowest thermal stability of the copolymer 196 among all investigated polymers as it contains only azo-crown ether units that are sensitive to thermal degradation. The insertion of fluorene moieties into a polymer chain significantly enhances thermal stability. The trans to cis isomerization of the polymers in DMSO occurs after irradiation with UV light (at 375 nm). The rate constant of the first order photoisomerization increases with decreasing the number of azobenzene units. It was also found that complexation of K+ inside the macrocyclic cavity increases trans to cis isomerization rate. Photo-induced structural transitions of azo compound bearing dibenzo-24-crown-8 (DB24C8) moiety, dibenzylammonium salt (DBA), and 1,2,3-triazole groups were tested by Dong et  al. [ 265 ]. Due to host–guest interactions between DB24C8 and DBA from separated molecules linear supramolecular polymers of 1:1 threaded structures (pseudorotaxanes) are formed. The presence of azobenzene moiety allows to control the complex formation, as trans-azobenzene-appended DBA interacts with DB24C8 stronger than its cis isomer. After addition of [PdCl2(PhCN)2], 1,2,3-triazole rings of different polymer chains are linked together by the metal coordination, what leads to the formation of cross-linked supramolecular polymers. In dichloromethane the cross-linked assemblies have a form of red gel. UV irradiation (365 nm) of linear and branched polymers induces trans to cis isomerization, resulting in weaker host–guest interactions and, in a consequence, dissociation of supramolecular polymers. The structural change of cross-linked supramolecular polymer is manifested by naked eye observable decrease of viscosity. The reformation to the gel state is achieved by exposing the solution to visible light (430 nm). The photoisomerization of a series of macrocyclic oligomers containing azobenzene moiety in the main chain and their linear analogs was studied by Zhu and co-workers N3 N N N O O O O 4 Fig. 90 Structure of poly(azodibenzo-18-crown-6ether)s 194 and 195 [ 263 ] _ O 5 O O O O H2N H2N O O O O O O O O O 1-x n-1 O O O O N N N O 4 5 nN N 194 mN N 195 O O O O O O N N O O O O N N O O O O n O O O O NH2 4',4" NH2 4',5" _ O x_ n N N Cu(I)-catalyzed azide-alkyne cycloaddition N N N N O S O O O O O O O O O O O O Scheme 23 Linear 190 and 191 and cyclic 192 and 193 photoresponsive polymers described by Zhu and co-workers [ 261 ] [ 266 ] (Scheme 24). Tetraethylene glycol (TEG) was chosen as the building block for the preparation of amphiphilic polymers of good solubility. According to the UV–Vis spectrophotometry it was shown that the trans to cis and reverse process are the first order reactions for both linear 201 and cyclic 202 compounds. The estimated values of rate constants for macrocyclic oligomers are distinctly higher (for trans to cis isomerization) and slightly higher (for cis to trans isomerization) in comparison with results for linear ones, especially for n = 1. This can be explained by the more stable conformation of cyclic cis-azobenzene than linear trans analog. In turn, poly(ethylene glycol)methyl ether was used as building block for other amphiphilic copolymers with cyclic azobenzene unit 203 [ 267 ]. For comparative purposes a linear analog 204 was also obtained. The synthetic route is shown in Scheme 25. The obtained copolymers assemble in phosphate buffer solution (pH 7.4) into stable vesicles with hydrophobic blocks containing the azobenzene moieties aggregated in the membranes of the vesicles, and the hydrophilic PEG arrangements on the outer and inner surface of the vesicles. Due to presence of azo moiety the obtained polymers are not only photoresponsive, but also sensitive towards reducing reagents. These properties were used for the investigation of the encapsulation and release of Nile Red (NR- a model compound for drug delivery system) and anticancer drug doxorubicin (DOX). NR-loaded vesicles are fluorescent. The intensity of fluorescence can be controlled by illumination with UV light (365 nm). The reverse cis–trans process occurs upon irradiation with visible light at 435 nm. Azo compounds can be reduced by azoreductase or popular reducing reagents, such as for example sodium dithionite. The result is azo bond cleavage, which can be used in effective drug transport. DOX-loaded vesicles were investigated in reductant-release of the encapsulated substance. The release rate of DOX from cyclic polymer 203 is higher compared with the linear analog 204. This points out the importance of investigated copolymers—particularly cyclic compounds—as potential agents in the treatment of colon disease. Combination of cyclodextrin and azobenzene units bearing polymer were used by Winnik and co-workers [ 268 ] to obtain molecular “charm bracelets”. The cyclic poly(Nisopropylacrylamide) with azobenzene inserted in the main chain 205 (Fig. 92) was synthesized by the “click” ring closure of α-azobenzene ω-azido poly(N-isopropylacrylamide) in the form of inclusion complex with α-cyclodextrin. UVlight irradiation of aqueous solution of 205 (at 365  nm) induces motion within the molecule as cis-azobenzene unit obtained upon photoisomerization, due to its size, is expelled from the α-cyclodextrin cavity pushing the host to the other sections of the polymeric ring. The trans to cis photoisomerization does not affect the temperature of phase transition of the polymer, whereas in the case of the analog without cyclodextrin the temperature increases by 1.7 °C. This may be explained assuming that the enhanced polarity due to trans to cis isomerization in polymer 205 is overshadowed by the strong hydrophilicity of α-cyclodextrin moiety interlocked along the polymer ring and no change of the phase transition temperature is observed. O N N 203 N N 204 O O O 11 O O 11 O O O45 O 17 Br Br 19 O 202 O 2 2O O O O 2 2 O OH O OH O Doxorubicin (DOX) N NN n-1 N N O O Nile Red (NR) O N N N N O PEG-Br 45O Scheme 25 Poly(ethylene glycol)methyl ether based amphiphilic copolymers with cyclic azobenzene unit 203 and its linear analog 204 [ 267 ]. The chemical formulas of doxorubicin and Nile Red are also shown O O O 5 O O O O 201 N NN n-1 N N Cu, TMEDA,Et3N, acetone high dilution cond. N N N N Scheme 24 The synthesis of the molecularly-defined linear 201 and cyclic oligomers 202 (n = 1–6) [ 266 ] O Br CuBr, PMDETA O O O11 O NN O CH3O NN O 11 O N N N N N N CH3O Miscellaneous macrocyclic systems bearing azo group(s) Molecular containers such as pillarenes are promising blocks for building of photoswichable assemblies. Ogoshi et al. [ 269 ] obtained supramolecular polymers consisting of trans-azobenzene-bridged pillar[ 5 ]arene  dimer 206 and bispyridinium cations linked by hexamethylene unit 207 (Fig. 93). On the basis of 1H NMR spectroscopy it was stated that in dichloromethane at low concentration (2 mM) complexes of 1:1 stoichiometry are formed, in which pyridinium cation moiety 207 is included inside the cavity of 206. At higher concentration (100 mM) supramolecular assemblies were detected according to DOSY 1H NMR experiments. Irradiation of diluted solution with UV light induces trans to cis isomerization of component 206. At the photostationary state the ratio of trans to cis isomer is 26:74. At high concentration nearly half of trans206 does not convert into the cis form. Efficient reverse process occurs after exposure to visible light (436 nm). Under equilibrium the ratio of trans to cis isomer is 93:7. It was demonstrated that photoisomerization from trans to cis form weakens the host–guest interactions, probably due to the steric hindrance caused by the cis isomer of 206. As a consequence, the created at high concentration, supramolecular polymers disassembly after UV-light irradiation. Photo-switching between assembly and disassembly of supramolecular system looks completely reversible by alternating irradiation between visible and UV light. Cavitands 208 and 209 (Scheme 26) bearing azo moiety integrated with macrocyclic [ 270 ] structure undergo trans–cis photoisomerization upon illumination with UV light (365 nm). Cis–trans conversion proceeds by heating to 164 °C for 5 min or irradiating with 450 nm light for 20 min. Trans–cis and cis–trans cycles can be repeated 5 times without degradation of the system. Both the trans isomers of 208 and 209 have deep cavities able to bind guest molecules. In fact, 208 and 209 were found to form complexes with small molecules of adamantane series in d12-mesitylene. The highest values of stability constants were found for 1-adamantanecarbonitrile and 2-adamantanone. It was explained assuming the possibility of stabilization of formed complexes by hydrogen bonding and polar interactions with the upper rim of the cavitands. The complexation of adamantane guests can be light controlled, namely irradiation controls uptake and release of guest for 208. Azo moieties can constitute part of macrocyclic Schiff bases, as for example fluorescent product of [2+ 2] condensation of N,N′-bis-(2-hydroxybenzaldehyde-5-yl)-benzeneFig. 92 Cyclic azo-poly(N-isopropylacrylamide) with interlocked α-cyclodextrin 205 and photoinduced molecular motion within the polymer [ 268 ] 1,3-diazene and benzene-1,2-diamine 210 (Fig. 94) [ 271 ]. The stoichiometry of complexes of 210 with zinc(II), copper(II) and nickel(II) is 1:2 (L:M) as showed by elemental analyses and spectral studies. Fluorescence spectra registered in DMSO showed quenching of fluorescence of Schiff base upon metal binding. Another example of azo derivative of Schiff base type can be chiral macrocycle 211 (Fig. 95), with three azobenzene residues [ 272 ]. This compound was obtained by [3 + 3] condensation reaction of enantiomerically pure trans 1,2-diaminocyclohexane with azobenzene-4,4′-dicarbaldehyde in dichloromethane. The subsequent sodium borohydride reduction of 211 produces macrocyclic hexaamine 212, also with three azobenzene units. Irradiation of chloroform solution of (R,R,R,R,R,R)-211 with 365 nm light for 30 min, causes the decrease of absorption peak at 348  nm and increase of bands intensity at 273 and 450 nm. The back process occurs upon leaving the solution at room temperature for 48 h. Similar observations were made for reduced analog (R,R,R,R,R,R)-212. Interesting properties were found for 211 dissolved in benzene. In this solvent a translucent and orange colored gel was obtained. Scanning electron microscopy O R' H N R' O N H R' HN HN R' O O O O Scheme 26 Synthesis of azo derivatives of cavitands 208, 209 and their complexation properties [ 270 ] O O R" R" R" R" O O O O O NH2 NH R' O N H O N R AcOH, 6 days, rt. R' R' = C H 2 5 R" = C11H23 O R' H N R' O N H R' HN HN R' O O O O O O R" R" R" R" O O O O O HOST MOLECULE N N 208. R = t-Bu 209. R = H NH R' O N H R H H (SEM) measurements of the obtained material showed the presence of elongated fibers with diameters of around 1 μm in the dried gel. The illumination of the gel with UV light for several hours led to gel–sol transformation. The reverse process occurs upon heating the sol. Macrocycle 212 forms inclusion complexes with various aromatic organic guest molecules. Complexes of 1:1 stoichiometry were found for benzene and toluene as the guests and 2:1 (212:guest) when o-, m-, and p-xylenes were complexed. This can indicate better complementarity of the host and benzene or toluene than in the case of larger xylene molecules. Not always photoinduced transformations are reversible. Red colored compound 213 (Fig. 96) obtained in reaction of 3,3′-dihydroxy-4,4′-bipyridine and azobenzene-2,2′dicarboxylic acid in dichloromethane is a highly-strained cyclophane 213 comprising azobenzene and methyl viologen units [ 273 ]. Cyclic voltammperometric measurements showed its unique irreversible electrochemical behavior. Trans–cis isomerisation upon visible light illumination of 213 is also irreversible. The photo- and redox properties of azo compounds can be extended to more sophisticated systems due to incorporation of transition metal cations into their structure. In such cases both the photoisomerization of azo compounds and chemical and physical properties of transition metal cations (optical, redox, magnetic etc.) can be utilized for construction of functional systems. Among the others, tetranuclear macrocyclic gold(I) alkynyl phosphine complexes with two azobenzene moieties, were obtained (shown schematically in Fig. 97) and investigated as photoswichable system [ 274 ]. It was found that the photo switching of gold(I) complex could be locked or unlocked with a second input: by the addition or removal of silver(I) ions. The conformational change of the molecule which is a consequence of the reversible trans–cis isomerization and the red-ox properties of iron are good examples of construction block for multi-stimuli molecular devices. The interlocking of a ferrocene-based rotary module with a photochromic azo unit of molecular machines operating via power-conversion mechanisms can be constructed. Such systems resemble daily used devices such as pliers 214 shown in Fig. 98 [ 275 ]. Azov et  al. [ 276 ] investigated macrocyclization of tetrathiafulvalene dithiolates with bis-bromomethylazobenzenes (Ab) under high dilution conditions (Fig. 99). The reaction afforded [1+ 1] cyclization product 215 with m-Ab and [2 + 2] cyclization product 216 with p-Ab in good yields N N N N N N OH HO OH HO N N 210 Fig. 94 Macrocyclic Schiff base 210 bearing inherent azo groups [ 271 ] 1:2 (L:M) complexes M = zinc(II), copper(II), nickiel(II) R,R-(-) NH2 NH2 H N N NH NH N N N N N N N N HN HN N H N H N N R,R,R,R,R,R-(-)-212 O H NaBH4 N N N O Me N + N N O O 213 O N Me 2CF3SO3+ Fig. 96 Highly-strained cyclophane 213 comprising azobenzene and methyl viologen units [ 273 ] (above 66%). Irradiation of p-Ab with UV light (365 nm, 0 °C) before reaction results in obtainment of cis-azobenzene bearing product 217 (1 + 1 cyclization type). Analysis of cyclic voltammograms registered in dichloromethane/0.1 M Bu4NClO4 showed that the electrochemical properties of tetrathiafulvalene moiety strongly depend on configuration (trans or cis) of azobenzene unit. Banerjee and co-workers [ 277 ] synthesized and compared properties of two covalent organic frameworks (COFs) (Fig. 100) being derivatives of triformylphloroglucinol and 4,4′-azodianiline (Tp-Azo) or 4,4′-diaminostilbene (TpStb). Azo-functionalized COF Tp-Azo exhibits better stability, porosity and crystallinity than stilbene-bearing analogoue Tp-Stb. The analysis of N2 absorption isotherm of Tp-Azo treated with 9 M HCl indicates the retention of intrinsic porosity of the azo-functionalized COF, whereas in the case of Tp-Stb decrease of porosity after the acid treatment was observed. According to TGA experiments it was stated that Tp-Azo possesses higher acid loading (5.4 wt%) than Tp-Stb (2.8 wt%). Doping of H3PO4 to the azo-functionalized COF leads to immobilization of the acid inside the framework pores, what enables proton transfer in both the anhydrous (σ = 6.7 × 10−5 S cm−1 at 340 K) and hydrated state (σ = 9.9 × 10−4 S cm−1 at 332 K under 98% relative humidity). Stilbene-bearing COF shows almost zero proton conductivity in anhydrous milieu and a poor proton conductivity value (σ = 2.3 × 10−5 S cm−1) at 332 K under 98% relative humidity. Azobenzene isomerization has been also utilized to drive functional changes in biomolecules such as: peptides, proteins, lipids, nucleic acids and carbohydrates. Comprehensive review of such applications can be found in the work of Beharry and Woolley from 2011 [ 72 ]. To apply azobenzene to direct protein conformational change in biological systems several requirements need to be fulfilled such as: (i) substantial structural change of azo bearing unit upon isomerization that can be coupled to protein conformational change, (ii) stability of the azo unit in a cellular environment, (iii) a suitable for cells and tissues irradiation wavelength and rate of thermal relaxation. Photocontrol of cyclic peptides was investigated among the others by Schutt et al. [ 278 ]. The authors described cyclization of a heptapeptide containing the Arg-Gly-Asp (RGD) sequence with 4-aminomethylphenylazobenzoic acid (AMPB). Studies of the cyclic peptide affinity to the cell surface receptor αVβ3 integrin show that RGD binds target protein stronger when azo unit is in its trans form. The cell adhesion can be also controlled by tethering of RGD peptide to a surface via azobenzene Fig. 97 Schematic diagram demonstrating the “locking” and “unlocking” mechanism brought about by the addition and removal of Ag+ ions in preventing and facilitating trans–cis isomerization of [Au4(P^P)2(C≡C–L–C≡C)2]. Reprinted with permission from [ 274 ]. Copyright 2007 American Chemical Society (214S,214'S')-cisII FeII FeIII UV N N UV N N FeII oxidation FeIII (214S,214'S')-transIII (214S,214'S')-cisIII Fig. 98 Left: the operation of molecular pliers 214 by light and redox stimuli. Right: schematic illustration of molecular pliers. Reprinted from [ 275 ]. Copyright 2008 with permission from Royal Society of Chemistry S H13C6S N S N S S S N N H13C6 S S S S S 216 trans or cis S S S S C6H13 S S N N S S C6H13 S H13C6S N N S S S S 217 trans or cis S SC6H13 Fig. 99 Macrocyclic azocompounds bearing tetrathiafulvalene units 215–217 [ 276 ] linker. 3-((4′-aminomethyl)phenylazo)benzoic acid was used to control the conformation of a cyclic peptide based on nNOS β-finger [ 279 ]. The trans isomer shows binding affinity towards target protein—α-1-syntrophin. Irradiation of the system with light at 330 nm enables the protein recognition. According to FTIR and NMR experiments, the isomerization azobenzene unit induces the formation of secondary, antiparallel β-type structure of the peptide ensuring the efficient interactions with α-1-syntrophin. Incorporation of azobenzene unit into protein disulfide isomerase via bis-cysteinyl active site was used to the obtainment of a simple model for allosteric conformational rearrangements [ 280 ]. It was stated, that the geometric changes accompanying isomerization of the azo group induce a rearrangement of peptide sequence changing energy landscape of the peptide and both isomers trans and cis exist in defined conformational states stabilized by disulfide bridge. Derda et al. [ 281 ] proposed bis(allenamide) functionalized azobenzene reagents for conversion of cysteine containing peptides to light responsive macrocycles. In comparison with typically used bis-alkyl halides containing azobenzene the allenyl amide derivatives ensure 2–3 order of magnitude faster macrocyclization by cysteine ligation in model peptide and those displayed on M13 phage. Woolley and co-workers [ 282 ] incorporated a thiol reactive azobenzene cross linker 218 into peptide backbone receiving cyclic azopeptides 219–221 (Fig. 101). Upon irradiation of the peptides in aqueous solution with blue light at 400–450 nm trans to cis isomerization occurs. Obtained cis isomers relax thermally with a half-life of about 1 s. It was stated, that azobenene linker 218 can be used to control of helical content of attached peptide, as in its trans form the linker bridges Cys residues spaced i, i + 15 (peptide Fig. 100 Left: a crystal structure of 4-[(E)-phenyl-diazenyl]anilinium dihydrogen phosphate. b schematic of Tp-Azo and Tp-Stb synthesis. Right: a schematic of H3PO4 doping of COFs. Proton conductivity of PA@Tp-Azo in b anhydrous and c wet conditions. d Proton con219) in an α-helix. Switching 218 to its cis isomer causes the decrease of the helix content of 219 and the increase of the helix content of 221. After photoisomerization no helix content of 220 is detected. Jaeschke and co-workers [ 283 ] described carbohydratebased macrocycles obtained from isothiocyanate-armed bis-azobenzene glycosides and piperazine. Isomerization of glycoazobenzene precursor molecules before the reaction ensured more efficient macrocyclization (yields: 48–65%). Obtained trans macrocycles isomerize into their cis forms upon UV-light irradiation what results in tremendous change of chirality with a strong helical induction in the cis state. The isomerization process is fully reversible by thermal relaxation, whereas upon irradiation with blue light only partially recovery of trans isomer is obtained. ductivity of PA@Tp-Stb in wet conditions. e Arrhenius plot for PA@ Tp-Azo in hydrous conditions. Reprinted with permission from [ 277 ]. Copyright 2014 American Chemical Society Summary The above review article is a subjective point of view on the current state of art in the synthesis and properties of selected azomacrocyclic compounds. It covers mainly the last 10 years, however, many of the previous works were also cited, to give more comprehensive background of the subject. Our intention was to underline the importance of very simple, seemingly tiny, functional –N=N– group, which can be incorporated into almost any molecule (material) giving extraordinary properties, especially when macrocyclic compounds are regarded. The presence of macrocyclic scaffold can have an enormous influence on switching properties of azo group due to ring strain and substituent effects. Photochemical characteristic of cyclic azobenzenes depends also on other factors, such as the number of azo units in the Cl O N Azo linker N N N 218 Azo linker Glu 219 i, i + 15 Azo linker 220 i, i + 14 Ala 221 i, i + 7 N N O Cl Ac Trp Gly Cys Ala Glu Ala Ala Ala Arg Ala Ala Ala Arg Glu Ala Ala Cys Arg Gln Ac Trp Gly Ala Cys Glu Ala Ala Ala Arg Glu Ala Ala Ala Arg Glu Ala Ala Cys Arg Gln NH2 NH2 Ac Glu Ala Cys Ala Arg Val Aib Ala Cys Glu Ala Ala Ala Arg Gln NH2 macrocycle, the symmetry of total molecule and the degree of conjugation, what makes the design of azomacrocyclic compounds a challenging task. Reversible trans–cis isomerization gives an opportunity to control the macrocycles structures at the molecular level what can be utilized for instance in the development of light-induced assembly/disassembly processes of supramolecular systems or in morphological transformation of assemblies. Binding properties of macrocyclic hosts e.g. crown ethers or cyclodextrins can be regulated by photoswitching of azo moiety, what finds applications among the others in ion transport through membranes and controlled drug release systems. Chromogenic and electroactive properties of azo group enable effective macrocycle use in optical and electrochemical sensors development. In the above manuscript we wanted to signalize the multifarious areas of science, technology and medicine where macrocyclic azo compounds can find applications. We believe the review will be helpful for readers interested in organic, analytical and practical aspects of supramolecular chemistry. 1. Izatt , R.M. : Charles J. Pedersen 's legacy to chemistry . Chem. Soc. Rev . 46 , 2380 - 2384 ( 2017 ) 2. Pedersen , C.J.: Cyclic polyethers and their complexes with metal salts . J. Am. Chem. Soc . 89 , 2495 - 2496 ( 1967 ) 3. Pedersen , C.J.: Cyclic polyethers and their complexes with metal salts . J. Am. Chem. 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Ewa Wagner-Wysiecka, Natalia Łukasik, Jan F. Biernat, Elżbieta Luboch. Azo group(s) in selected macrocyclic compounds, Journal of Inclusion Phenomena and Macrocyclic Chemistry, 2018, 1-69, DOI: 10.1007/s10847-017-0779-4