Photoactive Azobenzene-Containing Supramolecular Complexes and Related Interlocked Molecular Compounds

Abstract

Two acyclic and three macrocyclic polyethers, three [2]catenanes, and one [2]rotaxane, each containing one 4,4'-azobiphenoxy unit, have been synthesized. In solution, the azobenzene-based acyclic polyethers are bound by cyclobis(paraquat-p-phenylene)Ða tetracationic cyclophaneÐin their trans forms only. On irradiation (l 360 nm) of an equimolar solution of the tetracationic cyclophane host and one of the guests containing a trans-4,4'-azobiphenoxy unit, the trans double bond isomerizes to its cis form and the supramolecular complex dissociates into its molecular components. The trans isomer of the guest and, as a result, the complex are reformed, either by irradiation (l 440 nm) or by warming the solution in the dark. Variable temperature 1 H NMR spectroscopic investigations of the [2]catenanes and the [2]rotaxane revealed that, in all cases, the 4,4'-azobiphenoxy unit resides preferentially alongside the cavities of their tetracationic cyclophane components, which are occupied either by a 1,4-dioxybenzene or by a 1,5-dioxynaphthalene unit. In the acyclic and macrocyclic polyethers containing 1,4-dioxybenzene or 1,5-dioxynaphthalene chromophoric groups and a 4,4'-azobiphenoxy moiety, the fluorescence of the former units is quenched by the latter. Fluorescence quenching is accompanied by photosensitization of the isomerization. The rate of the energy-transfer process is different for trans and cis isomers. In the [2]rotaxane and the [2]catenanes, the photoisomerization is quenched to an extent that depends on the specific structure of the compound. Only in one of the three [2]catenanes and in the [2]rotaxane was an efficient photoisomerization (l 360 nm) from the trans to the cis isomer of the 4,4'-azobiphenoxy unit observed. Single crystal X-ray structural analysis of one of the [2]catenanes showed that, in the solid state, the 4,4'-azobiphenoxy unit in the macrocyclic polyether component also resides exclusively alongside. The cavity of the tetracationic cyclophane component of the [2]catenane is filled by a 1,5-dioxynaphthalene unit, and infinite donor ± acceptor stacks between adjacent [2]catenanes are formed in the crystal. These supramolecular complexes and their mechanically interlocked molecular counterparts can be regarded as potential photoactive nanoscale devices.

Artificial Molecular Machines

“…transfer complexes with pseudorotaxane geometries. Indeed, the compound trans-47 self-assembles with 25 4 to give [130] a pseudorotaxane (K a 470 m À1 in MeCN at 298 K) both in solution and in the solid state, as shown by 1 H NMR spectroscopy and X-ray crystallography, respectively. On irradiation at 360 nm of an equimolar MeCN solution of 25 4 and trans-47, which are in part associated to give a pseudorotaxane superstructure, the NN bond isomerizes to the cis form and the pseudorotaxane dethreads.…”

“…Threadlike species containing a p-electron-rich azobiphenoxy unit have been used [130] (Figure 6), in conjunction with electron-accepting hosts such as 25 4 or 46 4 , to obtain charge- Figure 6. The change in intensity of the luminescence (l exc 411 nm) associated [133] with the tetracationic cyclophane 46 4 for an equimolar solution (1 Â 10 À4 m, MeCN, 298 K) of 46 4 and 47 upon consecutive trans 3cis (irradiation at 365 nm, cyan areas) and cis 3trans (irradiation at 436 nm and rest in the dark, green areas) isomerization cycles.…”

Künstliche molekulare Maschinen

“…[130] So entsteht aus trans-47 und 25 4 ein Pseudorotaxan (K a 470 m À1 in MeCN bei 298 K), wie durch 1 H-NMR-Spektroskopie in Lösung und durch Röntgenstrukturuntersuchungen im Festkörper gezeigt wurde. [130] Bei Bestrahlung einer äquimolaren MeCN-Lösung von 25 4 und trans-47, die bereits zum Teil zum Pseudorotaxan assoziiert sind, mit Licht der Wellenlänge 360 nm isomerisiert die NN-Bindung zur cis-Form, und das Pseudorotaxan wird entfädelt. Durch Bestrahlen (440 nm) oder durch Erwärmen der Lösung im Dunkeln kann das trans-Isomer regeneriert werden, das sich wieder in das Cyclophan einfädelt.…”

Azobenzene Polymers for Photonic Applications

“…In these experiments, illumination causes contraction of an azobenzene polymer, showing that each chromophore can exert pN molecular forces on demand. A pseudorotaxane that can be reversibly threaded-dethreaded using light has been called an ''artificial molecular-level machine'' (Balzani et al, 2001;Asakawa et al, 1999). The ability to activate and power molecular-level devices using light is of course attractive since it circumvents the limitations inherent to diffusion or wiring.…”