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) - a tetracationic cyclophane - in their trans forms only. On irradiation (λ = 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 (λ = 440 nm) or by warming the solution in the dark. Variable temperature 1H 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 (λ = 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.