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 (lambda = 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 (lambda = 440 nm) or by warming the solution in the dark. Variable temperature H-1 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 (lambda=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.
Asakawa, M., Ashton, P.r., Balzani, V., Brown, C.l., Credi, A., Matthews, O.a., et al. (1999). Photoactive azobenzene-containing supramolecular complexes and related interlocked molecular compounds. CHEMISTRY-A EUROPEAN JOURNAL, 5(3), 860-875 [10.1002/(SICI)1521-3765(19990301)5:3<860::AID-CHEM860>3.0.CO;2-K].
Photoactive azobenzene-containing supramolecular complexes and related interlocked molecular compounds
Balzani, V;Credi, A;
1999
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 (lambda = 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 (lambda = 440 nm) or by warming the solution in the dark. Variable temperature H-1 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 (lambda=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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
