ИСТИНА

Photosensitive bis(calixarenes) of photo-variable length were prepared by a direct distal alkylation of parent calix[4]arene tetrols with (E)-4,4′-bis(bromomethyl)azobenzene or (E)-4,4′-bis(bromomethyl)stilbene using potassium carbonate as a base. Alkylation of four remaining phenolic OH-groups in two calix[4]arene macrorings allowed further derivatization of the biscalixarene cores. When exposed to soft UV light, the (E)-azobenzene-derived bis(calixarenes) underwent stepwise isomerization of both linkers from the (E)- to the (Z)-form, resulting in shrunken (Z,Z)-bis(calixarenes) as the major products. These (Z,Z)-bis(calixarenes) are thermally unstable and can be stretched back into their (E,E)-shape upon heating. Photo-shrinking of an (E)-stilbene-containing bis(calixarene) was achieved in the presence of 1,2-benzanthracene as a photosensitizer and the stable (Z,Z)-isomer of the bis(calixarene) was separated. Upon irradiation, the (E)-stilbene-linked bis(calix[4]arene) with no additional functional groups in its structure undergoes an intramolecular [2 + 2]-cycloaddition, which occurs much faster than E-to-Z isomerization and results in a mixture of isomeric biscalixarene cyclobutanes with rtct- and rctt-configurations. The X-ray diffraction data as well as the results of quantum-chemical calculations, show that E/Z-isomerization of two azobenzene or stilbene linkers enables longitudinal shrinking/stretching of the bis(calixarenes), while maintaining the tail-to-tail arrangement of the calix[4]arene macrorings and their co-axiality. Along with the change in the molecular length, the shrinking/stretching affects the shapes of the internal cavities of the bis(calixarenes), as well as the arrangement of the functional groups, thus showing the applicability of the azobenzene-/stilbene-linked bis(calixarenes) as cores for the design of photoswitchable host molecules and larger assemblies capable of sophisticated supramolecular interactions controlled by light.