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Shrinkable/stretchable bis(calix[4]arenes) comprising photoreactive azobenzene or stilbene linkers
Semi-tubular bis(calixarenes) of photo-variable length were shown to be readily available by linking together two calix[4]arene macrorings by means of two bifunctional azobenzene or stilbene units. These biscalixarene cores 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. As expected, these (Z,Z)-bis(calixarenes) are thermally unstable and can be stretched back into their (E,E)-shape upon heating. Successful photo-shrinking of an (E)-stilbene-containing bis(calixarene) was achieved only in the presence of 1,2-benzanthracene as a photosensitizer. In this case, due to sufficient thermal stability, the (Z,Z)-isomer of the bis(calixarene) was separated (along with the intermediate (E,Z)-isomer), and its molecular structure was obtained by X-ray crystallography and compared with the molecular structure of the initial (E,E)-isomer. The results unambiguously prove the substantial shrinkage of bis(calixarene) upon irradiation, with retention of the ‘co-axiality’ of the calix[4]arene macrorings. For the stilbene-containing bis(calixarene) with no additional substituents in its structure, another photochemical transformation was observed. Upon irradiation, irrespective of the presence of a photosensitizer in the system, this compound rapidly formed stable cyclobutane-linked bis(calixarenes) with rtct- and rctt-structures due to the intramolecular [2 + 2]-cycloaddition between two stilbene units, which were preorganized by two calix[4]arene macrocycles.