CALIZ[4]CROWNS INCLUDING PHOTOISOMERIZABLE AZOBENZENE UNITS - DEPENDENCE OF THE CIS TRANS GEOMETRY ON CROWN LENGTH AND CALIXARENE CONFORMATION/

The crystal structures of four calix[4]crowns including a photoisomeri zable azobenzene unit in the ether bridge are reported. The calixarene unit is in the cone conformation in 1 and 2, with six and eight oxyge n atoms in the ether part of the bridge respectively, whereas it is in the 1,3-alternate one in 3 and 4, with four and six oxygen atoms resp ectively, due to the presence of two glycolic chains on the phenolic r ings which are not bridged. In each group, the diazo cis/trans geometr y appears dependant on both the bridge length and the calixarene confo rmation: the constraints in the shorter bridges, particularly when the calixarene conformation is the cone one, stabilize the less stable ci s form.