Three conformational polymorphs and order-disorder phase transition in a calix[4]arene bis(crown)

Three polymorphs of a calix[4]arene fixed in the 1,3-alternate conformation by two bridges, a crown-6 on one side and a crown-6 including a photoisome rizable azobenzene unit on the other, 8,11,14,28,31,34,37,40,43,57,60,63-do decaoxa-2,3-diazanonacyclo[62.2.2.2(4,7).1(16,45).1(26,55).0(15,22),0(22,27 ).0(44.49).0(51,56)] (66),2,4,6,15,17,19,22,24,26,44,46,48,51,53,55,64,67,6 9-nonadecene, have been characterized by single-crystal X-ray diffraction. In all the polymorphs the azobenzene group is in the more stable trans conf ormation. The polymorphism arises from the high conformational flexibility of the bridges, which are arranged differently and are highly disordered in the two monoclinic forms (1) and (2a). Form (2a) presents a phase transiti on near 268 (2) K, leading to a triclinic form (2b), differing from the hig h-temperature form by some ordering of the chains and a subsequent reductio n in crystallographic site multiplicity. The evolution of the cell paramete rs and peak width with temperature has been investigated down to 173 (2) K. Adiabatic calorimetry measurements indicate a smooth transition centered a t 265.0 (1) K, with a heat capacity jump of 60 mJ g(-1) K-1, suggesting a s econd-order nature for the transition.