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.