Comparative study of the vibrational spectra of carboxylate azocalix[4]arenes and azothiacalix[4]arenes

Abstract

The vibrational spectra of calix[4]arene (p-CAC) and thiacalix[4]arenes with azobenzene units having carboxylate groups in the para (p-CATC) and meta position (m-CATC) were recorded and analyzed. The calculated structures of molecules p-CAC, p-CATC, and m-CATC are consistent with experimental X-ray data. The most stable and polar is the cone conformation with four intramolecular cooperative hydrogen bonds. In classical calixarenes, the hydrogen bonds are shorter and, therefore, more durable than in thiacalixarenes. The cone conformation retains for all the studied calixarene molecules. The energy differences between E- and Z-forms of azobenzene groups in p-CAC, p-CATC, and m-CATC are 254.4, 260.2, and 249.4 kJ/mol, respectively. The shape of the molecules changes noticeably upon isomerization of azobenzene groups. Upon passing from thiacalixarenes to classical calixarenes, the dipole moment of the molecule increases. The polarity of calixarene molecules changes upon isomerization of azobenzene units. Ionization energy electron affinity and electrophilicity are higher in classical calixarenes compared to thiacalixarenes. There are active sites of reaction and interaction in the narrow and wide rims of the calixarene molecules. Carboxylate groups are most active in the formation of complexes with metals.