Conformational isomerism of electroactive calix[4]arenes: influence of the electronic state in the flexibility of thiophene-containing calix[4]arene.

The cone-to-paco conformational isomerism of 11,23-bis(thiophen-5-yl)-26,28-dimethoxycalix[4]arene-25,27-diol, a calix[4]arene with thiophene substituents in para position with respect to the hydroxyl groups, has been investigated using ab initio and DFT quantum mechanical methods. This compound models a molecular device constituted by small oligomers of thiophene and calix[4]arene units, whose actuation mechanism is promoted by the conformational flexibility of the latter. To examine the influence of the electronic structure of this electroactive calix[4]arene, three different states have been considered: (i) neutral state; (ii) oxidized state, in which one electron is extracted from each thiophene ring; and (iii) oxidized-deprotonated state, in which the two hydroxyl groups of the oxidized compound are deprotonated. Results are discussed and compared with those obtained for the same molecule but without thiophene substituents, 25,27-dihydroxy-26,28-dimethoxycalix[4]arene. Although the influence of the thiophene substituents is negligible in the neutral state, they play a crucial role in the rotational isomerism of both the oxidized and deprotonated-oxidized states.