Thermodynamic study of azobenzene inclusion complexes : effect of the nature of receptors

Host-guest interactions based on cyclodextrins (CDs) and calixarenes (Cals) play a remarkable role in the field of supramolecular chemistry. CDs are cyclic oligomers and possess a hydrophilic outside and a hydrophobic hollow ... [ view full abstract ]

Host-guest interactions based on cyclodextrins (CDs) and calixarenes (Cals) play a remarkable role in the field of supramolecular chemistry. CDs are cyclic oligomers and possess a hydrophilic outside and a hydrophobic hollow due to the cyclic arrangement of the glucopyranose units. Due to this structure, CDs form inclusion complexes with a variety of appropriately sized guest molecules in aqueous solution. The p-sulfonatocalixarene has similar properties of CDs with both a hydrophobic environment  (the benzene rings) and also hydrophilic heads (the sulfonic acid groups). These systems are widely used as starting materials to built  supramolecular architectures.

 Among the various families of guest molecules, the chromophoric azobenzene and its derivatives have particular advantages. Due to the very weak solubility in water of these compounds and their used in various applications, the inclusion behavior of azobenzene derivatives is required to increase the aqueous solubility of these molecules. Since noticeable spectral changes can be measured upon inclusion complexation of the host molecule in solution, spectrocopic study was chosen to characterized host-guest complexation with these chromophoric guests. Azo dyes have also attracted much attention because of their potential to obtain photo-driven molecular machines. It is well known that azobenzene can reversibly switch between trans and cis conformation upon external stimuli such as heat or visible light. It is generally reported that the photoisomerization of azo moiety results in the dissociation of the host-guest assembly.

  The present work is focussed on the fully thermodynamical characterization of the association of aminoazobenzene derivatives with both p-sulfonatocalix4arene and b-cyclodextrins. For each system the thermodynamic parameters were determined by UV-visible spectroscopy (K, D_rG°, D_rH° and TD_rS°). To gain information of the host-guest process, an molecular dynamic approach was applied to complete the experimental features.  Our purpose is to compare the complexing tendencies of these cage-like molecules towards aminoazobenzene.