Synthesis and Binding Studies of a Tetra-α Aryl-Extended Photoresponsive Calix[4]pyrrole Receptor Bearing meso-Alkyl Substituents

We report the synthesis of the tetra-α isomer of a meso-azobenzene-extended calix[4]pyrrole receptor bearing alkyl substituents at its lower rim. In solution, the photo-irradiation of the all-transreceptor induces the isomerization of its azobenzene units producing at the photostationary state a mixture of cis-enriched counterparts in which the all-cis receptor is the major component (85 %). Simple molecular modelling studies show that the cone conformation of the all-trans isomer possesses a deep aromatic cavity open at one end. Otherwise, the isomers containing cis-azobenzene groups display a significant reduction of the cavity size and a diminution of the number of CH–π and π–π interactions that can be established with the included guest. The interaction of the all-trans isomer, as well as of the mixture mainly containing the all-cis receptor, with electron-rich molecular guests (neutral and anionic) was probed, in dichloromethane and acetonitrile solutions, using 1H NMR spectroscopic titrations. Isothermal titration calorimetry (ITC) experiments enabled the accurate determination of the binding constants of the inclusion complexes.