Reversible Light-Controlled Cargo Release in Hydrogen-Bonded Dimeric Capsules

We describe the synthesis of three tetraurea-calix[4]arenes having four appended terminal azobenzene groups. In CD2Cl2 solution and at millimolar concentration, the thermally equilibrated all-trans-tetraureas dimerize quantitatively, encapsulating one Me4P+ cation. The light-induced isomerization of the all-trans encapsulation complexes produced a plethora of isomeric cis-enriched counterparts displaying a reduction in cavity size. cis-enriched dimers not suitable for the encapsulation of the cation or a solvent molecule are also produced, leading to partial release of the cargo (Me4P+) to the bulk solution. The substitution of the terminal phenyl in the azobenzene groups plays a key role in controlling the amount of released cargo in quantities up to 70%. The switching between the two states (all-trans and cis-enriched capsules) proceeds with no detectable photodegradation, even when it is repeated multiple times.