Functionalized calix[4]resorcinarene cavitands. Versatile platforms for the modular construction of extended molecular switches

We report the synthesis and investigation of the vase-kite conformational switching of bridged calix[4]resorcinarene-based cavitands bearing iodo or ethynyl substituents on the upper rim of the cavity. These reactive centers make the cavitands versatile platforms for further modular extension by means of Sonogashira cross-couplings, thereby enabling the construction of diverse functional architectures that are switchable between two highly restricted conformations and that undergo geometrically defined expansions/contractions on the multi-nanometer scale. This is illustrated by attachment of oligo(phenyleneethynyl) fragments, bearing terminal donor and acceptor BODIPY (dipyrrometheneboron difluoride) dyes, to a diiodo-substituted cavitand. The multi-nanometer-sized switching motion between the expanded kite and the contracted vase states, induced by temperature or pH changes, was proven by (HNMR)-H-1 spectroscopy and FRET (fluorescence resonance energy transfer) measurements. The cavitand system, with its modular construction scheme, holds promise to become a general tool for controlled on/off switching of excimer and exciplex formation, PET (photoinduced electron-transfer), or other processes and for investigating the distance dependence of the underlying intermolecular interactions.