Rationally Designed Calix[4]arene–Pyrrolotetrathiafulvalene Receptors for Electron-Deficient Neutral Guests

Four
upper rim bis-monopyrrolotetrathiafulvalene-calix[4]­arene
conjugates 2a,b and 3a,b have been efficiently synthesized using a modular construction
approach. The new compounds feature a molecular tweezer architecture
with a quasi-parallel arrangement of redox-active tetrathiafulvalene
(TTF) arms, which serve as the guest binding centers. Complexation
studies using UV/vis binding titrations revealed a high affinity of
the calixarene–TTF receptors for planar electron-deficient
guests, leading to formation of deeply colored charge-transfer complexes
in solution. The binding efficiency of the receptors depends on the
flexibility of the calixarene scaffolds and the electronic nature
of the TTF arms: the highest binding efficiency is shown by receptor 2b, featuring a highly preorganized molecular structure and
an electron-rich TTF moiety.