Aromatic Ring-Fused Amidine Based Allosteric Receptors Activated by Guest-Induced π-Conjugation Switching

Amidine-substituted allosteric receptors 2 a and 2 b for benzenediols were synthesized. Receptor 2 a with five-membered amidines exhibited greater allostericity than the amide-substituted receptor 1, while 2 b with six-membered amidines exhibited less allostericity. NMR titration experiments revealed that a significant enthalpic factor was involved in the allostericity of these receptors. X-ray and DFT optimized structures of 2 a and 2 b revealed that 2 a adopted a coplanar conformation with π-conjugation between the amidines and the phenylene ring of the hydrindacene framework, resulting in high allostericity due to inactivation of the initial binding.