New Insights into the Geometry of Resorc[4]arenes:  Solvent-Mediated Supramolecular Conformational and Chiroptical Control

The conformations of inherently chiral resorc[4]arenes were studied by circular dichroism (CD) spectroscopy. Whereas in aprotic solvents the crown conformation (C 4) is preferred, protic solvents favor the boat conformation (C 2). As a result of electronic coupling of the lowest L b state of the resorcinol unit in the resorc[4]arene, the CD spectra show a strong dependence on the conformation of the macrocycle. For the first time the solvent dependence of the CD spectra was qualitatively analyzed and simulated by using theoretical methods. We have thus demonstrated not only that the conformation of the calixarene is dramatically manipulated by the solvent but also that the joint use of chiroptical measurements and theoretical calculations is a powerful and versatile tool for elucidating structural variations in supramolecular chemistry.