Rigid Molecular Tweezers: Preorganized Hosts for Electron Donor-Acceptor Complexation in Organic Solvents

Novel nonmacrocyclic receptor molecules 1 and 7 have been synthesized which are capable of acting as molecular tweezers. These molecules are 7-aryl-2,12-bis(9-acridinyl)dibenz[c,h]acridines, which when unsymmetrically substituted exhibit atropisomerism. In addition to being rigid and providing a ca. 7 Å interchromophore distance, the dibenz[c,h]acridine spacer is the first to enforce a syn cofacial orientation of the complexing chromophores. ¹H NMR studies indicate that 1 and 7 have open clefts. These preorganized clefts form highly oriented inclusion complexes with 2,4,7-trinitrofluorenone (TNF) in CDC1₃ as monitored by NMR. The complexation has a strong electron donor-acceptor component. As such, the association constants for TNF are dependent on the π-basicity of the host and range from 149 M^-1 for tert-butyl-substituted 7b to 697 M^-1 for dimethylamino-substituted 7f (CDCl₃). Monoacridines and a flexible diacridine, under identical conditions, bind TNF very weakly (K_assoc < 5 M^-1).