Quinoxaline-bridged porphyrinoids

Quinoxaline-bridged porphyrinoids (3), the first macrocycles containing dipyrrolylquinoxaline (DPQ, 1) subunits, were synthesized from the condensation of the diformyl-substituted DPQ derivatives (2) and 1,8-diaminoanthracene. The resulting structures were confirmed by x-ray analyses, which showed encapsulation of CHCl₃ molecules within the columnar channels established by the stacked arrangement of the individual macrocycles. The solution phase interactions with fluoride and dihydrogenphosphate anions were studied in the case of the unsubstituted system 3a in CH₂Cl₂. The binding affinities for these anions, studied at the tetrabutylammonium salts, were found to be enhanced relative to those of the simple, unsubstituted monomeric DPQ "parent" system (1a), presumably as the result of the combined effects of preorganization and cooperative binding permitted by the pyrrole NH donor groups. Positive homotropic allosteric anion binding was observed and is ascribed to the structurally coupled nature of the two binding cavities present in the macrocycles. Support for this latter contention came from energy minimization studies.