Molecular recognition of chiral diporphyrin receptor with a macrocyclic cavity for intercalation of aromatic compounds

Chiral diporphyrin receptor 1, which has a macrocyclic cavity for the intercalation of aromatic guest molecules, was designed and synthesized from pyrrole in five steps. The binding constants (Ka) revealed the greater affinity of 1 for more electron-deficient aromatic guests. The complexation between 1 and 1,3,5-trinitrobenzene (G8) (Ka = 1850M ^-1) was much stronger than that between porphyrin monomer 3 and G8 (Ka = 50M ^-1). This result strongly suggests that G8 was intercalated into the cavity of 1 via cooperative double π-π stacking. Interestingly, 1 enabled the naked-eye detection of an aromatic explosive G8; a dark-red solution of 1 in CHCl3 turned into a colloidal suspension upon addition of G8, and the light was scattered. Fluorescence spectroscopy was also useful for the selective detection of G8; fluorescence of 1 was quenched by complexation with G8, which was visible with the naked eye. Despite modest binding constants for dinitrobenzene derivatives, 1 showed a good ability to discriminate the enantiomers of twelve chiral compounds bearing a dinitrophenyl group by NMR spectroscopy. The MM calculations with the MM3 force field reproduced inclusion complexes between 1 and nitroaromatic compounds. The mechanism of chiral discrimination is proposed.