Unprecedented formation of bis-calix[5]phyrins centrally linked with pyrene subunits

Abstract

A [3+2] acid-catalyzed condensation of a tetra-functionalized pyrene fluorophore (4) with dipyrromethane dicarbinol (5) resulted in the unprecedented formation of two bis-calix[5]phyrins (bis-calix[5]-1 and bis-calix[5]-2). The solution state structure of both bis-calix[5]phyrins was found to be similar with slight variations as confirmed by NMR spectral studies. Interestingly, two different linkages between the pyrene subunits with dipyrromethane dicarbinols led to two different calix[5]phyrin systems. Although the optical features seem to be similar, the single crystal X-ray structure obtained for bis-calix[5]-1 provided the structural proof showing a non-planar, half-chair conformation with two meso-sp3 carbons. DFT studies were carried out to compare the HOMO-LUMO energy levels of both bis-calix[5]phyrins with the targetted conjugated bis-macrocycle (bis-N[Formula: see text]. Further, both the calix[5]-phyrins showed high Stokes shift (5404 cm[Formula: see text] and fluorescence quantum yield efficiency of around 90%.