Temperature effect on supramolecular chirality induction in bis(zinc porphyrin)

The achiral syn folded conformer (face-to-face) of the ethane-bridged bis(Z n porphyrin) gradually transforms into the chiral extended anti form in the presence of enantiopure guest molecules (alcohols or amines) upon lowering the temperature from 293 to 183 K. The mechanism of the supramolecular chi rality induction is based upon the formation of right- or left-handed screw diastereomers of the anti form. The split absorption maxima which are caus ed by the exciton coupling of the corresponding B transitions match the bis igned Cotton effects. The amplitude of the CD bands is found to be dependen t on the bulk and ligation strength of the chiral guest, while the sign of the couplets is observed to be determined by the absolute configuration of the external ligand. The formation of the screw structure in the anti confo rmation is also confirmed by H-1 NMR.