Substituent effects on a binaphthyl-linked zinc bisporphyrinate host: Modulating chirality sensing and circular dichroism response in the enantioselective recognition of amino acid esters

A new binaphthyl-linked zinc bisporphyrinate host, bearing ethoxy substituents, was designed and synthesized to investigate the substituent effect on chiral recognition. This host was evaluated for its enantioselective binding towards various amino acid ethyl esters using circular dichroism (CD), UV-Vis spectroscopy, and 1 H NMR titration. The host demonstrated effective enantiomeric discrimination of amino acid esters via CD signals. However, the intensity ratio between the L- and D-enantiomers of PhgOEt was significantly lower than that observed for the previously reported methoxy-substituted analogue. X-ray crystallographic analysis of the racemic host-guest complex revealed a multifaceted recognition mechanism involving axial coordination to zinc, hydrogen bonding, and stabilizing C-H[Formula: see text], and O[Formula: see text] interactions. The diminished chiroptical response in the ethoxy derivative is likely due to a smaller dihedral angle between the transition dipole moments of the porphyrin subunits ([Formula: see text] vs. [Formula: see text] in the methoxy analogue), which reduces the strength of exciton coupling, a conclusion further supported by time-dependent density functional theory calculations. This study elucidates the critical role of substituents in modulating the geometry and chiroptical properties of bisporphyrin hosts, providing valuable insights for the rational design of supramolecular sensors with tailored enantioselectivity.