Dibenzodiaza-30-crown-10-appended bis(zinc porphyrin) tweezers: synthesis and crown-assisted chiroptical behaviour

Abstract

In our program for developing chirality manipulation systems, we synthesized bis(zinc porphyrin) 1, with a dibenzodiaza-30-crown-10 as a linker unit. Two structural features were examined. The aza-crown segment exhibited an intermolecular interaction with the zinc(II) of the porphyrin, capable of causing aggregation to form spherical nanostructures, as inferred by concentration-dependency of ¹H NMR as well as scanning electron microscopy (SEM) observation. We also consider the crown-based conformation flexibility, in which accommodated K⁺ tunes the porphyrin orientation into the tweezers conformation, assisting chirality induction upon complexation with chiral diamine 2. The circular dichroism (CD) intensity change essentially reached a plateau at a [(1R,2R)-2] : [1] ratio of 2 : 1 for which a 45% enhancement in the amplitude of CD spectra was observed compared to the K⁺-free conditions. Use of the crown linker of 1 is not limited to promoting chirality induction with diamines in the presence of K⁺; chiroptical probing of unprotected amino acids (Lys, His, Trp, and Phe) using 1 was attained through liquid (1 in CH₂Cl₂)–liquid (the amino acids in 1 N KOH) two-phase extraction. The amphiphilic properties of the crown segment, as well as the K⁺-assisted tweezers conformation, make it possible to explore a potent way to develop chirality sensors for amino acids in water.