Cyclo[2]carbazole[2]pyrrole: a Preorganized Calix[4]pyrrole Analogue

Overview

Journal Chem Sci

Publisher Royal Society of Chemistry

Specialty Chemistry

Date 2023 Feb 9

PMID 36756337

Authors

Areum Lee

Ju Ho Yang

Ju Hyun Oh

Benjamin P Hay

Kyounghoon Lee

Vincent M Lynch

Jonathan L Sessler

Sung Kuk Kim

Affiliations

Soon will be listed here.

Abstract

A cyclo[2]carbazole[2]pyrrole (2) consisting of two carbazoles and two pyrroles has been synthesized by directly linking the carbazole 1- and 8-carbon atoms to the pyrrole α-carbon atoms. Macrocycle 2 is an extensively conjugated 16-membered macrocyclic ring that is fixed in a pseudo-1,3-alternate conformation. This provides a preorganized anion binding site consisting of two pyrrole subunits. H NMR spectroscopic analysis revealed that only the two diagonally opposed pyrrole NH protons, as opposed to the carbazole protons, take part in anion binding. Nevertheless, cyclo[2]carbazole[2]pyrrole 2 binds representative anions with higher affinity in CDCl than calix[4]pyrrole (1), a well-studied non-conjugated tetrapyrrole macrocycle that binds anions four pyrrolic NH hydrogen bond interactions. On the basis of computational studies, the higher chloride anion affinity of receptor 2 relative to 1 is rationalized in terms of a larger binding energy and a lower host strain energy associated with anion complexation. In the presence of excess fluoride or bicarbonate anions, compound 2 loses two pyrrolic NH protons to produce a stable dianionic macrocycle [2-2H] displaying a quenched fluorescence.

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