Pyrroles are found in several natural anion binding motifs. However, their use as artificial recognition motifs antedates an appreciation of their role in biological anion binding. The recognition that pyrrole-containing systems can bind anions dates back to 1990 and early work with expanded porphyrins. However, to date, expanded porphyrins have only proved useful for anion binding when studied in their protonated forms. An ongoing challenge has thus been to create neutral pyrrole-based anion recognition systems. Such a desire led to the discovery of calix[n]pyrroles as anion binding agents. Calix[n]pyrroles are synthetic compounds containing four or more pyrrole or pyrrole-like heterocyclic subunits within their non-conjugated frameworks. Inspired by earlier studies of the venerable calix[4]pyrrole system discovered by Baeyer in the 18th century, the chemistry of calixpyrrole-type compounds has grown to include systems built up from bipyrrole, bis(pyrrolyl)benzene, biimidazole, and a number of other heterocyclic subunits.

In this lecture, calix[n]pyrroles will be discussed in the context of recent efforts to develop them as “tunable” systems that are capable of acting as anion-triggered “molecular switches”. This has made them useful as “logic gates”, as extractants capable of recognizing and releasing ion pairs, and as precursors for environmentally responsive, self-assembled materials. It is these newer applications of calix[n]pyrroles that will be presented in detail.

This project has benefited from a number of ongoing collaborations, including with the groups of Philip A. Gale, Bruce A. Moyer, Franz Schmidtchen, Jan Jeppesen, Shunichi Fukuzumi, Chang-Hee Lee, Vincent Lynch, Chris Bielawski, Dongho Kim, Injae Shin, and Jong Seung Kim. Thanks is also given to current coworkers on this project: Sung Kuk Kim, Jung Su Park, Dong Sub Kim, and Steffen Bähring, and Christina Davis. Financial Support from the NSF and DOE is gratefully acknowledged