Chirality Sensing, Halochromism and Solvatochromism in a Conjugated Calix[4]Pyrrole

Abstract

The conjugated calix[4]pyrrole tetrakis(3,5-di-t-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) is a highly coloured macrocycle with excellent potential as a sensing element in various applications. The first of these is chiral sensing where guest binding at the macrocyclic NH groups allows for estimation of enantiomeric excesses in non-racemic mixtures of chiral analytes. This feature is permitted by the relatively weak binding of the guest resulting in its rapid exchange so that the molecule essentially acts as a probe of average solution chirality. Chirality sensing in OxP is complicated by tautomeric processes when an acid analyte is used. This has allowed us also to develop an acidity sensor for use in non-polar media. More recently, we have designed a series of OxP type molecules that undergo modulation of halochromic (sensitivity to acid) properties and the development of colour scales based on the blocking of OxP binding sites and involving allied protic tautomerization of the OxP chromophore. We will discuss the sensing properties of OxP in relation to chirality, tautomeric processes and the complicating factor of solvatochromism.