Structural Modulation of Chromic Response: Effects of Binding‐Site Blocking in a Conjugated Calix[4]pyrrole Chromophore

Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5‐di‐tert‐butyl‐4‐oxocyclohexadien‐2,5‐yl)porphyrinogen (OxP) by structural modification. N‐Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole‐type binding sites of OxP and serial variation of the chromic properties of the products, double N‐benzylated Bz2OxP and tetra N‐benzylated Bz4OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz2OxP, and Bz4OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding‐site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators. Varycolour calixpyrrole: Modification of a calixpyrrole macrocycle by N‐alkylation strongly modulates its halochromic and solvatochromic responses. This is achieved by selective blocking of calixpyrrole binding sites, which leads to a family of chromophores suitable for acidity and polarity sensing in nonpolar solvents.