Increasing the complexity of oxoporphyrinogen colorimetric sensing chromophores: N-alkylation and β-substitution

Meso-5,10,15,20-tetrakis-3,5-di-tert-butyl-4-oxocyclohexadienylideneporphyrinogen, OxP, is a versatile, highly colored chromophore derived from meso-5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin. It exhibits a wide range of chromogenic responses to solvents (solvatochromism), anions and acidic media (halochromism) making it potentially useful as an analytical reagent. The chromogenic responses of OxP can be modulated by varying its chemical structure, and this is reviewed here based on the introduction of substituents at central nitrogen atoms or pyrrolic beta-positions. OxP and its N-alkylated derivates Bn(2)OxP and Bn(4)OxP have been used to estimate acidity in non-polar solvents. Bn(2)OxP can also be used to determine enantiomeric excesses of chiral substances. N-alkylation has also been used to introduce higher functional groups such as porphyrins to prepare self-assembling systems. beta-Substitution has been used to introduce selectivity of anion interactions including towards basic anions (fluoride, cyanide) and polyoxoanions (nitrate, perchlorate, etc.). These aspects make OxP a highly adaptable tetrapyrrole molecule for sensing and other applications.