Introduction

Compounds capable of light absorption in the visible region as well as host-guest complexation with variety of ligands are perfect candidates for molecular sensors. These conditions are fulfilled by oxoporphyrinogen (OxP) and its derivatives, which are studied in this work. Oxoporphyrinogens are flat macrocyclic molecules possessing multiple binding and protonation sites. They have been previously investigated from the perspective of colorimetric detection of various molecules, anions and acids in nonpolar media. Here, we extend the current knowledge and use nontrivial analyses of UV/vis spectra to qualitatively and quantitatively explain the mechanism of color change of oxoporphyrinogens due to the presence of organic acids. Solvatochromic properties are also studied.