Abstract
Porphyrins have long been recognized as sensors for various analytes. However, this involves mostly metalloporphyrins and there are still limitations with regard to discrimination and sensitivity. Here we illustrate that metal-free metal-free ('free base') porphyrins can be re-engineered via conformational design to make the core N-H units accessible for interaction with substrates or analytes. This allows to utilize metal-free porphyrin free bases as organocatalysts. Highly conjugated porphyrin macrocyclic systems offer spectrophotometric sensitivity towards geometrical and/or electronic changes and thus, utilizing the porphyrin core for selective detection of substrates in solution offers significant potential for applications. As a proof-of-concept we describe how a combination of macrocycle distortion in conjunction with suitable functional groups in the porphyrin periphery allowed the design of the binding pocket in an enzyme-like fashion and facilitated the highly selective detection of the pyrophosphate anion in a switchable sensor system.