Chemical sensitivity of porphyrin assemblies

Porphyrins play an important role in our world that is essential for life and colours. It is astonishing to consider the various and complex chemistry that nature is able to perform through these molecules, modulating their properties by subtle changes in the macrocyle framework and in their biological environment. Iron complexes for example are exploited to bind, store and activate oxygen in animals, while reduced porphyrins, such as magnesium chlorin complexes, take part in the photosynthetic processes for the solar energy conversion. This richness of properties has attracted the interest of researchers in the chemical sensor fields, because in these devices the binding of the target analyte by the sensing materials in large part mimic the porphyrin role in biological systems. For this reason in the last few years porphyrins have assumed a protagonist role in the scenario of chemical sensors(1). The porphyrin molecular framework offers a wide range of interaction mechanisms for analyte binding, spanning from the weak Van der Waals forces to hydrogen bond, to p interactions and finally to the coordination to the central metal ion. Studying the interaction of chemically modifed gold tips of a scanning tunneling microscope with porphyrin layers evidenced that all these interactions may contemporaneously be present and cooperate in the total guest molecule binding(2).