Study of multiporphyrin compounds as colorimetric sitting-atop metal complexes: Synthesis and photophysical studies

Mono-, di-, tri-, and hexaporphyrin derivatives were synthesized and their sensorial ability toward Na⁺, Li⁺, Ca²⁺, Cu²⁺, Ni²⁺, Co²⁺, Fe³⁺, Zn²⁺, Cd²⁺, Hg²⁺, Ag⁺, Cr³⁺, Pb²⁺, and Al³⁺ was explored by absorption and emission spectroscopy and ¹H NMR spectroscopy. For all of the studied porphyrin derivatives the most significant spectral changes were observed upon addition of Zn²⁺, Cd²⁺, Cu²⁺, Co²⁺, Pb²⁺, Hg²⁺, and Fe³⁺, which resulted in a color change from red to green. When the remaining metal ions were tested, no significant changes were observed. The results reveal, as expected, the formation of mononuclear complexes for porphyrin 4, dinuclear complexes for diporphyrin 7, and trinuclear complexes for triporphyrins 9 and 10. In addition, complexes of type M₆L were determined for hexaporphyrin 8. The NMR spectroscopy studies suggest that the interaction with the metals occurs with the formation of sitting-atop complexes. This study also shows that the sensitivity increases with increasing number of coordination sites, with the most pronounced case observed for Hg²⁺. The lowest value of detectable concentration of 0.05 μm for Hg²⁺ was achieved for hexaporphyrin 8. Sensitive subjects: Mono-, di-, tri-, and hexaporphyrin derivatives (see compound 4 in the figure) were synthesized and their sensorial ability towards metal ions was studied. It was found that they showed an increased sensitivity to Hg²⁺ ions. The NMR spectroscopy studies suggest that the interaction with the metal ions occurs with the formation of sitting-atop complexes.