Hexaphyrin-Based Mercuric Ion Sensors

Significance

The synthesis and ion sensing properties of a new expanded porphyrin are reported. A condensation of pyrrole and pentafluorobenzaldehyde under acidic conditions followed by dehydration with DDQ afforded macrocycle 1 in poor yields (8-12%). X-ray crystallography revealed the non-planar structure of 1. Additionally, this compound was found to have absorption bands at 543, 704, and 768 nm and exhibit NIR emission at 959 and 1085 nm with photoexcitation at 514 nm. Complexation studies with various metal ions (Mn²⁺, Co²⁺, Cd²⁺, Zn²⁺, Fe³⁺, Ag⁺, Hg²⁺, Li⁺, K⁺, Ca²⁺, and Ba²⁺) in methanol indicated a high affinity of 1 for Hg²⁺ to the virtual exclusion of all the others. A complexation ratio of 2:1 of Hg²⁺ to macrocycle was determined via titration experiments. The interaction with Hg²⁺ ions had pronounced effects on the absorption spectrum, with a large bathochromic shift, and on the emission spectrum in the form of marked fluorescence quenching. Spectroscopic detection of Hg²⁺ was possible in the ppb range.