A New 3,5-Bisporphyrinylpyridine Derivative as a Fluorescent Ratiometric Probe for Zinc Ions

A new 3,5‐disubstituted pyridine with two porphyrin moieties was prepared through an efficient synthetic approach involving 2‐formyl‐5,10,15,20‐tetraphenylporphyrin (1), piperidine, and catalytic amounts of [La(OTf)3]. 3,5‐Bis(5,10,15,20‐tetraphenylporphyrin‐2‐ylmethyl)pyridine (2) was fully characterized and its sensing ability towards Zn2+, Cu2+, Hg2+, Cd2+, and Ag+ was evaluated in solution by absorption and fluorescence spectroscopy and in gas phase by using matrix‐assisted laser desorption/ionization (MALDI)‐TOF mass spectrometry. Strong changes in the ground and excited state were detected in the case of the soft metal ions Zn2+, Cd2+, Hg2+, and Cu2+. A three‐metal‐per‐ligand molar ratio was obtained in all cases and a significant ratiometric behavior was observed in the presence of Zn2+ with the appearance of a new band at 608 nm, which can be assigned to a metal‐to‐ligand charge transfer. The system was able to quantify 79 ppb of Zn2+ and the theoretical calculations are in accordance with the stoichiometry observed in solution. The gas‐phase sensorial ability of compound 2 towards all metal ions was confirmed by using MALDI‐TOF MS and in solid state by using polymeric films of polymethylmethacrylate (PMMA) doped with ligand 2. The results showed that compound 2 can be analytically used to develop new colorimetric molecular devices that are able to discriminate between Hg2+ and Zn2+ in solid phase. The crystal structure of ZnII complex of 3,5‐bisporphyrinylpyridine was unequivocally elucidated by using single‐crystal X‐ray diffraction studies. Selective sensing: The treatment of 2‐formyl‐5,10,15,20‐tetraphenylporphyrin (1) with piperidine catalyzed by [La(OTf)3], affords 2 as a new ratiometric Zn2+ molecular probe (see figure). Compound 2, when supported in polymethylmethacrylate (PMMA) films, is able to distinguish between Zn2+ and Hg2+.