N‐Confused Porphyrin – A Unique “Turn‐On” Chemosensor for CN− and F− ions and “Turn‐Off” Sensor for ClO4− ions

Abstract

N-Confused meso-tetrakis(4-carbomethoxyphenyl)porphyrin (1) and its Ni(II) complex (1 a) have been synthesized and utilized for anion sensing studies, and the results are compared with N-confused meso-tetraphenylporphyrin (NCTPP). Anion susceptibilities of 1 and 1 a were investigated using spectroscopic, electrochemical, and DFT studies. Porphyrins 1 and 1 a were able to detect CN- , F- , and ClO4 - ions selectively over the tested set of anions even at ppm level. Interestingly, the addition of ClO4 - ions resulted in fluorescence quenching (turn off) whereas the addition of F- or CN- resulted in fluorescence enhancement (turn on). Notably, the TFA addition resulted in fluorescence quenching, whereas the fluorescence enhancement was observed while adding TBAOH. The higher association constant (Ka ) values with anions, lower detection limit, and shifts in redox potentials are due to the electron-withdrawing effect of the -COOCH3 group at the para-position of the meso-phenyl ring. This electron-withdrawing nature is crucial for the higher affinity towards anions. The anion sensing description in this article may not only unveil the built-in nature of N-confused porphyrins, but may also provide a general proposal for the development of novel anion sensors based on porphyrinoids. The electron-deficient porphyrin framework, large polarisable π-system, and anion binding through the outer NH or a combination of the above factors serve as a foundation for N-confused porphyrin to act as an anion sensor.