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.