Porphyrin Chemodosimeters: Synthesis, Electrochemical Redox Properties and Selective ‘Naked-eye’ Detection of Cyanide Ions

Dicyanovinyl appended β-pyrrole substituted porphyrins have been synthesized and utilized as chemodosimeters for ratiometric and colorimetric sensing of cyanide ions. In addition to meso-tetraphenylporphyrin (TPP) based system, we have designed and synthesized octaphenylporphyrin (OPP) based moderately nonplanar systems. These systems also depicted effective π-conjugation between the pyrrole ring and its olefinic substituent which is reflected in the UV-vis absorption spectra as the red-shifting of both Soret and Q-bands and possibly the splitting of the absorption Soret band. This article describes the single crystal X-ray structure, electronic spectral and electrochemical redox properties of these porphyrin chemodosimeters. The stable anionic-porphyrin species formed after addition of CN‒ ions has a blue shifted unperturbed absorption spectrum due to the disruption of electronic resonance and charge-transfer interactions between the β-dicyanovinyl acceptor and porphyrin core. The probes exhibited excellent selectivity towards CN‒ over other anions such as F‒ , Cl‒ , Br‒ , NO3 ‒ , H2PO4 ‒ , AcO‒ , I‒ , HSO4 ‒ , ClO4 ‒ and PF6 ‒ . The action of the chemodosimeters were investigated by 1 H-NMR titrations, ESI-mass spectrometry, electrochemical studies and DFT calculations which supported the formation of cyanide addition adduct (porphyrin–CN)‒ . These chemodosimeters depicted lowest detection limit (LOD) of 0.023-0.082 ppm, which is very much lower than the permissible limit (0.2 ppm) and other porphyrinoids reported till date. These porphyrins are also able to detect cyanide ions in (9:1, v/v) CH3CN:H2O mixture with low detection limits thereby providing practical applicability of these chemodosimeters.