Porphyrin chemodosimeters: synthesis, electrochemical redox properties and selective 'naked-eye' detection of cyanide ions

Dicyanovinyl appended beta-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 pi-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 beta-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 H-1-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.