Colorimetric 'Naked-Eye' Chemodosimeter for the Detection of CN^- Ion: Experimental and Theoretical Studies

An azo-based chemodosimeter (N'E)-3-hydroxy-N'-(2-hydroxy-5-((4-nitrophenyl)diazenyl) benzylidene)-2-naphthohydrazide (hhnbn) has been synthesized by a two-step method and characterized by FT-IR and ¹H NMR spectroscopies and ESI-MS. Upon addition of CN^- ion, hhnbn produces a new absorption band at ∼510 nm and, subsequently, red color in H₂O/DMSO (7:3 v/v) medium, which is noticeable under naked eye condition selectively. Moreover, Whatman filter paper test strips, previously soaked in hhnbn solution and dried, are detected to generate red color in the presence of CN^- ion selectively. Nucleophilic addition reaction of the CN^- ion to the imine carbon (CH=N) of hhnbn is substantiated by ¹H NMR titration. The functioning of hhnbn with CN^- as a chemodosimeter is also validated by ESI-MS data. In order to supplement the experimental results, DFT and TD-DFT calculations are performed to explore the spectroscopic signature detection of the CN^- ion by hhnbn and [hhnbn-CN]^- adduct. TD-DFT results revealed that the appearance of an additional band (∼504 nm) in the absorption spectrum of the [hhnbn-CN]^- adduct is assigned to the electronic transition from the HOMO-3 orbital to the LUMO orbital. In addition, hhnbn is capable for serving as a practical colorimetric probe for the detection of CN^- ion in the real-time analysis of water samples.