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 (<b>hhnbn</b>) 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, <b>hhnbn</b> 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 <b>hhnbn</b> 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 <b>hhnbn</b> is substantiated by ¹H NMR titration. The functioning of <b>hhnbn</b> 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 <b>hhnbn</b> and [<b>hhnbn-CN</b>]^- adduct. TD-DFT results revealed that the appearance of an additional band (∼504 nm) in the absorption spectrum of the [<b>hhnbn-CN</b>]^- adduct is assigned to the electronic transition from the HOMO-3 orbital to the LUMO orbital. In addition, <b>hhnbn</b> is capable for serving as a practical colorimetric probe for the detection of CN^- ion in the real-time analysis of water samples.