Exploring Colorimetric Real-Time Sensing Behavior of a Newly Designed CT Complex toward Nitrobenzene and Co2+: Spectrophotometric, DFT/TD-DFT, and Mechanistic Insights

An exceptionally unique, easy-to-prepare, and economic charge transfer complex (CTC), (IMH)+(PA)-, was synthesized as a highly selective real-time colorimetric chemosensor material for nitro explosive nitrobenzene (NB) and Co2+ ion. Co2+ and NB are highly potential toxic and hazardous beyond the exposure limits and also classified as carcinogens (group 2B) by IARS and United States Environmental Protection Agency. Unusual sensing ability with appreciatively low detection limits of 0.114 and 0.589 ppb for NB and Co2+ ion, respectively, in the aqueous medium of dimethyl sulfoxide has been reported for the first time among this class of complexes reported so far. The mechanism of the tremendous sensing behavior of this material as chemosensor was ascertained by static quenching mechanism, Dexter electron transfer, and Forster resonance energy transfer dynamic quenching mechanism, which was supported by spectral overlapping and density functional theory (DFT) (B-3LYP/def2-SVP) calculations. Real-time colorimetric sensing behavior of chemosensor was demonstrated by the naked eye test and prestained paper Co2+ strip test. Job's plot and comparative Fourier transform infrared (FTIR) study between CTC and CTC-Co2+ complex revealed the coordination mode between CTC and Co2+ ion and 2:1 stoichiometry. This sensing material (IMH)+(PA)- was synthesized with donor imidazole (IM) and acceptor picric acid (PA), and its characterization was achieved by experimental (single-crystal X-ray diffraction, thermal gravimetric analysis-differential thermal analysis, FTIR, and UV-vis studies) and theoretical methods DFT/TD-DFT calculations, comparing experimental-theoretical data and obtaining MEP map along with electronic energy gap of HOMO → LUMO (ΔE = 3.545 eV) and Hirshfeld surfaces analysis. The SC-XRD confirms the composition and bonding features, which show hydrogen bond via N+-H···O- between IM and PA. This N+-H···O- interaction plays a significant role in Co2+ binding, proving this method of synthesizing CTC as a chemosensor to be a novel approach.