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.一种极具独特性、易于制备且成本低廉的电荷转移配合物（charge transfer complex, CTC）[(IMH)+(PA)−]被成功合成，可作为高选择性的实时比色化学传感材料，用于检测硝基爆炸物硝基苯（nitrobenzene, NB）与钴离子（Co²+）。钴离子与硝基苯均具有极高毒性，超出安全暴露限值，且被IARS与美国环境保护署归类为2B类致癌物。在该类已报道的配合物中，本研究首次报道了该材料在二甲基亚砜水溶液中对硝基苯与钴离子的非凡传感性能，其检测下限分别低至0.114 ppb与0.589 ppb。该材料作为化学传感器的优异传感机制通过静态猝灭机制、德克斯特电子转移以及福斯特共振能量转移动态猝灭机制得以阐明，上述结论得到了光谱重叠实验与密度泛函理论（density functional theory, DFT）（B3LYP/def2-SVP）计算的支持。该化学传感器的实时比色传感性能可通过裸眼测试与预染色试纸钴离子检测条实验得以验证。通过Job曲线与电荷转移配合物及其与钴离子形成的配合物之间的对比傅里叶变换红外光谱（Fourier transform infrared, FTIR）研究，阐明了该电荷转移配合物与钴离子的配位模式及2:1的化学计量比。本传感材料[(IMH)+(PA)−]以给体咪唑（imidazole, IM）与受体苦味酸（picric acid, PA）合成得到，其表征通过实验手段（单晶X射线衍射、热重分析-差热分析、傅里叶变换红外光谱与紫外-可见光谱研究）与理论方法[密度泛函理论/含时密度泛函理论（DFT/TD-DFT）计算、实验与理论数据比对、获取分子静电势图（MEP map）以及最高占据分子轨道→最低未占据分子轨道（HOMO→LUMO）的电子能隙（ΔE=3.545 eV）、希尔施菲尔德表面分析]完成。单晶X射线衍射（SC-XRD）证实了该配合物的组成与成键特征，其显示咪唑与苦味酸之间通过N+–H···O–形成氢键。该N+–H···O–相互作用在钴离子结合过程中发挥关键作用，证明了本合成化学传感器用电荷转移配合物的方法是一种全新的策略。