A series of 2‐arylbenzothiazoles has been designed and synthesized as chromogenic and fluorescent probes for the detection of picric acid (PA) over other nitro analytes. A straightforward condensation of a variety of aryl nitriles with 2‐aminobenzenethiol under metal‐ and base‐free conditions afforded the corresponding products in 26–98% yields. All 2‐arylbenzothiazoles could efficiently sense PA at micromolar levels. The highly sensitive compound 2‐(4‐((2‐methyl‐1H‐imidazol‐1‐yl)methyl)phenyl)benzo[d]thiazole (3c) showed 95% fluorescence quenching on addition of 1.0 eq of PA which can be attributed to hydrogen bonding and ground state charge transfer complex formation with a detection limit of 19.0 μM. The high sensitivity of 3 c for PA detection was also established by test strips experiment. The sensing mechanism was explored by 1H NMR titrations and Job's plot.

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A series of dicationic N, N’‐disubstituted benzimidazolium salts with Br− and PF6− as the counter anions were synthesized in 75–98 % yields. The single crystal X‐ray diffraction studies of one of the benzimidazolium salts confirmed the structure of these dicationic compounds. These salts exhibited a selective “turn‐off” fluorescence response toward Fe3+ ions in aqueous solution over the other competitive metal ions such as Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe2+, K+, Mg2+, Na+, Ni2+, Pb2+, and Zn2+. Upon addition of 5 equivalents Fe3+ ions, emission intensity was quenched by 94–99 % in pure aqueous media. Detection limits for all probes with Fe3+ ions were found in micromolar range. Further, drug combination analysis was conducted by evaluating the combinatorial treatment effects of two benzimidazolium salts with doxorubicin on breast cancer cells using the Chou‐Talalay method and the Highest Single Agent (HSA) model, and it was found that these compounds showed synergism with doxorubicin. Further, molecular docking studies revealed the best docking score (−6.11) for one of the benzimidazolium salts, and explained its binding affinity with checkpoint kinase 2 (chk2) protein.

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