1-(p-Terphenyl)-benzimidazolium (TRIPOD-TP) molecules undergo self-assembly to form rodlike structures in aqueous medium, as shown by field-emission scanning electron microscopy, transmission electron microscopy, and dynamic light scattering studies. Upon gradual addition of picric acid (PA), these aggregates undergo an aggregation/disaggregation process to complex morphological structures (10(-12)-10(-10) M PA) and spherical aggregates (10(-9)-10(-8) M PA). These spherical aggregates undergo further dissolution to well-dispersed spheres between 10(-7)-10(-6) M PA. During fluorescence studies, these aggregates demonstrate superamplified fluorescence quenching (>97%) in the presence of 10(-5) to 0.2 equiv of the probe concentration, an unprecedented process with PA. The lowest detection limits by solution of TRIPOD-TP are 5 × 10(-13) PA, 50 × 10(-12) M 2,4-dinitrophenol, 200 × 10(-12) M 2,4,6-trinitrotoluene, and 1 nM 1-chloro-2,4-dinitrobenzene. Paper strips dipped in the solution of TRIPOD-TP demonstrate quantitative fluorescence quenching between 10(-17) and 10(-6) M PA using front-surface steady state studies and can measure as low as 2.29 × 10(-20) g/cm(2) PA.

The combination of steric gearing in substituted triethylbenzene and linear structures of 1‐(4‐biphenyl)(benz)imidazolium moieties have been used to design and synthesize tripodal chemosensors with preorganized cavities for detection of picric acid (PA) in (4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES) buffer. The two chemosensors undergo highly selective fluorescence quenching with PA and lead an approximate tenfold increase in KSV values (3.57×105 M−1 and 2.67×105 M−1) and greater than 2000 times higher sensitivity than earlier reported benzimidazolium‐based chemosensors. The upfield shift of PA protons (Δδ≈0.8) in 1H NMR titration studies reveals encapsulation of PA in the cavity of the sensors. In water, one of the sensors can detect 1.0 nM PA as the lowest detection limit and TLC strips doped with the same sensor can detect 137 ag cm‐2 of PA under UV illumination. Thin films of this sensor undergo fluorescence quenching in the presence of PA vapor in less than 120 seconds.

Dipod 1 bearing two pyrenyl tethered benzimidazolium moieties gives a fluorescence spectrum in aqueous medium which reveals a structured emission band between 330-400 nm and a broad emission band centered at 475 nm, respectively due to monomer and excimer emission of the pyrene moieties. The presence of an excimer emission band points to the formation of a pseudocyclic structure. Dipod 1 undergoes highly selective fluorescence quenching of the excimer emission band in the presence of iodide ions, whereas the fluorescence intensity of the monomer emission band remains stable. The ratio of fluorescence intensity I395 nm/I475 nmvs. log [I(-)] undergoes a linear change over a broad iodide concentration range of 10(-9) to 10(-5) M with KSV 3.7 × 10(5) M(-1). Dipod 1 can be used to determine iodide ions in urine samples, tap water and sea water conditions with 1 nM iodide as the lowest detection limit. On using paper strips coated with dipod 1, ∼1.7 pg cm(-2) iodide ions could be detected. Dipod 1 shows a fluorescence quenching response to 100 nM iodide ions in C6 glioma cells using confocal microscopy. The life time of the dipod 1 shows a linear decrease with log [I(-)] and points to coordination based recognition of iodide ion.

The imidazolium derivatives due to their positive charge possess one of the most polarized and positively charged proton at C2-H to form strong ionic hydrogen bond (also termed as double ionic hydrogen bond) with anions and also provide opportunities for anion - π interactions with electron-deficient imidazolium ring. In the present review article, imidazolium based molecular probes for their ability to recognize inorganic anions like halides, cyanide, perchlorate, carboxylic acids, phosphate, sulfate etc. and their derived molecules viz. nucleotides, DNA, RNA, surfactants, proteins, etc have been discussed. The review covers the literature published after year 2009 and has > 130 references. The previous literature has already been discussed by Yoon et al. in two review articles published in Chem. Soc. Rev. 2006 and 2010.

Aggregation of the molecular probe increases its fluorescence sensitivity towards nitroaromatic explosives by the order of 103to 105.

An aggregation–disaggregation based molecular probe allows the detection of 10 pM ClO4−in solution and 6 × 10−18g cm−2ClO4−in the solid state by a contact mode method with a selectivity of ∼10 000 over other inorganic anions.

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