Traditional analyte-specific synthetic receptors or sensors have been developed on the basis of supramolecular interactions (e.g., hydrogen bonding, electrostatics, weak coordinative bonds). Unfortunately, this approach is often subject to limitations. As a result, increasing attention within the chemical sensor community is turning to the use of analyte-specific molecular indicators, wherein substrate-triggered reactions are used to signal the presence of a given analyte. This tutorial review highlights recent reaction-based indicator systems that have been used to detect selected anions, cations, reactive oxygen species, and neutral substrates.

A new set of spectroscopic tools is proposed that may be used to distinguish antiaromatic compounds from their corresponding aromatic congeners. This prediction is based on a detailed analysis of the optical and photophysical properties of a matched set of expanded porphyrins. In particular, the antiaromatic porphyrinoids having [4n] π-electrons within their conjugation pathway exhibit distinct photophysical features that differ dramatically from what is observed for the corresponding aromatic congeners. The clear diagnostic differences seen between the antiaromatic and aromatic compounds leads us to propose that the spectroscopic methods detailed in this Perspective could emerge as general tools that may be used to characterize the electronic characteristics of complex systems for which a number of potential electronic states can be envisioned on the basis of simple line formulas or analyses of π-electron populations. W ith a storied history dating back to the days of early interest in benzene, the concept of aromaticity continues to excite the imagination of chemists, even as its importance has become increasingly apparent in fields as diverse of biomedicine and materials science. Not surprisingly, therefore, considerable effort has been devoted to exploring and understanding aromaticity. As a consequence, aromaticity has come to be defined in terms of five key experimental parameters, namely, energetics, structure, reactivity, magnetism, and spectroscopic features. 1 While the chemical and physical properties of many canonical aromatic systems have been investigated in detail, a complete understanding of the underlying structure-property relationships, including H€ uckel's [4n þ 2] rule and other types of quantitative indices of aromaticity, 2-7 still remains elusive. 8,9 This is even more true in the case of antiaromaticity.

The synthesis of a new series of 2,3-diindol-3'yl quinoxalines (DIQ), as well as a comparison of their anion recognition properties to those of our previously reported pyrrole based sensors, 2,3-dipyrrol-2'yl quinoxalines (DPQ) is reported. To the best of our knowledge, this new DIQ system represents the first example of a free-standing indole-based small molecule receptor for which evidence of anion binding is available both in solution and in the solid-state. It also provides one of the few structurally characterized neutral receptor-dihydrogen phosphate complexes. This work thus serves to demonstrate the utility of indoles as an anion recognition motif.Anions are ubiquitous in biological milieus. Among the most important physiologically relevant anions is inorganic phosphate. Not only does it play a critical role in energy storage and signal transduction, 1 it recognized as essential structural element in teeth and bones. 2 On the other hand, elevated serum phosphate levels can result in calcification of tissues 3 and is a recognized problem in patients suffering from chronic kidney failure. Phosphate is also a key pollutant whose role in the eutrophication of waterways is well recognized. 4 This diversity of function has stimulated considerable current interest devoted to the recognition and sensing of phosphate anions. 5 Increasingly, efforts in this area have focused on the use of acyclic small molecule receptors, 4 including those based on amide, sulfonamide, urea, and pyrrole recognition units. While each of these motifs presents certain advantages, there is incentive to explore additional putative binding subunits that could be used to generate new receptor systems. One such motif is indole, a potential hydrogen bond donor that has yet to be exploited extensively in the area of anion sensor development. Indeed, currently only indolocarbazoles 1 6 and bis(indolyl)methane, 2 7 are known to the best of our knowledge, and, in the case of the latter system, quantitative binding studies were not fully carried out. Thus, there remains a need to explore the fundamental anion binding properties (e.g., selectivity, affinity) of indolebased receptors. Towards this end, we report here the synthesis of a new series of 2,3-diindol -3'yl quinoxalines (DIQ, 6 and 7), as well as a comparison of their anion recognition properties to those of our previously reported pyrrole based sensors, 2,3-dipyrrol-2'yl quinoxalines (DPQ, 3). 8 To the best of our knowledge, this new DIQ system, which relies on β-connectivity, represents the first example of an indole-based small molecule receptor for which evidence of anion binding is available both in solution and in the solid-state. It also provides one of the few structurally characterized neutral receptor-dihydrogen phosphate complexes. 9 sessler@mail.utexas.edu. Supporting Information Available: Details describing the synthesis and characterization of compound 6 and 7, details of fitting binding curves, and crystallographic data (CIF). This material is available free of ch...

On the basis of two-photon absorption and time-resolved spectroscopic measurements, as supported by theoretical calculations of quantitative aromaticity, a relationship between the nonlinear optical properties and aromaticity index has been established for a series of four fully conjugated pentapyrrolic expanded porphyrins, namely pentaphyrin (1.1.1.1.1), sapphyrin (1.1.1.1.0), isosmaragdyrin (1.1.1.0.0), and orangarin (1.0.1.0.0), all of which proved amenable to study in dichloromethane.

The benzil-cyanide reaction is a cyanide-specific reaction that has been exploited to produce a colorimetric indicator for this toxic anion. This was done by producing a pi-extended analogue of benzil, 7, which is soluble in a 70:30 (v/v) mixture of methanol-water. In this medium, dilute solutions of 7 are yellow but produce colorless products when exposed to low concentrations of cyanide anion (> or = 1.7 microM; added as an aqueous NaCN solution), but no other common anions (e.g., OH(-), F(-), N3(-), benzoate(-), and H2PO4(-)). On the basis of these observations and supporting mechanistic analyses, it is concluded that the modified benzil system 7 is a promising cyanide anion indicator that is attractive in terms of its selectivity, ease-of-use, water compatibility, and the low, naked-eye discernible cyanide detection limit it provides.

The synthesis of an inverted, methoxylated sapphyrin derivative is described. This system, wherein inversion of pyrrolic nitrogen atoms is configurationally enforced via the use of a 3,3‘-bipyrrolic precursor, displays what is best described as “weak aromaticity” as judged from its spectroscopic features and supporting theoretical calculations.

Single wall carbon nanotubes (SWNTs) bind strongly to sapphyrins, quintessential pentapyrrolic "expanded porphyrin" macrocycles, through donor-acceptor stacking interactions. The specific use of a functionalized sapphyrin diol yields stable water-suspendable nanotubes and also permits the formation of well-defined assemblies in ionic liquids. The absorption and steady-state fluorescence spectra of the resulting noncovalently functionalized nanotube complexes have been analyzed in aqueous media and ionic liquids, yielding a description of the photophysical properties of the nanotube-sapphyrin complexes as donor-acceptor species for light-harvesting.

The first synthesis of meso-fused carbaporphyrin via a premodification method was accomplished by substituting two pyrrole moieties and one meso-carbon with 2-(naphthalen-1-yl)thiophene. The obtained global π-conjugation pathway of the macrocycle noticeably disturbs the 10π local aromaticity of naphthalene, and its aromatic nature was supported by NMR spectroscopy together with nucleus-independent chemical shift, anisotropy of the induced current density, and harmonic oscillator stabilization energy calculations. In addition, the meso-fused carbaporphyrin also allowed the formation of a square planar Pd(II) complex.