Development of Ion Chemosensors Based on Porphyrin Analogues

Review pubs.acs.org/CR

Development of Ion Chemosensors Based on Porphyrin Analogues Yubin Ding,†,‡ Wei-Hong Zhu,† and Yongshu Xie*,† †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ Department of Biomedical Engineering, College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, China ABSTRACT: Sensing of metal ions and anions is of great importance because of their widespread distribution in environmental systems and biological processes. Colorimetric and fluorescent chemosensors based on organic molecular species have been demonstrated to be effective for the detection of various ions and possess the significant advantages of low cost, high sensitivity, and convenient implementation. Of the available classes of organic molecules, porphyrin analogues possess inherently many advantageous features, making them suitable for the design of ion chemosensors, with the targeted sensing behavior achieved and easily modulated based on their following characteristics: (1) NH moieties properly disposed for binding of anions through cooperative hydrogen-bonding interactions; (2) multiple pyrrolic N atoms or other heteroatoms for selectively chelating metal ions; (3) variability of macrocycle size and peripheral substitution for modulation of ion selectivity and sensitivity; and (4) tunable near-infrared emission and good biocompatibility. In this Review, design strategies, sensing mechanisms, and sensing performance of ion chemosensors based on porphyrin analogues are described by use of extensive examples. Ion chemosensors based on normal porphyrins and linear oligopyrroles are also briefly described. This Review provides valuable information for researchers of related areas and thus may inspire the development of more practical and effective approaches for designing high-performance ion chemosensors based on porphyrin analogues and other relevant compounds.

CONTENTS 1. Introduction 2. General design strategies for pyrrole-based chemosensors 3. Porphyrin analogues for ion sensing 3.1. Corrole-based ion chemosensors 3.1.1. Corroles for anion sensing 3.1.2. Corroles for metal ion sensing 3.2. Expanded porphyrin-based chemosensors 3.2.1. Expanded porphyrins for anion sensing 3.2.2. Expanded porphyrins for metal ion sensing 3.3. Calix[4]pyrrole-based chemosensors 3.3.1. Anion-binding properties of calix[4]pyrroles 3.3.2. Calix[4]pyrroles for anion sensing 3.3.3. Calix[4]pyrroles for the recognition of metal ions and ion pairs 3.4. N-Confused porphyrin-based chemosensors 3.4.1. N-Confused porphyrins for anion sensing 3.4.2. N-confused porphyrins for metal ion sensing 3.5. Calixphyrin-based chemosensors 3.6. Other porphyrin analogues for ion sensing 4. Porphyrin-based ion chemosensors 4.1. Porphyrins for anion sensing © 2016 American Chemical Society

4.2. Porphyrins for metal ion sensing 5. Linear oligopyrroles for ion sensing 5.1. Linear oligopyrroles for anion sensing 5.2. Linear oligopyrroles for metal ion sensing 6. Conclusions and Future Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION The detection of environmental pollutants or biologically important species has attracted substantial attention because the reliable and quantitative determination of these analytes provides essential information for properly disposing contaminated environmental and biomedical materials.1,2 In this respect, the development of small organic molecule based sensors has been demonstrated to be promising due to their advantages of low cost, wide applicability, and ease of manipulation. Thus, a

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: January 11, 2016 Published: April 14, 2016 2203

DOI: 10.1021/acs.chemrev.6b00021 Chem. Rev. 2017, 117, 2203−2256

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typical synthetic precursors for porphyrins and porphyrin analogues and their application is a simple matter for porphyrin chemists. Subsequent to these sections, brief conclusions and perspectives will be given in section 6. As mentioned above, porphyrin analogues exhibit greater diversity for the design of ion chemosensors relative to simple porphyrins and linear oligopyrroles, and no review so far published has systematically described ion chemosensors based on porphyrin analogues. With this in mind, here we emphasize the ion-sensing properties of porphyrin analogues as the main topic of this Review, and we have attempted to comprehensively review the excellent available examples of these materials.

number of high-performance sensors have been developed to detect a wide range of important environmental and biomedical target analytes, including metal ions, anions, small organic molecules, DNA, RNA, proteins, and even cells, bacteria, and viruses.3−11 From this point-of-view, the development of sensors with near-infrared excitation/emission wavelengths, excellent tolerance of complex detection environments, good stabilities, and high reproducibility is of significant current interest. Porphyrins are naturally occurring macrocyclic compounds containing four pyrroles connected in macrocyclic fashion through four methine carbons at their α-positions.12 Synthetic porphyrins have been developed as powerful tools for applications in various fields owing to their advantageous properties of strong light absorption, high emission, and rich coordination chemistry.13−20 The family of porphyrin macrocycles has been expanded by investigations of porphyrin analogues, which have attracted increasing attention.21−27 Compared to normal porphyrins, porphyrin analogues show greater structural diversity, unprecedented coordination properties, light absorption/emission, aromaticity, etc.28−33 The unique properties of porphyrin analogues thus enable their applications in various fields, including sensing.34−46 The large family of porphyrin analogues consists of diverse structures that may be roughly divided into ring-expanded porphyrins, ring-contracted porphyrins, porphyrin isomers, and heteroporphyrins, according to the numbers of pyrrolic units in the molecules or the presence of hetero-donor atoms, such as O, S, Se, and Te instead of pyrrolic N within the porphyrin core. Due to this structural diversity, porphyrin analogues show inherent advantages for sensing target analytes with the targeted sensing behavior achieved and easily modulated in the following ways: first, conformations of the molecules can be modulated through structural variation so that the pyrrolic NH moieties are properly arranged for selective anion binding through cooperative hydrogen-bonding interactions; second, multiple pyrrolic N atoms can be employed to selectively chelate metal ions, including by the replacement of pyrrolic N with other heteroatoms, leading to unique metal ion binding affinity and selectivity; third, high selectivity and sensitivity toward target anions and metal ions could be further achieved by the modification of binding affinities through variation of the macrocycle size. Additionally, porphyrin analogues such as expanded porphyrins may undergo fluorescence emission at wavelengths longer than 600 nm, even reaching into the nearinfrared region, making them more suitable for applications involving biological samples because autofluorescence emission from biological tissues can be avoided.47−49 This Review consists of six sections. Following this Introduction, the general design strategies for pyrrolic chemosensors will be briefly described in section 2. After that, we intend to emphasize the ion-sensing properties of porphyrin analogues in section 3, considering their structural diversity suitable for designing ion chemosensors. Thus, ion chemosensors based on porphyrin analogues will be described in detail with extensive examples illustrating the design strategies, sensing mechanisms, and sensing performance. In section 4, ion chemosensors using normal porphyrins will be briefly described considering the fact that normal porphyrins share many common physical and chemical properties with porphyrin analogues, and the design strategies for porphyrin-based ion chemosensors are insightful for the future design of ion chemosensors based on porphyrin analogues. In section 5, examples of ion chemosensors based on linear oligopyrroles are briefly introduced, because they are

2. GENERAL DESIGN STRATEGIES FOR PYRROLE-BASED CHEMOSENSORS The selective recognition of target analytes using molecular chemosensors can be achieved either by supramolecular interactions or through covalent bond formation/cleavage.50−54 If a selective recognition process is accompanied by vivid changes in the photophysical properties, then the system can be employed for sensing applications.55,56 In the past decade, various fluorophores, such as cyanine dyes,57 boron dipyrromethene (BODIPY),58−60 porphyrin,61 coumarin,62,63 anthracene,64 naphthalimide,65 fluorescein,66−68 and rhodamine,69,70 have been incorporated as the signal-reporting unit of organic molecule based chemosensors. The framework of organic molecule platforms for sensing applications can be roughly classified into two main types, as shown in Scheme 1. Type 1 Scheme 1. Two Commonly Employed Types of Organic Molecule Based Sensing Platforms: (a) General Working Scheme of Sensing Platform Type 1 (Note, the Linker between the Recognition Unit and the Reporter May Be Omitted); (b) General Working Scheme of Sensing Platform Type 2

consists of two basic working units, namely, the reporter unit and the recognition unit, and in some cases, a linker between the reporter and recognition unit is needed. For this type, the recognition unit is incorporated to selectively interact with the target analyte. During interaction processes, the photophysical properties of the reporting moiety are perturbed and a signal will be reported. Several approaches have been developed for the design of this type of chemosensor based on various photophysical processes, such as photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence/ Förster resonance energy transfer (FRET), aggregation-induced emission (AIE), and excimer/exciplex formation.71−74 Obviously, it is essential to precisely control the electron-, charge-, or 2204


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energy-transfer processes between the recognition unit and the reporter unit. To improve the sensing behavior of this type of chemosensor, it is necessary to design functionalized molecular systems with properly organized recognition and reporter moieties. In contrast, type 2 systems consist of only one component, which acts simultaneously as the recognition and the reporting unit (Scheme 1). For this reason, synthesis is relatively simple, and there is no need to modulate complicated electron-, charge-, or energy-transfer processes. The most commonly developed chemosensors of this type are based on so-called chelationenhanced fluorescence (CHEF).75,76 In these cases, the molecular structures of the chemosensors are usually flexible, which causes the loss of excited-state energy via free rotation of C−C single bonds, thus resulting in weak fluorescence. Upon binding with the target metal ion or anion, the fluorescence could be enhanced due to the formation of more rigid molecular structures or assemblies. For successfully developing such chemosensors, it is vital to employ a suitable building block that may be utilized to construct chromophores with inherent binding moieties for target analytes. In this respect, pyrrole is a versatile and promising candidate. Pyrrole is a N-containing five-membered heterocycle, which tends to be condensed to afford linear and cyclic oligopyrrolic compounds that exhibit attractive and tunable color and fluorescence.77 The multiple imino N and amino NH moieties of these oligopyrrolic compounds can be utilized for cooperatively binding metal ions and anions through metal coordination and hydrogen bonds, respectively.78−81 Thanks to these structural and photophysical characteristics, oligopyrrolic compounds can play the roles of both the recognition unit and the reporter in sensor design. Thus, sensors of type 1 can be constructed either by combining an oligopyrrolic recognition moiety with a chromophore or by introducing a recognition unit to an oligopyrrolic chromophore. More importantly, the inherent light absorption/emission and ion-binding properties of oligopyrroles enable their applications as type 2 chemosensors, with the oligopyrrolic moiety functioning simultaneously as the reporter and recognition unit. Thus, we will herein describe ion chemosensors based on diverse pyrrole-derived compounds.

Scheme 2. Framework Structures of Representative Porphyrin Analogues

dianionic form, and thus they show unique metal ion coordination capabilities. For example, they are able to stabilize unusual high oxidation states of transition metals.85 Shortly after the discovery of corrole, an expanded porphyrin analogue named sapphyrin was serendipitously obtained by Woodward and further developed by Woodward, Broadhurst, Johnson, and coworkers.86−88 In 1990, a more efficient synthetic method for sapphyrin was developed by Sessler and co-workers. With the aid of X-ray diffraction and 19F NMR analyses, they even accidentally discovered that sapphyrin can bind a F− anion in its protonated core.89,90 In addition to the ring-contracted and -expanded porphyrinoids, porphyrin isomers are also an important class of porphyrin analogues. In 1986, a porphyrin isomer, named porphycene, was first reported by Vogel and co-workers.91 The planar and aromatic structure of porphycene consists of two 2,2′bipyrrole subunits linked by two ethylene moieties. Since then, porphycenes have been demonstrated to be very stable, and thus they have been widely used for various application purposes because of their unique coordination and optical properties.92−96 In 1994, another important porphyrin isomer, N-confused porphyrin, which contains an α,β′-linked pyrrole (N-confused pyrrole) in the tetrapyrrolic macrocycle, was first reported independently and almost simultaneously by the Furuta and Latos-Grażyński groups.97,98 The pyrrolic NH from the Nconfused pyrrole lies at the periphery of the macrocycle, thus leading to their unique anion-binding abilities using the peripheral NH.99 On the other hand, the N atoms within the porphyrin macrocycle could be systematically replaced with other heteroatoms to afford another family of conjugated porphyrin analogues called heteroporphyrin.100,101 Since the first report in 1969, a number of heteroporphyrins have been developed by the replacement of the N atoms with heterodonor atoms such as O, S, Se, and Te.102−107 In addition, heteroporphyrins can be modified by combination with ring-expanded and -contracted porphyrins to further manipulate the metal ion coordination properties.108 Compared with the aforementioned conjugated porphyrin analogues, a nonconjugated porphyrin analogue, calix[4]pyrrole, was actually synthesized much earlier in 1886 by Baeyer through acid-catalyzed condensation of pyrrole with acetone.109 However, further usage of this unique nonconjugated structure had

3. PORPHYRIN ANALOGUES FOR ION SENSING The detection of a certain ion is of great importance because of the fact that a number of metal ions and anions could be toxic or biologically important.82 As mentioned above, pyrrole-based compounds show diverse structures and properties suitable for sensing both metal ions and anions. Among these compounds, porphyrin analogues are a family of macrocyclic oligopyrroles that show unique and tunable properties in light absorption/ emission, metal coordination, and anion binding that have attracted intensive attention in designing ion chemosensors. The family of porphyrin analogues is quite large, which includes both conjugated oligopyrrolic macrocycles and nonconjugated oligopyrrolic macrocycles (Scheme 2). The advantages of using porphyrin analogues as ion chemosensors were gradually revealed accompanied with the synthetic history of porphyrin analogues, which dated back to the artificial synthesis of vitamin B12, during which period several conjugated porphyrin analogues were discovered.83 One representative of these compounds is a contracted porphyrin analogue, named corrole, which was first synthesized by Johnson et al. in 1960.84 Different from porphyrins, corroles exhibit smaller macrocyclic core sizes and prefer to coordinate as trianionic ligands rather than in the 2205


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Figure 1. (a) Chemical structure of cobalt(III) corrole-based NO2− sensing molecule 1; (b) absorption spectral changes of the NO2− optical sensing film that contains 20 mmol/kg of 1 as the ionophore under varying NO2− levels (0.1 μM to 0.1 M) in 50 mM phosphate buffer solutions at pH = 4.5. Reprinted with permission from ref 131. Copyright 2014 Elsevier.

ionophore is ∼5 μM. Later, 1 was further incorporated into plasticized poly(vinyl chloride) films for fabricating a bulk optode for sensitive, fast, and fully reversible colorimetric detection of NO2− (Figure 1b).131 The obtained chemosensor film based on 1 shows a dramatically enhanced selectivity toward NO2− over other anions including lipophilic anions like thiocyanate and perchlorate. In addition to the applications in plasticized polymeric membrane electrodes and optodes, corroles can also be directly solubilized in solvents for anion sensing. As demonstrated by Lodeiro, Neves, and their co-workers, both corrole 2 and the Ga(III) complexes 3 and 4 (Figure 2a) could be used as chemosensors for anion detection.132 Anion-binding studies of the free-base corrole 2 in toluene revealed that it shows fluorescent response to CN−, F−, CH3COO−, and H2PO4− with binding affinities in the sequence of CN− > CH3COO−, H2PO4− > F−. The binding stoichiometry of these anions was found to be 1:2 (ligand/anion), with the inner NH moieties acting as the binding sites. The binding of CN−, F−, CH3COO−, and H2PO4− leads to enhanced emission intensity by 89−93% at ∼628 nm, with a new emission peak developing at ∼680 nm (Figure 2b). The detection limit of 2 toward F− is 0.35 ppm, and 0.69 ppm of F− can be quantified. Similarly, 0.54 ppm of CN− could be detected by 2, and the minimal quantifiable amount of CN− was found to be 1.43 ppm. Moreover, using poly(methyl methacrylate) (PMMA) and polyacrylamide as the solid supports, a low concentration of 70 ppb of CN− could be detected in water using 2 as the chemosensor. Different from the 1:2 binding mode observed for 2, the metallocorrole 3 binds CN−, F−, CH3COO−, and H2PO4− in a stoichiometry of 1:1, and 4 binds only CN− and F− in a stoichiometry of 2:1 (ligand/ anion), through axial coordination at the central gallium in 3 and 4, inducing red-shifts of the emission bands. The sensing behavior of corroles may be modulated by introducing another component with well-established photophysical or other desired properties. Lodeiro, Neves, and their co-workers thus synthesized gallium(III) corrole−coumarin conjugates 5 and 6 (Figure 3a) for anion recognition.133 The introduction of a coumarin moiety to the corrole skeleton of 5 and 6 can enhance their fluorescence emission intensity and increase their solubility in ethanol. Upon addition of F− to the toluene solution of 5, a quenching of 40% fluorescence intensity at 602 nm was observed, whereas the titration of CN− and CH3COO− to 5 resulted in a red-shift of the fluorescence peak from 602 to 614 nm. The binding of 5 with F−, CN−, and CH3COO− has a stoichiometry of 1:1, showing a binding affinity

not attracted enough attention until Sessler et al. discovered that it shows promising anion-binding properties.110 Moreover, Sessler, Gale, Moyer, and co-workers later found that calix[4]pyrroles are even ideal receptors for ion-pairs due to their allosteric effects upon anion binding.111 From the brief description of the synthetic history of porphyrin analogues, it is obvious that unique properties have always been found accompanying each step forward in their synthetic chemistry. Meanwhile, the applications of this large family of compounds have been extended into various areas, such as photosensitizers, chemosensors, drug delivery, etc.112−118 As we have mentioned, during the synthesis of sapphyrins, Sessler et al. accidentally found that sapphyrin shows the ability of anion binding.89,90 Since then, the anion-binding properties of pyrrolic compounds have been extensively investigated. Considering the structural diversity of porphyrin analogues, and their unique properties in metal ion coordination, anion binding, and intense light absorption/emission, porphyrin analogues indeed show great potential for sensing applications. Representative examples will be described below. 3.1. Corrole-based ion chemosensors

Corroles are tetrapyrrolic macrocycles that bear a skeleton identical with the cobalt-chelating corrin of vitamin B12. They have a smaller ring size than that of porphyrins, containing a directly linked bipyrrole unit in the macrocycle. These macrocycles absorb and emit light in the visible region, with high extinction coefficients, high fluorescent quantum yields, and good photostability.119,120 Another interesting feature of corroles is that they are trianionic ligands and are thus able to stabilize transition metals in high oxidation states.85 The unique capabilities of corroles in metal ion coordination and anion binding endorse their applications in energy conversion, sensing, and imaging.119,121−124 Especially after the report of improved synthetic method by Gross, Paolesse, and co-workers, intensive attention has been paid to this specific class of porphyrin analogues.125−128 3.1.1. Corroles for anion sensing. The anion-sensing capability of metallocorroles was discovered in 1985.129 With the consideration that cobalt(III) corrole complexes may exhibit high binding affinities toward NO2−, the Meyerhoff group incorporated a cobalt(III) corrole complex 1 (Figure 1a) into plasticized polymeric membrane electrodes as an ionophore for NO2− detection.130 The NO2− selectivity of the obtained chemosensor system could be enhanced by adding an appropriate amount of lipophilic cationic sites to the membrane phase. The achieved detection limit for NO2− using 1 as the 2206


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Figure 3. (a) Chemical structures of gallium(III) corrole-based anion chemosensors 5−8; (b) absorption and fluorescence (inset) spectral response of 7 (10 μM) upon addition of CN− in toluene (λex = 590 nm). Inset images show the naked eye detection of CN− with 7 under ambient light and UV lamp, respectively. Reprinted with permission from ref 134. Copyright 2014 Elsevier.

Figure 2. (a) Chemical structures of corrole-based chemosensors 2−4 (Ar = C6F5) and their corresponding anion-binding modes; (b) absorption and emission (inset) spectral changes upon titration of 2 with F− in toluene. Reprinted with permission from ref 132. Copyright 2012 Royal Society of Chemistry.

weak metal coordination and hydrogen-bonding ability because of its low charge density and large ionic radius, and thus it is challenging to develop iodide-selective chemosensors by using the metal coordination or hydrogen-bonding interactions. To address this problem, the Sankar group reported a unique example of an iodide-selective chemosensor achieved through a one-electron redox couple between iodide and Cu(III)−corrole 9 (Figure 4a).135 Upon addition of tested anions, F−, Cl−, Br−, I−, SO42−, NO3−, CH3COO−, and PO43−, to the dichloromethane solution of 9, only the addition of I− dramatically changed the solution color from yellow−brown to green (Figure 4b, c). NMR titration studies revealed that the addition of I− to 9 generated a paramagnetic species in the solution, which was further elucidated by electronic paramagnetic resonance (EPR) measurements to be [Cu(II)−9]−. Possibly, the strong electron-withdrawing character of eight β-bromine atoms at the corrole scaffold resulted in a highly electron-deficient Cu(III) center, which can oxidize I− by a one-electron redox reaction. The detection limit of chemosensor 9 for I− was evaluated to be 7 μM. From the aforementioned examples, it can be concluded that effective corrole-based anion chemosensors can be developed by employing either the coordinated central metal or the pyrrolic NH moieties as the anion-binding site, and the reduction of the high-valent central metal ion may be utilized for sensing reductive anions based on a redox mechanism. In all these cases, the periphery modification of the corrole macrocycle is

sequence of CN− > F− > CH3COO−, with the corresponding log Kas of 6.11, 5.28, and 4.32, respectively. Thus, 5 shows the highest sensitivity for CN−, with a detection limit of 0.18 ppm and a quantification limit of 0.27 ppm. Similarly, corrole 6 was able to detect 0.19 ppm of CN− in toluene. Using the similar gallium(III) corrole core, they also demonstrated that Ga(III) complex 7 (Figure 3a) shows a colorimetric response to CN− in toluene.134 Upon addition of CN− to the toluene solution of 7, red-shifts of absorption and fluorescence emission bands were observed, accompanied by a color change from green to colorless and a fluorescence quenching (Figure 3b). A similar spectral response was also observed for gallium(III) corrole 8 (Figure 3a) upon interaction with CN−. For both compounds 7 and 8, a stoichiometry of 2:1 (sensor/anion) was postulated. Using 7 and 8 as the chemosensors, the authors were able to detect and quantify CN− at low concentrations of 0.33−0.34 and 1.00−1.01 μM, respectively. By comparison of the structures of gallium(III) corrole based anion chemosensors 3−8, it can be concluded that the modification at the periphery of the metal corrole skeleton greatly influences its anion-sensing behavior. Iodide plays vital roles in many physiological functions such as thyroid function, brain function, and cell metabolism, and thus it is an essential micronutrient for human health. Hence, the quantitative detection of I− is meaningful for the evaluation of I− involved biological processes. However, iodide exhibits rather 2207


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Figure 4. (a) Synthesis of the electron-deficient Cu(III)−corrole chemosensor 9 and the proposed sensing mechanism of 9 upon addition of I−; (b) selectivity of 9 (1.7 μM) for I− in the presence of various anions in dichloromethane (DCM); (c) proposed sensing mechanism of 9 for I− and the corresponding colorimetric response. Reprinted with permission from ref 135. Copyright 2014 Royal Society of Chemistry.

essential for improving the anion-sensing selectivity and sensitivity. 3.1.2. Corroles for metal ion sensing. As mentioned above, corroles show unique metal ion coordination properties, which may enable their applications in metal ion sensing. In this respect, Zhang and co-workers used the simple free-base corrole 2 to construct a fluorescence “turn-off” sensing membrane for Hg2+ detection.136 Of the tested alkali, alkaline earth, and transition metal ions, only addition of Hg2+ effectively quenched the fluorescence of 2 based sensing membrane. The fluorescence-quenching response to Hg2+ in a concentration range from 1.2 × 10−7 to 1.0 × 10−4 M was observed to be within 5 min, and 2 can be used in a wide pH range covering 5.0−8.0 as well as in real water samples. Interestingly, Lodeiro, Neves, Oliveira, and co-workers found that corrole 2 can also be applied as a highly selective colorimetric chemosensor toward Hg2+ both in acetonitrile and in toluene, with totally different color changes (Figure 5).137 The Bandyopadhyay group synthesized four A2B-corroles 10− 13 bearing different meso-substituents (Figure 6a) and used them as fluorescence turn-off chemosensors for Hg2+.138 Among these compounds, 13 shows the best performance for Hg2+ sensing. It is strongly emissive in toluene with the maximum emission wavelength of 697 nm and a quantum yield of 8.2%. The addition of Hg2+ quenched the fluorescence of 13 through coordination and subsequent exciplex formation. Similar fluorescence quenching upon Hg2+ addition was observed for corroles 10− 12, with the quenching efficacy lying in the order of 10 < 11 < 12 < 13. The highest sensitivity observed for 13 may be related to the presence of the tridecyloxy long-chain moiety, which may improve the surface area of the molecule and thus reduce the propensity for molecular stacking. Selectivity studies revealed that 13 shows high selectivity toward Hg2+ over other tested metal ions like Co2+, Cd2+, Zn2+, Mn2+, Cu2+, Ag+, Na+, Pb2+, Li+, and K+ (Figure 6b). The replacement of pyrrole subunits in the corrole macrocycle with other heterocycles may afford novel corrole analogues with unique photophysical and coordination properties, which may find applications in metal ion sensing. Recently, the Srinivasan group synthesized a corrole analogue by introducing a biphenyl

Figure 5. (a) Absorption and emission (inset) spectral changes upon titration of corrole 2 with Hg2+ in CH3CN; (b) selectivity of 2 toward Hg2+ in CH3CN and images showing the corresponding solution colors before and after the addition of Hg2+. Reprinted with permission from ref 137. Copyright 2013 American Chemical Society.

moiety instead of the bipyrrole unit, providing a trianionic CCNN core for the stabilization of Cu3+.139 On this basis, the Srinivasan group further synthesized a corrole analogue 14 by introducing a bipyridine subunit (Figure 7a), which can be used for Zn2+ detection.140 14 itself is weakly emissive at 699 nm with a quantum yield of 0.011, whereas red-shifted and enhanced fluorescence was observed for 14−Zn at 709 nm with a quantum yield of 0.089 (Figure 7b). High-resolution mass spectrometry (HRMS) and single-crystal diffraction analysis data indicated that Zn2+ was bound within the monoanionic core of 14 with a counteranion coordinated at the axial position. Gradual addition of Zn(ClO4)2 to the methanol solution of 14 resulted in dramatic fluorescence enhancement, which can be ascribed to the CHEF effect. The association constant between Zn2+ and 14 was found to be 5.47 × 104 M−1. The detection limit of 14 for Zn2+ was calculated to be 1.5 ppm, and 14 was found to be highly selective toward Zn2+ even in the presence of 100 equiv of other 2208


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Figure 7. (a) Synthesis of corrole analogue 14 and the proposed Zn2+ sensing mechanism; (b) absorption and emission spectral differences between 14 and 14−Zn in CH3OH. Reprinted with permission from ref 140. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

fluorescence intensity, and the detection limit for Hg2+ using 16 as the indicator was determined to be 10 nM. Apart from the roles as metal ion coordination site, the inner core nitrogen atoms of corroles are capable of being protonated or deprotonated, which may be employed for designing optical pH chemosensors. Especially, introduction of a 4-aminophenyl group to the corrole skeleton could afford a more proton-active center, which favors the construction of a corrole-based chemosensor to detect a wide range of pH values.142 As shown in Figure 10, there are four possible protonated or deprotonated forms of corrole 17, and each state shows different fluorescence intensity. After being immobilized into a sol−gel glass matrix, the corrole 17-based optical chemosensor shows a linear response to pH changes in the range of 2.17−10.30 with good reproducibility and reversibility. These examples demonstrated that the unique metal coordination properties of corroles may be utilized for the design of corrole-based metal ion chemosensors, and core modification of corroles is an effective strategy to improve their metal ion sensing performance.

Figure 6. (a) Chemical structures of corrole chemosensors 10−13; (b) selectivity of 13 to Hg2+ over various tested metal ions. Reprinted with permission from ref 138. Copyright 2012 Royal Society of Chemistry.

competing metal ions like Ag+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, Mg2+, Mn2+, and Ni2+. Lodeiro, Neves, Oliveira, and co-workers also reported that silica nanoparticles coated with corrole 15 (Figure 8a) were able to detect Ag+ via the formation of satellite AgNPs.137 Of the tested metal ions, Cu2+, Hg2+, and Ag+, only the presence of Ag+ generated the satellite AgNPs around the corrole-coated silica nanoparticles (Figure 8b, c), resulting in vivid solution color change from green to yellow. The formation of the satellite AgNPs was supported by transmission electron microscopy (TEM) observations (Figure 8b). On the basis of the fact that the secondary G-quadruplex structure of single-strand phosphorothioate sequence T4G4-S3 (5′-TPSTPSTPSTGGGG-3′) could be stabilized by Hg2+ through forming S−Hg−S pairs, Zhou, Deng, and co-workers designed a fluorescence turn-on assay for sensing Hg2+ using triazatetrabenzocorrole 16 as the signal reporter (Figure 9).141 Bearing four positively charged piperidinium groups, triazatetrabenzocorrole 16 is fluorescent and soluble in water. In the buffer solution, the fluorescence of 16 can be quenched via single-strand T4G4-S3induced aggregation. However, the presence of Hg2+ resulted in the formation of all-parallel G-quadruplex of T4G4-S3, which induced only negligible aggregation of 16, and thus its fluorescence is still strong. Hence, the presence of Hg2+ could be detected in the fluorescence turn-on manner. The concentration of Hg2+ shows a linear relationship with the

3.2. Expanded porphyrin-based chemosensors

The term of expanded porphyrins refers to all oligopyrrolic macrocycles whose internal cavity is larger than that of porphyrins. The macrocycle of an expanded porphyrin consists of heterocyclic subunits (pyrrole, furan, thiophene, etc.), interlinked directly or through spacers, with the internal ring pathway containing at least 17 atoms. The development of novel expanded porphyrins is of great interest to chemists because expanded porphyrins show many unique properties in metal ion coordination and anion binding and transport.143−146 In addition, some expanded porphyrins show distinct optical features such as near-infrared light absorption and emission, and the photophysical properties of expanded porphyrins could be systematically modulated by metal coordination or anion 2209


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Figure 8. Silica nanoparticles coated with corrole 15 for Ag+ sensing. (a) Chemical structure of the coated corrole 15; (b) TEM images before (left) and after (right) the addition of Ag+ to the corrole coated silica nanoparticle; (c) illustration of the formation of satellite AgNPs around the coated silica nanoparticles. Reprinted with permission from ref 137. Copyright 2013 American Chemical Society.

Figure 9. Illustration of the fluorescence turn-on sensing mechanism of the triazatetrabenzcorrole-DNA-based chemosensor for Hg2+. (a) Fluorescence quenching of triazatetrabenzcorrole 16 via binding with ssDNA T4G4-S3; (b) presence of Hg2+ induced the formation of G-quadruplex of T4G4-S3, which cannot bind triazatetrabenzcorrole 16.

3.2.1. Expanded porphyrins for anion sensing. As mentioned above, as a class of aromatic pentapyrrolic porphyrin analogue, sapphyrin is the first reported example of expanded

binding. Thus, the design and synthesis of expanded porphyrinbased chemosensors is a reasonable and promising task.147 2210


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105 M−1, which is much larger than that obtained for Cl− and Br− (≤102 M−1). The high selectivity of 18 toward F− was rationalized by NMR and X-ray diffraction analysis of the binding processes and the resulting adducts, which indicated that F− binds 18 in a mode different from those for Cl− and Br−. To be more specific, F− could be bound within the diprotonated sapphyrin core, whereas both Cl− and Br− were bound with an out-of-plane, ion-pair-like fashion. On this basis, Tabata et al. reported sapphyrin 19 (Figure 11) as a fluorescent chemosensor for F−.150,151 The fluorescence intensity at 680 nm was enhanced upon binding F−, and thus F− could be detected at the ppb concentration level even in the presence of a large excess of Al3+ and Fe3+ ions. Apart from spherical halide anions, tetrahedrally shaped anions like phosphate and pertechnetate anions also exhibit strong binding affinities with sapphyrin.152,153 In this respect, Sessler et al. demonstrated that water-soluble sapphyrins 20−24 (Figure 11) could be used as fluorescent chemosensors for the phosphate anion.154 They found that, in aqueous media, the aggregation behavior of water-soluble sapphyrins 20−24 was influenced by the addition of anions, especially the phosphate anions. Thus, addition of sodium phosphate to the solutions of 20−24 (pH = 7) increased the ratio of the monomeric form of sapphyrins, leading to a dramatic increase in the fluorescence emission intensity. These observations may be rationalized by the fact that the binding of phosphate to the protonated sapphyrin core stabilized the monomeric form, allowing the liberation of π-stacked sapphyrins as highly fluorescent anionbound monomers. The anion-binding ability of expanded porphyrins can be utilized for the construction of supramolecular architectures with excellent sensing behavior. Very recently, Sessler, Anslyn, and coworkers reported that supramolecular assemblies of expanded porphyrins 25 and 26 (Figure 12) with anions could be used as

Figure 10. Chemical structure of corrole 17 and its protonated and deprotonated forms.

porphyrins. With a larger core size than the porphyrins, sapphyrins exhibit stronger capability for anion binding.148 The anion-binding behavior first discovered by Sessler et al. in 1990 promoted the anion-recognition chemistry of pyrrole-based macrocycles.89 For example, the same group reported 18 (Figure 11) as a F− selective chemosensor.149 Titration studies indicated that the binding constant between diprotonated 18 and F− is 1 ×

Figure 12. Chemical structures of expanded porphyrins 25 and 26. The cartoon illustrates the construction of supramolecular assemblies using 25, 26, and diacids as the building blocks.

chemosensors for organic solvents and anions.155 Diacids such as 4,4′-biphenyldisulfonic acid, tetrafluoroterephthalic acid, octafluorobiphenyldicarboxylic acid, oxalic acid, and acetylene dicarboxylic acid can be used as the linker units; thus, expanded porphyrins 25 or 26 can be successfully linked to afford supramolecular assemblies. Exposure of the obtained supramolecular assemblies to polar solvents, such as methanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), leads to the visible color or fluorescence changes, or solubility

Figure 11. Chemical structures of the diprotonated sapphyrin 18, freebase sapphyrin 19, and water-soluble monoprotonated sapphyrins 19− 24. 2211


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Because of the presence of a large conjugated π-system, some expanded porphyrins show intense fluorescence with emission wavelengths reaching the near-infrared region. As an example, [26]hexaphyrin(1.1.1.1.1.0) 30 synthesized (Figure 14) by the

differences, which may be related to the competition of the polar solvent for the hydrogen-bonding sites and changes in the degree of interaction between the expanded porphyrins and the diacids. By using cross-reactivity discrimination analyses, these supramolecular assemblies can be used to differentiate between common solvents and identify complex solvent mixtures. Similarly, the diacid supramolecular assemblies of 25 and 26 can also be used to distinguish between F−, Cl−, Br−, NO3−, and SO42−. From these descriptions, we can see that the anion-binding properties of expanded porphyrins have attracted much attention. However, research on the anion sensing of expanded porphyrins is still rather limited. Further research in this respect is highly desired. 3.2.2. Expanded porphyrins for metal ion sensing. With a larger macrocyclic framework, expanded porphyrins exhibit some unique properties unreachable for normal porphyrins. For example, expanded porphyrins may show near-infrared emission and multimetal coordination capability. Considering these characters, expanded porphyrins can be regarded as promising candidates as metal ion chemosensors. An early example of an expanded porphyrin-based metal ion chemosensor was developed by Sessler et al.156,157 Hexaphyrin(1.0.1.0.0.0) 27 (isoamethyrin) exhibits colorimetric response to dioxo actinide cations such as UO22+, PuO22+, and NpO22+, showing high selectivity over most of the transition metal ions (Figure 13). The

Figure 14. Chemical structures of expanded porphyrin chemosensors 30 for Hg2+ and 31 for Ag+.

Wong group shows intense fluorescence with emission wavelengths longer than 900 nm.159 Moreover, the fluorescence can be quenched distinctly upon addition of Hg2+ to its methanol solution. This observation is related to the formation of the corresponding Hg2+ complex in a ligand/metal ratio of 1:2. The binding constant was calculated to be 1.62 × 109 M−2. In addition, 30 shows a high selectivity toward Hg2+ over other tested metal ions like Li+, Na+, K+, Rb2+, Mg2+, Ca2+, Sr2+, and Ba2+, and a concentration down to 10−7 M of Hg2+ could be detected. Later, [26]hexaphyrin(1.1.1.1.1.1) 31 was further investigated by the same group as an Ag+ chemosensor (Figure 14).160 In terms of the structures, 31 possesses one more methine bridge (CH−) than 30. Because of this small structural difference, 31 shows high selectivity toward Ag+ instead of Hg2+, with only a slight response to Hg2+. The intense emission observed for 31 in the near-infrared (NIR) region at 1030 nm can be dramatically quenched upon interaction with Ag+. The binding stoichiometry between 31 and Ag+ was also observed to be 1:2 (ligand/Ag+), and the binding constant was estimated to be 7.24 × 1010 M−2. The detection of Ag+ is selective over other metal ions, with a high sensitivity to 10−7 M. The introduction of nonpyrrolic heterocycles into an expanded porphyrin may bring forth unique metal ion binding features that are totally different from all-pyrrole macrocycles. In this respect, Shen, You, Rurack, and co-workers reported that the incorporation of four thiophene units into a rubyrin macrocycle afforded 32 (Figure 15a), which may be used as a Hg2+ selective chemosensor.161 The four thiophene units were introduced as “soft” donors that show high affinities toward thiophilic target ions such as Hg2+. Using polyurethane hydrogel D4 membrane as the solid support, 32 shows high selectivity toward Hg2+ in water over other tested metal ions like Zn2+, Cd2+, Cu2+, Co2+, Ni2+, Pb2+, Ag+, Ca2+, Mg2+, K+, and Na+. Addition of different amounts of Hg2+ to the 32-doped membrane resulted in a redshift of the Soret-band absorption from 597 to 638 nm, with a clear isosbestic point at 617 nm (Figure 15b). The binding between 32 and Hg2+ was found to be in a ratio of 1:1 with an association constant of 3 × 104 M−1, and the detection limit of 32 for Hg2+ was determined to be 1 ppm. Using a similar thiophilic approach, Ganapathi, Lee, and Ravikanth developed a series of dithiaporphyrin (2.1.1.1) macrocycles 33−38 (Figure 16a) for colorimetric and fluorescent sensing of Hg 2+.162 Compared with normal porphyrins, an additional meso-carbon atom exists between the

Figure 13. Chemical structures and UO22+ sensing of isoamethyrin-type expanded porphyrins 27−29. The image shows the colorimetric response of 27 upon sequential addition of Et3N and UO22+. Reprinted with permission from ref 156. Copyright 2004 Elsevier Ltd.

detection limit of 27 toward UO22+ was found to be 39, Figure 17). On the other hand, calix[4]pyrroles with electron-donating groups at the β-positions show lower anion-binding affinities (anion-binding affinities: 42 < 40, Figure 17).168 These observations can be easily rationalized: the introduction of electron-withdrawing groups at the β-positions increases the acidity of the pyrrolic NH and thus enhances the stability of the calix[4]pyrrole−anion hydrogen-bonded complex. Accordingly, bromination at the βpositions of the calix[4]pyrrole increases the binding affinities of 41 toward F−, Cl−, and H2PO4−. Surprisingly, the introduction of fluorine substituents at the β-positions dramatically increases the binding affinity of 43 (Figure 17) toward Cl− and H2PO4− instead of F−.169 On the other hand, modification at the mesopositions of calix[4]pyrrole can also effectively modulate its anion-binding behavior.170−173 For example, the introduction of meso-aryl groups to calix[4]pyrroles may afford walled calix[4]pyrroles with a deep cavity for anion binding (44, Figure 17).174 Ballester, Matile, and co-workers found that the anion-binding affinities of walled calix[4]pyrroles are highly dependent on the electronic nature of the aromatic walls, similar to that observed for β-substituted calix[4]pyrroles, i.e., calix[4]pyrroles with electron-deficient aromatic walls show stronger binding with anions.175 Because of the tunable anion-binding properties, calix[4]pyrroles show promising applications for anion sensing, extraction, and transport.176−183 In the following subsection, we will be focused on the development of anion chemosensors using calix[4]pyrroles. 2214


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noteworthy that efficient binding of HP2O73− to 46 was even observed in the presence of 30% water, with the association constant of 3.63 × 106 M−1. The detection limit of 46−fluo1 complex toward HP2O73− was found to be 2 ppb. For HP2O73− sensing, a colorimetric-displacement approach was further reported by the Lee group using the same bispyridinium picket calix[4]pyrrole 46 as the chemosensor and a colored azophenol as the indicator (Figure 21a).191 The pink color of the azophenol indicator was changed into yellow upon binding 46 in a stoichiometry of 1:1. The association constant (Ka) of 46−azophenol was determined to be 1.80 × 106 M−1. Upon addition of HP2O73− to the acetonitrile solution of the 46−azophenol complex, a vivid color change from yellow to red was observed (Figure 21b). Titration studies revealed that the binding of HP2O73− to 46 has an association constant of 2.45 × 107 M−1, which is more than 13-fold larger than that of the azophenol indicator. Thus, the addition of HP2O73− to the solution of 46−azophenol complex displaced the azophenol indicator, resulting in vivid solution color changes. The 46− azophenol complex is quite selective toward HP2O73− over other tested anions like F−, Cl−, Br−, I−, CN−, CH3COO−, and H2PO4−, with a detection limit of 5.6 μM. Similar to the supramolecular displacement approach, anioninduced disassembly of a supramolecular polymer constructed from calix[4]pyrroles could also be used for sensing application. Considering the fact that tetra-tetrathiafulvalene (TTF) calix[4]pyrroles are ideal receptors for electron-deficient neutral guests,192,193 Sessler, Johnston, Bielawski, and co-workers designed two complementary types of calix[4]pyrroles as the building blocks to construct stimuli-responsive supramolecular coploymers.194 For example, calix[4]pyrrole 47 containing electron-rich tetrathiafulvalene units was supposed to interact with calix[4]pyrrole 48 that bears electron-deficient dinitrophenyl groups through a combination of oriented hydrogen bonding and π−π donor−acceptor interactions to afford a supramolecularly assembled copolymer [47−48]n (Figure 22). Interestingly, [47−48]n shows a response to external stimuli such as Cl−. To be more specific, the addition of 2 equiv of Cl− to the chloroform solution of copolymer [47−48]n resulted in deaggregation of the copolymer by forming the corresponding

Figure 19. Schematic representation of the formation of nonfluorescent 45−fluo1 complex and reversible fluorescence off−on−off switching of 45−fluo1 via alternate addition of F− and Li+.

switching of 45−fluo1 can be repeated for several cycles by alternate addition of F− and Li+. Sessler, Lee, and co-workers later modified the calix[4]pyrrole meso-positions with two pyridinium groups, and the afforded calix[4]pyrrole 46 was found to be a specific receptor for pyrophosphate anion (HP2O73−) (Figure 20a).190 With fluo1 used as the supramolecular displacement indicator, HP2O73− can be detected in a fluorescence turn-on manner. Similar with previous reports, a 1:1 binding between the indicator fluo1 and calix[4]pyrrole 46 completely quenched the fluorescence of fluo1, and the binding constant was determined to be 7.25 × 106 M−1. Subsequent addition of HP2O73− to the 46−fluo1 complex solution fully recovered the fluorescence intensity of fluo1, indicating the release of the fluorescent indicator. The binding constant of HP2O73− with calix[4]pyrrole 46 was calculated to be 2.55 × 107 M−1, ∼3.5-fold magnitude of that for fluo1. Other tested anions including F− only induced negligible displacement of fluo1 from the complex; thus, the nonfluorescent 46−fluo1 complex can be used as a highly selective fluorescence turn-on chemosensor for the detection of HP2O73− (Figure 20b). It is

Figure 20. (a) Schematic representation of the formation of nonfluorescent 46−fluo1 complex and subsequent release of fluorescent fluo1 upon addition of HP2O73−; (b) fluorescence changes of 46−fluo1 complex upon addition of various anions in acetonitrile. The inset images show the observed fluorescence of fluo1 (left), 46−fluo1 (middle), and 46−fluo1 after addition of HP2O73− (right). Reprinted with permission from ref 190. Copyright 2012 Royal Society of Chemistry. 2215


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interaction. Other tested anions like Cl− and H2PO4− exhibited much weaker binding, and Br−, I−, and HSO4− showed almost no binding. Similarly, compound 50 (Figure 23a) also can be used as a colorimetric chemosensor toward F−, with anion-binding affinities lying in the sequence of F− > Cl− > H2PO4− ≫Br− ≈ I− ≈ HSO4−.198 The introduction of nonchromophoric dye precursors to the calix[4]pyrrole may be effective for obtaining the reporter chromophores. Thus, the Anzenbacher group reported 51−53 (Figure 24) for colorimetric anion sensing in aqueous solutions.199 Upon introducing the colorless electron-withdrawing unit to calix[4]pyrrole, the resulting compounds 51− 53 exhibit intense colors. Due to the electron-deficient character, chemosensors 51−53 show high anion-binding affinities, with vivid colorimetric responses to F−, CH3COO−, HP2O73−, and H2PO4− solutions in a mixture of DMSO/0.5% water, while addition of other tested anions such as Cl−, Br−, I−, and NO3− resulted in no color changes. 1H NMR titration studies revealed that the anions are bound to the calix[4]pyrrole NH moieties through hydrogen bonds. Chemosensor 51 was then successfully applied for carboxylate sensing in an aqueous solution whose ionic strength and pH are fixed in accordance with blood plasma. Moreover, 52 was successfully embedded in a polyurethane film for anion sensing. Nishiyabu and Anzenbacher further reported that the incorporation of various electron-withdrawing units at the calix[4]pyrrole β-position is effective for modulating the binding affinities and selectivities of the resulting anion chemosensors.200 Incorporated with different numbers of dicyanovinyl moieties, 54−56 (Figure 24) exhibit anion-binding affinities in the sequence of 54 < 55 < 56. These observations can be rationalized by the fact that the electron-withdrawing nature of the dicyanovinyl moiety can increase the acidity of the pyrrolic NH, thus enhancing the anion-binding affinities. In addition, dramatic changes in anion selectivity were also observed. From 54 to 56, the selectivity toward CH3COO− over Cl− was greatly improved with the increasing numbers of the dicyanovinyl moieties. Thus, chemosensors 54 and 55 show anion-binding affinities in a sequence of F− > CH3COO− > H2PO4− > Cl−, and the sequence for 56 was determined to be F− ≈ CH3COO− > H2PO4− > Cl−. Similar calix[4]pyrrole-based anion chemosensors 57 and 58 (Figure 24) with intramolecular chargetransfer properties were reported by Tomé and co-workers.201 The Chauhan group reported that the incorporation of different azo substituents at the β-positions of the calix[4]pyrrole affects its anion-binding affinity and sensing behavior.202 The azo group in 59 is appended with an electron-donating methoxyphenyl group (Figure 25). In contrast, an electron-withdrawing chlorophenyl substituent is present in 60. This structural difference resulted in a higher anion-binding affinity of 60 relative to that of 59. Both calix[4]pyrroles 59 and 60 show selective colorimetric response to F− in DMSO. The addition of 10 equiv of F− to the solution of 59 changed the solution color from light yellow to dark brown, while the addition of the same amount of F− to the solution of 60 only induced a color change to light brown. The detection of F− using 59 and 60 is highly selective over other tested anions like Cl−, Br−, I−, HSO4−, and H2PO4−. Later, the Chauhan group introduced two 4-nitrophenylazo substituents at the β-positions of the calix[4]pyrrole to obtain compounds 61 and 62, which show selective colorimetric response to F−, CH3COO−, and H2PO4− (Figure 26).203 Compared with the 4-chlorophenylazo substituent in 60, the

Figure 21. Schematic representation of the formation of yellow-colored 46−azophenol complex and subsequent release of the azophenol indicator upon addition of HP2O73−. Absorption changes of the 46− azophenol complex upon titration with HP2O73− in CH3CN. The inset image shows the corresponding solution color changes. Reprinted with permission from ref 191. Copyright 2013 Elsevier.

Cl− complexes of 47−Cl and 48−Cl, which was clearly evidenced by NMR and single-crystal X-ray diffraction analysis (Figure 22b). This Cl− induced disassembly process was accompanied by vivid spectral changes, indicative of the potential application of this supramolecular assembly system for Cl− sensing. Because calix[4]pyrrole itself is an ideal receptor for anions, another strategy for designing calix[4]pyrroles-based chemosensors is the functionalization with an optically sensitive chromophore as the reporter unit.195−197 Thus, compound 49 was synthesized by attaching an anthraquinone at the pyrrolic βposition of the calix[4]pyrrole using a modified Sonogashira coupling reaction in a yield of 73% (Figure 23a).198 The anthraquinone chromophore may act as an optically sensitive indicator of anion binding to the calix[4]pyrrole moiety. Upon addition of F− to the dichloromethane solution of 49, the absorption peak was red-shifted from 467 to 518 nm, thus resulting in a vivid solution color change from yellow to red (Figure 23b). Only 6 equiv of F− were needed to saturate the 2216


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Figure 22. (a) Chemical structures of building blocks 47 and 48 for constructing calix[4]pyrrole-based supramolecular assemblies; (b) scheme illustrating the Cl− induced disassembly of the supramolecular polymer of [47−48]n.

of 60. Hence, higher anion-binding affinities were observed for 61 and 62. They not only respond to F− effectively but also show intense response to CH3COO− and H2PO4−. Actually, the addition of F− to the solutions of 61 and 62 resulted in the deprotonation of the pyrrolic NH, as evidenced by UV−vis and 1 H NMR studies. This observation is in sharp contrast to the fact that only hydrogen bonding with F− was observed for the lessacidic pyrrolic NH moieties in 59 and 60. Interestingly, the addition of F−, CH3COO−, and H2PO4− to the DMSO solution of 61 changed the solution color from orange to blue, greenishblack, and deep red, respectively (Figure 26b). Althrough it is very common to use the calix[4]pyrrole unit as the anion-recognition unit, the introduction of another recognition unit to the calix[4]pyrrole macrocycle is also an effective approach for developing anion chemosensors. As we know, a dicyanovinyl group is extensively used as the recognition moiety for CN− and could be incorporated into various chromophores to develop CN− selective chemosensors.204,205 Thus, a β-dicyanovinyl-substituted calix[4]pyrrole 63 was synthesized by the Lee group as a colorimetric chemosensor for CN− (Figure 27a).206 Compared with the colorless parent βfree octamethylcalix[4]pyrrole 39, compound 63 exhibits a yellow color with a maximum absorption peak at 374 nm in CH3CN−DMSO. The addition of CN− to the solution of 63 completely bleached the yellow color to colorless (Figure 27b).

Figure 23. (a) Chemical structures of calix[4]pyrrole−anthraquinone conjugated colorimetric chemosensors 49 and 50 for F−, Cl−, and H2PO4−; (b) images showing the colorimetric changes upon addition of various anions (100 equiv) to the CH2Cl2 solution of 49 (50 μM). Reprinted with permission from ref 198. Copyright 2000 Wiley-VCH Verlag GmbH, Weinheim.

introduction of two nitrophenyl groups in 61 and 62 resulted in much stronger electron-withdrawing effect, and thus the pyrrolic NH moieties in 61 and 62 exhibit much stronger acidity than that 2217


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Figure 24. Chemical structures of calix[4]pyrrole-based anion chemosensors 51−58 with electron-withdrawing moieties attached at the β-positions.

of the vinyl group. It is noteworthy that chemosensor 63 is highly selective toward CN− even in the coexistence of competing anions. By the introduction of a fluorophore to the β-position of the calix[4]pyrrole, it is also possible to develop calix[4]pyrrolebased fluorescent chemosensors. One early example is the conjugates of calix[4]pyrrole with anthracene, which could be used as fluorescent anion chemosensors.207 Thus, calix[4]pyrrole−anthracence conjugates 64−66 (Figure 28) were synthesized with the linker lengths varying in the sequence of 64 < 65 < 66. Fluorescence titration studies revealed that the addition of F−, Cl−, Br−, or H2PO4− to the respective dichloromethane solutions of 64−66 quenched their fluorescence to varying extents. Among these anions, F− shows the highest fluorescence quenching efficiency. The binding constants of 64−66 toward the tested anions were determined, which indicated that F− shows the strongest binding strengths with the log K of 4.94, 4.52, and 4.49 for 64−66, respectively. Thus, compounds 64−66 could be used as selective F− chemosensors in organic solvents. Anion-binding abilities of these three compounds were found to follow the sequence of 64 > 65 > 66, which may be ascribed to the different electronic communication pathways in 64−66. To be more specific, the linker between the calix[4]pyrrole and the anthracene for 64 is conjugated, while for 65 and 66 the linkers are nonconjugated. Thus, it may be concluded that a conjugated linker between the calix[4]pyrrole and the fluorophore is better than the nonconjugated ones. As an improvement, the Sessler group later introduced a rigid aromatic spacer between the calix[4]pyrrole and the fluorophore to synthesize compounds 67−69 (Figure 29).208 In these molecules, a phenylene spacer and either a sulfonamide or thiourea group were incorporated, which might provide additional hydrogen-bonding donor sites that cooperate with the calix[4]pyrrole NH to enhance the overall anion-binding abilities. 1H NMR and fluorescence titration studies revealed that chemosensors 67−69 can bind anions in a 1:1 stoichiometry and F− induced the largest fluorescence response. It is noteworthy

Figure 25. Chemical structures of chemosensors 59 and 60.

Figure 26. (a) Chemical structures of the calix[4]pyrrole-based colorimetric chemosensors 61 and 62 for F−, H2PO4−, and AcO−; (b) images showing the colorimetric changes upon addition of various anions to the DMSO solutions of 61 (above) and 62 (below), respectively. Reprinted with permission from ref 203. Copyright 2010 Royal Society of Chemistry. 1

H NMR and matrix-assisted laser desorption/ionization timeof-flight (MALDI-TOF) MS studies indicate that the addition of CN− afforded the corresponding cyanide adduct 63−CN, which reduced the extent of conjugation by disrupting the CC bond 2218


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Figure 27. (a) Synthesis of the dicyanovinyl substituted calix[4]pyrrole colorimetric chemosensor 63 for CN−; (b) images showing the colorimetric changes upon addition of various anions to the CH3CN−DMSO (3%) solution of 63. Reprinted with permission from ref 206. Copyright 2009 Royal Society of Chemistry.

Figure 28. Chemical structures of calix[4]pyrrole-based fluorescent chemosensors 64−66 with an incorporated anthracene unit.

Figure 29. Chemical structures of calix[4]pyrrole-based fluorescent chemosensors 67−69 incorporating an additional hydrogen-bonding site.

that 69 shows the highest binding affinity toward HP2O73−, with an association constant larger than 2 × 106 M−1, which may be ascribed to the effective binding of both the calix[4]pyrrole and the thiourea moieties with the pyrophosphate. Thus, 69 can be used to detect HP2O73− successfully in the presence of water at the physiological pH. The above examples indicate that modification at the βpositions of calix[4]pyrrole is an effective approach for designing colorimetric and fluorescent anion chemosensors. The electronwithdrawing or -donating effects of the β-substituents can greatly influence the anion-binding behavior and may even reverse the anion-binding selectivity sequence. In addition to the approach of modification at the β-positions of a single calix[4]pyrrole core, the combination of two

calix[4]pyrrole moieties in one chemosensor molecule may be another effective approach for developing anion chemosensors, which may enable a cooperative anion-binding effect, thus resulting in a totally different sensing behavior. In this respect, Lee and co-workers used the anthracene units as the linkers to synthesize a tetracationic calix[4]pyrrole homodimer 70 (Figure 30a), which displays distinctive cooperativity on selective F− binding in an allosteric fashion.209 Compound 70 is weakly emissive at 421 nm in acetonitrile. Anion-binding studies revealed that only the addition of F− switched on the fluorescence, and thus 70 can be used as a highly selective fluorescence turn-on chemosensor for F− (Figure 30b). The binding of F− to 70 shows a sigmoidal response behavior, indicating the binding of F− to 70 is a cooperative process. 2219


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Figure 30. (a) Chemical structure of the anthracene-linked calix[4]pyrrole homodimer 70; (b) fluorescence changes of 70 upon gradual addition of F−; (c) energy-minimized structures of chemosensor 70 and the [70−2F] complex, and the proposed F− binding-induced inhibition of the PET processes from the calix[4]pyrrole moieties to the anthracene moieties. Reprinted with permission from ref 209. Copyright 2015 Royal Society of Chemistry.

Actually, the binding of the first F− to 70 favors the second F− binding, as evidenced by a much larger binding constant of 5.95 × 107 M−1 for the second binding process than that of the first one (1.91 × 105 M−1). The well-defined emission peaks of the 70−2F complex show the characteristics of anthracene emission. The fluorescence enhancement upon F− binding may be related to increased rigidity of the molecule, as well as recovery of the radioactive deactivation pathway through suppressing the PET processes from the electron-rich calix[4]pyrroles to the excited state of the anthracene moieties (Figure 30c). Impressively, this designed allosteric binding may be favorable for improving the selectivity for F−, because the rigid cavity formed upon binding the first F− is only suitable for accommodating another small F− anion. Using BODIPY as the bridge, a sandwich-type anion receptor 71 was developed by the Shao group (Figure 31).210 Different from the parent calix[4]pyrrole, compound 71 is promising to bind anions in the form of a sandwich-type complex and thus shows higher anion-binding affinity. Job plot analysis indicates that compound 71 binds anions in a 1:1 mode. The binding constant of F− with 71 was determined to be 3.19 × 105 M−1, which is larger than those of Cl−, Br−, CH3COO−, and H2PO4−. The addition of F− to the acetonitrile solution of 71 red-shifted its absorption spectra and almost completely quenched the original fluorescence of 71, which could be ascribed to an enhanced intramolecular charge transfer (ICT) process from the electron-rich calix[4]pyrrole units to the electron-withdrawing BODIPY moiety upon binding F−. The binding of Cl−, Br−, CH3COO−, and H2PO4− to 71 also caused similar spectral changes, but to a much smaller extent than that of F−. Later, the same group further reported a BODIPY−calix[4]pyrrole conjugate 72 for F− sensing.211 As shown in Figure 32,

Figure 31. Chemical structure of BODIPY-bridged calix[4]pyrrole dimer 71 as a fluorescent F− chemosensor and its proposed F− binding mode.

only the addition of F− to the acetonitrile solution of 72 changed the solution color from pink to blue, accompanied by vivid fluorescence quenching. The binding of F− to 72 also exhibits a stoichiometry of 1:1, with a binding constant of 104 M−1, which is smaller than that of the BODIPY-bridged calix[4]pyrrole dimer 71 (3.19 × 105 M−1). Apart from F−, Costero and co-workers discovered that BODIPY-bridged biscalix[4]pyrrole dimer 73 (Figure 33) can be used for detection of dicarboxylates via a similar fluorescencequenching response.212 Of the tested aliphatic and aromatic dicarboxylates, compound 73 shows stronger binding with the dicarboxylates containing a chain length of 7 or 8 methylene groups, which may be ascribed to their best fit with the space between the two calix[4]pyrrole moieties, and thus a cyclic 1:1 2220


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unstrapped calix[4]pyrroles, endorsing their promising applications in anion extraction and membrane transport.218−221 Moreover, when an enantiomeric strap is incorporated, the resulting calix[4]pyrrole, such as 75 (Figure 34), will be an enantiomer, which could be used as a chirogenic chemosensor that shows higher binding affinity toward (S)-2-phenylbutyrate than the corresponding (R)-form,222 with association constants of 1.0 × 105 and 9.8 × 103 M−1, respectively. On the basis of the tunable anion-binding behavior, strapped calix[4]pyrroles have been extensively used for developing fluorescent and colorimetric anion chemosensors. Lee, Sessler, and co-workers synthesized a dipyrrolylquinoxaline-strapped calix[4]pyrrole 76 as a colorimetric chemosensor for F− detection (Figure 35a).223 NMR studies indicate that F− was

Figure 32. (a) Chemical structure of BODIPY−calix[4]pyrrole conjugated fluorescent chemosensor 72; (b) images showing the color (above) and fluorescence (below) changes of 72 upon addition of various anions in acetonitrile. Reprinted with permission from ref 211. Copyright 2011 Springer Science+Business Media B.V.

Figure 33. Chemical structure of the BODIPY-bridged calix[4]pyrrole dimer 73 as a fluorescent dicarboxylate chemosensor and the proposed dicarboxylate-binding mode.

complex with 73 can be formed by simultaneous coordination of the two carboxylate groups to both of the calix[4]pyrrole units of 73. Another effective approach of modulating the anion-binding strength and selectivity of calix[4]pyrrole-based chemosensors is to introduce a “strap” into the molecule, affording the so-called “strapped calix[4]pyrrole”.213,214 The anion-binding properties of strapped calix[4]pyrroles are influenced by both the size and electronic character of the strap.215,216 As reported by Lee, Sessler, and co-workers, the strapped calix[4]pyrrole 74 (Figure 34) can bind F− in a 1:1 mode, and an equilibrium between 1:1

Figure 35. (a) Chemical structure of dipyrrolylquinoxaline-strapped calix[4]pyrrole 76 and the postulated F− binding mode; (b) image showing the colorimetric response of 76 upon addition of various anions in CH3CN/DMSO (97:3 v/v). Reprinted with permission from ref 223. Copyright 2009 American Chemical Society.

bound to the NH moieties of the calix[4]pyrrole through hydrogen bonds. Meanwhile, F− was also involved in anion···π interactions with two pyrrole rings of the dipyrrolylquinoxaline strap. The binding constant between 76 and F− is 8.97 × 106 M−1, >822-fold higher than Cl− binding. Interestingly, the addition of H2PO4− to the CH3CN/DMSO (97:3 v/v) solution of 76 also vividly changed the solution color, although the binding constant of 76 with H2PO4− is only 1.13 × 103 M−1, much smaller than that of F− (Figure 35b). Considering the largest binding constant for F−, the strapped calix[4]pyrrole 76 can be regarded as a highly selective chemosensor toward F−. Sessler and co-workers further reported a cavity-control strategy to improve the F− binding selectivity by strapping calix[4]pyrrole with a bulky calix[4]arene diester through the linkage of two rigid phenyl groups.224 The resulting compound 77 has a calix[4]pyrrole core with an increased rigidity (Figure 36). Possibly, the combination of the rigid calix[4]pyrrole core with the bulky calix[4]arene diester moiety prevents anions larger than F− from either reaching the interior of the cavity or interacting with the calix[4]pyrrole NH moieties. Thus, chemosensor 77 shows a high selectivity toward F− over other tested anions like Cl−, Br−, I−, NO3−, SO42−, H2PO4−, and

Figure 34. Chemical structures of strapped calix[4]pyrroles 74 and 75 showing tunable anion-binding affinities.

and 2:1 (sensor/anion) complexes was observed for Cl− binding, while a binding stoichiometry of 2:1 (sensor/anion) was observed to be the predominant form for Br− binding.217 It is obvious that strapped calix[4]pyrroles with a unique strap incorporated with binding moieties could show higher binding affinities toward the target anion compared with the parent 2221


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Figure 38. Chemical structures of 1,3-indanedione containing calix[4]pyrroles 82 and 83 for CN− sensing.

Figure 36. Chemical structure of the calix[4]arene diethyl ester strapped calix[4]pyrrole 77 and the single-crystal structure of 77·TEAF·H2O. Reprinted with permission from ref 224. Copyright 2014 American Chemical Society.

colorimetric chemosensors for CN−.228 Anion-binding studies using 1H NMR, UV−vis, and isothermal titration calorimetry (ITC) revealed that compounds 82 and 83 bind strongly with anions like F−, HP2O73−, Cl−, CH3COO−, and CN− and moderately with H2PO4− and Br−. The binding of F−, HP2O73−, Cl−, CH3COO−, H2PO 4−, and Br− induced red-shifted absorption spectra of 82 and 83, while only the addition of CN− bleached the color of 82 and 83, which may be ascribed to a reduced extent of conjugation resulting from a second step of nucleophilic addition of CN− to the 1,3-indanedione group. The colorimetric response to CN− is insensitive to the presence of other interfering anions, indicative of the practical capability of using 82 and 83 as highly selective CN− chemosensors. Another effective approach for modulating the anion-binding behavior of calix[4]pyrroles is to vary the ring size of the calix[4]pyrrole core, and thus the synthesis of ring-expanded calix[4]pyrroles has attracted attention from chemists.229−237 Panda and co-workers synthesized an expanded calix[4]pyrrole 84 bearing two ethylene moieties (Figure 39a) by a McMurry

HP2O73−. The binding constant for F− with 77 was determined to be >104 M−1. To capture small anions, the Panda group proposed another strategy for designing calix[4]pyrrole-based chemosensors by introducing a strap that is as short as possible.225 The authors thus synthesized four calix[4]pyrroles 78−81 that bear the shortest strap reported to date (Figure 37). Indeed, 1H NMR

Figure 39. (a) Proposed binding mechanism of expanded calix[4]pyrrole 84 for F− sensing; (b) colorimetric response of 84 upon addition of various anions in CH3CN. Reprinted with permission from ref 238. Copyright 2012 American Chemical Society.

Figure 37. Chemical structures of calix[4]pyrroles 78−81 that bear a short strap for F− sensing.

titration studies carried out in CD3CN revealed that calix[4]pyrroles 78−81 respond only to F−, the smallest one among the tested anions of F−, Cl−, Br−, I−, CN−, HSO4−, H2PO4−, CH3COO−, N3−, NO2−, NO3−, and ClO4−. However, the F− binding constants for 78−81 lie in the 104−105 M−1 range, which is smaller than that calix[4]pyrroles with longer straps.226,227 These observations may be related to the repulsion between F− and the lone pairs of the oxygen atoms present in the strap due to the small cavity size. The introduction of a specific anion-recognition group at the β-position of a strapped calix[4]pyrrole may also afford highly selective anion chemosensors. Sessler, Lee, and co-workers synthesized strapped calix[4]pyrroles 82 and 83 (Figure 38) bearing a 1,3-indanedione moiety at the β-pyrrolic position as

reaction using a formylated dipyrromethane as the precursor. It was found that 84 can be involved in an anion···π interaction specifically with F− in polar aprotic solvents.238 To be more specific, upon addition of F− to the acetonitrile solution of 84, the absorption peak at 316 nm was attenuated, with two new peaks developing at 337 and 515 nm. Thus, the solution color was changed vividly from colorless to red upon addition of F− (Figure 39b). Job plot analysis indicates that F− binds to 84 with a stoichiometry of 1:1. Interestingly, the 1H NMR spectra for 84 showed no shift in the NH peak during addition of F−. Further electronic paramagnetic resonance (EPR) studies indicated that a charge transfer process occurred during the complex formation. Consistently, density functional theory (DFT) calculations 2222


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suggested that the charge transfer occurs from F− to the CC bonds through anion···π interactions. Later, Panda and co-workers further synthesized expanded calix[4]pyrroles 85 and 86 via Suzuki coupling reactions (Figure 40a) and used them as fluorometric chemosensors for F− and

Figure 41. (a) Chemical structures of N-confused calix[4]pyrroles 87 and 88 and the tautomerization of 88 to its cyclic form 88-cyclic; (b, c) absorption spectra and color changes of 87 and 88-cyclic upon addition of F− and Cl−, respectively. Reprinted with permission from ref 242. Copyright 2006 American Chemical Society.

Figure 40. (a) Chemical structures of expanded calix[4]pyrroles 85 and 86 incorporated with lateral aryl rings; (b) fluorescence turn-on response of 85 and (c) turn-off response of 86 to F−. Reprinted with permission from ref 239. Copyright 2013 American Chemical Society.

family of compounds shows a prospective future for anionsensing applications. From these descriptions, we can briefly conclude that calix[4]pyrroles are indeed powerful tools for anion sensing, with the sensitivity and selectivity easily modulated by the βsubstituents, the incorporation of calix[4]pyrrole dimers, incorporated straps, macrocycle ring size, and the incorporation of N-confused pyrrolic units. 3.3.3. Calix[4]pyrroles for the recognition of metal ions and ion pairs. It was found that the binding of an anion to calix[4]pyrrole drives its conformational conversion from the 1,3-alternate to the cone conformation. This allosteric process created a cavity suitable for recognition of target cations through cation-π interactions, and thus calix[4]pyrroles can be used as ion-pair receptors.111,243 The capability of calix[4]pyrroles to bind both anion and its counter metal ion endorsed their usage for extraction of metal salts.244,245 It was also reported that calix[4]pyrroles with large internal cavities could be used as binucleating ligands for two metal ions.246 These facts indicate that functionalized calix[4]pyrroles can be used for metal ion and ion-pair recognition.247 The recognition of Cs+ is of great importance because of its wide industrial use, especially in radioactive occasions. Sessler and co-workers reported a calix[4]pyrrole strapped with calix[4]arene-crown-6 (89, Figure 42a), which was able to accommodate CsF by forming an ion-pair complex even in the solid state.248 The addition of cesium perchlorate to 89 in a mixed solvent of CDCl3 containing 10% (v/v) CD3OD resulted in the formation of the corresponding metal complex with Cs+ encapsulated in the calix[4]arene-crown-6 ring, while the perchlorate anion was almost not bound to 89. However, the addition of CsF to 89 in the same solvent induced chemical shift changes of both calix[4]arene-crown-6 and calix[4]pyrrole moieties, consistent with the formation of the ion pair complex 89·CsF (Figure 42b). Anion-binding studies revealed that 89 is unable to bind F− in a mixed solvent of CDCl3/CD3OD when F−

CH3COO−.239 The incorporation of phenyl or naphthalene moieties at the calix[4]pyrrole periphery extended the πconjugation in 85 and 86, resulting in fluorescence enhancement and red-shifted absorption and emission peaks. Interestingly, addition of F− or CH3COO− separately to the acetonitrile solution of 85 enhanced its fluorescence (Figure 40b), while fluorescence quenching was observed upon addition of F− or CH3COO− to the solution of 86 (Figure 40c). Because other tested anions like Cl−, Br−, I−, H2PO4−, HSO4−, ClO4−, and PF6− cannot induce obvious fluorescence changes, expanded calix[4]pyrroles 85 and 86 can be used for fluorescent detection of F− and CH3COO−. For regular calix[4]pyrroles, the pyrrolic units are linked by meso-carbon atoms at the pyrrolic α,α′-positions. If one of the pyrroles is linked at the α,β′-positions, the obtained tetrapyrrolic macrocycle is then called a N-confused calix[4]pyrrole. This kind of novel macrocycle exhibits not only unique anion-binding properties but also special chemical reaction activities at the free α-position.240,241 Anzenbacher and co-workers reported that Nconfused calix[4]pyrroles 87 and 88 (Figure 41) show anionbinding modes different from those of normal calix[4]pyrroles.242 To be more specific, the three inner pyrrolic NH moieties are hydrogen-bonded with the anion, and the inner βCH of the confused pyrrole appears to be in close contact with the anion. This unique binding mode resulted in changes in anion selectivity relative to corresponding normal calix[4]pyrroles. The normal calix[4]pyrroles bearing the same 4nitrophenylazo or tricyanoethylene substituent show an anionbinding affinities in the order of F− > HP2O73− > CH3COO− > H2PO4− > Cl−, while for N-confused calix[4]pyrroles 87 and 88, the affinity order is changed into CH3COO− > F− > HP2O73− > H2PO4− > Cl−. Because anion binding to α-substituted Nconfused calix[4]pyrroles easily induces vivid color changes, this 2223


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Figure 42. (a) Chemical structures of ion-pair receptors 89 and 90 containing a calix[4]pyrrole strapped with calix[4]arene-crown ether; (b) single-crystal structure of 89−CsF. Reprinted with permission from ref 248. Copyright 2008 American Chemical Society.

is added alone as tetra-n-butylammonium fluoride (TBAF), although it is a highly selective receptor for F− in CDCl3. In contrast, the addition of TBAF to the 89·Cs complex in CDCl3/ CD3OD could afford the corresponding 89·CsF complex. These results indicate that the binding of Cs+ to the crown etherstrapped calix[4]arene moiety in 89 can promote the second step F− binding to form an ion-pair complex. Later, Sessler and co-workers synthesized compound 90 by using a smaller calix[4]arene-crown-5 unit to replace the calix[4]arene-crown-6 moiety in compound 89 (Figure 42a).249 Thus, 90 was expected to provide a dedicated binding site for the smaller K+ ion rather than Cs+. In fact, 90 was able to recognize KF and CsF ion pairs with different binding strengths.250 1H NMR titration studies revealed that both K+ and Cs+ coordinate with 90 at the calix[4]arene-crown-5 moiety, with binding constants of 6.5 × 106 and 3.3 × 104 M−1, respectively. As expected, the coordinated Cs+ can be replaced by subsequent exposure to K+ to form a more stable KF ion-pair complex of 90. The higher affinity of chemosensor 90 toward KF is also reflected in its ability to extract the KF salt effectively in an aqueous nitrobenzene two-phase system. Compared with the calix[4]arene-crown-strapped calix[4]pyrroles, the crown-free calix[4]arene-strapped calix[4]pyrrole 91 (Figure 43) shows different ion-binding behavior.251 In a mixed solvent of CDCl3/CD3OD (9:1, v:v), 91 binds neither Cs+ nor F− when exposed separately to these two species with other counterions. Interestingly, when Cs+ and F− were present simultaneously, 91 was able to coordinate the CsF ion pair by forming a stable 1:1 CsF complex. For the aforementioned calix[4]arene-crown-strapped calix[4]pyrroles 89 and 90, there are two distinct strong ion-binding sites, with the calix[4]pyrrole and the calix[4]arene-crown-6 subunits suitable for binding anions and cations, respectively. However, the crown-free ionpair receptor 91 lacks the crown moiety for cation recognition. Thus, 91 displays an anion-dependent ion-pair binding behavior with the CsF ion pair encapsulated within the receptor cavity, as evidenced by both single-crystal X-ray diffraction analyses and 1 H NMR spectroscopic studies. 91 shows selectivity for CsF over Cl−, Br−, and NO3− salts of Cs+. It is also noteworthy that the ionpair binding modes can be modulated by varying the counteranion of Cs+. The Lee group reported a coumarin-strapped calix[4]pyrrole 92, which effectively responds to water, Na+, and Cl− (Figure 44).252 The fluorescence of 92 in acetonitrile was enhanced upon

Figure 43. Ion-binding behavior of calix[4]arene-strapped calix[4]pyrrole 91.

Figure 44. Chemical structure of coumarin-strapped calix[4]pyrrole 92 and schematic representation of the sequential interactions of 92 with Na+ and Cl−.

adding water, which may be related to the hydrogen-bonding interactions between the water molecule(s) and the carbonyl unit of the coumarin moiety or the phenoxy group, thus inhibiting the photoinduced electron transfer (PET) quenching process in 92. They found that the addition of Na+ to the solution of 92 could also enhance its fluorescence, which is also related to the inhibition of the PET quenching process through coordination with the coumarin oxygen atoms. Interestingly, subsequent addition of Cl− to the fluorescent complex of 92−Na quenched the fluorescence. 1H NMR studies revealed that Cl− effectively binds the pyrrolic NH in 92, which activates a different PET pathway, thus quenching the fluorescence. Because the fluorescence of 92 can be effectively modulated by the addition of a specific metal ion, anion, and water, it is thus possible to construct “smart” logic devices based on 92. The ion-pair binding capability of calix[4]pyrroles could also be used to construct supramolecular polymers with different structures by varying the choice of ion pairs and solvents. Sessler 2224


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Figure 45. (a) Chemical structures of calix[4]pyrroles 93 and 94 with different strapped modes; (b) view of the cyclic hexamer structure obtained from the crystallization of 93 with CsF in chloroform/methanol; (c) view of the linear supramolecular polymer structure obtained from the crystallization of 93 with CsF in chloroform/ethanol; (d) view of the linear supramolecular polymer structure obtained from the crystallization of 94 with CsF in chloroform/ethanol. Reprinted with permission from ref 253. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

the coordination of the S atoms with Hg2+. Other tested cations such as Li+, Na+, K+, Cs+, Sr2+, Ca2+, Ba2+, Pb2+, Mg2+, Zn2+, and Cd2+ do not interact with 95. Therefore, calix[4]pyrrole 95 can be used as a highly selective Hg2+ chemosensor. The Chauhan group synthesized two core-modified calix[4]pyrroles 96 and 97 (Figure 47a), which are able to coordinate with Hg2+ effectively in dry acetonitrile.255 The addition of Hg2+ to 96 resulted in the formation of the corresponding 96−Hg2+ complex in a stoichiometry of 1:1, accompanied by a bathochromic shift of the absorption band from 284 to 296 nm (Figure 47b). The binding constant (Ka) of 96−Hg2+ was calculated to be 1.45 × 105 M−1. As we know, common calix[4]pyrroles are unable to bind Hg2+; the Hg2+ binding capability of 96 could be ascribed to the introduction of the tellurium and selenium donor atoms to the calix[4]pyrrole macrocycle. 1H NMR, 125Te, and 77Se NMR studies revealed that the interaction between Hg2+ and 96 and 97 takes place only at the tellurium and selenium atoms sites, because no obvious chemical shift change could be observed for the NH moieties. As a brief summary for this subsection, it can be concluded that the strong ion-binding capabilities of the calix[4]pyrroles strapped with additional metal binding sites enable their usage in the selective recognition and extraction of ion pairs. Furthermore, the introduction of other donor atoms like S and Te to the macrocycle adds additional means of modulating the ion-binding character of calix[4]pyrrole, which may generate chemosensors suitable for highly selective detection of target metal ions.

and co-workers synthesized strapped calix[4]pyrrole 93 (Figure 45a) for crystallization with CsF in chloroform/methanol, thus affording a cyclic hexamer (Figure 45b), and the crystallization in chloroform/ethanol resulted in the formation of a linear supramolecular polymer (Figure 45c).253 The use of different ion pairs was also important to obtain different polymers. In the same solvent of chloroform/methanol, the use of CsCl instead of CsF resulted in the formation of a linear supramolecular polymer. Besides, a small modification at the calix[4]pyrrole’s strap may dramatically influence the structure of the ion-pair complex based supramolecular polymers. For example, compound 94 is an analogue of 93 containing a linker with a different substitution mode, which afforded only linear supramolecular polymers regardless of the crystallization solvent (Figure 45d). Fused with 2,5-dihydrothiophene, calix[4]pyrrole 95 (Figure 46a) not only shows the ability of ion-pair recognition but also displays exceptional selectivity toward Hg2+.254 Both the solid− liquid extraction and titration studies in organic solvents revealed that a tight ion-pair complex of 95·CsF was formed with the host 95 adopting the cone conformation. However, titration of Hg2+ to the solutions of either 95 or the 95·CsF ion-pair complex resulted in the formation of an insoluble Hg2+ complex. NMR studies revealed that calix[4]pyrrole 95 adopts the 1,3-alternate conformation to coordinate with Hg2+, affording a linear polymeric structure (Figure 46b). The complete decomplexation of the 95·CsF ion-pair complex by addition of Hg2+ may be related to the fact that the cation···π interaction between Cs+ and the cone-shaped cavity of 95 is relatively weak as compared with 2225


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Figure 48. Chemical structure of the N-confused porphyrin Pt(II) complex 98 and the proposed F− binding and deprotonation processes.

results indicate that small amounts of F− may bind the outer NH of 98 through hydrogen bonding, while larger amounts of F− caused deprotonation of the peripheral NH (Figure 48). The binding and deprotonation processes were accompanied by vivid absorption changes, suggesting that metal complexes of Nconfused porphyrins are promising candidates to be used as anion chemosensors. Using the peripheral NH of N-confused porphyrin as the recognition site, the Furuta group reported a dyad chemosensor 99 constructed from a conjugate of porphyrin and N-confused porphyrin (Figure 49a).257 In this case, the N-confused porphyrin moiety was used as the anion-recognition unit, and the porphyrin moiety functioned as the reporter unit. Upon addition of Bu4NF to the dichloromethane solution of 99, the 1:1 binding between F− and the peripheral NH was observed by vivid absorption spectral changes, with isosbestic points at 446 and 702 nm (Figure 49b). Vivid fluorescence spectral changes were also observed during the addition of F−. When excited at 421 nm, the emission peaks of 99 at 730 and 803 nm decreased upon gradual addition of F−, while new and stronger emission peaks were observed at 703 and 765 nm, with the emission quantum yield dramatically improved from 0.0053 to 0.0153 (Figure 49c). This vivid fluorescence spectral change can be ascribed to an enhanced excitation energy transfer from the porphyrin unit to the N-confused porphyrin unit upon F− binding. In addition to outer NH moiety, the axial positions of metal centers in N-confused porphyrin complexes could also be used as the anion-binding site.258 Xie, Morimoto, and Furuta synthesized an SnIV complex of N-confused oxoporphyrin 100 (Figure 50a, b), which shows weak emission in dichloromethane. However, the addition of halides such as Cl−, Br−, and I− to its solution vividly enhanced the fluorescence (Figure 50c). These observations can be ascribed to the halide binding at the axial position of 100, which results in better rigidity and planarity of the obtained complex, and thus the fluorescence was dramatically enhanced. The binding constants were found to be 1.15 × 105, 2.1 × 104, and 6.1 × 102 M−1 for Cl−, Br−, and I−, respectively. From these examples, it may be concluded that N-confused porphyrins show unique anion binding at the periphery NH. In addition, some of their unique metal complexes may provide an additional anion-binding site at the axial position of the metal center. Considering that N-confused porphyrins are also effective chromophores, they can be combined with some recognition units for developing anion chemosensors, which may be developed as a promising topic of research. 3.4.2. N-confused porphyrins for metal ion sensing. The presence of the α,β′-linked pyrrole in N-confused porphyrins not only results in their anion-binding capabilities but also brings forth rich metal ion coordination behavior. Tetrapyrrolic N-confused porphyrins usually coordinate with metal ions in a 1:1 fashion, and expanded N-confused porphyrins

Figure 46. (a) Chemical structure of 2,5-dihydrothiophene-fused calix[4]pyrrole 95; (b) schematic representation of the formation of the CsF ion-pair complex of 95, and the formation of the polymeric complex induced by addition of Hg2+ to the solutions of either 95 or 95−CsF in CDCl3. Reprinted with permission from ref 254. Copyright 2014 American Chemical Society.

Figure 47. (a) Proposed binding mode of the core-modified calix[4]pyrroles 96 and 97 with Hg2+; (b) absorption changes of 96 (4.2 μM) upon gradual addition of Hg(ClO4)2 in acetonitrile. Reprinted with permission from ref 255. Copyright 2013 Royal Society of Chemistry.

3.4. N-Confused porphyrin-based chemosensors

3.4.1. N-Confused porphyrins for anion sensing. With a unique α,β′-linked pyrrole, N-confused porphyrins can bind anions with their peripheral pyrrolic NH, which is similar to the pyrrole molecule itself. Hence, in terms of anion binding, Nconfused porphyrin was regarded as an expanded pyrrole by Furuta. Thus, the Furuta group reported a simple Pt(II) complex of N-confused porphyrin (98, Figure 48) for anion binding.256 It was found that 98 can bind Cl−, Br−, and I− at its peripheral N− H, while the addition of F−, H2PO4−, and OH− to the dichloromethane solution of 98 resulted in the deprotonation of the peripheral N−H. Upon addition of Cl−, Br−, and I−, downfield shifts of the outer NH 1H NMR signal were observed, accompanied by enhanced absorption of the Soret band and weakened absorption of the Q bands. In contrast, F− induced two observable processes. Upon addition of F− within 0.1 equiv, the 1 H NMR signal for the outer NH displayed a downfield shift; the signal was then attenuated with further addition of F−, and finally completely disappeared after addition of 0.4 equiv of F−. These 2226


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Figure 49. (a) Chemical structure of the conjugated dyad of porphyrin and N-confused porphyrin 99 and the proposed effect of F− binding at the peripheral NH upon the excitation energy transfer from the porphyrin unit to the N-confused porphyrin unit; (b) absorption spectral changes of 99 (6.6 μM) upon addition of F− (0−18 equiv) in CH2Cl2; (c) fluorescence spectral changes of 99 (0.69 μM) upon addition of F− (0−32 equiv) in CH2Cl2 (λex = 421 nm). Reprinted with permission from ref 257. Copyright 2009 Royal Society of Chemistry.

attractive light absorption/emission properties such as NIR fluorescence emission. Considering these characteristics, expanded N-confused porphyrins are promising candidates suitable for designing NIR chemosensors that can avoid biological autofluorescence and eliminate biological damage. In this respect, the Furuta group synthesized a doubly Nconfused hexaphyrin 101 (Figure 51a), which was used to selectively detect Zn2+ in water.261 In this case, the hexaphyrin macrocycle was used as both the analyte-recognition group and the signal-reporting group, and two highly hydrophilic octaarginine peptides were introduced to ensure its water solubility. The large cavity within the macrocycle can coordinate with two Zn2+ ions to form a stable complex, which shows an enhanced fluorescence around 1050 nm, with the quantum yield enhanced by 14-fold, and the selective detection of Zn2+ is not obviously interfered by other competing metal ions like Na+, K+, Mg2+, Ca2+, Ag+, Ba2+, Li+, Mn2+, Ni2+, Pb2+, Cr3+, Co2+, Cd2+, Cu2+, Fe3+, Hg2+, Pd2+, and Sn2+ (Figure 51b). Notably, chemosensor 101 can be used to detect Zn2+ at neutral to weakly alkaline pH values with the NIR emission, and thus its biomedical applicability would be of interest. More recently, the Furuta group reported another doubly Nconfused hexaphyrin 102 bearing six pyridinium groups (Figure 52), which also showed fluorescence turn-on response to Zn2+ in water.262 In this case, the authors discovered that the enhanced fluorescence of 102−Zn2 can be quenched by subsequent addition of double-stranded DNA (dsDNA) due to the further formation of a ternary complex of 102, Zn2+, and dsDNA. This result suggests that expanded porphyrins with large π-planes

Figure 50. (a) Chemical structure of SnIV complex 100 and the proposed axial Cl− binding mode; (b) perspective view of the X-ray crystal structure of 100; (c) fluorescence enhancement of 100 (6.63 μM) upon addition of increasing amounts of Bu4NCl in CH2Cl2 (λex = 465 nm). Reprinted with permission from ref 258. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

are able to coordinate with two metal ions simultaneously.259 Especially, singly N-confused [26]hexaphyrin was even able to coordinate with two different metal ions in variable oxidation states.260 Besides, expanded N-confused porphyrins show 2227


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enhanced by 31-fold (Figure 53, inset), which can be assigned as the chelation enhanced fluorescence (CHEF). Ring fusion is an effective approach for developing novel porphyrin analogues. Upon heating and/or base treatment, 3-BrN-confused porphyrin can be converted into the corresponding N-fused porphyrin, which is stable and shows distinct absorption and emission in the near-IR region.265 Interestingly, the tricyclic moiety of N-fused porphyrins can be attacked by thiophenol, resulting in a skeletal rearrangement to recover the corresponding N-confused porphyrin frameworks.266 Considering the fact that N-fused porphyrins and N-confused porphyrins have a large difference in their optical properties, Furuta et al. further investigated the possibility of using N-fused porphyrins as colorimetric and fluorogenic chemosensors for thiol detection in water (Figure 54).267 Of the eight tested nucleophilic amino acids, only the addition of cysteine (Cys) induced vivid absorption and solution color changes of 106 (Figure 54b). The reaction between 106 and Cys can occur in the pH range from 5.0 to 8.5. 1H NMR and circular dichroism (CD) analysis of the Cys-adduct of 106 revealed that the addition of Cys to 106 resulted in the formation of 106−Cys, an N-confused porphyrin bearing a Cys group at the C3 position. Similar with Cys, the other two tested biothiols, homocysteine (Hcy) and glutathione (GSH), were also reactive to 106, and the nucleophilic addition of GSH to 106 was observed to be faster than that of Cys and Hcy under either neutral or basic conditions (Figure 54c). Because the addition of biothiols to 106 induced vivid changes in solution color and fluorescence-emission wavelengths and intensity, 106 can be used as a selective chemosensor for biothiol detection in water. In summary, few examples of N-confused porphyrin based chemosensors are presented in this subsection. It is obvious that the application of N-confused porphyrins as metal ion chemosensors is still in the infancy stage, despite the fact that expanded N-confused porphyrins are promising platforms for designing near-infrared chemosensors. We believe that the sensing properties of N-confused porphyrins will attract more attention with the development of the synthetic methods.

Figure 51. (a) Chemical structure of the doubly N-confused hexaphyrin modified with octa-arginine peptides (101) and the proposed binding mechanism of 101 with added Zn2+; (b) absorption changes and relative fluorescence intensity of 101 (0.53 μM) in H2O upon addition of various metal ions. Reprinted with permission from ref 261. Copyright 2010 Royal Society of Chemistry.

could interact with higher-order nucleic acids and thus may be developed as near-infrared chemosensors for DNA. Xie, Furuta, and co-workers recently synthesized novel porphyrinoids with unprecedented structures and interesting properties by the direct linkage of two confused pyrrole units and subsequent macrocycle interconversion reactions.263,264 Thus, they found that a dihydrosapphyrin isomer 103 with the confused pyrroles linked in a unique β,α−α,β mode is very reactive, which can readily rearrange to a ring-contracted pyrrolyl norrole 104 under basic conditions (Figure 53).263 Interestingly, upon treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in CH2Cl2, 104 further underwent a ringexpansion rearrangement to an isosmaragdyrin analogue 105, with the two confused pyrroles linked in an unusual N,α−α,β mode. All of these porphyrinoids 103−105 are weakly emissive in CH2Cl2. However, upon Zn2+ coordination in methanol, the fluorescence of the pyrrolyl norrole 104 at ∼736 nm was

3.5. Calixphyrin-based chemosensors

The hybridization of porphyrins and calixpyrroles may afford another type of porphyrin analogues called calixphyrins, which contain at least one sp3-hybridized bridging carbon atom.

Figure 52. Chemical structure of doubly N-confused hexaphyrin 102 modified with six pyridinium groups and the proposed binding mode of 102 with added Zn2+. 2228


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Figure 53. Syntheses of porphyrin analogues 103−105 with two directly linked confused pyrrole units, and Zn2+ coordination mode of 104. Inset image shows the fluorescence enhancement of 104 upon addition of Zn2+ based on the CHEF mechanism. Reprinted with permission from ref 263. Copyright 2013 American Chemical Society.

Figure 54. (a) Chemical structure of the N-fused porphyrin 106 and the proposed sensing mechanism of 106 for Cys; (b) selectivity of 106 toward Cys over other tested nucleophilic amino acids; (c) time course of the reactions of 106 with 1.0 equiv of Cys, Hcy, and GSH at pH 5.0. Inset shows the corresponding reaction solution colors after the addition of the biothiols for 1 h. Reprinted with permission from ref 267. Copyright 2013 Elsevier Ltd.

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HSO4−. Of the tested anions of F−, Cl−, Br−, I−, ClO4−, CH3COO−, H2PO4−, HPO42−, HSO4−, SO42−, SCN−, S2O3−, NO3−, and N3−, only the addition of HSO4− to the chloroform solution of 107 induced red-shift of the absorption peak from 445 to 476 nm, accompanied by vivid solution color change from green to yellow (Figure 55b). The binding between 107 and HSO4− exhibits a stoichiometry of 1:1, with a binding constant of 1.4 × 103 M−1. The selective binding of HSO4− to 107 may be ascribed to the presence of multiple hydrogen-bonding interactions between HSO4− and the inner NH moieties of 107. The Ravikanth group further reported an expanded dithiacalixphyrin 109 (Figure 56a, b), which exhibits a highly distorted and flexible conformation due to the presence of two sp3-meso bridging carbon atoms. 109 turns out to be incapable of metal ion coordination, but it shows selective colorimetric response to F−.269 Of the tested anions of F−, Cl−, Br−, I−, OH−, N3−, ClO4−, AcO−, and H2PO4−, only the addition of F− to 109 in CH3CN induced a vivid color change from yellow orange to peach (Figure 56c). UV−vis titration studies revealed that the gradual addition of F− to 109 decreased the original absorption peak at 454 nm, and a new peak developed at 530 nm with a clear isosbestic point observed at 398 nm. The binding stoichiometry of 109 and F− was found to be 1:1, as indicated by Job plot analysis and the HRMS peak corresponding to the 1:1 109···F− complex. The binding constant was calculated to be 3.53 × 104 M−1. 1H NMR titration studies revealed that the F− anion was bound to the inner NH, as evidenced by ∼0.8 ppm downfield shift of the inner NH signal observed upon addition of F−. Similar to the skeleton of calixoxasmaragdyrin, the Ravikanth group later reported a calixazasmaragdyrin 110 (Figure 57a), which shows high selectivity toward Hg2+.270 Of all tested metal ions and anions, only the addition of Hg2+ to the chloroform solution of 110 induced significant absorption spectra and solution color changes. Titration of 110 with increasing amounts of Hg2+ resulted in the decrease of the absorption bands at 425 and 685 nm, accompanied by new bands developing at 470, 790, and 889 nm, and two clear isosbestic points were observed at 616 and 600 nm (Figure 57b). These observations may be related to the formation of the Hg2+ complex of 110, which was further confirmed to be 110−Hg (1:1), considering the molecular ion peak at 772.2373 in the HRMS and a 1:1 binding stoichiometry indicated by the Job plot. Density functional theory (DFT)

Compared with porphyrins and calixpyrroles, the study on calixphyrins is relatively limited due to their limited synthetic methods and unstable nature. The Ravikanth group reported the successful synthesis of calixphyrins 107 and 108 (Figure 55a) by [3 + 2]-type

Figure 55. (a) Chemical structures of calixphyrins 107 and 108; (b) absorption changes of 107 (10 μM) upon gradual addition of HSO4− (0−20 equiv) in CHCl3, with the inset showing the color change of 107 upon binding HSO4−. Reprinted with permission from ref 268. Copyright 2014 Royal Society of Chemistry.

condensation of the dipyrromethane under acid-catalyzed conditions with oxatripyrrane and thiatripyrrane, respectively.268 Interestingly, the resulting calixoxasmaragdyrin 107 showed selective response toward HSO4− in both its neutral and protonated forms, whereas calixthiasmaragdyrin 108 did not show any noticeable response to all tested anions including

Figure 56. Expanded dithiacalixphyrin 109 for F− sensing. (a) Chemical structure of 109; (b) single-crystal structure of 109, with thermal ellipsoids shown at 50% probability; (c) images showing the selective colorimetric response of 109 toward F− over other tested anions. Reprinted with permission from ref 269. Copyright 2015 Royal Society of Chemistry. 2230


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Figure 58. (a) Chemical structure of a 1,3-phenylene-containing porphyrin analogue 111 and the proposed binding mode of 111 upon addition of Zn2+; (b) absorption changes of 111 (10 μM) upon gradual addition of Zn2+ in acetonitrile; (c) corresponding fluorescence spectral changes under the same conditions as those of (b) (λex = 564 nm). Reprinted with permission from ref 272. Copyright 2007 Royal Society of Chemistry.

Figure 57. (a) Chemical structure of calixazasmaragdyrin 110 and its proposed sensing mechanism toward Hg2+; (b) absorption changes of the chloroform solution of 110 upon guadual addition of Hg2+. Inset: Hg2+ induced solution color change. (c) Optimized structure of complex 110−Hg. Reprinted with permission from ref 270. Copyright 2015 American Chemical Society.

yield of 0.34, ∼3-fold higher than that of Cd2+ and 17-fold higher than that of Hg2+. The titration studies, NMR, MS, Job plot, and X-ray analyses unambiguously indicated that a 1:1 complex was formed between 111 and Zn2+ with a stability constant of 2.05 × 105 M−1. The fluorescence enhancement upon Zn2+ coordination can be ascribed to the more rigid structure of the resulting complex 111−Zn. Mg2+ ions play important roles in manipulating many polyphosphate compounds and enzyme functions in biological systems. Compared with other divalent metal ions, Mg2+ has a smaller ionic radius (∼0.72 Å). Taking this advantage, Naruta and co-workers designed a 1,10-phenanthroline moiety embedded porphyrin analogue 112 (Figure 59a), which acts as a ratiometric fluorescent chemosensor for Mg2+.273 The titration of Mg2+ to the acetonitrile solution of 112 red-shifted its fluorescence from 572 to 639 nm, with ∼16-fold enhancement of the emission intensity ratio between 572 and 639 nm, while in aqueous solutions, a similar red-shift of the emission peak from 583 to 631 nm was observed upon gradual addition of Mg2+ (Figure 59b, c). The mass spectrum peak observed at m/z = 759.2285 indicates that the Mg2+ complex of 112 was formed in a 1:1 ratio. The binding constants of 112 with Mg2+ were calculated to be 4.4 × 106 and 37.3 M−1 in acetonitrile and aqueous solutions, respectively. Due to the designed central cavity size, other biologically abundant metal ions such as Na+, K+, and Ca2+ almost did not interfere with the ratiometric detection of Mg2+ using 112 as the chemosensor. Further X-ray diffraction and fluorescence studies revealed that the Mg2+ complex of 112 shows a distorted square-pyramidal geometry with Cl− coordinated as an axial counteranion.274 The red-shift of the emission wavelength upon Mg2+ coordination is governed by the donor-excited photoinduced electron transfer (d-PET) processes from the dipyrrin subunit to the 1,10-phenanthroline moiety. Bearing essential structural similarities with calix[4]pyrroles, 4-oxocyclohexadienylidene-substituted porphyrinogen 113 (Figure 60) is also an ideal anion receptor. Furthermore, compared

calculations and 1H NMR data further indicated that Hg2+ is coordinated with the two pyrrolic imino nitrogen atoms, with the NH protons left intact (Figure 57c). The association constant between 110 and Hg2+ was determined to be 1.9 × 104 M−1, which is moderate and might enable chemosensor 110 to be reused by further treatment with water or ethylenediaminetetraacetic acid (EDTA) after Hg2+ sensing. As mentioned above, reports on calixphyrins are still very limited compared with its parent compounds of porphyrins and calix[4]pyrroles. However, we believe that the development of calixphyrins is an attractive field of research because of their unique ion-binding and optical-sensitive properties. In addition, the ion sensing selectivity of calixphyrins can be manipulated by varying the ring size and types of incorporated donor atoms. 3.6. Other porphyrin analogues for ion sensing

From the aforementioned examples, we can see that the rich synthetic chemistry and novel structures of porphyrin analogues provide various strategies for the design of ion chemosensors. Nowadays, porphyrin chemists are still endeavoring to develop novel porphyrin analogues and investigate their sensing applications. In this respect, the term of carbaporphyrin is referred to those porphyrin analogues with at least one carbon atom within the macrocyclic cavity, which also show unique metal ion coordination properties compared with normal porphyrins.271 With a structure similar to that of carbaporphyrins, a porphyrin analogue 111 containing a 1,3-phenylene moiety was synthesized by Hung et al. and used for detecting Zn2+ (Figure 58a).272 The structure of 111 is relatively flexible, and it emits only very weak fluorescence in acetonitrile. However, upon addition of Zn2+ to the solution of 111, bright red fluorescence was observed at 672 nm (Figure 58b, c). Of all the tested metal ions, only Zn2+, Cd2+, and Hg2+ enhanced the fluorescence of 111. To be more specific, the Zn2+ complex shows the strongest emission with a quantum 2231


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istics: (1) intense light absorption/emission; (2) easy peripheral modification; and (3) easy modification of the macrocycle size and incorporation of unique donor atoms within the macrocycle core. Hence, diverse design strategies have been developed to construct various porphyrin analogues for sensing both anions and metal ions. However, examples of detecting target ions in real environmental or biological samples using porphyrin analogue based chemosensors are still very limited. It can be anticipated that more attention will be focused on this promising field of research. Considering that porphyrins and porphyrin analogues exhibit quite similar structures and many common properties, we believe that the knowledge of ion chemosensors based on porphyrins would be favorable for expanding the design strategies for ion chemosensors based on porphyrin analogues. Thus, porphyrin-based ion chemosensors will be described concisely in the following section.

4. PORPHYRIN-BASED ION CHEMOSENSORS Compared with the diverse synthetic chemistry and novel structures of porphyrin analogues, the term porphyrin only refers to the aromatic tetrapyrrolic compounds with four pyrrolic subunits interconnected at the α-positions by four methine ( CH−) bridges. Because porphyrins are chemically stable and exhibit excellent photophysical properties such as intense absorption at the Soret and Q bands and red to near-infrared emission, the sensing applications of porphyrins have also attracted intensive attention.77 With the fast development of effective synthetic methods for porphyrins, nowadays porphyrins can be functionalized at both the meso-positions and the β-positions of the pyrrolic subunits. This synthetic convenience provides the possibility that porphyrins can be incorporated into molecular structures with sensing properties, say, by the introduction of a recognition unit to the porphyrin macrocycle, the porphyrin moiety can then act as the reporting unit of the resulting chemosensors. There are also some reported examples of using metal ion coordination properties of porphyrins for sensing applications.277,278

Figure 59. (a) Chemical structure of a 1,10-phenanthroline moiety embedded porphyrin analogue 112 and the proposed Mg2+ sensing mechanism; (b) UV/vis absorption spectral changes of 112 (20 μM) upon gradual addition of MgCl2 in MeCN; and (c) corresponding fluorescence spectral changes of (b). Reprinted with permission from ref 273. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 60. Chemical structures of porphyrinogens 113 and 114 for anion sensing.

4.1. Porphyrins for anion sensing

Porphyrins can be protonated at the inner imino N atoms and the protonated porphyrins can be considered as a large cationic ion for anion sensing.279,280 In addition, the protonation of porphyrins can be promoted by the counteranion, like Cl−, as reported by Yang and co-workers.281 However, a more effective way to develop porphyrin-based anion chemosensors is to functionalize it with an anion-recognition unit.282 As a Lewis acid, triarylborane shows a strong anion-binding capability. Shinkai, Takeuchi, and co-workers reported that the attachment of a triarylborane at the meso-position of a porphyrin afforded a colorimetric and ratiometric fluorescent chemosensor 115 for F− (Figure 61).283 The addition of F− to the THF solution of 115 resulted in enhanced fluorescence at 356 and 692 nm and attenuated fluorescence at 670 nm, which is accompanied by solution color change from purple to green. These observations can be ascribed to the 1:1 binding of F− to the boron center of the triarylborane unit, which perturbed the πconjugation pathway and the energy transfer from the triarylborane donor to the porphyrin acceptor (Figure 61). The binding constant of 115 with F− was determined to be 99 700 M−1. Recently, Thilagar and co-workers synthesized a zinc porphyrin 116 bearing four triarylborane units at its periphery (Figure 62a) for the discrimination of F− and CN− via different

with calix[4]pyrroles, porphyrinogen 113 has a more rigid macrocycle with a conjugated π-electronic system, whose intense visible light absorption ability can be utilized for colorimetric anion sensing. Hill, D’Souza, and co-workers reported that compound 113 and its di-N-benzylated derivative 114 (Figure 60) are both solvatochromic dyes that are capable of binding various anions through hydrogen bonding.275 UV−vis titration and Job plot data indicated that the binding of anions with 113 and 114 has a stoichiometry of 2:1 (113/anion) and 1:1, respectively. Especially, both 113 and 114 show higher binding affinities toward F− relative to other tested anions such as Cl−, Br−, I−, PF6−, ClO4−, NO3−, CH3COO−, and H2PO4−. The binding of anions resulted in vivid solution color changes for compounds 113 and 114 that enable their applications in colorimetric anion sensing. Besides, the anion-sensing selectivity can be improved by combining with an appropriate solvent. Later, Hill, D’Souza, and co-workers discovered that the binding of anions at the inner pyrrolic NH hydrogens of 114 can induce large cathodic shifts, which enables its application as an excellent electrochemical anion chemosensor.276 From the examples illustrated in this section, it is obvious that porphyrin analogues indeed show many advantages for developing ion chemosensors based on the following character2232


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Figure 61. Chemical structure of triarylborane−porphyrin conjugate chemosensor 115 and the proposed F− sensing mechanism.

Figure 62. (a) Chemical structure and proposed F− and CN− binding sites of chemosensor 116; (b) plausible mechanism of energy transfer from the triarylborane unit to porphyrin. Reprinted with permission from ref 284. Copyright 2014 American Chemical Society.

optical signal changes.284 Compared with the earlier example reported by Shinkai, Takeuchi, and co-workers, there are two different Lewis acid anion-binding sites in the structure of 116, namely, the boron center and the inner Zn2+. The presence of four triarylborane units in 116 was able to modulate the Lewis acidity of the Zn2+ center, enabling 116 to discriminate F− and CN− via different color and emission changes. Thus, the addition of F− to the solution of 116 resulted in only slight changes in the absorption spectra but almost completely quenched the fluorescence when excited at the boryl absorption dominated band. Considering the fact that the addition of F− cannot quench the fluorescence of 116 if the excitation wavelength is fixed at 425 nm other than at the boryl absorption dominated band, it is postulated that F− binding occurred at the peripheral borane moieties and perturbed the energy transfer from the triarylborane unit to the porphyrin unit (Figure 62b). In contrast, the addition of CN− to 116 resulted in vivid changes in the absorption spectrum, with the Soret band at 430 red-shifted to

445 nm and the Q-bands at 550 and 595 nm shifted to 580 and 625 nm, respectively. The CN− binding process was accompanied by a solution color change from pale purple to dark green, indicating that 116 can be used for colorimetric detection of CN−. Meanwhile, the addition of CN− to 116 resulted in only slight fluorescence changes when excited at the boryl absorption dominated band, while vivid fluorescence enhancement was observed if the excitation wavelength was fixed at the porphyrin Soret band of 425 nm. These absorption and emission spectral changes indicated that CN− was bound to the Zn2+ center. Upon addition of other anions such as Cl−, Br−, I−, NO3−, CH3COO−, and ClO4−, no obvious spectral changes were observed, indicative of the high selectivity of 116 toward F− and CN−. The detection limits of 116 for F− and CN− in dichloromethane were determined to be 0.10 and 1.5 ppm, respectively. Tian and co-workers reported a naphthalimides-winged zinc porphyrin 117 for dual-mode fluorescent sensing of F− (Figure 63a).285 In the molecule of 117, two naphthalimide chromo2233


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wavelengths. On the other hand, the [Naph]− species exhibit a broad absorption band at 508 nm, showing no absorption at 365 nm. Thus, the fluorescence response of chemosensor 117 toward F− was observed to be dependent on the selected excitation wavelength. Other tested anions such as Cl−, Br−, I−, and CH3COO− show no or only very weak influences on the emission of 117 under the same conditions. When diamine substituents were incorporated at the periphery of the porphyrin macrocycle, the obtained porphyrins may be used for sensing phosphate anions. For example, three porphyrins 118−120 containing different diamino substituents were synthesized by Tomé, Sessler, and co-workers (Figure 64a).286 They found that only the addition of H2PO4− to the chloroform solution of 118−120 resulted in vivid UV−vis spectral changes, while other tested anions like F−, Cl−, Br−, CH 3 COO − , NO 3 − , and NO 2 − cannot induce dramatic absorption changes. The single-crystal structure of the 118−F− complex indicates that porphyrin 118 binds F− at the periphery diamino NH (Figure 64b). Although these porphyrins are insoluble in water, they can be incorporated into a piezoelectric chemosensor for the detection of HPO42− in aqueous media. Several “picket fence” porphyrins were developed by Beer and co-workers for the purpose of anion recognition. One of their key strategies for modulating the anion-binding behavior of such porphyrins was to introduce suitable anion-recognition units as the “arms”. For example, the incorporation of four imidazolium amide arms to the zinc porphyrin meso-positions afforded compound 121 for sulfate recognition (Figure 65).287 NMR studies indicate that both the positively charged imidazolium and the amide groups in compound 121 participate in anion binding. In DMSO, 121 was able to bind anions in a stoichiometry of 1:1, showing higher binding affinities toward tetrahedral anions like H2PO4− and HSO4−. When the test solution was changed into a water/DMSO mixture, the binding of HSO4− and SO42− was found to be much stronger than other tested anions. Two porphyrin cages 122 and 123 containing triazole and triazolium groups were also synthesized for anion sensing (Figure 65).288 The neutral tetratriazole zinc porphyrin cage 122 shows colorimetric responses upon anion binding in acetone. Anions were bound with 122 in a stoichiometry of 1:1 within the cage cavity by coordinating to both the triazole and phenyl C−H moieties. Among tested anions, SO42− shows the highest binding affinity with 122. Compared with the neutral cage 122, the positively charged porphyrin cage 123 is a superior anion receptor in the presence of water. With an anion-binding mode similar to that of 122, cage 123 also shows a high selectivity

Figure 63. (a) Chemical structure of porphyrin 117 as a F− chemosensor; (b) images showing the turn-off and turn-on fluorescence responses of 117 upon addition of F− (excited at 365 and 504 nm, respectively). Reprinted with permission from ref 285. Copyright 2006 American Chemical Society.

phores and the zinc porphyrin were used as the energy donor and acceptor, respectively, and these two components were conjugated through two amide linkers. The addition of F− to the solution of 117 resulted in a proton transfer from the amides to F−, which is accompaned with decreased absorption of 117 at 365 nm and enhanced absorption at 508 nm, which could be ascribed to the generation of negatively charged naphthalimide ([Naph]−) species. When excited at 365 nm, a fluorescence quenching was observed after addition of F−, due to decreased excitation energy transfer from the naphthalimide moiety to zinc porphyrin (Figure 63b). However, when the excitation wavelength was fixed at 508 nm, the addition of F− to the solution of 117 enhanced the fluorescence intensity at 608 nm vividly due to the enhanced excitation energy transfer from the naphthalimide moiety to zinc porphyrin (Figure 63b). For both excitation wavelengths of 365 and 508 nm, the porphyrin moiety cannot be directly excited because it exhibits no absorption at these

Figure 64. (a) Chemical structures of porphyrin-based anion chemosensors 118−120; (b) single-crystal structure of the 118−F− complex. Reprinted with permission from ref 286. Copyright 2013 Royal Society of Chemistry. 2234


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Figure 66. (a) Chemical structure of the doubly strapped porphyrin chemosensor 125; (b) energy-minimized structure of 125−2F−. Yellow, sulfur; red, oxygen; blue, nitrogen; and light blue, fluoride. Reprinted with permission from ref 291. Copyright 2001 Wiley-VCH Verlag GmbH, Weinheim.

cooperative in a stoichiometry of 1:2 (125/F−, Figure 66b). Moreover, compound 125 can be constructed into a conducting polymer for sensing F− through electrochemical and conductivity responses. Xie and co-workers reported that the introduction of a typical CN− recognition unit, dicyanovinyl (DCV), to the porphyrin fluorophore afforded a highly selective fluorescence turn-on chemosensor 126 for CN− (Figure 67).292 126 is nonfluorescent,

Figure 65. Chemical structures of picket fence porphyrins 121−124 suitable for anion binding and/or sensing.

toward SO42− over other tested anions, with the 123−SO42− association constant in 5% water−acetone about 5-fold larger than that in acetone. When the test solvent was changed into 15% water−acetone, only the addition of SO42− induced significant absorption changes. When iodotriazoles were incorporated into the four arms of the “picket fence” zinc porphyrins, the afforded compound 124 was able to bind anions through halogen bonding (Figure 65).289 As revealed by 1H NMR titration studies, the Cl− was bound with 124 by four C−I···Cl− halogen bonds within the pocket formed by the four arms of the picket fence. In addition to varying the meso-substituents, the Beer group further investigated the influence of metal centers on the anionbinding properties of the picket fence porphyrins.290 These results revealed that the combination of suitable recognition arms and metal centers may afford desired high-performance anion chemosensors based on the picket fence porphyrin structure. By introducing two straps to the porphyrin meso-positions, the Swager group synthesized a porphyrin-based chemosensor 125 for F− (Figure 66a).291 Calculation of the energy-minimized geometry of compound 125 indicated that the two cavities in 125 are contracted with a π-stacking distance of 4.0 Å between the strap bands and the porphyrin. Because of the limited size of the two cavities, only F− is small enough to be accommodated in 125. Thus, chemosensor 125 shows high selectivity toward F− over other tested anions like Cl−, Br−, I−, CN−, CH3COO−, and H2PO4−. The addition of F− to the DMSO solution of 125 induced significant changes in the absorption spectra. Analysis of the spectral data revealed that the binding of F− to 125 is

Figure 67. Sensing mechanism of 126 for CN−. Images show the fluorescence turn-on response of 126 in CH2Cl2 upon addition of CN−. Reprinted with permission from ref 292. Copyright 2013 Royal Society of Chemistry.

while the addition of CN− to its dichloromethane solution significantly enhanced its fluorescence intensity. This observation can be ascribed to the nucleophilic attack of CN− at the DCV unit, which may result in better planarity of the porphyrin skeleton as well as inhibition of the twisted intramolecular charge transfer (TICT) process. Beer, Cormode, and Davis reported that the zinc center of porphyrin 127 (Figure 68) shows Lewis acidity for anion binding, and the assembly of 127 on the Au nanoparticles dramatically enhanced its anion-binding affinity.293 127 was able to bind anions in a stoichiometry of 1:1 using the zinc center and amide as the Lewis acid and hydrogen-bonding sites, respectively. Depending on the nanoparticle size, ∼30−80 porphyrin molecules can be adsorbed onto one Au nanoparticle to form 127−AuNP. UV−vis titration studies indicate that both 127 and 127−AuNP show high binding affinities toward Cl− and H2PO4−. However, the binding of anions to 127−AuNP in dichloromethane seems to be stronger than that of the individual porphyrin 127, especially for Cl−, Br−, and NO3−. In DMSO, the association constants of 127−AuNP with Cl− and H2PO4− are 2235


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Figure 68. Chemical structure of amide−disulfide zinc porphyrin 127 and the corresponding adsorbed gold nanoparticle 125−AuNP for anion binding.

Figure 69. (a) Proposed sensing mechanism of a zinc porphyrin-linker−quinone structured dyad 128 for the detection of Y3+; (b) fluorescence response (I/I0 at 610 nm) of 128 (3 μM) upon addition of various metal ions (λem = 560 nm). Inset shows the optimized structure of complex 128−Y3+. Reprinted with permission from ref 294. Copyright 2004 American Chemical Society.

much larger than those of porphyrin 127. This observation was rationalized by the fact that the assembly of metalloporphyrins

onto the AuNP surface may increase the Lewis acidity of the central metal ion and thus enhance its anion-binding affinity. 2236


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Figure 70. Proposed sensing mechanism of a terpyridine−porphyrin dyad 129 for the detection of Cd2+.

Figure 71. Synthesis of 130−Mn and Zn2+ sensing mechanisms of 130 and 130−Mn.

observed by electrochemistry and ESR measurements. This donor-excited photoinduced electron transfer (d-PET) leads to the fluorescence quenching and resulting weak fluorescence of compound 128. Interestingly, Y3+ could bind the two carbonyl oxygens in 128, which suppresses the aforementioned d-PET process and enhances the fluorescence intensity. Dyad 128 was able to detect Y3+ with a high selectivity over other tested metal ions (Figure 69b). The binding constant between 128 and Y3+ was estimated to be ∼3 900 M−1 in deaerated PhCN. Similar with quinone, pyridine can also be used as the electron acceptor in the construction of a donor−acceptor system. Using terpyridine as the recognition unit and the electron acceptor, Yu, Zhang, and co-workers reported that the TPP−terpyridine dyad 129 can be used as a fluorescence turn-on chemosensor for Cd2+ based on the d-PET mechanism (Figure 70).295 Upon excitation at 417 nm, 129 is weakly emissive, showing an emission peak at

From these examples, it can be concluded that the design of porphyrin-based anion chemosensors can be achieved through the modification of porphyrins with a recognition unit and/or a suitable cooperative metal ion center for anion binding. 4.2. Porphyrins for metal ion sensing

To date, porphyrins functionalized with various recognition units have been reported for the detection of different metal ions like Zn2+, Cu2+, Cd2+, Pb2+, Hg2+, and Y3+. In addition, the metal ion coordination ability of porphyrins can also be utilized for sensing metal ions. Early in 2004, Okamoto and Fukuzumi reported that fluorescence of zinc porphyrin-linker−quinone structured dyad 128 can be selectively enhanced by Y3+ in deaerated PhCN (Figure 69a).294 When excited at 560 nm, electron transfer from the singlet excited state (1ZnP*) to the quinone unit was 2237


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Figure 72. (a) Chemical structures of DPA-appended porphyrin chemosensors 131 and 132; (b) absorption and solution color changes of 131 upon addition of Pb2+ and Cu2+; (c) fluorescence changes of 131 upon addition of Pb2+ and Cu2+. Reprinted with permission from ref 297. Copyright 2012 Royal Society of Chemistry.

643 nm. Gradual addition of Cd2+ led to its coordination at the terpyridine site and thus switched on the fluorescence vividly due to the inhibition of the d-PET process from porphyrin to the terpyridine unit. 129 selectively responds to Cd(II) in a linear range of 3.2 × 10−6 to 3.2 × 10−4 M with a detection limit of 1.2 μM. Dipicolylamine (DPA) has been used widely as the recognition unit for designing Zn2+ chemosensors because it shows high binding affinity to Zn2+. Lippard, Jasanoff, and coworkers reported that the incorporation of two DPA units to the porphyrin macrocycle bearing three sulfonatophenyl groups afforded compound 130 and its manganese complex 130−Mn (Figure 71), which can be used for Zn2+ sensing in water using fluorescence and magnetic resonance (MR) as the reporting signal.296 Fluorescence of compound 130 is very weak in water due to the PET process between the porphyrin core and the DPA units. Upon addition of Zn2+ to the solution of 130, the PET process was interrupted, accompanied by vivid fluorescence turnon at 648 and 715 nm. The detection of Zn2+ with 130 is moderately selective and applicable in a wide pH range from 4.5 to 10.1. In 130−Mn, the axial coordination sites of Mn3+ were initially occupied by the intramolecular pyridyl groups from the DPA units, thus protecting the Mn(III) center from access of water. Upon addition of Zn2+, the DPA pyridines coordinate with Zn2+, leaving bared Mn3+ center for water coordination. Thus, Zn2+ can be detected by the resulting MR signal changes, because both the T1- and T2-weighted images decreased drastically after

Zn2+ coordination. Moreover, both 130 and 130−Mn were successfully applied for Zn2+ detection in living HEK-293 cells. Interestingly, instead of Zn2+, the DPA-appended porphyrins can be occasionally used for sensing other metal ions, say, Pb2+ and Cu2+. In this respect, Chen and Jiang reported that the addition of Pb2+ to the solution of 131 resulted in vivid changes in the solution color and fluorescence-emission wavelengths (Figure 72).297 Thus, 131 can be used as a ratiometric Pb2+ chemosensor. Other tested metal ions including Zn2+ show negligible interference for the detection of Pb2+, except that the binding of Cu2+ to 131 is stronger than that of Pb2+, and the subsequent addition of Cu2+ to the solution of 131−Pb replaced Pb2+ to form a nonfluorescent 131−Cu complex, resulting in the observed fluorescence quenching (Figure 72c). Compound 132 shows a ratiometric Pb2+ sensing behavior similar to that of 131. However, the presence of four DPA units in 132 resulted in the formation of a network structure after Pb2+ coordination. Similar to dipicolylamine, 2,2′-dipyridylamine can also be used to construct metal ion chemosensors. Differently, 2,2′-dipyridylamine is capable of selectively binding Cu2+. Weng et al. reported that the incorporation of the 2,2′-dipyridylamine unit to the porphyrin macrocycle afforded highly selective fluorescent chemosensors 133 and 134 for Cu2+ detection (Figure 73a).298,299 Both 133 and 134 are strongly emissive in CHCl3 with quantum yields of 3.6% and 5.8%, respectively. The addition of Cu2+ almost fully quenched their fluorescence due to the formation of nonfluorescent Cu2+ complexes. Interestingly, 133 bears four 2,2′-dipyridylamine moieties and thus is able to form a 2238


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Figure 73. (a) Chemical structures of 2,2′-dipyridylamine-substituted porphyrin chemosensors 133 and 134; (b) plausible network structure formed after addition of Cu2+ to the solution of 133. The gray balls represent Cu2+ ions; (c) fluorescence response of 133 toward various tested metal ions; (d) images showing the fluorescence of 133 (25 μM) in CHCl3 after addition of various metal ions (160 μM) in MeOH. Reprinted with permission from ref 298. Copyright 2007 American Chemical Society.

supramolecular network structure via Cu2+ coordination in a stoichiometry of 1:2 (porphyrin/Cu2+) (Figure 73b), while 134 bears only one 2,2′-dipyridylamine unit, which can bind Cu2+ in a 1:1 mode. Both 133 and 134 show high selectivity toward Cu2+ (Figure 73c, d), and the detection limits were calculated to be 3.3 × 10−7 and 1.5 × 10−6 M for 133 and 134, respectively. Moreover, the fluorescence quenched by Cu2+ coordination can be recovered by subsequent treatment with EDTA, indicating that 133 and 134 can be regenerated for the sensing application. Metal coordination-induced changes in the ICT process may be systematically employed for designing the so-called ratiometric chemosensors. Wang, Lv, and co-workers reported that the incorporation of a triamino unit to the porphyrin macrocycle not only improved its water solubility but also provided a binding site for selective Zn2+ detection (Figure 74a).300 The addition of Zn2+ to the EtOH/H2O solution of 135 blue-shifted the maximum emission wavelength from 650 to 610 nm (Figure 74b), which may be ascribed to a suppressed ICT process from the porphyrin moiety to the triamino recognition site after Zn2+ coordination. The binding stoichiometry of 135 with Zn2+ was found to be 1:1, and chemosensor 135 can be used to detect Zn2+ in the physiological pH range with a detection limit of 1.8 μM. Although the functionalization of ICT molecules with recognition units is an effective strategy for the development of

Figure 74. (a) Chemical structure of chemosensor 135; (b) ratiometric response of 135 (10 μM) upon gradual addition of Zn2+ in EtOH/H2O (1:1, v:v). Reprinted with permission from ref 300, which is an open access article distributed under the Creative Commons Attribution License (CC BY).

ratiometric chemosensors, it is difficult to design ICT-based chemosensors with a large Stokes shift. Another design strategy that may overcome this issue is to synthesize a chemosensor molecule bearing two moieties capable of independently sensing one common analyte. As shown in Figure 75, the structure of chemosensor 136 consists of two parts, i.e., the porphyrin macrocycle and the naphthalimide−pyridine−piperazine moiety, linked through a four-carbon alkyl chain. These two subunits in 136 can work independently but also “cooperatively” for ratiometric Hg2+ detection.301 Upon addition of Hg2+ to the 2239


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Figure 75. Proposed Hg2+ sensing mechanism of chemosensor 136, which contains a porphyrin moiety and a naphthalimide−pyridine−piperazine moiety.

Figure 76. Chemical structure and proposed Zn2+ sensing mechanism of chemosensor 137 containing two 2-(oxymethyl)pyridine units.

Figure 77. (a) Chemical structure of chemosensor 138 containing four thymine groups at the porphyrin meso-positions; (b) fluorescence spectral response of 138 (2 μM) upon addition of 1 equiv of Hg2+ in DMF/H2O (7/3, v/v). Inset image shows the solution of 138 (100 μM) in the absence (left) and presence (right) of 2 equiv of Hg2+. Reprinted with permission from ref 306. Copyright 2015 John Wiley & Sons, Ltd.

water/EtOH solution of 136, a fluorescence decrease was observed at 650 nm, accompanied by enhanced emission at 525 nm. This observation can be ascribed to the fact that both the porphyrin and pyridine−piperazine moieties can effectively bind Hg2+, and the coordination of Hg2+ to the porphyrin core quenched its fluorescence at 650 nm, while the binding of Hg2+ to the pyridine−piperazine site enhanced the fluorescence of coumarin at 525 nm. The binding constants for the pyridine− piperazine moiety and the porphyrin core were determined to be 4.26 × 105 and 6.31 × 105 M−1, respectively. Chemosensor 136 shows a high selectivity toward Hg2+ over other tested metal ions like Na+, K+, Mg2+, Ca2+, Zn2+, Fe3+, Fe2+, Cu2+, Mn2+, Co2+, Ni2+, Ag+, Pb2+, Cr3+, and Cd2+, and it can be applied for Hg2+ sensing in living HeLa cells. The detection limit of 136 for Hg2+ was calculated to be 2 × 10−8 M.

For the above example, Hg2+ can be captured within the porphyrin core. In fact, the binding of the target metal ion within the porphyrin core could be further assisted by adding cooperative binding units. With two 2-(oxymethyl)pyridine units as the cooperative binding site, porphyrin 137 was able to bind Zn2+ selectively within its inner core in a 1:1 binding mode (Figure 76).302 Upon addition of Zn2+ to the EtOH/H2O (1:1, v/v) solution of 137, a vivid blue-shift in the fluorescence emission spectra from 650 to 605 nm was observed. The binding constant was calculated to be 1.04 × 105 M−1, and 137 shows a linear response to Zn2+ in the concentration range from 3.2 × 10−7 to 1.8 × 10−4 M with a detection limit of 5.5 × 10−8 M. In addition, chemosensor 137 was found to be selective toward Zn2+ over other tested metal ions in a wide pH range from 4.0 to 8.0. 2240


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It has been reported that thymine shows high affinity toward Hg2+ by forming thymine−Hg2+−thymine complexes, and thus some fluorescent chemosensors have been developed using this selective binding.303−305 By combining this knowledge with porphyrin chemistry, a fluorescent chemosensor 138 bearing four thymine groups at the meso-positions was reported by Chen, Li, and co-workers for Hg2+ sensing (Figure 77a).306 The fluorescence of 138 at 614 nm in DMF/H2O (7/3, v/v) can be quenched upon addition of Hg2+, with high selectivity over other tested metal ions like Co2+, K+, Sn2+, Zn2+, Cu2+, Ni2+, Mn2+, Na+, Ca2+, Mg2+, Pb2+, and Cd2+. The fluorescence quenching may be ascribed to Hg2+ induced aggregation (Figure 77b). The detection limit of 138 for Hg2+ was reported to be 6.7 nM under the tested condition, and 138 can be regenerated by treatment with hydrochloric acid. Although thymine shows high binding affinity toward Hg2+, the presence of another strong Hg2+ binding moiety like pyridine in the porphyrin chemosensor molecule may alter the final coordination mode of the resulting complex. In this respect, Balaz and co-workers reported that both of the porphyrin−DNA conjugates 139 and 140 can be used for Hg2+ detection in water (Figure 78).307 The addition of Hg2+ to the buffered solution of

Figure 79. (a) Chemical structure of porphyrin 141. Blue and red colors represent the hydrophilic and hydrophobic units, respectively; (b) proposed structure of the self-assembled nanoparticle of 141 and its binding with organic mercury (yellow and green colors represent the phenyl and mercury moieties, respectively). Reprinted with permission from ref 308. Copyright 2011 Royal Society of Chemistry.

pink to bright green after PhHg2+ capture. Interestingly, the selfassembled 141 also shows colorimetric response to Hg2+ with a solution color change from light pink to olive yellow; thus, the self-assembled 141 can be used for the discrimination of organic and inorganic mercury species in water. Rotating-frame nuclear Overhauser effect correlation spectroscopy (ROESY) studies revealed that the added PhHg2+ molecules were included in the hydrophobic interior of the assembly (Figure 79b). Besides, the binding constant for the self-assembled 141 with PhHg2+ was found to be larger than that of the free 141, and the detection limit of the assembled 141 for PhHg2+ was determined to be 6.7 × 10−7 M. These results indicate that the porphyrin-assembly approach is effective for the design of highly sensitive PhHg2+ chemosensors. The combination of porphyrins with other support materials is also an effective strategy for the design of high-performance ion chemosensors based on porphyrins. In this respect, Jung and coworkers reported that the attachment of porphyrin 142 onto the surface of Au@SiO2 core/shell nanoparticles afforded the nanosensor Au@SiO2−142 for colorimetric and fluorimetric Hg2+ detection (Figure 80).309 The obtained nanosensor Au@ SiO2−142 exhibited strong fluorescence at 650 nm with a quantum yield of 0.039. Upon addition of increasing amounts of Hg2+ to the water solution of Au@SiO2−142, vivid fluorescence quenching was observed, accompanied with a solution color change from red to green, which may be ascribed to the coordination of Hg2+ within the porphyrin core in a 1:1 mode. Other tested metal ions like Li+, Na+, Ca2+, Cu2+, Cd2+, Co2+, Mn2+, Cd2+, Ag+, and Pb2+ cannot induce obvious spectral changes. Au@SiO2−142 was able to detect Hg2+ in a pH range of 4−10 with a detection limit lower than 1.2 ppb. Besides, the Hg2+ captured nanosensor Au@SiO2−142 can be regenerated by EDTA treatment. By doping meso-tetrakis(p-carboxyphenyl) porphyrin 143 into a silica monolith, Buntem et al. reported a solid chemosensor for spectrometric detection of Cu2+, Zn2+, Pb2+, and Ni2+.310 As

Figure 78. (a) Chemical structures of porphyrin−DNA conjugates 139 and 140 for Hg2+ detection; (b) UV−vis absorption spectral changes of 139 (2 μM) upon addition of Hg2+ in water. Inset: the linear relationship between the absorption changes of 139 and the concentration of Hg2+. Reprinted with permission from ref 307. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

139 resulted in the red-shift of the porphyrin Soret band absorption, accompanied by dramatic fluorescence quenching. Similar response behavior was observed for 140, which contains eight adenosine units instead of the thymidine moieties in 139. Thus, the tripyridylporphyrin unit in 139 and 140 plays an important role in Hg2+ sensing. Because the zinc porphyrin core is stable enough to prevent the replacement of Zn2+ with Hg2+, Hg2+ was able to bind to the pyridyl nitrogens and quench the fluorescence. Considering the fact that the binding constant of 139 with Hg2+ is 6-fold higher than that of 140, the combination of the pyridyl binding site with thymidine may be an ideal strategy for the design of highly sensitive and selective Hg2+ chemosensors. The obtained detection limit of 139 for Hg2+ is 21 nM. Liu et al. developed a supramolecular strategy for the design of porphyrin-based Hg2+ chemosensors.308 An amphiphilic porphyrin 141 (Figure 79a) was synthesized and constructed into a supramolecular self-assembled architecture, which shows selective colorimetric response to PhHg2+ in water. The addition of PhHg2+ to the self-assembled 141 resulted in the decrease of the absorption band at 404 nm, accompanied by a new peak developing at 465 nm. The solution color was changed from light 2241


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Figure 82. (a) Chemical structures of chemosensors 144 and 145; (b) images showing the PMMA film doped with compound 144 under the ambient light (left) and under a UV lamp (λ = 365 nm) (right). Reprinted with permission from ref 311. Copyright 2014 American Chemical Society.

Figure 80. Synthetic route for porphyrin 142-coated nanosensor Au@ SiO2−142.

shown in Figure 81, upon separate treatment with Cu2+, Zn2+, Pb2+, and Ni2+ for 2 days, the resulting silica monolith colors are

peak developing at 608 nm. Thus, chemosensor 144 acts as a ratiometric fluorescent chemosensor for Zn2+. The ratiometric fluorescence changes can be ascribed to a metal-to-ligand charge transfer (MLCT) process upon Zn2+ coordination. When titrated with Hg2+, the initial emission peak at 659 nm also decreased dramatically, with a new peak developing at 704 nm. The addition of Cu2+ quenched 99% of the initial fluorescence, and thus chemosensor 144 can be considered as an on−off type chemosensor for Cu2+. Theoretical calculations revealed that 144 coordinates with Zn2+, Hg2+, and Cu2+ in a metal-to-ligand ratio of 1:2. However, if the phenyl group at the 3-position of pyrazole in 144 was replaced with a 2-pyridyl group to afford ligand 145, the metal coordination mode will be different. Especially, ligand 145 coordinates with Zn2+, Hg2+, and Cu2+ in a metal-to-ligand ratio of 2:1. However, the spectral changes of 145 upon titration of Zn2+ are similar to those observed for chemosensor 144, while the addition of either Hg2+ or Cu2+ to 145 quenched its fluorescence. Furthermore, the authors tried to design a solidphase chemosensor with ligand 144 by doping it into a low-cost poly(methyl methacrylate) (PMMA) thin film (Figure 82b), and fluorescence turn-on at 719 nm was observed when the thin-film chemosensor was treated with an aqueous Zn2+ solution. Immobilized in plasticized poly(vinyl chloride) (PVC) membrane, an optical chemosensor for Hg2+ using porphyrin 146 as the sensing reagent was developed by Yang, Yu, and coworkers (Figure 83).312 The sensing membrane shows a linear fluorescence-quenching response to Hg2+ in a range of 4.0 × 10−8 to 4.0 × 10−6 M, with a detection limit of 8.0 × 10−9 M. The fluorescence quenching is a result of the formation of a Hg2+− 146 complex. The membrane is selective toward Hg2+ over tested alkali, alkaline earth, and other heavy metal ions and can be used for Hg2+ detection in real water samples. Interestingly, Gupta et al. found that the PVC membrane containing porphyrin 147 (Figure 83) and (sal)2trien as the electroactive material and dioctyl phthalate, tri-n-butylphosphate, chloronaphthalene, dibutyl phthalate, and dibutyl(butyl) phosphonate as the plasticizing solvent mediators can be used as an electrochemical chemosensor for Ni2+.313 By introducing both metal ion and anion-binding sites, it is also possible to develop porphyrin-based chemosensors for metal

Figure 81. Porphyrin 143-doped silica monolith for spectrometrically sensing Cu2+, Zn2+, Pb2+, and Ni2+. Reprinted with permission from ref 310. Copyright 2010 Elsevier.

different for each metal ion. Importantly, the chemosensor can be regenerated by subsequent treatment with 1 M HNO3 for 24 h. Although the preparation and detection processes of the chemosensor based on 143 are time-consuming, solid chemosensors are much easier to carry and operate compared with solution ones. Lodeiro, Neves, and co-workers synthesized four pyrazole− porphyrin conjugates that can detect Zn2+, Hg2+, and Cu2+ in different signal pathways.311 For example, upon addition of Zn2+ to a chloroform solution of chemosensor 144 (Figure 82a), the initial emission peak at 659 nm decreased dramatically with a new 2242


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Figure 83. Chemical structures of porphyrins 146 and 147 for the construction of PVC membrane supported chemosensors for Hg2+ or Ni2+, respectively.

5. LINEAR OLIGOPYRROLES FOR ION SENSING Similar to porphyrins and porphyrin analogues, linear oligopyrroles also show high binding affinities toward various anions and metal ions and can be utilized for ion-sensing application.77,315−318 Considering that linear oligopyrroles are widely used as the synthetic precursors for the synthesis of novel porphyrins and porphyrin analogues, we will briefly describe the ion-sensing properties and mechanisms of linear oligopyrrolic compounds in this section. We believe that descriptions of the design strategies and sensing mechanisms of ion chemosensors based on linear oligopyrroles will be beneficial for the design of ion chemosensors based on porphyrin analogues.

salts. With a Lewis acidic central metal ion as the anion-binding site, the further introduction of a metal binding strap to the metalloporphyrin core afforded ditopic chemosensors 148 and 149 (Figure 84) for metal salts.314 The difference between 148

5.1. Linear oligopyrroles for anion sensing

The pyrrolic NH moieties of linear oligopyrroles are promising hydrogen-bonding donors; thus linear oligopyrroles have been investigated for anion binding and transport.144,319,320 However, only limited numbers of linear oligopyrroles have been synthesized as optical chemosensors for anions, and thus approaches for the design of linear oligopyrrole-based anion chemosensors are still very limited.321−328 The Xie group discovered that the incorporation of 3,5-di-tertbutyl-4-hydroxyphenyl groups at the meso-positions of di- and tripyrrins resulted in the formation of pyrrole−hemiquinone compounds 150 and 151, which show selective colorimetric response toward F− in DMSO (Figure 85a).329 Of tested anions F−, Cl−, Br−, I−, CH3COO−, H2PO4−, and CN−, only the addition of F− to the DMSO solution of 150 induced a significant color change from orange to bright blue (Figure 85b). The high selectivity of 150 for F− was ascribed to the fact that F− is the most electronegative atom, and thus it is able to induce deprotonation of the N−H moiety in 150. Similarly, chemosensor 151 also exhibits high selectivity for F−, showing two-step deprotonation processes upon addition of F−. The detection limits of chemosensors 150 and 151 for F− were calculated to be 2 × 10−4 and 2 × 10−5 M, respectively. Recently, the Sessler group reported that tetrakis(1H-pyrrole2-carbaldehyde) compounds 152−154 (Figure 86a) show high binding affinities toward dihydrogen phosphate and pyrophosphate anions in chloroform.330 As shown in Figure 86b, the turbid suspension of compound 152 in CHCl3 becomes transparent after addition of 1.2 equiv of H2PO4− or HP2O73−, and the addition of the same amount of other anions such as

Figure 84. Chemical structures of ditopic porphyrin chemosensors 148 and 149 strapped with a diaza crown ether moiety.

and 149 lies in the size of the strap, with incorporated diaza-15crown-5 and diaza-18-crown-6 moieties, respectively. Hence, 148 and 149 show different binding selectivities toward metal salts. 148 and 149 show NaCN binding affinities higher than those for KCN by 56-fold and 12-fold, respectively, with the metal ion and the counteranion CN− bound at the crown site and the porphyrin zinc center, respectively. From the examples described in this section, it can be concluded that porphyrin-based ion chemosensors can be obtained via three general design strategies: (1) functionalization at the meso- and pyrrolic-β-positions of porphyrins with ionrecognition units; (2) using the porphyrin core as the ionrecognition unit and the porphyrin itself as the reporter; (3) immobilizing porphyrin dyes into other supporting materials to afford chemosensors applicable in the solid or the nanoparticle form. Inspired by these design strategies for porphyrin-based ion chemosensors, it is envisioned that the approach of immobilizing porphyrins into supporting materials may also be practicable for designing high-performance ion chemosensors based on porphyrin analogues, and more investigations are desirable to incorporate a recognition unit into the porphyrin analogue macrocycle to construct high-performance ion chemosensors. 2243


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Figure 87. (a) Proposed sensing mechanism of dipyrrin 155 for CN−; (b) solution color changes of 155 in DMSO−H2O upon addition of various anions. Reprinted with permission from ref 332. Copyright 2012 Royal Society of Chemistry.

Figure 85. (a) Chemical structures of linear di- and tripyrrolic hemiquinone compounds 150 and 151 for F− sensing; (b) image showing the colorimetric response of 150 upon addition of various anions in DMSO. Reprinted with permission from ref 329. Copyright 2010 Royal Society of Chemistry.

different interaction mechanisms. Only hydrogen bonding was observed between 155 and F−, while CN− was able to form an adduct with 155. The detection limit of 155 toward CN− was calculated to be 3.6 × 10−6 M. From these examples, it can be summarized that mainly two strategies have been used for the design of anion chemosensors based on oligopyrroles. First, the NH moieties of oligopyrroles are excellent anion-binding sites, and thus highly selective anion chemosensors can be developed by introducing suitable substituents to oligopyrroles to modulate the acidity and binding affinities of the pyrrolic NH moieties. Another strategy is to incorporate an additional recognition unit to the oligopyrrolic chromophore/fluorophore. 5.2. Linear oligopyrroles for metal ion sensing

Generally, linear oligopyrroles are flexible in structure, which tends to induce nonradiative energy loss due to free rotation of the pyrrolic units; thus, linear oligopyrroles are usually not or very weakly emissive. However, they may become strongly emissive after forming metal or boron complexes.77,333 These facts indicate that linear oligopyrroles may be developed as fluorescence turn-on metal ion chemosensors through careful molecular design. Dipyrrins have been extensively used as ligands to construct supramolecular coordination architectures, while their abilities in metal ion sensing remain relatively unexplored,334−338 which may be ascribed to the fact that the fluorescence properties of dipyrrin complexes are influenced by several complicated factors. In fact, chemists have been interested in developing fluorescent dipyrrin metal complexes. As early as 2004, Bocian, Lindsey, Holten, and co-workers reported that dipyrrins with bulky mesityl groups at the meso-positions lead to steric constraints on intramolecular rotations, and the corresponding Zn2+ complex 156 is highly fluorescent with a quantum yield of 36% (Figure 88).339 Then, it was found that dipyrrins with the same mesityl groups at the meso-positions were also able to form fluorescent complexes with various metal ions including Sn2+, Ga3+, and In3+ (compounds 157−159, Figure 88).340,341 Using the similar steric constraint strategy, Boyle, Archibald, and co-workers synthesized a 2-pyridyl-substituted dipyrrin and demonstrated that its Zn2+ complex 160 is also strongly emissive (Figure 88).342 In 2010, Cheprakov, Vinogradov, and co-workers discovered that the fluorescence of π-extended dipyrrin zinc complexes can be reversibly modulated by changing the metal coordination modes between 1:1 and 1:2 (metal/ligand).343 From the variation of the quantum yields for these examples, we can see that the luminescent properties of dipyrrin metal complexes are greatly dependent on the substituents of the dipyrrin ligand, the

Figure 86. (a) Structures of linear oligopyrrolic compounds 152−154 containing α-formyl substituents; (b) image showing the suspension/ solution of 152 in CHCl3 upon addition of 1.2 equiv of various anions. Reprinted with permission from ref 330. Copyright 2014 Royal Society of Chemistry.

HSO4−, PhCO2−, Cl−, and NO3− did not induce any visible changes. The simplest conjugated linear oligopyrroles are the so-called dipyrrins, which are dipyrrolic compounds with two pyrrolic units linked by a methine bridge (CH−) at their αpositions.331 Considering that dipyrrins show intense absorption bands in the visible-light region and are thus deeply colored, the Xie group reported that the introduction of a carbonyl group at the dipyrrin pyrrolic α-position is an effective approach for developing colorimetric CN− chemosensors.332 Thus, dipyrrin 155 containing a pentafluorobenzoyl substituent at the pyrrolic α-position was synthesized for CN− detection (Figure 87). Of tested anions of CN−, F−, Cl−, Br−, I−, CH3COO−, and H2PO4−, only the addition of CN− to the DMSO−H2O solution of 155 resulted in dramatic color change from light yellow to pink (Figure 87b), which could be ascribed to the nucleophilic attack of CN− to the carbonyl carbon to afford a stable cyanide− dipyrrin adduct 155−CN (Figure 87a), as evidenced by HRMS, UV−vis, and 1H NMR data. Interestingly, when tested in pure organic solvents such as CH2Cl 2, 155 shows different colorimetric responses to CN− and F−: The addition of CN− changed the solution color from light yellow to pink while F− induced a color change to orange. Thus, 155 was able to differentiate CN− and F− in organic solutions. The different color changes induced by CN− and F− in CH2Cl2 resulted from 2244


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With the pentafluorophenyl group incorporated as the mesoaryl substituent, the Xie group developed a series of fluorescence turn-on Zn2+ chemosensors 162−169 based on dipyrrins and conjugated tripyrrolic ligands (Figure 89).350−352 The chemosensor structures, Zn2+ coordination modes, and fluorescenceemission properties of the corresponding Zn2+ complexes are summarized in Figure 89. They found that the modification at the α- and β-positions of the dipyrrin’s pyrrolic units is an effective approach for manipulating the emission wavelengths of the corresponding Zn2+ complexes. The obtained chemosensors 162−169 were able to detect Zn2+ with tunable emission wavelengths that cover a visible light region from 514 to 637 nm. DFT calculations revealed that the tunable and red-shifted emission wavelengths of these dipyrrin chemosensors can be ascribed to the enlarged π-conjugation frameworks and thus diminished highest occupied molecular orbital (HOMO)− lowest unoccupied molecular orbital (LUMO) energy gaps by introducing the substituents at the pyrrolic units. It is also noteworthy that the modification at the pyrrolic α-positions seems to be more effective for enlarging the π-conjugation frameworks than that at the β-position, as evidenced by the fact that chemosensor 167 with two α-substituents shows a longer emission wavelength than that of 166 with an α-,β′-substitution mode. The fluorescence enhancement of chemosensors 162− 169 upon Zn2+ coordination could be ascribed to the CHEF mechanism. All the synthesized dipyrrin and tripyrrolic compounds 162−169 are selective and sensitive for the detection of Zn2+ in aqueous solutions, and chemosensor 169 was successfully applied for Zn2+ imaging in living cells. By incorporating two more phenolic oxygen atoms as the binding sites, Yoon, Thangadurai, and co-workers developed a N2O2 type dipyrrin 170 (Figure 90) for fluorescence turn-on detection of Zn2+.353 Compound 170 is weakly emissive at 630 nm, while the addition of 0−16 equiv of Zn2+ to its methanol solution resulted in sharper, blue-shifted, and vividly enhanced fluorescence at 615 nm. Actually, there are mainly two binding steps upon addition of Zn2+ to 170. Fluorescence spectroscopy and LC-MS data indicate that the initial addition of Zn2+ to 170 resulted in the formation of the 1:1 complex 170−Zn, while the

Figure 88. Representative examples of fluorescent dipyrrin metal complexes.

coordinated metal ion, and the coordination modes. It is thus possible to develop highly selective fluorescent chemosensors based on dipyrrins for metal ion detection.344−347 Bentley and co-workers reported that the selective binding of Zn2+ to dipyrrin 161 (Figure 89) weakened the ICT process from the electron-donating dipyrrin moiety to the electron-withdrawing 8-hydroxyquinoline unit, resulting in blue-shifts of the fluorescence spectra from 672 to 616 nm.348 By using pyrene as the meso-aryl group, the Bentley group demonstrated that the obtained 5-(pyren-1-yl)-4,6-dipyrrin shows a selective fluorescence turn-on response toward Zn2+ and can be used to image Zn2+ concentration in neuronal vesicles.349

Figure 89. Chemical structures of chemosensors 161−169 based on dipyrrins and their tripyrrolic analogues. Note that the indicated emission wavelengths refer to their corresponding Zn2+ complexes. 2245


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dipyrromethane skeletons are present, different binding stoichiometries for Zn2+ coordination were observed for chemosensors 171−176. As evidenced by 1H NMR and HRMS data, 171−173 were oxidized to the corresponding dipyrrins, which coordinate with Zn2+ in a 1:2 (dipyrrin/metal) mode, while the corresponding zinc(II) complexes of 174−176 were found to be in a 1:1 mode, as revealed by single-crystal structures and Job plot analyses. These observations may be ascribed to the unique electron-withdrawing character of the pcyanophenyl substituents in 174−176. These examples clearly indicate that the easy synthesis and modification of linear oligopyrroles endorsed their applications in ion sensing, and they share many similar design strategies with porphyrins and porphyrin analogues, such as the utilization of pyrrolic NH and imino N moieties for anion binding and metal coordination, respectively, and the effective incorporation of a recognition unit. It is noteworthy that dipyrrins and their tripyrrolic analogues provide the possibility of effectively modulating the emission wavelengths in a wide wavelength range. In addition, the presence of Zn2+ can trigger the oxidation of meso-OH-substituted dipyrromethanes and subsequent coordination, which can be employed for Zn2+ sensing. This observation is in sharp contrast to the fact that ion chemosensors based on target ion-induced redox reactions still remain relatively unexplored for porphyrins and their analogues.

Figure 90. Scheme illustrating the two-step responses of the N2O2 type dipyrrin chemosensor 170 to Zn2+. Below: images showing the fluorescence changes of 170 upon addition of 1 or 2 equiv of Zn2+ in CH3OH. Reprinted with permission from ref 353. Copyright 2012 Elsevier B.V.

presence of higher concentrations of Zn2+ resulted in the formation of 170−Zn2−Cl2. Chemosensor 170 was demonstrated to be selective toward Zn2+ with little interference from Cd2+. Dipyrromethanes are also typical dipyrrolic compounds, with a lesser degree of conjugation than dipyrrins. Different from dipyrrins, dipyrromethanes are usually colorless and nonfluorescent due to the interruption of π conjugation by an sp3 carbon between the two pyrrolic units. However, the Xie group reported that the presence of Zn2+ led to the oxidation of mesoOH-substituted dipyrromethanes 171−176 (Figure 91) to the corresponding dipyrrins with simultaneous formation of the corresponding zinc complexes, resulting in the appearance of bright fluorescence.354,355 Because there is no background fluorescence for chemosensors 171−176, this oxidationcoordination induced fluorescence turn-on sensing mechanism provided a strategy to improve the signal-to-noise ratio for Zn2+ detection. Interestingly, although the same meso-OH-substituted

6. CONCLUSIONS AND FUTURE PERSPECTIVES In this Review, we have attempted to describe developments in colorimetric and fluorescent ion chemosensors based on porphyrin analogues. As mentioned in the Introduction of this Review, the diverse chemistry of porphyrin analogues provides various design strategies for developing porphyrin analogues for both metal ion and anion detection. First, porphyrin analogues show promising photophysical properties suitable for application as optical chemosensors, such as strong light absorption in the visible light region and red to near-infrared fluorescence emission. Thus, porphyrin analogues can be used as the reporting unit in the design of small organic molecule based

Figure 91. Chemical structures of chemosensors 171−176 based on meso-OH-substituted dipyrromethanes. The indicated emission wavelengths refer to their corresponding Zn2+ complexes. 2246


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optical chemosensors. By incorporating a suitable recognition unit to a porphyrin analogue reporter, ion chemosensors with desired sensing behavior may be conveniently developed. Second, the unique metal coordination properties of porphyrin analogues can be utilized for metal ion sensing. In this respect, porphyrin analogues may simultaneously act as the recognition and reporting units. The ring sizes and inner donor atoms may be systematically varied to selectively accommodate a unique target metal ion, and the coordination of the target ion will alter the photophysical properties, thus producing a reporting signal. For example, contracted porphyrins may be suitable for coordinating small-sized metal ions while expanded porphyrins provide a larger inner cavity for large metal ions, and in some cases, even two metal ions can be simultaneously accommodated. Third, the ion-binding affinities of porphyrin analogues may be strongly influenced by the substituents present at the meso-, pyrrolic α-, and β-positions, and ion selectivity may be effectively modulated. Finally, a cooperative recognition strategy can also be employed by introducing an additional recognition unit to operate cooperatively with the oligopyrrolic macrocycle to further enhance binding affinity and/or modulate selectivity. For example, the introduction of a strap may result in steric hindrance that prevents the binding of large ions by the chemosensor, and the presence of binding sites within the strap may further modulate the ion affinities and selectivities, thus providing an additional way to improve the ion selectivity. Similar to porphyrin analogues, porphyrins have also been extensively used for ion sensing due to their promising photophysical and coordination properties. The chemistry of porphyrins is relatively simple compared with their analogues. However, porphyrins have been demonstrated to be very powerful in the field of ion sensing. A wide range of metal ions such as Zn2+, Cu2+, Hg2+, Pb2+, Cd2+, and Y3+ and anions including F−, CN−, SO42−, HSO4−, and HPO42− have been successfully detected using porphyrin-based chemosensors. In addition, as important synthetic intermediates for porphyrins and their analogues, linear oligopyrroles have also been used for sensing of both metal ions and anions based on their color and/ or fluorescence changes. The development of ion chemosensors based on porphyrins and linear oligopyrroles provides useful information and insights for the further design of highperformance ion chemosensors based on porphyrin analogues. In spite of the recent achievements in the development of porphyrin-related ion chemosensors, this research field is still at a nascent stage. Most of the reported ion chemosensors have been applied in organic media. However, real-world environmental and biochemical analyte samples are most often available in aqueous media. Thus, further research on improving water solubility and sensing behavior in aqueous systems is required. On the other hand, polymer-supported sensing devices are also strong requirements to make practical applications more convenient.

China University of Science & Technology (ECUST), obtaining his Ph.D. degree in 2013. He is currently a research scientist in Nanjing University. His research interests are focused on the development of fluorescent organic materials. Prof. Wei-Hong Zhu received his Ph.D. degree in 1999 from East China University of Science &Technology (ECUST), China. He worked in AIST Central 5, Tsukuba (Postdoctoral), and in Tsukuba University (visiting professor), Japan, from 2001 to 2005. He became a full professor in 2004 and deputy director in 2006. He has published 171 papers in international journals and received several awards, such as Oriental Scholar (2009) and NSFC for Distinguished Young Scholars (2013). His current research interest is focused on functional chromophores, including fluorescent sensors, photochromism, and metal-free solar cell sensitizers. Prof. Yongshu Xie received his Ph.D. degree from Zhejiang University. Following postdoctoral research and associate professorship in University of Science & Technology of China, he successively joined Prof. Xuming Peng group at National Taiwan University, Prof. Hiroyuki Furuta group at Kyushu University, and Prof. Katsuhiko Ariga and Prof. Jonathan P. Hill group in NIMS (Japan) as a research fellow. Since 2007, he has been a professor at East China University of Science & Technology. He has received several awards, including Oriental Scholar (2011) and New Century Excellent Talents in University (MOE, China, 2011). His research interests include porphyrin chemistry, ion sensing, and optoelectronic materials chemistry.

ACKNOWLEDGMENTS This work was supported by the Science Fund for Creative Research Groups (21421004), National Basic Research 973 Program (2013CB733700), NSFC/China (21472047, 91227201), the Oriental Scholarship, and the Programme of Introducing Talents of Discipline to Universities (B16017). Y.D. thanks the Natural Science Foundation of Jiangsu Province (BK20140593) and Open Funds of the State Key Laboratory for Chemo/Biosensing and Chemometrics (2014002) for financial support. We thank Professor Hui Wei at NJU for insightful discussions. REFERENCES (1) Ueno, T.; Nagano, T. Fluorescent Probes for Sensing and Imaging. Nat. Methods 2011, 8, 642−645. (2) Nolan, E. M.; Lippard, S. J. Small-Molecule Fluorescent Sensors for Investigating Zinc Metalloneurochemistry. Acc. Chem. Res. 2009, 42, 193−203. (3) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G. E. M.; Sandanayake, K. R. A. S. Molecular Fluorescent Signalling with ’Fluor-spacer-receptor’ Systems: Approaches to Sensing and Switching Devices via Supramolecular Photophysics. Chem. Soc. Rev. 1992, 21, 187−195. (4) Galbraith, E.; James, T. D. Boron Based Anion Receptors as Sensors. Chem. Soc. Rev. 2010, 39, 3831−3842. (5) Jiang, P.; Guo, Z. Fluorescent Detection of Zinc in Biological Systems: Recent Development on the Design of Chemosensors and Biosensors. Coord. Chem. Rev. 2004, 248, 205−229. (6) Yang, Y.; Zhao, Q.; Feng, W.; Li, F. Luminescent Chemodosimeters for Bioimaging. Chem. Rev. 2013, 113, 192−270. (7) Shie, J. J.; Liu, Y. C.; Lee, Y. M.; Lim, C.; Fang, J. M.; Wong, C. H. An Azido-BODIPY Probe for Glycosylation: Initiation of Strong Fluorescence upon Triazole Formation. J. Am. Chem. Soc. 2014, 136, 9953−9961. (8) Rong, L.; Liu, L. H.; Chen, S.; Cheng, H.; Chen, C. S.; Li, Z. Y.; Qin, S. Y.; Zhang, X. Z. A Coumarin Derivative as a Fluorogenic Glycoproteomic Probe for Biological Imaging. Chem. Commun. 2014, 50, 667−669.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Dr. Yubin Ding received his B.S. degree in chemistry from Shanxi University in 2008. He then joined Prof. Yongshu Xie group in East 2247


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