Optical sensing of anions using C3v-symmetric tripodal receptors

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 27 (2016) 30–53

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Review

Optical sensing of anions using C3v -symmetric tripodal receptors Suban K. Sahoo a,b,∗ , Gi-Dong Kim b , Heung-Jin Choi b,∗ a b

Department of Applied Chemistry, SV National Institute of Technology, Surat, India Department of Applied Chemistry, Kyungpook National University, Daegu, South Korea

a r t i c l e

i n f o

Article history: Received 29 December 2015 Accepted 5 April 2016 Available online 20 May 2016 Keywords: Tripodal anion receptors Fluorogenic receptors Chromogenic receptors Sensing mechanisms

a b s t r a c t The ubiquitous nature of anions has gained a bourgeoning interest among the supramolecular chemists to develop artificial receptors for the selective recognition and sensing of bioactive anions. Many excellent linear, dipodal, tripodal and multipodal anion receptors have been reported in the last two decades to recognize and detect anions of diverse geometry selectively. This review was narrated with the aim to summarize the C3v -symmetric tripodal receptors reported for the chromogenic and fluorogenic sensing of anions using various flexible and rigid tripodal frameworks. Sensing mechanisms like ICT, PET, excimermonomer formation, push-pull, rigidity effect etc., and the design concepts applied for the development of tripodal anion sensors have been discussed. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 1.1. Anion sensing and mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Neutral tripodal anion receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.1. Tripodal receptors with urea/thiourea binding sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2. Tripodal receptors with other binding sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Positively-charged tripodal anion receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Metal complex based tripodal anion receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Dr. Suban K. Sahoo is working as an Assistant Professor (since 2009) in the Department of Applied Chemistry, SV National Institute of Technology, Surat, India. He is presently on a research visit to Prof. Choi Lab as Brain Korea 21+ fellow. He has published more than 90 research papers in peer-reviewed international journals in the fields of design and molecular modeling studies of biomimetic ligands and bio-inspired fluorosensing materials, molecular recognition and sensing, metallo-supramolecular chemistry, synthetic coordination chemistry and studies of metal complexes in solution.

Gi-Dong Kim is a MSc student with Prof. Choi at the Department of Applied Chemistry, Kyungpook National University, Korea working in the field of design and synthesis of tripodal receptors for the selective recognition and sensing of bioactive anions.

∗ Corresponding authors at: Department of Applied Chemistry, Kyungpook National University, Daegu, South Korea. E-mail addresses: suban [email protected] (S.K. Sahoo), [email protected] (H.-J. Choi). http://dx.doi.org/10.1016/j.jphotochemrev.2016.04.004 1389-5567/© 2016 Elsevier B.V. All rights reserved.

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Prof. Heung-Jin Choi is a Professor (since 1993) in the Department of Applied Chemistry, Kyungpook National University, Korea. He has received Ph. D. from State University of New York at Stony Brook in organic synthesis with Professor Thomas Bell, and was a postdoctoral fellow with the late Professors Donald J. Cram and late Orville L. Chapman at the University of California, Los Angeles (UCLA). His major research projects include basic and applied supramolecular chemistry: (i) design of container molecules such as self-folding cavitands and self-assembled dimeric molecular capsules (molecular recognition, kinetics, thermodynamics on supramolecular phenomena), and (ii) molecular receptors for ion selective electrodes (macrocyclic tetradentate amide receptors for cation binding), and for chemosensors (molecular scaffold for anion binding) built on C3v -symmetric tribenzyltrindane-tricaboxylic acid scaffold designed based on structurally and symmetrically complementary for various types of anions, and (iii) self-assembling molecular linkers on gold for protein chip which is analyzed by SPR (surface plasmon resonance).

1. Introduction After the first ion receptors reported by Park and Simmons [1], it took more than 20 years by the scientific world to give attention towards the design of suitable abiotic anion receptors for various purposes like recognition, isolation, extraction, transport, sensing in the biological systems or in the nature [2]. The main reasons behind the delayed interest in the development of effective anion receptors is that the requirement of more design parameters than for cations due to their size, complex geometry, highly hydrated, pH sensitivity or the influence of the counterions [3]. With the popularization of the concept of preorganization and shape complementarity from supramolecular chemistry, there is expedite growth from the last two decades in the development of anion receptors with high selectivity and stronger anion binding ability by designing multi-dimensional receptors with increased number of binding sites and binding dimensions. Among the various supramolecular approaches, the C3v -symmetric tripodal anion receptors have shown highly effective anion recognition ability because the tripodal conformation with multiple binding sites has the ability to form spheroidal cavities to encapsulate anions of perfect fitting. Recently, many review articles related to the selective recognition of anions by tripodal receptors have been reported [4–7] but no particular review emphasized on their applications towards optical sensing of anions. Therefore, this review was narrated with the aim to summarize the tripodal anion receptors applied for the fluorogenic and chromogenic sensing of anions. The tripodal receptors are summarized in three different categories: neutral, positively charged and metal complex based anion receptors. We have also given a comprehensive analysis about the selectivity and the signaling mechanisms for the screening of a wide range of anions of environmental, biological, and medical importance using the fluorescence and/or color changes. 1.1. Anion sensing and mechanisms The design of chemosensors based on optical (colorimetric/fluorescent) changes upon anion recognition have gained a wide interest in the current research due to the high sensitivity, selectivity, simplicity in detecting low concentrations of anions. This methods also allowed to monitor anion in biological milieu either spectroscopically or with the naked-eye (bioimaging) [8]. In general, most of the optical sensors consist of a receptor for the selective recognition of anion and a reporter (fluorophore/chromophore) to give detectable optical signals upon the recognition of anion. The artificial anion sensors can be categorized into one of the design approaches shown in Fig. 1. (a) The receptor may be the integrated part of the conjugated-␲ system of the reporter and the optical signals observed mainly due to the internal charge transfer (ICT) upon anion binding. (b) The anion binding site is covalently linked to a reporter through a short spacer. The optical sensors based on reporter-spacer-receptor model are most widely used approach and operated with the mechanisms like photo-induced electron transfer (PET), excimer/exciplex formation, photo-induced charge transfer (PCT), the rigidity effect, fluorescence resonance energy transfer (FRET), excited-state proton transfer (ESPT) etc. (c) The irreversible reaction based strategy which occurred between the anion and the “chemodosisensor” to create a new molecule with different optical properties. Many excellent examples for the sensing of anions like fluoride [9], cyanide [10] have been reported using the “Chemodosimeter” type approach. (d) The indicator-displacement approach (IDA) for the optical sensing of anion is a supramolecular approach having many advantages over the single-molecule-based sensors. In IDA approach (Fig. 2a), the anion binding site of the receptor was first occupied by the reporter (chromogenic/fluoregenic dye) through the reversible non-covalent interactions. Addition of target anion into the solution containing the ‘molecular ensembles’ of receptor and dye, the dye displaced from the receptor and the spectroscopic behavior of non-coordinating dye was recovered in solution. The receptordye affinity should be lower compared to that of the receptor-anion complex, and also the dye showed different optical behavior during bound and unbound state in solution. The ease of the synthesis (i.e., no need of further functionalization of the receptor with reporter), depending on the need variety of dyes may be used for one type of receptor, and the wider availability of dyes make this approach very advantageous. However, in order to circumvent the need of excess of the indicator dye which lowers the sensitivity to low concentration of anions, the intramolecular indicator displacement approach (IIDA) was introduced for the selective detection of anion (Fig. 2b). In IIDA, the sensor comprises a receptor and spacer with an attached anionic dye. In the absence of target anion, the anionic dye is bound by the receptor and the photophysical properties of the dye change when it is displaced by the analyte anion. In this review, the tripodal chromogenic sensors discussed are mainly operated with the charge transfer mechanism, whereas the fluorescent tripodal sensors followed the mechanisms like PET, ICT and excimer-monomer formation. The PET based sensors are of the type fluorophore-spacer-receptor model (Fig. 3), where the emission intensity (or quantum yield) is altered (switched ‘On–Off’ or ‘Off–On’) with little or no spectral shift. The PET from the receptor to the fluorophore quenched the emission, but enhanced when it is inhibited. So, the emission switching from ‘On-to-Off’ or ‘Off-to-On’ upon recognition of analyte will ideally depend on the changes that occur in the reduction or the oxidation potential of the receptor compared to that of the fluorophore. The PET based anion sensors are either turn-Off or turn-On types based on the design, which allowed to monitor anion at a particular wavelength. Also, the turn-Off anion sensors are not suitable in the biological matrices due to the pronounced autofluorescence between 400 and 600 nm. These problems can be adequately circumvented by anion selective ratiometric sensors based on the ICT mechanism. The functioning of the ICT based sensors is diagrammatically shown in Fig. 4. ICT based anion sensors most often contain an electron donating

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Fig. 1. Four different approaches for the design of anion selective optical chemosensors: (a) binding site is an integral part of the signaling unit, (b) binding site and signaling subunit connected covalently by a short spacer, (c) chemodosimeter and (d) indicator-displacement approach. These approaches can give anion-induced both chromogenic and fluorogenic signals (here only the ‘switch-on’ fluorescent sensors are shown).

Fig. 2. Illustrated concept of the (a) indicator-displacement approach (IDA) and (b) intramolecular indicator displacement approach (IIDA) for the optical sensing of anions.

(also proton donating) group such as an amino, amide, or hydroxy group as binding site. This group is conjugated without spacer to the fluorophore/chromophore containing preferably an electron-withdrawing group, resulting in an electron donor-acceptor (D-A) system. The sensor then undergoes internal charge transfer from the donor to the acceptor upon interaction with anion. Binding of anions (anioninduced deprotonation of receptor) increase the electron donating character of the receptor group, resulting in an increased ICT which is expected as red-shift in the absorption and emission spectrum together with an enhancement. In contrast to PET and ICT based sensors, the formation of excimer/exciplex based sensors required interaction of at least two fluorophores in close proximity. The functioning of excimer/exciplex-monomer based sensors for the detection of anion is demonstrated in Fig. 5. The excited fluorophore can formed a dimeric complex with the same ground state molecule (excimer) or different molecules (exciplex) through non-covalent interactions like ␲-␲ stacking, electrostatic interactions. This dimeric complex formation is a reversible process i.e., after emission they break apart to the initial ground state molecules (monomer). The emission of an excimer/exciplex is usually broad, structureless and appeared at longer wavelength than that of the monomer emission. For example, the well-studied pyrene fluorophore showed monomer and excimer emission respectively at 370–380 nm and 460–480 nm. In excimer/exciplex-monomer based sensors, binding of analyte promote or disrupt the excimer/exciplex formation to afford a ratiometric sensors. The analyte can be monitored by the fluorescent intensity ratio of both monomer and excimer/exciplex. The intermolecular excimer/exciplex formation is concentration dependent i.e., dilution can shifts the equilibrium towards monomer, but the intramolecular excimer/exciplex formation is independent of concentration where the excited and the ground states are tethered in one molecule. The flat aromatic moieties like pyrene, anthracene or naphthalene etc. are mostly used for the designing of excimer/exciplex based anion sensors because of their flat structure allowing for strong ␲-stacking interactions and showed pronounced excited state lifetimes.

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Fig. 3. Diagram for the PET mechanism for anion sensing by fluorescence turn-Off process.

Fig. 4. Diagrams for the ICT mechanism for anion sensing.

Fig. 5. Design for the (a) intramolecular and (b) intermolecular excimer-monomer based ratiometric sensors for anion.

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Fig. 6. PET sensing of H2 PO4 − anion using the receptor 2.

Fig. 7. The proposed binding model of the receptors (a) 5 and (b) 14a–c with H2 PO4 − .

2. Neutral tripodal anion receptors 2.1. Tripodal receptors with urea/thiourea binding sites The urea and thiourea based receptors has been widely investigated for the selective recognition, sensing and transport of anions after the first anion receptors reported by Smith et al. [11]. Urea and thiourea moiety have hydrogen atoms on the N-atoms which can be donated directionally to form two parallel H-bonds: with the oxygen atoms on oxoanions formed an eight-membered ring whereas the bifurcated H-bonds with the spherical anion form a six-membered ring. The naphthylthiourea receptor 1 and naphthylurea receptor 2 derived from the flexible tri(2-aminoethyl)amine (TREN) are the initial examples of tripodal urea/thiourea based anion receptors, which showed the ability to form 1:1 host–guest complex with the oxo-anions (H2 PO4 − and HSO4 − ) in DMF through multiple hydrogen bonds [12–14]. The intensity of absorption band of 1 at 294 nm was decreased and concomitantly a new absorbance band appeared at ∼340 nm. Receptor 2 showed weak fluorescence at ∼377 nm (␭exc = 310 nm) due to the PET process from the tertiary amine of TREN and a broad band at 410 nm assigned to the structureless emission of a naphthyl excimer in DMF. Upon addition of oxo-anions (H2 PO4 − and HSO4 − ) resulted in the formation of hydrogen bonds with tertiary amine group of 2, which inhibited the intramolecular PET to the fluorophore and leading to an enhancement in the fluorescence emission (Fig. 6). Encourage from the fact that the tripodal architecture with a preorganized structure having multiple hydrogen bond donors can effectively recognize the tetrahedral H2 PO4 − , Sasaki et al. [15] have studied two tripodal fluororeceptors 3 and 4. Receptor 3 has a pyrene moiety adjacent to the thiourea binding site whereas receptor 4 connected to anthracene groups via methylene units. Receptor 3 showed the typical pyrene emission along with a weak broad band at around 500 nm due to the intramolecular interaction of the pyrene rings in CH3 CN. The fluorescence intensity of receptor 3 was increased significantly at 500 nm in the order H2 PO4 − > CH3 COO− > Cl− . In contrast, the fluorescence of the anthracene moiety of 4 is quenched in the same order due to a PET process. Basaran et al. [16,17] have introduced the quinoline-based tripodal thiourea receptor 5, which exclusively binds F− and H2 PO4 − anion in DMSO and formed complex in 1:1 stoichiometry among the other tested anions. The fluoride anion known to show strong basicity from other halides and also high hydration energy due to the considerably high bond strength (569 kJ/mol) of its conjugated acid (HF). Addition of F− to the receptor solution resulted in a gradual decrease in the absorbance at 335 nm with a new peak at 400 nm due to the intramolecular charge transfer (ICT), whereas addition of H2 PO4 − caused a slight decrease in the absorption at 335 nm. The density functional theory (DFT) study of 5·H2 PO4 − /F− suggest that the anions are encapsulated within the host’s cavity via multiple hydrogen bonds (Fig. 7a). Also, 1 H NMR results suggest that the quinoline groups are protonated by fluoride-induced proton transfer from the solution to the host molecule. Wei et al. [18] have introduced the tripodal colorimetric sensor 6 containing thiourea unit, amido unit and a chromophore nitro-benzene for the selective detection of anions in DMSO. Receptor 6 showed selective color changes for AcO− and F− by forming hostguest complex through multiple hydrogen-bonding interactions. The tripodal receptor 7 containing the fluorescein chromophore with the thiourea as anion recognition is designed to open the lactone ring to induce anion-responsive optical responses [19]. The optical response of 7 towards various anions is clearly related with the basicity of the tested anions (CN− ≈ F− > AcO− > BzO− > H2 PO4 − ) in DMSO. With the increase in the basicity of the anion, a larger enhancement of the intensity of the visible bands at 520 nm and the enhancement of the fluorescence was observed. In addition, the chromogenic response was also detected in mixed DMSO–water (90:10, v/v, pH 7.0) medium.

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Subsequently, Hundal and his coworkers [20] have reported several tripodal receptors 8a–d having mesitylene/triethylbenzene as core moiety, urea/thiourea as binding groups and p-nitrobenzene as signaling unit. The receptors act as selective colorimetric, naked-eye sensors for F− with some interference from tetrahedral H2 PO4 − . Thiourea derivatives form a stable 1:1 hydrogen bonded complexes with F− and to some extent with H2 PO4 − , whereas the anion recognition with the urea derivatives is simply based on acid–base reaction between ureidic protons and basic F− . Addition of F− resulted a gradual decrease in the absorption band at 355 nm of 8b with a simultaneous increase in the absorption at 410 nm. The preorganisation of thiourea derivatives coupled with their high-intrinsic acidity is supposed to help them in the formation of strong H-bonded complexes with F− . The more acidic thiourea (pKa of thiourea and urea is 21.1 and 26.9 in DMSO, respectively) receptors are more flexible and adapt well to form H-bonded complexes. The urea-based receptors do not respond at lower concentrations of the anion, but at higher concentrations, they undergo completely reversible deprotonation concomitant with a colorimetric change, with the production of stable [HF2 ]− anion.

The receptor 9 derived from calix[6]crown-3 framework with pyrene as signaling unit and urea as anion binding site exhibited a remarkable selectivity for the SO4 2− in DMSO by forming a host-guest complex in 1:1 stoichiometry (log ˇ = 3.9) [21]. Upon addition of SO4 2− , the monomer emission of 9 (402 and 420 nm) was quenched and slightly red-shifted while the excimer emission (506 nm) enhanced slightly (␭exc = 348 nm). Anion coordination by the urea groups of 9 resulting in a greater proximity between the pyrene moieties. It was stated that the high selectivity for SO4 2− could be due to the fact that this anion is large and doubly charged and displays good complementarity with the tris(urea) binding site. However, the monomer emission (395 and 415 nm) of 9 was enhanced while the excimer emission (484 nm) quenched upon successive additions of SO4 2− in CDCl3 . These spectral changes are in accordance with a separation of the intramolecularly self-associated urea groups bearing the pyrene moieties upon anion coordination. In CDCl3 , 9 was able to bind ammonium ions efficiently only in association with the sulfate anion. This cooperative binding of ammonium sulfate salts was also observed in a protic environment. In another work [22], the heteroditopic receptor 10 derived from calix[5]crown-3 framework with naphthylureido moieties was reported for the selective complexation of alkylammonium halide salts. Such receptors act by placing each ion into its specific binding site, i.e., n-butylammonium cation into the calixarene cavity, and the halide anion into the pocket generated by the ureido-bearing pendant chains. 1 H NMR studies have provided clear-cut evidence that the binding of the entire salt species by these heteroditopic receptors is more efficient than that observed for the single ions. The absorption band of 10 centered at 284 nm decreased in intensity and concomitantly a new band at 307 nm appeared upon addition of n-Bu4 N+ Cl− or n-BuNH3 + PF6 − in CH2 Cl2 . The emission band centered at 380 nm underwent a half-fold increase in intensity, suggesting that both radiative and non-radiative constants were affected. These spectral changes likely depend on a series of electronic effects caused by the conformational change of the host molecule upon substrate binding and/or the interaction of the n-butylammonium cation with the oxygen atoms of the calixcrown. Fang et al. [23] have developed an ion-controlled on–off switch using the tripodal tris(porphyrinato-urea) receptor 11. The porphyrin trimer create a preorganized triangular cone-shaped cavity for the selective encapsulation of C70 , which resulted in a fluorescence quenching (Fig. 8a).

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Fig. 8. (a) Fluorescence changes of 11 in toluene and ion-controlled on-off switch excited at 429 nm: (a) 11 (1.0 ␮M) + 0–10 equiv of C70 , (b) 11 (1.0 ␮M) + 10 equiv of C70 and then 0–2 equiv of H2 PO4 − , and (c) 11 (1.0 ␮M) + 10 equiv of C70 + 2 equiv H2 PO4 − and then 0–3 equiv of Ca2+ . (d) Four continuous association–disassociation cycles were recorded at 657 nm (Adapted from Ref. [23]).

Subsequent addition of H2 PO4 − (as the tetrabutylammonium salt) into the mixture of 11 and C70 , the quenched emission of porphyrin by C70 was significantly recovered (Fig. 8b). In contrast, 11 without binding fullerene shows little fluorescence quenching even when an excess amount of H2 PO4 − was added. Subsequent addition of 3 equiv of Ca2+ (as perchlorate salt) can withdraw H2 PO4 − and regenerates the C70 @11 complex (Fig. 8c). The association–dissociation process was found to be repeatable in several cycles by feeding H2 PO4 − and Ca2+ alternately, which indicates an ion-controlled inclusion and release of fullerene (Fig. 8d). Recently, Choi et al. [24] have introduced the fluorescent C3v -symmetric tris(coumarin-urea) anion receptor 12 using the trindane as tripodal framework with urea group as recognition unit and coumarin as signaling unit. Receptor 12 showed naked-eye detectable turn-on fluorescence selectively in the presence of H2 PO4 − and F− in CH3 CN. The emission band of 12 was shifted from 395 nm to 415 nm. Similar anion selectivity by 12 was observed in DMSO, but addition of excess F− resulted new charge transfer absorption at 439 nm and emission band at 469 nm. 1 H NMR titration experiments supported the formation of a hydrogen bonded 1:1 caviplex 12·H2 PO4 − , whereas F− ion triggered the deprotonation of urea-NH protons. The abstraction of urea-NH protons of 12 resulted F− selective and specific colorimetric and fluorescence changes in DMSO. Son et al. [25,26] have reported the preorganized tripodal thiourea/urea based receptors 13a–d which showed the selective color change to light or dark orange upon binding with H2 PO4 − , AcO− and F− in DMSO. The receptor with acidic thiourea receptors showed more prominent color change then the urea based receptors. UV–vis and 1 H NMR spectroscopic studies of the receptors suggested two steps mechanism for the color change: formation of hydrogen bonded host-guest complex followed by the deprotonation of the thiourea/urea groups to thioureido/ureido anion. The P-bridgehead urea-based tripodal anion receptors 14a–c have one methylene group between phosphine oxide and each phenyl group to increase the flexibility of the ligand so as to obtain the suitable preorganized conformation for anion recognition [27]. The receptors 14a–c showed a better binding affinity, sensitivity, and selectivity for H2 PO4 − by forming host-guest complex in 1:1 stoichiometry by multiple hydrogen bonding interactions through the two NH of urea/thiourea group act as hydrogen bond donors to recognize anionic oxygen atom and the oxygen atom of hydroxyl group of H2 PO4 − , and the P O group of tripodal receptors at the bridgehead site could act as hydrogen bond acceptor to recognize the other hydroxyl proton of H2 PO4 − (Fig. 7b). Among 14a–c, the receptor 14b has shown the best binding affinity for H2 PO4 − due to the presence of stronger electron-withdrawing groups (4-nitrophenyl groups) in each urea subunit. In DMF, the recognition of 14b with H2 PO4 − resulted an observable color change and a notable bathochromic shift of 12 nm from 352 nm. The tripodal tris(urea) ligand 15 linked to a Ru-bipyridyl complex showed selective recognition of SO4 2− or H2 PO4 − ions in the 1:1 binding mode [28]. Upon excitation at 447 nm, complex 15 showed the characteristic metal-to-ligand charge transfer (MLCT) emission band of Ru–bipyridyl complex at 600 nm in CH3 CN. The decrease of emission intensity could be tentatively explained by the binding interactions of the anions, especially SO4 2− or H2 PO4 − ions, to the urea units affecting the excited states of bipyridyl ligands and disfavoring the MLCT process of the Ru(bpy)3 2+ complex. In addition, upon titration of SO4 2− and H2 PO4 − ions, the absorption at 450 nm increased gradually with a bathochromic shift of ∼ 8.0 nm and the band at 288 nm decreased gradually with a slight bathochromic shift of ∼5.0 nm.

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Anzenbacher and his coworkers [29,30] have developed a series of tripodal receptors 16a–d and 17a–d linked with various signaling units. These receptors showed low fluorescence in the resting state due to deactivation by the random intramolecular collisional quenching with the neighbouring side-arms, as well as rotational/vibrational quenching. Upon addition of phosphate and phosphonate like anion, these receptors formed a 1:1 complex where all three arms are equally involved in the formation of the host-guest complex without any deprotonation, and displayed a turn-on fluorescence response. The fluorescence amplification suggests that the anions binding results in the restriction of their conformational freedom, thus improving the electronic coupling between the chromophore and the hydrogen donors while limiting the rotational and vibrational modes, which would otherwise result in nonradiative decay without strong fluorescence output. These sensors in polyurethane films were used for the fabrication of turn-on arrays for analysis of phosphate-type anions in blood serum. In addition, these receptors are applied in a sensor microarray suitable for high-throughput screening of phosphonates, products of hydrolysis of the nerve gas sarin. Subsequently, Minami et al. [31] have developed the IIDA approach for the selective detection of anion. The IIDA based sensors 18 and 19 formed hydrogen bonded complex with the anions. The fluorescence quantum yields of 18 and 19 in aqueous DMSO solution (H2 O:DMSO = 5:95, v/v) are 10.4% and 6.0%, respectively. In the case of 18, fluorescence quenching of the naphthalene carboxylate was observed upon addition of dihydrogen phosphate. This observation was explained with fact that the fluorescent naphthyl carboxylate moiety is fixed in position through intramolecular hydrogen bonding in the resting state. The displacement of fluorophore upon the addition of guests dramatically increases the degree of freedom of the naphthyl carboxylate moiety. As a consequence, the

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excited state deactivation via rotation and vibration are increased resulting in an attenuation of fluorescence, and a “turn-off” signaling is observed with the sensor 18. In contrast, the fluorescence arising from the naphthalimide groups of 19 is increased with the increase of anion concentration. This is due to the fact that the formation of the complex with suitable guests is associated with increased rigidity of the receptor and limited rotational/vibrational modes that would otherwise cause nonradiative decay. Moreover, fluorescence could be quenched by the formation of an excited charge-transfer complex between the naphthalene and naphthalimide moieties. Once a guest (PPi and glyphosate) displaces the naphthalene from the cavity the charge-transfer complex ceases to exist, inducing an increase in fluorescence. The IIDA sensor was incorporated in a simple microarray for phosphates sensing in pure water. The sensing process using 18 and 19 as well as the microtiter arrays is entirely reversible, and the microarray chips are reusable.

2.2. Tripodal receptors with other binding sites The neutral anion receptors can recognize anions by different non-covalent interactions like H-bond, CH· · ·anion, anion· · ·␲. Therefore, in addition to the urea/thiourea binding sites, a large number of neutral receptors using groups, such as pyrrole, amide, phenol, amine, sulfonamide, squaramide, imidazole and indole have been reported coupled with different signaling units. Lee and his coworkers [32] have introduced a tripodal receptor 20 appended with three 2-aminobenzimidazole from 1,3,5-triethylbenzene platform for the selective sensing of I− in CH3 CN/H2 O (99:1, v/v, HEPES buffer pH = 7.91) medium. The absorption spectrum of 20 at 258 nm of the benzimidazole moiety was shifted to 248 nm upon addition of I− . Upon excitation of 20 at 258 nm, a dual channel emission was observed at 318 and 408 nm. The emission at higher wavelength is most likely due to an excimer formation. Binding of I− in the receptor’s pseudocavity through hydrogen bonding of benzimidazole-NH2 protons resulted quenching

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Fig. 9. Fluorescence spectra of 28 in the presence of Na3 PO4 in THF at 298 K. [Na3 PO4 ]: (1) 0.0, (2) 1.0, (3) 2.0, (4) 3.0, (5) 6.0, (6) 10.0, (7) 15.0, (8) 20.0, (9) 25.0, and (10) 30.0 mM. Inset: Fluorescence spectra of 28 in the presence of H3 PO4 in THF solution at 298 K. [H3 PO4 ]: (1) 0.0, (2) 1.0, (3) 2.0, (4) 3.0, (5) 4.0, (6) 6.0, (7) 8.0, (8) 12.0, (9) 15.0, and (10) 20.0 mM (Adapted from Ref. [40]).

in the fluorescence intensity of 20. Receptor 20 formed a host-guest complex in 1:1 ratio with I− with the detection limit of 7.45 × 10−6 M. Following this work, a similar tripodal fluorescent receptor 21 with benzimidazole motifs as signaling unit was reported for the chromogenic and fluorescence detection of I− in CH3 CN/H2 O (99:1, v/v, HEPES buffer pH = 7.91) medium [33]. The absorption band of 21 at 255 nm was shifted to 245 nm, whereas the fluorescence of 21 at 260 nm and 315 nm was quenched in the presence of I− . In compared to receptor 20, 21 designed using the flexible tripod TREN showed the detection limit down to 8.2 × 10−9 M. The benzimidazole based tripodal receptor 22 also created a pseudocavity compatible to the size of I− to form host-guest complex in 1:1 ratio through multiple hydrogen bonds, which resulted a dramatic fluorescence quenching at 495 nm (␭exc = 346 nm) in CH3 CN/H2 O (9:1, v/v) at neutral pH (HEPES buffer) [34]. Receptor 22 can detect I− up to a low concentration of 2.1 ␮M. Receptor 20 was further derivatized to get the receptor 23 bearing phenol as a hydrogen bond donor site and azo dye as the signaling subunit [35]. Receptor 23 showed a high binding affinity for CN− and showed a colorimetric response. Addition of CN− induced a decrease in the absorbance at 375 nm of 23 and an increase in the absorbance at 520 nm in CH3 CN:DMSO:HEPES (93:1:6, v/v/v) medium. 1 H NMR results indicated that the CN− is coordinated in the pseudocavity of the receptor and the color changed is not the consequence of anion-mediated deprotonation. The tripodal receptors (24a–e) containing electron withdrawing and donating groups appended to the azophenol moiety were applied for the chromogenic sensing of anions in CH3 CN [36]. These receptors showed a distinct color change only when treated with F− ions by two steps mechanism: F− recognition through H-bonding interactions employing phenolic-OH groups followed by the deprotonation facilitated by the high intrinsic acidity of phenolic-OH groups. All the tripodal hosts 24a–e gave birth to F− -induced dramatic color changes from light pale yellow to orange (24a and 24c–e) and tripodal host 24b displays noticeable deep color change from yellow to deep purple with the formation of a new red shifted charge transfer absorption band at 532 nm. With the deprotonation mechanism, tripodal Schiff base receptor 25 having catechol as end groups showed a visually detectable optical sensor for F− ions in DMSO [37]. Receptor 25 showed a new absorption band at max 433 nm with a detectable color change from very pale yellow to bright yellow selectively in the presence of F− . Tripodal Schiff base 26 based on hydrazone CH N NH groups have been reported for the colorimetric sensing of anionic guests in DMSO [38]. Receptors 26a and 26b showed colorimetric responses for F− , H2 PO4 − and AcO− in DMSO, while in DMSO/H2 O (9:1, v/v) solutions, sensor 26a showed selective response towards AcO− . 1 H NMR study delineated that the tripodal sensors bind anions through the collaboration of three hydrazone groups and anions residing in the central cavity of the sensors. Similar to receptor 26a, tris–hydrazine receptor 27 was introduced from a benzene platform for the colorimetric sensing of anions such as F− , AcO− and H2 PO4 − in DMSO [39]. Addition of basic anions, the absorbance of 27 at 397 nm was significantly decreased and a new absorbance peak at 521 nm was observed due to the ICT occurred between the bonded anion and the electron deficient NO2 moiety. The quinoline based tripodal receptor 28 showed monomer emission at 310 nm (␭exc = 270 nm) and an intermolecular excimer emission band centered at 475 nm in THF [40]. Addition of unprotonated phosphate i.e., Na3 PO4 leads to a decrease in the emission intensity of the excimer band only, whereas protonated phosphate H3 PO4 quenched both the monomer and excimer emission (Fig. 9). The selective protonation of the amine and/or quinoline nitrogen atoms by the unprotonated/protonated phosphate prevents the intermolecular excimer formation due to charge repulsion and also quenched the monomer fluorescence. However, the structurally similar naphthalene based tripodal receptor 29 showed only a locally excited broad emission with maximum at 410 nm (exc = 330 nm) in THF [41]. Addition of nitrate ions in to THF solution of 29 resulted significant fluorescence quenching among the other tested anions. In another work, tripodal receptor 30 containing phenalenone moiety showed I− selective fluorescence quenching at 478 nm in THF by forming a complex in 1:1 ratio [42]. Detection limit of 30 for I− was 1.8 × 10−7 M. It was proposed that the three flexible pods of 30 create a pseudocavity with compatible core size to selective accommodate I− . 1 H NMR study of 30 revealed that the host–guest complex formed due to the phenalenone· · ·I− interaction but not through N H· · ·I− . With this receptor, the detection of iodide can be possible down to 1.8 × 10−7 M.

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The advent of “click” chemistry has gained interest among supramolecular chemists to exploit 1,2,3-triazoles as polarized donors to stabilize neutral CH.... anion interactions. Also, the synthetic simplicity in getting various supramolecular architectures, this chemistry has emerged as an important approach to anion recognition and sensing. The tripodal receptor 31 with the azaindole-1,2,3-triazole conjugate selectively recognizes Cl− and H2 PO4 − in CH3 CN containing 0.01% DMSO by exhibiting a significant change in emission [43]. The emission of 31 at 369 nm (exc = 310 nm) for the azaindole motif enhanced significantly in the presence of Cl− without any other change in the spectrum, whereas the fluorescence quenched at 369 nm and enhanced at 475 nm in the presence of H2 PO4 − . Receptor 31 formed hydrogen

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Fig. 10. DFT optimized geometries of the complexes of 31 with (A) H2 PO4 − (a = 1.70 Å, b = 1.64 Å, c = 1.58 Å, d = 2.82 Å, e = 2.45 Å, f = 2.03 Å, g = 1.54 Å, h = 2.87 Å and i = 1.84 Å) and (B) Cl− (a = 1.83 Å, b = 2.32 Å, c = 3.01 Å, d = 2.88 Å) ions. (C) Bright field and fluorescence microscopic images of HeLa cell: (a) bright field image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and subsequently until 3 h; (b) fluorescence image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and subsequently until 3 h; (c) fluorescence image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and subsequently treated with 5 mM ATP for 3 h; (d) fluorescence image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and subsequently treated with 5 mM ADP for 3 h; (e) fluorescence image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and subsequently treated with 5 mM AMP for 3 h; (f) fluorescence image of HeLa cells incubated with receptor 31 (10 mM) for 30 min and after addition of ALP and ATP subsequently until 3 h (Adapted from Ref. [43]).

Fig. 11. (a) Changes in the emission spectrum of 33 (2 × 10−6 M) in CH3 CN H2 O (1:1, v/v) upon addition of increasing amounts of citrate anions up to 80 equiv.; (b) visual changes observed in the fluorescence of 33 (left) upon addition of citrate anions (right); (c) Job’s plot indicating the formation of a 1:1 complex (Adapted from Ref. [44]).

bonded host-guest complex with Cl− and H2 PO4 − , where the triazole-CH protons actively participate in the recognition of anions (Fig. 10). In addition, receptor 31 selectivity recognizes phosphate based biomolecule ATP over ADP and AMP in semi-aqueous solvent (CH3 CN containing 0.01% DMSO:H2 O, 1:1, v/v) at pH 7.3. The tripod 31 is cell permeable and detects ATP by showing quenching of emission (Fig. 10C). In contrast, the indole-based tripod 32 failed to show any binding-selectivity with the same anions. The pyrene appended

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tripodal receptor 33 showed two distinct emission bands at 386 and 405 nm which are attributed to the pyrene monomeric emission and a red-shifted structureless maximum at 486 nm, typical of pyrene excimer fluorescence in CH3 CN H2 O (1:1, v/v) solution [44]. Addition of citrate ion resulted a slight decrease in the intensity of the pyrene excimer emission band along with pronounced intensity enhancement of the pyrene monomer emission (Fig. 11). Receptor 33 formed a host-guest complex with citrate ions in 1:1 stoichiometry by the participation of the triazole-CH protons in hydrogen bonding with this anion. With receptor 33, citrate ion can be detected down to 3.7 × 10−6 M without any interference from structurally very similar carboxylate anions such as malate or tartrate. The tripodal triamide receptor 34 bearing dinitroaryl functions selectively sense F− by encapsulation within the tripodal pseudocavity in polar aprotic solvents and displayed a visible color change with a remarkable solvatochromism and solvatomorphism in various polar aprotic solvents [45]. In DMSO, receptor 34 showed three new charge-transfer (CT) absorption bands at 388 nm, 537 nm and 665 nm in the presence of F− . It was found that the F− is encapsulated within the tripodal cavity governed by six strong intramolecular H-bonds involving the amide-NH protons and three aryl-CH protons (Fig. 12). Shao and coworkers have studied the anion sensing ability of a series of tris(indolyl)methene receptors 35a-b [46] and 36a–c [47]. The 2-linked receptor 35a showed the ability to bind F− , AcO− and H2 PO4 − in CH3 CN whereas the 3-linked receptor 35b exhibited insignificant spectral response to anions. This selectivity will be attributed to suitable steric array of NH binding sites of 2-linked receptor 35a, which helps to form multiple hydrogen bonding interactions with anions. Hydrogen bonding interactions of 35a with anions destroyed the previous planarity and resulted the obvious fluorescence quenching effects at 356 nm (␭exc = 283 nm). However, the tris(indolyl)methene receptors 36a–c containing conjugated bisindole skeletons showed colorimetric response towards the basic anions, such as F− , AcO− and H2 PO4 − due to the deprotonation [47]. Also, it was explained that the anion selectivity and binding affinity of 36 can be tuned by the introduction of the electron donating or withdrawing groups. With the receptor 36a, the LOD estimated of 9.28 ppm for F− , 459 ppm for AcO− , and 558 ppm for H2 PO4 − .

The tripodal receptor 37 containing carbazole-2-carboxamide showed a selective response towards the biologically important HP2 O7 3− anion in DMSO H2 O mixed medium by forming host-guest complex in 1:1 ratio through multiple hydrogen bonds [48]. The absorption spectrum of 37 resembled that of the carbazole unit with an intense band at 307 nm, a weaker band at 341 and a shoulder at 358 nm. The addition of HP2 O7 3− caused a general decrease in the absorbance accompanied by a red shift of the bands. The fluorescence spectrum of 37 at 389 nm with a subtle shoulder at 380 nm is quenched upon addition of HP2 O7 3− . Curiously, an incipient growth of the fluorescence could be detected in the range of 450–550 nm which is responsible for an analytically useful color change in the fluorescence of the receptor. Upon HP2 O7 3− complexation the anion sitting in the cavity of the receptor blocks its conformational freedom and makes the carbazole units point inwards, which improves the binding ability of the tripodal receptor 37. Hao et al. [49] have introduced tripodal

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receptors 38a-b having multiple hydroxyl and amide groups with long alkyl chains (N-dodecylamino and N,N-didodecylamino groups, respectively) for the selective sensing of anion. The receptors caused the drastic increase in fluorescence intensity upon addition of F− . Receptor 38a having two kinds of primary amide groups in one arm in a molecule can recognize anions even in a polar solvent such as DMSO and aqueous DMSO by showing enhanced fluorescence around 500 nm. Tripodand 38b can exhibit fluorescent F− sensing in CHCl3 , but not in DMSO because of insolubility. Recently, Singh and Sun [50] have introduced the C3 -symmetric acyclic artificial receptor 39a incorporating amide functionality for anion recognition and dansyl groups as signaling unit. The presence of SO3 H group protonated the conformationally flexible N-bridged tripodal podand 39a in situ and formed a cone shape conformation 39b through hydrogen bonding and C H· · ·␲ interactions. The cavity formed by 39a is suitable to encapsulate nitrate ion among the other tested anions, which resulted a selective fluorescent quenching in DMSO and aqueous DMSO.

3. Positively-charged tripodal anion receptors The first ion receptors reported by Park and Simmons [1] were positively charged ammonium macrocyclic hosts for halides recognition. Therefore, it is not surprising that many anion selective positively charged chemosensors have been reported in the recent years based on the charge–charge interaction along with other non-covalent forces [like (N H)+· · · anion and (C H)+ anion types hydrogen bonds]. In compared to neutral receptors, charged receptors showed stronger affinity towards anion provided the binding anion must compete easily with the associated counter anions. Most of the reported charged sensors contained the binding motifs like guanidinium, pyridinium, imidazolium, polyammonium, benzimidazolium and thiourenium for the recognition of anions. Steed and his coworkers [51,52] have introduced two tripodal receptors based on positively charged aminopyridinium anion binding sites substituted with anthracene (40) or pyrene (41) signaling units. 1 H NMR study of receptor 40 proved the selective AcO− recognition over spherical anions such as Cl− through the involvement of N H and C H hydrogen bond donation from the appended arms, whereas host 41 in particular showed high association constant for Cl− instead of AcO− , and marked tendency to dimerize. Surprisingly, the receptors 40 and 41 based on fluorophore-spacer-receptor model were failed to exhibit a fluorescence response to anion binding. The possibility of anioninduced excimer emission from the association of excited anthracene/pyrene unit with the ground state of another fluorophore and/or the exciplex emission from an excited-state anthracene/pyrene and pyridinium complex formation was also not observed. It was proposed that the charge-transfer interactions between the anthranyl/pyrenyl sensing unit and the aminopyridinium binding unit quenched the fluorescence of the receptors in the excited state, and therefore the receptors failed to exhibit any fluorescence changes upon anion binding. However, Gong and Hiratani [53] have reported an analogous tripodal fluorescent receptor 42 from flexible cyclohexane platform using aminopyridinium anion binding sites and anthracene moiety as the sensing subunit. Receptor 42 showed H2 PO4 − selective fluorescence response owing to the cooperation of multiple non-covalent interactions, such as hydrogen bonding, electrostatic interactions, as well as the dynamic conformational change via formation of unique anion binding-induced excimer. Receptor 42 alone displayed typical monomer emission of anthracene at 387, 413 and 436 nm, as well as a shoulder at 466 nm (␭exc = 368 nm). Similar to receptors 40 and 41, receptor 42 showed a weak fluorescent due to the quenching effect of photo-induced electron transfer (PET) process from the anthracene moieties to the charged pyridinium ring. Upon addition of H2 PO4 − resulted a small increase in the intensity of monomer fluorescence of 42 due to

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Fig. 12. (a) Crystal structure of 34 showing encapsulation of F− inside the tripodal cavity; (b) space filling representation depicting the formation of a F− encapsulated neutral molecular capsule in 34; (c) color changes observed upon addition of anions (25 equiv.) to DMSO solutions of 34 (L) (Adapted from Ref. [45]).

the inhibition of the PET along with a remarkably enhanced broad band was observed at 505 nm corresponded to the excimer emission between anthracenes (Fig. 13). The formation of excimer band delineated that the binding of H2 PO4 − resulted a conformational change of 42 which pulled the separated anthracene moieties closer. Fluorescent experiments of 42 with H2 PO4 − pointed to the 1:1 host-guest complexation with the binding constant, log K = 4.28. The formation of 42·H2 PO4 − also resulted an apparent bathochromic shift of the absorption bands of anthracene. In similar approach, the pyridinium-based tripodal chemosensor 43 was reported for the selective sensing of H2 PO4 − and ATP respectively in pure CH3 CN and CH3 CN/H2 O (1:1, v/v, pH = 6.5) medium [54]. The emission at 386 nm of 43 due to naphthalene unit was observed when excited at 290 nm. Sensor 43 selectively binds H2 PO4 − in 1:1 stoichiometry (K = 7.06 × 103 M−1 ) with the detection limit down to 1.46 × 10−6 M, and showed a naphthalene excimer emission at 455 nm. In contrast, addition of ATP resulted a broad emission at 390 nm with the formation of host-guest complex in 1:1 stoichiometric with K = 1.16 × 103 M−1 and LOD = 8.44 × 10−6 M. It was proposed that the adenine of ATP is located closely to naphthalene moiety and a naphthalene–adenine–naphthalene ␲-stacking interaction exists in solution which allowed the formation of exciplex emission at 390 nm. Sensor 43 was also successfully applied for the intracellular ATP detection through fluorescent confocal imaging in HEK293T cell line.

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Fig. 13. Changes in fluorescence spectra of 42 (50 ␮M) upon addition of H2 PO4 − in 9:1 CH3 CN–EtOH solution (Adapted from Ref. [53]).

O -

O

O

CO2

N N H

H

CO2N

H

N

O -

O H

H

O

N

N

N H

H

O

O OH

O

O

H N

N H

O

HO

H

O

H H

O

H

N

N

N H H N

N O

HN -

O

O

H N

N

O

O CO2-

HN 46

CO2Fig. 14. Citrate sensing by 46 using IDA approach.

Using the amide-pyridinium as recognition site and nitro-benzene as signal-reporting unit, Bao et al. [55] have reported the anion sensing ability of tripodal receptor 44 in CH3 CN. Receptor 44 displayed a new charge transfer absorption band at 375 nm and visually detectable color change from colorless to yellow selectively in the presence of AcO− and F− by forming a host-guest complex in 1:1 stoichiometry. The intramolecular charge transfer occurred from electron-rich anion binding amide pyridinium unit to relative electrondeficient nitro-benzene fragment resulted the binding-induced significant bathochromic shift. Subsequently, Ghosh et al. [56] have studied the pyridinium-based tripodal sensor 45 for the selective recognition of AMP over ATP and ADP through IDA approach. The absorption and emission intensity of fluorescein dye upon interaction with 45 in aqueous solution (10 mM Tris–HCl buffer, pH = 6.4) were significantly reduced and the color of the resulting solution changed from lemon yellow to light yellow. Also, the fluorescent green color of the dye changed to light green. Addition of AMP displaced the fluorescein dye from the binding cavity of 45, and its absorbance and fluorescence is restored. The good recognition of 45 is due to the better accommodation of AMP at the core of 45 as well as functional interaction involving hydrogen bonding and charge–charge interaction. In addition, the utility of 45 in recognizing the intercellular AMP in A549 cells as well as alkaline phosphatase (ALP) mediated conversion of ATP/ADP to the AMP via the IDA technique were explored. The IDA approach is a well-known approach, which was previously applied for the detection of anion by Metzge and Anslyn [57] (Fig. 14). In Anslyn IDA approach, the 1,3,5-trisubstitued-2,4,6-triethylbenzene scaffold was functionalized with three guanidinium moieties 46 which can form a molecular ensemble with the commercially available 5-carboxyfluorescein dye having two carboxylates for binding with host 46. In MeOH/H2 O (3:1, v/v) medium buffered at pH 7.4, the absorbance at 498 nm and fluorescence intensity at 525 nm of the dye increases with addition of host 46. Further addition of citrate ions to the ensemble of 46 and dye resulted decrease in the intensity. This cycling of absorbance and fluorescence could typically be repeated five times before any significant signal degradation occurred. This system was used to assay the concentration of citrate in soft drinks and sport drinks. The anthracene-coupled benzimidazolium-based tripodal tricationic fluorescent chemosensor 47 was designed for the selectivity detection of H2 PO4 − in CH3 CN through anion-induced quenching of emission along with the formation of a weak excimer complex in the excited states [58]. Receptor 47 exhibited fluorescence bands with maxima at 400, 418 and 440 nm upon excitation at 370 nm. Addition of H2 PO4 − resulted a significant decrease in emission (∼90%) and a unique excimer peak was observed at 500 nm due to the strong chelation of H2 PO4 − at the core of 47 that brings the pendant anthracenes closer to form the excimer. Also, the absorption band of 47 at 370 nm was decreased significantly with a red shift of ∼6 nm upon complexation of H2 PO4 − . Receptor 47 also showed selective sensing of ATP over ADP and AMP by exhibiting fluorescence enhancement in aqueous CH3 CN (CH3 CN:H2 O = 3:2 v/v). Considering the fact that the fluorescent

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naphthoimidazolium groups can form unique ionic hydrogen bonds with anions, tripodal receptor 48 bearing three quaternary ammonium groups was applied for the recognition of nucleotides [59]. Receptor 48 showed a large fluorescence enhancements with UTP, CTP and TTP and moderate fluorescence enhancements with ATP and pyrophosphate and a fluorescence quenching effect with GTP. In another study, Bai et al. [60] have developed tripodal receptor 49 comprising benzoimidazolium hydrogen bonding moieties and the naphthalene unit for the fluorescent ‘off-on’ sensing of halide anions. The design is similar to the receptors 40–43 based on fluorophore–spacer–receptor model. Receptor 49 exhibited an emission band around 340 nm with a very weak excimer fluorescence in the region of 400–700 nm, indicating that the three lumophore naphthyl rings are well separated i.e., no excimer emission (“off” state). The fluorescence intensity at 340 nm did not change significantly upon addition of Cl− anion, but the intensity of the excimer fluorescence at 450 nm was dramatically enhanced upon formation of host-guest complex in 1:1 ratio with the association constant of 4.1 × 103 M−1 . The hydrogen bonds between the arms and the Cl− induce the tripodal receptor to display a cone conformation with all three positive charged arms orientated inward direction which bring the three naphthalene lumophores closer for the excimer fluorescence (“on” state). Tripodal receptor 50 with aminonaphthalimide imidazolium podands exhibited a strong green emission at 548 nm due to the 1,8naphthalimide upon excitation at 465 nm [61]. Addition of ADP, caused fluorescence enhancement due to the formation of the host–guest complex in a 1:2 stoichiometry (log K = 10.44) (Fig. 15a). The enhancement of the fluorescence was due to a reduced electron-charge density at the imidazole site after the binding with diphosphate of the ADP, which reduced the “push–pull” nature of the ICT excited state of 50 (caused by the electron-donating amine and the electron-withdrawing imidazole). The selective response with ADP over other adenosine polyphosphates was ascribed the suitable length of the linked group for the diphosphate group. Further, to recognize the longer adenosine triphosphate, the receptor 51 was designed to enlarge the chain of each arm through the well-known “click” reaction [61]. The 1,8-naphthalimide fluorescence of 51 was observed at 550 nm when excited at 395 nm. Addition of ATP, caused fluorescence enhancement due to the formation of the host–guest complex in a 1: 2 stoichiometry (log K = 8.75) (Fig. 15b). Sensors 50 and 51 are also successfully applied to cell imaging for the corresponding nucleotide polyphosphates.

Tripodal receptor 52 [62] containing three fluorogenic 6-methoxy-1-methylquinolinium units showed fluorescence quenching when the inclusion of anion occurred within the host cavity through electrostatic interactions, and the quenching efficiency decreases in the order Br− » I− > NCS− » Cl− > NO3 − > HSO4 − . Structurally similar, the tripodal quinolinium-derived anion sensor 53 [63] showed a turn-off fluorescence response at 408 nm (exc = 317 nm) selectively in the presence of AcO− . Fluorescent quenching is due to both dynamic and static processes with charge transfer being the dominant mechanism. In another work, receptor 54 [64] give rise to an intense purple coloration with an absorption band centered at ∼540 nm upon the addition of carboxylates such as AcO− in CH3 CN (Fig. 16a). This absorption is assigned to an anion–viologen charge-transfer state. The particularly electron-deficient nature of the viologens in these compounds makes them excellent electron acceptors and charge-transfer behavior in viologens is well known. DFT optimised structure of 54 with AcO− (Fig. 16b), suggests a three-up conformation in which the anion is bound by three charge-assisted hydrogen bonds, one of the AcO− oxygen atoms is bound by two hydrogen bonds, whereas the second is bound by only one.

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4. Metal complex based tripodal anion receptors Metal-ligand complexes have been successfully used as anion binding sites for the development of optical chemosensors [65]. The complexes bearing vacant coordination sites on central metal ions can bind anionic guests and induced distinct optical changes. Alternately, the anionic guest can displaced the metal ions from the metal-ligand complex to recover the optical properties of coordinated ligand. The directional coordinative bond formed between the metal ions and anionic guest is generally stronger than the H-bonding or electrostatic interactions with the advantageous feature to detect anions in aqueous medium. The anthracene based anion chemosensor 55 reported by Czarnik in 1989 [66], which showed a remarkable emission intensity enhancement upon addition of phosphate at pH = 6. It was proposed that the fluorescence enhancement could be attributed to the PET process from the benzylic nitrogens to the excited fluorophore (anthryl group) was inhibited due to the partially protonation both the polyamine and the phosphate at pH = 6. Alternately, the PET may be inhibited due to the favorable formation of hydrogen bonds between one OH group of the phosphate with the lone pair of the benzylic nitrogen upon encapsulation of phosphate within the tripodal cavity. When the receptor 55 was treated with Zn2+ gave a fluorescent complex in methanol, which upon addition of N,N-dimethylaminobenzoate resulted an intramolecular electron transfer process from the benzoate to the photo-excited anthracene unit and thus the fluorescence was quenched [67]. With the similar mechanism, the fluorescence of complexes 56 [68] and 57 [69] was quenched respectively in the presence of aromatic carboxylates (benzoate, 4-nitrobenzoate, and 9-anthracenoate) and triphenylacetate. The [Cu·58]4+ complex formed a suitable cavity for citrate inclusion that is coordinated through the metal cation and the guanidinium groups [70]. The emission of [Cu·58]4+ increased dramatically upon addition of citrate. This emission enhancement could be attributed to a change in the oxidation-reduction potential of the metal upon citrate coordination, thus changing the extent of electron transfer from metal cation to the 1,10-phenanthroline fluorophore.

Fig. 15. (a) Fluorescence responses of 50 (20 ␮M) in CH3 CN upon addition of ADP (0–0.4 mM). Inset: fluorescence responses for several ribonucleotide di- and triphosphates (0.4 mM). The bars represent the value of [(F − F0 )/F0 ], where F and F0 are the emission intensities at 548 nm in the presence and absence of the ribonucleotides, respectively; and (b) mission spectra of 51 (TIA2, 20 ␮M) in aqueous solution upon addition of ribonucleotide polyphosphates (0.4 mM) (Adapted from Ref. [61]).

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The Cu2+ complex of 59 was applied for the anion sensing based on an IDA approach using the indicator 5-(and 6)-carboxyfluorescein [71]. The yellow color of the indicator in a MeOH:H2 O (50:50, v/v, TRIS buffer pH = 7.4) changed to light orange upon binding with 59 in a 1:1 stoichiometry. Subsequent addition of phosphate solution (as the sodium salt) to the host-dye complex, the light orange color reverts to the yellow color of free dye. The dye-displacement assay was successfully applied for the quantitative analysis of phosphate in protein-free samples of horse serum and saliva. Following the IDA approach, Ahn and his coworkers have introduced two trifurcated receptors 60 [72] and 61 [73] for the selective sensing of myo-inositol hexakis(phosphate) sodium salt (phytate) and myo-Inositol 1,4,5tris(phosphate) (IP3 ), respectively. The plant ingredient phytate is richly found in cereals, legumes, oil seeds, etc. It is also an important constituent of the human diet in the forms of chelates with bioactive metal ions such as Zn2+ , Ca2+ , Mg2+ , Cu2+ , Co3+ and Fe3+ , which adversely limit the bioavailability of such minerals [72]. The analyte IP3 is also an important signaling molecule involved in intracellular signal transduction [73]. The biological importance of phytate and IP3 makes them attractive targets for the development of chemosensors. The sensing ensemble 60 comprised of three Cu(II)-dipicolylamine ligands and eosinY as indicator. The addition of phytate to the chemical ensemble system in an aqueous medium of physiological pH resulted in the restoration of fluorescence of eosinY at 536 nm (␭exc = 517 nm) as it is displaced from the metal complex by the anion added. The ensemble system shows the maximum fluorescence change in the case of phytate and pyrophosphate ions, in an aqueous solution buffered at pH 7.0 (a HEPES buffer, 10 mM). The flexible tripodal receptor 61 was created to provide geometry wide enough to encapsulate IP3 in a 1:1 chelation mode [73]. In aqueous buffered medium (pH = 7.0, HEPES, 10 mM), the quenched fluorescence of eosinY at 536 nm (␭exc = 517 nm) on interaction with 61 was recovered upon addition of IP3 .

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The C3 -symmetric ratiometric fluorescent receptor 62 based on a pyrene-linked triazole-modified homooxacalix[3]arene was investigated for the selective detection of Zn2+ and H2 PO4 − ions [74]. Receptor 62 alone showed both the monomer and excimer emissions (␭exc = 343 nm) at 396 and 485 nm, respectively. The fluorescence intensity of the excimer emission of 62 gradually decreased and concomitantly the monomer emission was enhanced upon addition of Zn2+ in CH3 CN/CH2 Cl2 (1000:1, v/v) due to the formation of 62·Zn2+ complex in 1:1 ratio (Fig. 17a). Addition of H2 PO4 − anion in to the complex 62·Zn2+ solution resulted selective enhancement of the excimer fluorescence intensity among the other tested anions (Fig. 17b). The complex 62·Zn2+ is able to detect H2 PO4 − down to 1.52 × 10−7 M. Further, the optical response of 62 was applied for the construction of logic gates (INHIBIT and OR gate) at molecular level using Zn2+ and H2 PO4 − ions as chemical inputs. In another approach, Mahapatra et al. [75] have introduced C3 -symmetry tri-arm 8-hydroxyquinoline based fluorescent chemosensor 63 for the selective fluorescent sensing of Zn2+ and PPi. The receptor 63 selectively chelates Zn2+ in 1:3 ratio in (DMSO:H2 O = 4:1, v/v, 20 mM, HEPES buffer, pH 7.4), which resulted the disappearance of the fluorescent emission of 63 centered at 405 nm (exc = 267 nm) and enhancement in the fluorescent emission centered at 491 nm due to the mechanism of internal charge transfer. Receptor 63 was also successfully used for the intracellular detection of Zn2+ . Subsequent addition of different anions into 63·Zn2+ solution causes selective complexation of PPi with the Zn2+ that resulted in a gradual increase in the emission with 38 nm blue-shift from 492 to 453 nm. The Cu2+ complex with the trpodal receptor 64 bearing an anthracene moiety on one pod was developed for the fluorescent sensing iodide anion in CH3 CN/H2 O (95:5, v/v) medium [76]. The typical anthracene emission of 64 was selectively quenched without spectral shifts upon addition of I− in to 64·Cu2+ solution via a PET process. The added I− was selectively coordinated with the vacant anion binding site provide by the Cu2+ in 64·Cu2+ along with the multiple hydrogen bonds with the receptor 64. This system showed an I− detection limit down to 0.2 ␮M without any interference from other tested anions. The colorimetric detection of I− was also reported with Cu2+ based complex with tripodal receptor 65 [77]. Upon addition of I− , the blue color of 65·Cu2+ changed to greenish-yellow in CH3 CN H2 O (4:1, v/v) and the intensity of the absorption band of 65·Cu2+ at 291 nm was decrease along with new bands appeared in the 330–390 nm region as well as strong Soret bands in the 400–500 nm region because of the formation of a charge transfer complex in 1:1 stoichiometry. In addition to Cu2+ complex, the Co2+ complex with the tripodal amide based ligand 66 in DMSO H2 O (50/1, v/v) displayed selective color change from colorless to yellow for weak acid anions, such as CO3 2− , Ac− , HCO3 − , SO3 2− , and PO4 3− [78]. The mixture of the ligand and Co2+ ions exhibited selective colorimetric sensing properties for weak acid anions, because of the cooperative interaction of the anion with the carboxamide H via X· · ·H N or O· · ·H N interactions. Maity and Bharadwaj [79] have introduced a tripod triazole-ring appended rhodamine dye 67 selectively complexed with Al3+ in 1:1 ratio and showed a turn-on fluorescence at 577 nm (␭exc = 540 nm) in a mixed aqueous medium (CH3 OH H2 O, 9:1, v/v). Among the tested anions, the emission of the Al3+ complex of the dye is completely quenched upon addition of sodium salt of either F− or AcO− . In the presence of F− or AcO− , the Al3+ is abstracted and the open spirolactum ring is closed. Tripodal receptor 68 with two arms was appended with sulfobetaine group and third one linked to a fluorescein fluorophore [80]. The fluorescence of fluorescein/chelator/Cu2+ complex was

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Fig. 16. (a) Degassed solutions of 54 (1 × 10−4 M) in the presence of the TBA salts (1 equiv) of the following anions (from left to right): acetate, malonate, chloride, bromide, iodide and nitrate. (b) DFT optimised geometry for 54 with acetate (Adapted from Ref. [64]).

quenched due to the paramagnetic Cu2+ in pH 7.4 HEPES buffered water. Addition of sulfide anion selectively reverted the fluorescence of 68 at 521 nm (exc = 490 nm). The sensor displays high specificity and low detection limit of 1.1 ␮M, and can be used for sulfide monitoring in such real sample as running water. The sensor is successfully applied for monitoring and imaging sulfide anion level change inside live cells-HeLa (human cervical cancer cell) and L929 (murine aneuploid fibrosarcoma cell). In similar approach, the fluorescence of the benzimidazole based tripodal fluorescence receptor 21 was quenched on complexation with Cu2+ and subsequently regained in presence of sulfide anions in CH3 CN/H2 O (1:1, v/v, pH 7.3, HEPES buffer) [81]. The effective binding of Cu2+ in the three nitrogen coordination pocket of 21 resulted the fluorescence quenching at 315 nm due to the efficient energy/electron transfer (paramagnetic Cu2+ ) to the benzimidazole units (Fig. 18a). The separation of Cu2+ as CuS upon addition of sulfide ions resulted the regeneration of 21 fluorescence at 315 nm (Fig. 18b).

Fig. 17. (a) Fluorescence spectra of 62 (1.0 ␮M) upon addition of increasing concentrations of Zn(ClO4 )2 in CH3 CN/CH2 Cl2 (1000:1, v/v). (b) Fluorescence spectra of receptor (L = 62)·Zn2+ in solution (CH3 CN/CH2 Cl2 , 1000:1, v/v) upon addition of various anions as their tetrabutylammonium salts in aqueous solution (Adapted from Ref. [74]).

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5. Concluding remarks In conclusion, we have summarized the C3v -symmetric synthetic tripodal receptors applied for the optical sensing of anions. It is not surprised that despite the complex geometry of anions, the development of tripodal receptors allowed the highly selective recognition

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Fig. 18. (a) Fluorescence titration spectra of receptor 21 (10 ␮M) upon addition of copper perchlorate (2 equiv.) in a CH3 CN/H2 O (1:1, v/v, HEPES buffer, pH 7.3). (b) Emission of [21·Cu2+ ] in presence of 30 ␮M of other anions and after the addition of 30 ␮M S2− in the same media (Adapted from Ref. [81]).

and measurements of anions because of the ability to wrap anion of perfect fitting through multiple non-covalent interactions within the spheroid cavity formed by the tripods. Many excellent examples were presented showing chromogenic and/or fluorogenic response for the sensing of small anions. But most of the reported sensors are employed in pure organic medium or mixed organic-water medium, and deprotonated when the basic anions are added in excess, which might restrict their applicability. Therefore, novel and improved approaches are still required to monitor anions in complex real-life samples and will remain a main issue to the scientific world working in the field of anion sensing using tripodal structures. In addition, many excellent fluorophores and sensing mechanism like FRET are yet to be explore for the development of tripodal receptors. Therefore, there is a wide horizon open for the further development of tripodal anion receptors and we strongly believe that this review will witness the further progress in the design of receptors with novel optical properties and improved anion selectivity in aqueous medium. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

C.H. Park, H.E. Simmons, J. Am. Chem. Soc. 90 (1968) 2431. A. Bianchi, K. Bowman-James, E. García-Espana, Supramolecular Chemistry of Anions, John Wiley & Sons Inc., New York, 1997. J.L. Sessler, P.A. Gale, W.-S. Cho, Anion Receptor Chemistry, RSC Publishing, Cambridge, 2006. I. Ravikumar, P. Ghosh, Chem. Soc. Rev. 41 (2012) 3077. R. Dutta, P. Ghosh, Chem. Commun. 51 (2015) 9070. M. Cametti, K. Rissanen, Chem. Soc. Rev. 42 (2013) 2016. M. Arunachalam, P. Ghosh, Chem. Commun. 47 (2011) 8477. T.D. Ashton, K.A. Jolliffe, F.M. Pfeffer, Chem. Soc. Rev. 44 (2015) 4547. Y. Zhou, J.F. Zhang, J. Yoon, Chem. Rev. 114 (2004) 5511. F. Wang, L. Wang, X. Chen, J. Yoon, Chem. Soc. Rev. 43 (2014) 4312. P.J. Smith, M.V. Reddington, C.S. Wilcox, Tetrahedron Lett. 33 (1992) 6085. H. Xie, S. Yi, S. Wu, J. Chem. Soc. Perkin Trans. 2 (1999) 2751. H. Xie, S. Yi, X. Yang, S. Wu, New J. Chem. 23 (1999) 1105. H. Xie, S. Yi, X. Yang, S. Wu, Sci. China B 43 (2000) 201. S. Sasaki, D. Citterio, S. Ozawa, K. Suzuki, J. Chem. Soc. Perkin Trans. 2 (2001) 2309. I. Basaran, M.E. Khansari, A. Pramanik, B.M. Wong, M.A. Hossain, Tetrahedron Lett. 55 (2014) 1467. I. Basaran, M.E. Khansari, A. Pramanik, B.M. Wong, M.A. Hossain, Tetrahedron Lett. 56 (2015) 115. L.-H. Wei, Y.-B. He, J.-L. Wu, H.-J. Qin, K.-X. Xu, L.-Z. Meng, Chin. J. Chem. 23 (2005) 608. M.E. Moragues, L.E. Santos-Figueroa, T. Abalos, F. Sancenon, R. Martinez-Manez, Tetrahedron Lett. 53 (2012) 5110. V.K. Bhardwaj, S. Sharma, N. Singh, M.S. Hundal, G. Hundal, Supramol. Chem. 23 (2011) 790. E. Brunetti, J.-F. Picron, K. Flidrova, G. Bruylants, K. Bartik, I. Jabin, J. Org. Chem. 79 (2014) 6179. C. Capici, R.D. Zorzi, C. Gargiulli, G. Gattuso, S. Geremia, A. Notti, S. Pappalardo, M.F. Parisi, F. Puntoriero, Tetrahedron 66 (2010) 4987. X. Fang, Y.-Z. Zhu, J.-Y. Zheng, J. Org. Chem. 79 (2014) 1184. W. Kim, S.K. Sahoo, G.-D. Kim, H.-J. Choi, Tetrahedron 71 (2015) 8111. A.R. Jennings, D.Y. Son, Tetrahedron Lett. 53 (2012) 21814. A.R. Jennings, D.Y. Son, Tetrahedron 71 (2015) 3990. L. Wu, L. Liu, R. Fang, C. Deng, J. Han, H. Hu, C. Lin, L. Wang, Tetrahedron Lett. 53 (2012) 3637. Y. Hao, P. Yang, S. Li, X. Huang, X.-J. Yang, B. Wu, Dalton Trans. 41 (2012) 7689. G.V. Zyryanov, M.A. Palacios, P. Anzenbacher Jr., Angew. Chem. Int. Ed. 46 (2007) 7849. N.A. Esipenko, P. Koutnik, T. Minami, L. Mosca, V.M. Lynch, G.V. Zyryanov, Pavel Anzenbacher, Jr., Chem. Sci. 4 (2013) 3617. T. Minami, Y. Liu, A. Akdeniz, P. Koutnik, N.A. Esipenko, R. Nishiyabu, Y. Kubo, P. Anzenbacher Jr., J. Am. Chem. Soc. 136 (2014) 11396. D.Y. Lee, N. Singh, M.J. Kim, D.O. Jang, Org. Lett. 13 (2011) 3024. C. Kar, A. Basu, G. Das, Tetrahedron Lett. 53 (2012) 4754. N. Singh, D.O. Jang, Org. Lett. 9 (2007) 1991. D.Y. Lee, N. Singh, A. Satyender, D.O. Jang, Tetrahedron Lett. 52 (2011) 6919. A.K. Mahapatra, S.K. Manna, P. Sahoo, Talanta 85 (2011) 2673. V.K. Bhardwaj, M.S. Hundal, G. Hundal, Tetrahedron 65 (2009) 8556. Y. Zhang, Q. Li, Q. Zhang, Q. Lin, C. Cao, M. Liu, T. Wei, Chin. J. Chem. 29 (2011) 1529. J. Shao, Y. Qiao, H. Lin, H. Lin, Spectrochim. Acta A 71 (2009) 1736. A. Pramanik, G. Das, Tetrahedron 65 (2009) 2196. A. Pramanik, M. Bhuyan, R. Choudhury, G. Das, J. Mol. Struct. 879 (2008) 88. A. Mitra, A. Pariyar, S. Bose, P. Bandyopadhyay, A. Sarkar, Sens. Actuators B 210 (2015) 712. K. Ghosh, D. Kar, S. Joardar, A. Samadder, A.R. Khuda-Bukhsh, RSC Adv. 4 (2014) 11590. M.C. González, F. Otón, A. Espinosa, A. Tárraga, P. Molina, Org. Biomol. Chem. 13 (2015) 1429.

S.K. Sahoo et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 27 (2016) 30–53 [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81]

S.K. Dey, G. Das, Chem. Commun. 47 (2011) 4983. W. Wei, Y. Guo, J. Xu, S. Shao, Spectrochim. Acta A 77 (2010) 620. L. Wang, X. He, Y. Guo, J. Xu, S. Shao, Org. Biomol. Chem. 9 (2011) 752. D. Curiel, G. Sánchez, C.R. de Arellano, A. Tárraga, P. Molina, Org. Biomol. Chem. 10 (2012) 1896. J. Hao, K. Hiratani, N. Kameta, T. Oba, Supramol. Chem. 23 (2011) 319. A.S. Singh, S.-S. Sun, J. Org. Chem. 77 (2012) 1880. K.J. Wallace, W.J. Belcher, D.R. Turner, K.F. Syed, J.W. Steed, J. Am. Chem. Soc. 125 (2003) 9699. M.H. Filby, S.J. Dickson, N. Zaccheroni, L. Prodi, S. Bonacchi, M. Montalti, M.J. Paterson, T.D. Humphries, C. Chiorboli, J.W. Steed, J. Am. Chem. Soc. 130 (2008) 4105. W.-T. Gong, K. Hiratani, Tetrahedron Lett. 49 (2008) 5655. K. Ghosh, D. Tarafdar, A. Samadder, A.R. Khuda-Bukhsh, RSC Adv. 5 (2015) 35175. S. Bao, W.-T. Gong, W.-D. Chen, J.-W. Ye, Y. Lin, G.-L. Ning, J. Incl. Phenom. Macrocycl. Chem. 70 (2011) 115. K. Ghosh, S.S. Ali, A.R. Sarkar, A. Samadder, A.R. Khuda-Bukhsh, I.D. Petsalakis, G. Theodorakopoulos, Org. Biomol. Chem. 11 (2013) 5666. A. Metzger, E.V. Anslyn, Angew. Chem. Int. Ed. 37 (1998) 649. K. Ghosh, I. Saha, Supramol. Chem. 23 (2011) 518. Z. Xu, N.R. Song, J.H. Moon, J.Y. Lee, J. Yoon, Org. Biomol. Chem. 9 (2011) 8340. Y. Bai, B.-G. Zhang, J. Xu, C.-Y. Duan, D.-B. Dang, D.-J. Liu, Q.-J. Meng, New J. Chem. 29 (2005) 777. D. Wang, X. Zhang, C. He, C. Duan, Org. Biomol. Chem. 8 (2010) 2923. V. Amendola, L. Fabbrizzi, E. Monzani, J. Chem. Eur. 10 (2004) 76. A.N. Swinburne, M.J. Paterson, A. Beeby, J.W. Steed, Org. Biomol. Chem. 8 (2010) 1010–1016. A.N. Swinburne, M.J. Paterson, K.H. Fischer, S.J. Dickson, E.V.B. Wallace, W.J. Belcher, A. Beeby, J.W. Steed, Chem. Eur. J. 16 (2010) 1480. (a) R. Martınez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419; (b) L. Fabbrizzi, M. Licchelli, G. Rabaioli, A. Taglietti, Coord. Chem. Rev. 205 (2000) 85. M.E. Huston, E.U. Akkaya, A.W. Czarnik, J. Am. Chem. Soc. 111 (1989) 8735. G. De Santis, L. Fabbrizzi, M. Licchelli, A. Poggi, A. Taglietti, Angew. Chem. Int. Ed. Engl. 35 (1996) 202. L. Fabbrizzi, M. Licchelli, L. Parodi, A. Poggi, A. Taglietti, Eur. J. Inorg. Chem. (1999) 35. I. Bruseghini, L. Fabbrizzi, M. Licchelli, A. Taglietti, Chem. Commun. (2002) 1348. L.A. Cabell, M.D. Best, J.J. Lavigne, S.E. Schneider, D.M. Perreault, M.-K. Monahan, E.V. Anslyn, J. Chem. Soc. Perkin Trans. 2 (2001) 315. S.L. Tobey, E.V. Anslyn, Org. Lett. 5 (2003) 2029. D.J. Oh, M.S. Han, K.H. Ahn, Supramol. Chem. 19 (2007) 315. D.J. Oh, K.H. Ahn, Org. Lett. 10 (2008) 3539. X.-L. Ni, X. Zeng, C. Redshaw, T. Yamato, J. Org. Chem. 76 (2011) 5696. A.K. Mahapatra, S.K. Manna, C.D. Mukhopadhyay, D. Mandal, Sens. Actuators B 200 (2014) 123. N. Singh, H.J. Jung, D.O. Jang, Tetrahedron Lett. 50 (2009) 71. S.A. Haque, R.L. Bolhofner, B.M. Wong, M.A. Hossain, RSC Adv. 5 (2015) 38733. J.-R. Zhou, D.-P. Liu, Y. He, X.-J. Kong, Z.-M. Zhang, Y.-P. Ren, L.-S. Long, R.-B. Huang, L.-S. Zheng, Dalton Trans. 43 (2014) 11579. S.B. Maity, P.K. Bharadwaj, Inorg. Chem. 52 (2013) 1161. F. Zheng, M. Wen, F. Zeng, S. Wu, Sens. Actuators B 188 (2013) 1012. C. Kar, G. Das, J. Photochem. Photobiol. A Chem. 251 (2013) 128.

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