Anion sensors based on β,β′-disubstituted porphyrin derivatives

Synthesis, Spectral, Redox, and Sensing Studies of β-Dicyanovinyl-Appended Corroles and Their Metal Complexes

A new family of β-dicyanovinyl (DCV)-appended corroles represented as MTPC(MN) (where M = 3H, Cu, Ag, and Co(PPh₃) and MN = malononitrile and TPC = 5,10,15-triphenylcorrole) were synthesized starting from the free base mono β-formyl corrole, H₃TPC(CHO), and characterized along with their respective MTPC(CHO) and MTPC complexes as to their spectroscopic and electrochemical properties in nonaqueous media. Comparisons between the two series of corroles demonstrate a pronounced substituent effect of the β-DCV group on the physicochemical properties making the MTPC(MN) derivatives substantially easier to reduce and more difficult to oxidize than the formyl or unsubstituted corroles. In addition, the colorimetric and spectral detection of 11 different anions (X) in the form of tetrabutylammonium salts (TBAX, X = PF₆^-, OAc^-, H₂PO₄^-, CN^-, HSO₄^-, NO₃^-, ClO₄^-, F^-, Cl^-, Br^-, and I^-) were also investigated in nonaqueous media. Of the investigated anions, only CN^- was found to induce changes in the UV-vis and ¹H NMR spectra of the β-DCV metallocorroles. This data revealed that CuTPC(MN) and AgTPC(MN) act as chemodosimeters for selective cyanide ion detection via a nucleophilic attack at the vinylic carbon of the DCV substituent, while (PPh₃)CoTPC(MN) acts as a chemosensor for cyanide ion sensing via axial coordination to the cobalt metal center. A low-limit detection of cyanide ions was observed at 1.69 ppm for CuTPC(MN) and 1.17 ppm for AgTPC(MN) in toluene.

Tuning the recognition properties of urea based receptor towards cyanide in aqueous medium: Effect of fluorine substitution on strength and selectivity of the anion receptor

In this report, we have successfully tuned the selectivity and sensitivity of an anion receptor L¹ by substituting electron withdrawing natured fluorine atoms directly on to the rim of the phenyl ring. Despite the fact that, we have two electron withdrawing natured nitro substituents on the other side of receptor L¹; two fluorine substitutions made dramatic change in the sensing ability as well as the selectivity of the receptor L¹ towards anions. The acetonitrile solution contains L¹ with tetrabutyl ammonium salts of fluoride, cyanide, acetate and dihydrogen phosphate results sudden color changes from yellow to brown; almost negligible spectral/color change for azide and bifluoride, where as there is no color change observed with any other anionic guests with L¹. Solution state binding studies of L¹ are carried out by UV-Vis spectrometry titration in 100% acetonitrile and it is found to be the receptor L¹ selectively binds cyanide, phosphate and fluoride stronger than acetate; it is also found that receptor binds fluoride 100 times stronger than that of the receptor L and L¹ has almost similar efficiency in binding towards acetate ion (AcO^-). The strength of the receptor L¹ towards fluoride, cyanide, acetate and dihydrogen phosphate bindings are found to be in the order of 1.271 × 10⁵ M^-1 > 1.245 × 10⁵ M^-1 > 1.368 × 10³ M^-1, 1.23 × 10³ M^-1 respectively. When we used aqueous environment (10% of water in acetonitrile) as testing solvent system, the receptor L¹ selectively sense cyanide ion. Over all, strength of the receptor is increased towards anions with an increasing the number of fluorine atom onto the receptors. Here in we have also prepared a reference compound L² in which the receptor molecule is substituted with only one fluoride atom. The acetonitrile solution of reference receptor L² with help of naked-eye colorimetric experiment and spectrometric ammonium salts of fluoride, cyanide, acetate and bifluoride results sudden color changes from faint-yellow to brown in color. Unlike receptor L¹, receptor L² does not recognize H₂PO₄^-, but instead of H₂PO₄^-, it recognizes bifluoride as evidenced from UV-Vis spectroscopic and naked-eye colorimetric studies.

Simple and highly electron deficient Schiff-base host for anions: First turn-on colorimetric bifluoride sensor

Here in, we have designed, synthesized and isolated sensor L, as a turn-on colorimetric chemo sensor for bifluoride ion. The acetonitrile solution contains L with tetrabutyl ammonium salts of bifluoride, cyanide and fluoride results sudden color changes from yellow to dark brown where as there is no color change observed with any other anionic guests with L. Solution state binding studies of L are carried out by UV-Vis spectrometry titration and the strengths of the chemosensor L towards bifluoride, cyanide and fluoride bindings are found to be 2.67 × 10⁵ M^-1, 4.78 × 10⁵ M^-1_, 4.45 × 10⁵ M^-1 respectively. The strength and sensitivity of the absorbance based assay with L for bifluoride ion, are found to be 2.67 × 10⁵ M^-1 and 0.7 μM respectively, which is the best (highest binding constant and lowest LOD) ever reported in the literature. In order to use this sensor in practical application, we also prepared a cassette which is fabricated with sensor L and we succeeded to sense bifluoride ion.

Functionalized Quinoxaline for Chromogenic and Fluorogenic Anion Sensing

This Review article provides a comprehensive analysis of recent examples reported in the field of quinoxaline-based chromogenic and fluorogenic chemosensors for inorganic anions such as fluoride, cyanide, acetate, and phosphate, as well as their utility in biomolecular science. It commences with a discussion of the various structural motifs such as quinoxaline-based oligopyrroles, polymers, sulfonamides, cationic receptors, and miscellaneous receptors bearing mixed recognition sites in the same receptor. Advances are discussed in depth, where the focus of this review is to tackle mainly solution state anion sensing utilizing quinoxaline-based receptors using different spectroscopic techniques with reference to anion selectivity by colorimetric and fluorescence response. The various examples discussed in this Review illustrate how the integration of anion binding elements with the quinoxaline chromophore could result in anion responsive chemosensors. Over the years, it has been observed that structural modification of the quinoxaline moiety with different sets of signaling unit and recognition sites has resulted in a few anion specific chemosensors.

β-Dicyanovinyl substituted porphyrinogen: synthesis, a reversible sensor for picric acid among explosives and a unique sensor for cyanide and fluoride ions by switching between various porphyrinoid states

Oxp-MN (1) acts as a multifunctional sensor for the simultaneous colorimetric detection of picric acid (PA) among other nitroaromatics, F⁻ and CN⁻ ions when “hidden” within a mixture of other anions under unrestricted queue assay.

Synthesis and anion binding properties of porphyrins and related compounds

Over the last two decades the preparation of pyrrole-based receptors for anion recognition has attracted considerable attention. In this regard porphyrins, phthalocyanines and expanded porphyrins have been used as strong and selective receptors while the combination of those with different techniques and materials can boost their applicability in different applications as chemosensors and extracting systems. Improvements in the field, including the synthesis of this kind of compounds, can contribute to the development of efficient, cheap, and easy-to-prepare anion receptors. Extensive efforts have been made to improve the affinity and selectivity of these compounds and the continuous expansion of related research makes this chemistry even more promising. In this review, we summarize the most recent developments in anion binding studies while outlining the strategies that may be used to synthesize and functionalize these type of macrocycles.

Quinoline-annulated chlorins and chlorin-analogs

The OsO4-mediated dihydroxylation of quinoline-annulated porphyrin generates a quinoline-annulated dihydroxychlorin in a regioselective fashion. Its dihydroxypyrroline moiety, located at the opposite of the annulated pyrrole, is susceptible to the same functional group interconversions we previously demonstrated for non-annulated dihydroxychlorins: oxidations to the corresponding dione, lactone, and dialkoxymorpholine derivatives. The quinoline-annulated chlorin and derivatives are all characterized by absorption spectra that are much broadened and between 130 and 220 nm red-shifted compared to their non-annulated analogs. Absorbance maxima in the NIR up to well above 800 nm were recorded. We attribute the bathochromic shift to their extended [Formula: see text]-systems and inferred non-planarity, highlighting that quinoline-annulation is a particularly effective and simple strategy to red-shift the absorption spectra of chlorins and chlorin analogs.

Ratiometric and colorimetric "naked eye" selective detection of CN(-) ions by electron deficient Ni(II) porphyrins and their reversibility studies

Highly electron deficient β-substituted Ni(II) porphyrins (1-5) were synthesized and utilized as novel sensors for selective rapid visual detection of CN(-) ions. This article describes the single crystal X-ray structures, electronic spectral and electrochemical redox properties of these sensors. The ratiometric and colorimetric responses of these porphyrins were monitored by the change in optical absorption spectra. These sensors were found to be highly selective for cyanide ions with extremely high binding constants (10(16)-10(8) M(-2)) through axial ligation of CN(-) ions and are able to detect <0.11 ppm of CN(-) ions. 1-5 were recovered from 1-5·2CN(-) adducts by acid treatment and reused without loss of sensing ability. CN(-) binding strongly perturbs the redox properties of the parent porphyrin π-system. The applicability of 1-5 as practical visible colorimetric test kits for CN(-) ions in aqueous and non-aqueous media has also been explored. The mode of binding was confirmed by single crystal X-ray, spectroscopic studies and DFT calculations.

Switching between porphyrin, porphodimethene and porphyrinogen using cyanide and fluoride ions mimicking volatile molecular memory and the ‘NOR’ logic gate

β-Substituted porphyrins were developed as a quantitatively operating “lab-on-a-molecule” for the detection of F⁻and CN⁻ions, by switching between porphyrin, porphodimethene and porphyrinogen along with distinct solution colour changes and reversibility.

Mono- and Bisquinoline-Annulated Porphyrins from Porphyrin β,β'-Dione Oximes

An acid-induced reaction of meso-tetraphenyl-2-hydroxyimino-3-oxoporphyrin leads, with concomitant loss of water, to a formal electrophilic aromatic substitution of the ortho-position of the phenyl group adjacent to the oxime, forming a quinoline moiety. Owing in part to the presence of a π-extended chromophore, the resulting meso-triphenylmonoquinoline-annulated porphyrin (λmax = 750 nm) possesses a much altered optical spectrum from that of the starting oxime (λmax = 667 nm). An oxidative DDQ-induced ring-closure process is also possible, generating the corresponding meso-triphenylmonoquinoline-annulated porphyrin quinoline N-oxide, possessing a slightly shifted and sharpened UV-vis spectrum (λmax = 737 nm). The connectivity of the chromophores was conclusively shown by NMR spectroscopy. Both ketone functionalities in meso-tetraphenyl-2,3-dioxoporphyrin can be converted, via the oxime and using the acid- or oxidant-induced reaction pathways, either in one step or in a stepwise fashion, to bisquinoline-annulated porphyrin (λmax = 775 nm) and its N-oxide (λmax = 779 nm), respectively. This process is complementary to a previously established pathway toward bisquinoline-annulated porphyrins. Their zinc(II), nickel(II), and palladium(II) complexes are also described. Several examples of the quinoline-annulated porphyrins were crystallographically characterized, proving their connectivity and showing their conformations that are extremely distorted from planarity. The work presents a full account on the synthesis, structure, and spectroscopic properties of these classes of NIR-absorbing dyes.

Liquid crystalline assembly of coil-rod-coil molecules with lateral methyl groups into 3-D hexagonal and tetragonal assemblies

In this paper, we report the synthesis and self-assembly behavior of coil-rod-coil molecules, consisting of three biphenyls linked through a vinylene unit as a conjugated rod segment and poly(ethylene oxide) (PEO) with a degree of polymerization (DP) of 7, 12 and 17, incorporating lateral methyl groups between the rod and coil segments as the coil segment. Self-organized investigation of these molecules by means of differential scanning calorimetry (DSC), thermal polarized optical microscopy (POM) and X-ray diffraction (XRD) reveals that the lateral methyl groups attached to the surface of rod and coil segments, dramatically influence the self-assembling behavior in the liquid-crystalline mesophase. Molecule 1 with a relatively short PEO coil length (DP = 7) self-assembles into rectangular and oblique 2-dimensional columnar assemblies, whereas molecules 2 and 3 with DP of 12 and 17 respectively, spontaneously self-organize into unusual 3-dimensional hexagonal close-packed or body-centered tetragonal assemblies.

Porphyrin-based sensor nanoarchitectonics in diverse physical detection modes

Porphyrins and related families of molecules are important organic modules as has been reflected in the award of the Nobel Prizes in Chemistry in 1915, 1930, 1961, 1962, 1965, and 1988 for work on porphyrin-related biological functionalities. The porphyrin core can be synthetically modified by introduction of various functional groups and other elements, allowing creation of numerous types of porphyrin derivatives. This feature makes porphyrins extremely useful molecules especially in combination with their other interesting photonic, electronic and magnetic properties, which in turn is reflected in their diverse signal input-output functionalities based on interactions with other molecules and external stimuli. Therefore, porphyrins and related macrocycles play a preeminent role in sensing applications involving chromophores. In this review, we discuss recent developments in porphyrin-based sensing applications in conjunction with the new advanced concept of nanoarchitectonics, which creates functional nanostructures based on a profound understanding of mutual interactions between the individual nanostructures and their arbitrary arrangements. Following a brief explanation of the basics of porphyrin chemistry and physics, recent examples in the corresponding fields are discussed according to a classification based on physical modes of detection including optical detection (absorption/photoluminescence spectroscopy and energy and electron transfer processes), other spectral modes (circular dichroism, plasmon and nuclear magnetic resonance), electronic and electrochemical modes, and other sensing modes.

Highly specific and reversible fluoride sensor based on an organic semiconductor

A novel sulfonamide-conjugated benzo-[2,1-b:3,4-b']bithiophene semiconductor has been designed and synthetized in order to develop a probe for specific detection of anions both in the homogeneous (solution) and heterogeneous phase. Its photophysical and electrochemical data were reported in this study. On the basis of the optical and NMR titrations analysis, the chelator was found to be highly selective for fluoride compared to others anions (Ka = 1.6 × 10(4) M(-1) in dimethyl sulfoxide (DMSO)). In addition, from an intricate sample, the novel chelator shows exceptional specificity toward fluoride and reveals a complete reversibility after addition of trifluoroacetic acid (TFA). Sensing films were obtained by electrochemical polymerization of the probe on an electrode surface, which clearly show effective detection of fluoride.

β-Pyrazino-fused tetrarylporphyrins

A novel method for the preparation of β-fused porphyrin dyads was developed that exploits a one-pot reaction of 2,3-diaminoporphyrins with diethyl oxalate. This approach provides good yields of the zinc β-fused dyad and the corresponding free-base, opening the way for preparation of several metal derivatives to permit modulation of optoelectronic characteristics for commercial applications.

meso-Tetraphenylporphyrin-derived oxypyriporphyrin, oxypyrichlorin, and thiomorpholinochlorin, as their Ni(II) complexes

trans-Diolchlorin was prepared by nucleophilic addition of methyl-Grignard bromide to meso-tetraphenyl-2,3-dioxoporphyrin, as its free base or Ni(II) complex. The trans-configuration of the vic-diol functionality was shown by single crystal X-ray diffractometry. The nickel complex of the trans-dimethyldiol proved susceptible to Pb(IV) acetate-induced, oxidative diol cleavage, generating a meso-tetraphenylsecochlorin bismethylketone Ni(II) complex, the first example of this chromophore class. Under Brønsted-basic conditions, this bisketone cyclized via an intramolecular aldol condensation to provide a meso-tetraphenyloxypyriporphyrin. Reduction of this porphyrin analog saturated the double bond in the pyridinone moiety, generating an oxypyrichlorin. Reaction of the meso-tetraphenylsecochlorin bismethylketone Ni(II) complex with Lawesson's reagent induced the formation of a thiomorpholinochlorin substituted with two methylene groups, the first example of any porphyrin analog containing a thiomorpholine moiety.

Synthetic transformations of porphyrins – Advances 2002-2004

Contemporary methods for the modification of porphyrins are presented. In association with the Third International Conference on Porphyrins and Phthalocyanines (ICPP-3) a survey of current method developments and reactivity studies is made. The review focuses on synthetic transformations of porphyrins currently in use for various applications and on functional group transformations. A brief survey of important developments covers selectively the literature from late 2001 to early 2004.

Recognition, encapsulation, and selective fluorescence sensing of nitrate anion by neutral C3-symmetric tripodal podands bearing amide functionality

A series of neutral C(3)-symmetric acyclic artificial receptors incorporating amide functionality has been designed, synthesized, and fully characterized. Upon protonation, these conformationally flexible N-bridged tripodal podands 1-5 form in situ cone shape conformation through hydrogen bonding and C-H···π interactions. The protonation-induced interior preorganized cavity is capable of entrapping nitrate anions through the amide N-H bonds to form discrete nitrate complexes (1a-5a), which were fully characterized by NMR, HRESI mass spectra, and single crystal structures. By incorporating suitable fluorophores at each branch of the tripod receptor, the resulting fluorescent receptor 5 selectively recognized nitrate anions by fluorescent quenching in a DMSO solution and displayed one of the highest binding affinities for nitrate anions reported so far in polar media. Receptor 5 represents a unique example of a neutral receptor for the recognition of nitrate anions in polar solvent media by its zwitterionic form. The possible mechanism of proton-induced preorganization of these flexible, acyclic receptors in a convergent cone conformation followed by nitrate complexation has been proposed to rationalize the effective nitrate recognition.

A short route to chlorin-α-diones

Chlorin-α-diones and tetraones are useful intermediates for the formation of porphyrin arrays. We have developed a simple three-step route to chlorin-α-diones and tetraones. It was found that meso-tetraarylchlorins could be converted to hydroxychlorins when adsorbed on silica. The hydroxychlorins could then be oxidized with the Dess-Martin Periodinane to the corresponding chlorin-α-diones and tetraones. The parent 5,10,15,20-tetrakis(3,5-di-t-butylphenyl)porphyrin could be transformed to 17,18-dioxo-5,10,15,20-tetrakis(3,5-di-t-butylphenyl)chlorin in the three steps in an overall yield of 54% while a tetraazaanthracene linked bis-bacteriochlorin could be transformed to the bis-bacteriochlorin-tetraone in a yield of 16%.

Recognition properties of carboxylic acid bioisosteres: anion binding by tetrazoles, aryl sulfonamides, and acyl sulfonamides on a calix[4]arene scaffold

Tetrazoles and acyl sulfonamides are functional groups that are common in medicinal chemistry but virtually unexplored as recognition elements in supramolecular chemistry. We report here on the anion binding properties of these highly acidic N-H functional groups. We have prepared two new calixarene-based tetrazole-containing hosts, as well as new acetyl sulfonamide and benzoyl sulfonamide hosts. We also report on analogous hosts bearing the better-known aryl sulfonamide functional group as a point of comparison. We find that these hosts are competent anion binders and that the recognition of anions by these groups is highly dependent on their conformational preferences. We also report in detail on the preferred molecular shape of each acid bioisostere as determined by calculations and structural database surveys, and discuss how these shapes impact binding in the context of the reported hosts.

A core-substituted naphthalene diimide fluoride sensor

The synthesis and characterization of a highly fluorescent core-substituted naphthalene diimide sensor (varphi = 0.34) bearing a bis-sulfonamide group is described. The compound shows a unique selectivity and reactivity for the fluoride ion over other anions in CHCl(3) by a two-stage deprotonation process leading to a colorimetric response. In DMSO solution, the sensor is shown to be highly selective for fluoride (K(a) approximately 10(6) M(-1)) over other anions with more pronounced changes in absorption characteristics.

Metalloporphyrin anion sensors: the effect of the metal centre on the anion binding properties of amide-functionalised and tetraphenyl metalloporphyrins

This article describes the synthesis and anion binding properties of a series of 'picket fence' metalloporphyrin complexes, within which the metal centre is systematically varied. The porphyrin structure contains four amide bonds and is the same for each metal. The anion binding properties of these receptors are further contrasted with those of their tetraphenylporphyrin congeners to elucidate both the effect of the metal centre and the influence of the amide groups on the anion recognition process. Anion binding was demonstrated using UV/visible and (1)H NMR spectroscopies, electrochemistry and luminescence. The metal centre was found to be highly influential in the strength and selectivity of binding; for example, the cadmium and mercury complexes exhibited far greater affinities for anions than the zinc complexes in competitive solvents such as DMSO. The amide functionalities were found to enhance the anion binding process.