Fluorescent Anion Sensor Research Articles

Synthesis, Structural and Photophysical Evaluations of Urea Based Fluorescent PET Sensors for Anions

The design, synthesis and photophysical evaluation of four anthracene based photoinduced electron transfer (PET) sensors (3a–d) for anions is described. The [4π+4π] photodimerization product, 4, was also obtained from 3b, by slow evaporation from DMSO solution and its X-ray crystal structure determined. The structure of 4 showed the classical dimerization at the 9,10-positions of anthracene. Sensors 3a–d are all based on the use of charge neutral aryl urea receptors, where the recognition of anions such as acetate, phosphate and iodide, was due to the formation of strong hydrogen bonding interactions in DMSO. This anion recognition resulted in enhanced quenching of the anthracene excited state via electron transfer from the receptor; hence the emission was ‘switched on–off’. The sensing of fluoride was, however, found to be a two-step process, which involved initial hydrogen bonding interactions with the receptor, followed by deprotonation and the formation of bifluoride (). The changes in the emission spectra upon sensing of chloride and bromide were, however, minor. The photophysical properties of these sensors in the presence of various anions were further studied, including investigation of their excited state lifetimes and quantum yields, as well as detailed Stern–Volmer kinetic analysis with the aim of determining the dynamic and static quenching constants, k q and k s for the above anion recognition. These measurements indicated that while the anion dependant quenching was mostly by a dynamic process, some contribution from static quenching was also observed at lower anion concentrations.

Colorimetric ‘naked-eye’ and fluorescent sensors for anions based on amidourea functionalised 1,8-naphthalimide structures: anion recognition via either deprotonation or hydrogen bonding in DMSO

The synthesis and the spectroscopic studies of three new amidourea-based sensors for anions, 1–3, are described. These are based on the use of 4-hydrazine-1,8-naphthalimides which upon reaction with isocyanates give rise to the formation of the desired amidoureas. 1–3 absorb strongly in the visible region, due to the internal charge transfer excited state character of the naphthalimide moieties. Large colorimetric changes were observed upon the addition of various anions such as acetate, dihydrogenphosphate and fluoride to 1–3 in DMSO, and are brought about through either hydrogen bonding to, or deprotonation of, the amidourea. These changes were clearly visible to the naked eye, changing from yellow/green to purple, and were reversed upon addition of protic solvents. Moreover, each of the three anions gave rise to unique changes in the structure of the absorption spectra which can be considered as being a ‘fingerprint’ identity for each of them. The fluorescence emission spectra were also affected upon anion binding, being significantly red shifted upon excitation. Non-linear regression analysis of the ground and excited state changes showed the anions were recognized in either 1 : 1 or 2 : 1 stoichiometry, and that the aryl-urea substituents govern the sensitivity of the binding; which in the case of acetate was in the order of 3 > 1 > 2. The anion recognition was also monitored by 1H NMR spectroscopy in DMSO-d6.

Acridone Heterocycles as Fluorescent Sensors for Anions.

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Specific optical signalling of anions via intramolecular charge transfer pathway based on acridinedione fluorophore

We present a simple but highly specific acridinedione fluorophore (ADD-1) that acts both as a fluorescent and colorimetric sensor for anions in acetonitrile. The specific optical signalling of ADD-1 is due to the formation of new distinct intramolecular charge transfer (ICT) emitting states in the presence of AcO − (490 nm), H 2PO 4 − (440 nm), and F − (510 nm) over other anions. Presence of F − shows a colour change that is perceptible to the naked eye, from colourless to an intense fluorescent green due to the deprotonation of acridinedione ring amino hydrogen.

Evaluation of a simple and novel fluorescent anion sensor, 4-quinolone, and modification of the emission color by substitutions based on molecular orbital calculations

4-Quinolone (4-QO) was evaluated as a simple and novel fluorescent anion sensor, and the modification of its emission color was carried out. The series of 4-QO derivatives having molecular orbitals with different energy levels was designed by substitutions at the 6 and 7 positions based on the molecular orbital calculations. All derivatives showed drastic fluorescence enhancements in the presence of F − via the intramolecular charge transfer mechanism, and the successful modification of the emission color was achieved. The anion-induced emission colors of these derivatives as well as their binding affinities for F − could be predicted by ab initio quantum chemical calculations, indicating that the present calculations are useful in designing new anion sensors.

Fluorescent anion sensors based on 4-amino-1,8-naphthalimide that employ the 4-amino N–H

The new charge neutral 4-amino-1,8-naphthalimide based anion sensors 2 and 3 bind to both acetate and dihydrogenphosphate with 1:1 stoichiometry through hydrogen bonding to both thiourea N–H atoms and in the case of dihydrogenphosphate, the naphthalimide 4 amino N–H group as well. This was clearly established from 1H NMR titration experiments with H 2 PO 4 - in DMSO- d 6 where a substantial shift in the resonance for the naphthalimide N–H was observed concomitant with the expected migration of the thiourea N–H chemical shifts. The binding constants determined from the titration studies indicate that the new sensors bind H 2 PO 4 - more strongly than AcO −. Fluorescence titrations with sensor 3 indicate quenching of 59% and 36% upon addition of acetate and dihydrogenphosphate, respectively.

First Fluorescent Sensor for Fluoride Based on 2-Ureido-4[1H]-pyrimidinone Quadruple Hydrogen-Bonded AADD Supramolecular Assembly

A simple, highly selective, neutral, fluorescent sensor for fluoride anions is reported. It is based on 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonded AADD supramolecular assembly, and its assembling and disassembling processes are also able to respond to external stimuli reversibly.

Acridone Heterocycles as Fluorescent Sensors for Anions

An acridone heterocycle with a suitable functionalization can be used as a fluorescent sensor for anions. Large differences in fluorescence efficiency are observed when chlorides are associated in the receptor cavity. Chlorides form two different complexes in chloroform, with 2:1 and 1:1 stoichiometries. The combined presence of receptor (1) and 18-crown-6 ether allows the extraction of aqueous α-amino acids to the chloroform phase. This extraction is very sensitive to temperature.

Fluorescent Anion Sensor Derived from Cholic Acid: The Use of Flexible Side Chain

[structure: see text] The fluorescent photoinduced electron transfer (PET) chemosensors 1-3 were synthesized from cholic acid. 1 and 2 containing amidothiourea groups as anion receptive sites demonstrated much higher affinity toward anions than 3 containing traditional thiourea H-bond donating group. Comparative studies on their binding affinity toward carboxylates, dihydrogen phosphate, and halides revealed that the amidothiourea moiety on the C17 side chain could work cooperatively with H-bond donating groups on C7 and C12 to bind spherical halogen anions. An unexpected specific fluorescence enhancement of 1 by coordinating bromide ion was observed.

Fluorescent sensors for amino acid anions based on calix[4]arenes bearing two dansyl groups

Two novel types of chiral calix[4]arenes containing hydrazide and dansyl groups were synthesized and examined for their enantioselective recognition abilities by the fluorescence and 1H NMR spectra in CHCl 3. The results indicate that both 4a and 4b have excellent enantioselectivities to the N-protected alanine or phenylalanine anions.

A New Terpyrrolic Analogue of Dipyrrolylquinoxalines: An Efficient Optical‐Based Sensor for Anions in Organic Media.

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Ratiometric Dual Fluorescent Receptors for Anions under Intramolecular Charge Transfer Mechanism.

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Poly(amine) biphenyl derivatives as fluorescent sensors for anions and cations

Four new ligands, derived from tetramethylbenzidine and containing additional amino groups, are described. The influence of pH on the fluorescent properties of these ligands has been studied, and the effect of the dihedral angle between the aromatic rings on the fluorescent response has been established. The behaviour of the new ligands, and that of others previously described in both the complexation and sensing of different anions and cations, are also described.

Detection of anions using a fluorescent alizarin–phenylboronic acid ensemble

Selective anion-induced organization of phenylboronic acids and alizarin results in a new TURN-ON fluorescent sensor for anions in MeOH.

Ratiometric dual fluorescent receptors for anions under intramolecular charge transfer mechanism

A series of the intramolecular charge transfer (ICT) dual fluorescent receptors with anion binding site in the electron acceptor were designed and synthesized. These receptors exhibited dual fluorescence in acetonitrile and the charge transfer (CT) emission energy was found to correlate linearly with the Hammett constant of the substituent existing in the electron acceptor, which is the basis for anion sensing. Dual fluorescence of these receptors was found to be sensitive to the presence of anions such as fluoride and acetate and the receptors can be employed as ratiometric fluorescent sensors for anions.

A new terpyrrolic analogue of dipyrrolylquinoxalines: an efficient optical-based sensor for anions in organic media

The synthesis and X-ray structural characterization of a new terpyrrolic analog of dipyrrolylquinoxaline (DPQ) is described that contains two pyrrole anion recognition groups bridged by a central 1,2-linked pyrrole. Relative to the ‘parent’ DPQ system, this new terpyrrolic scaffold acts as an improved colorimetric and fluorescent sensor for halide and dihydrogen phosphate anions in organic media. This enhancement is particularly dramatic in the case of H 2PO 4 − (studied as its tetrabutylammonium salt); in CH 2Cl 2 it is bound with an affinity constant, K, of 17,500 M −1 by the terpyrrole of this report vs. 80 M −1 in the case of DPQ. The present findings thus support the emerging notion that a range of pyrrole-based anion recognition and sensing systems may be produced by replacing the central quinoxaline spacer in DPQs by other bridging subunits and that these new receptors may have properties that differ dramatically from their DPQ ‘parents’.

A concave fluorescent sensor for anions based on 6-methoxy-1-methylquinolinium.

The ligand 2, in which three fluorogenic 6-methoxy-1-methylquinolinium fragments are appended to a mesityl platform, in MeCN forms 1:1 adducts with halides and other inorganic anions. (1)H NMR studies and molecular modelling indicate that 2 provides a cavity for anion inclusion and establishes electrostatic interactions with the guest. Anion inclusion induces quenching of the fluorogenic fragments with an efficiency decreasing along the series Br(-)>>I(-)>NCS(-)>>Cl(-)>NO(3) (-)>HSO(4) (-). The fluorimetric response of 2 to anions is orders of magnitude more sensitive than that of just 6-methoxy-1-methylquinolinium, ligand 1.

Slow anion exchange, conformational equilibria, and fluorescent sensing in venus flytrap aminopyridinium-based anion hosts.

The synthesis, anion binding, and conformational properties of a series of 3-aminopyridinium-based, tripodal, tricationic hosts for anions are described. Slow anion and conformational exchange on the (1)H NMR time scale at low temperature, coupled with NMR titration, results in a high level of understanding of the anion-binding properties of the compounds, particularly with respect to significant conformational change resulting from induced fit complexation. Peak selectivity for halides, particularly Cl(-), is observed. The approach has been extended to dipodal and tripodal podands based on 3-aminopyridinium "arms" containing photoactive anthracenyl moieties. The 1,3,5-tripodal host shows a remarkable selectivity for acetate over other anions, in contrast to the analogous unsubstituted tris(3-aminopyridinium) analogue, despite the fact that low-temperature (1)H NMR experiments reveal a total of four acetate-binding conformations. Photodimerization of anthracene units results in the formation of potential fluorescent anion sensors.

The design of fluorescent sensors for anions: taking profit from the metal–ligand interaction and exploiting two distinct paradigms

Fluorescent sensors are molecular systems consisting of a receptor moiety and of a fluorogenic fragment, which are capable of recognising a given analyte and signalling recognition through a variation of the emission intensity. The fluorogenic fragment responsible of the signal can be associated to the receptor either covalently or non-covalently, giving rise to two well distinct classes of fluorosensors and sensing paradigms. The design of fluorescent sensors is described, with a special attention to the sensing of anionic groups (including those of amino acids). In any case, it seems convenient that the receptor moiety contains one or more metal centres, which establish strong coordinative interactions with the envisaged anionic substrate. Selectivity is related to the energy of the metal–analyte interaction and can be achieved by taking profit of the concepts developed in more than one hundred years of coordination chemistry. As an example, recognition and sensing of the amino acid histidine is considered in detail, which is based on the attitude of the imidazole residue to deprotonate and bridge two MII ions prepositioned at the right distance, within a defined coordinative framework (M = Cu, Zn).

A new fluorescent fluoride chemosensor based on conformational restriction of a biaryl fluorophore

A new fluorescent anion sensor 1 , based on a biaryl–thiourea system, exhibits a fluorescence emission enhancement via conformational restriction upon a hydrogen bond-mediated complexation of fluoride anions.