Switching of intramolecular charge transfer in cruciforms: metal ion sensing

Three different cruciforms containing the 1,4-bis-4'-pyridylethynyl-2,5-distyrylbenzene or the 1,4-bis-phenylethynyl-2,5-distyrylbenzene unit were synthesized and investigated with respect to their metal sensing properties. Upon addition of metal cations to these cruciforms, either a bathochromic or hypsochromic shift in emission and absorption is observed. The shift depends on whether the metal coordinates preferentially to the pyridine or to the dibutylaniline branches of the cruciforms. The three cruciforms were exposed to a selection of metal cations in dichloromethane, and their emission was examined. The cruciforms show differential sensing of metal cations, that is, different metals can be specifically discerned upon exposure to three cruciforms but not when exposed to only one. The system can discern calcium from magnesium cations and silver from mercury or lithium cations.     

A selective colorimetric chemosensor for thiols based on intramolecular charge transfer mechanism

Compound 1 as an electron donor-acceptor compound with N,N-dimethylaniline and quinone units was designed for a highly selective colorimetric determination of thiol-containing amino acids and peptides, by making use of the unique reactivity of thiol towards quinone. Compound 1 shows a strong intramolecular charge transfer (ICT) band around 582 nm; but, it decreased after addition of either cysteine (Cys) or glutathione (GSH). Moreover, the ICT band intensity at 582 nm decreased linearly with the increasing concentrations of Cys or GSH. The interference from other amino acids can be neglected. Therefore, compound 1 can be employed as a selective colorimetric visual chemosensor for thiol-containing amino acids and peptides.     

A phenolic Schiff base for highly selective sensing of fluoride and cyanide via different channels

A chemosensor based on phenolic Schiff base bearing a pyrene group (sensor 1) has been synthesized and demonstrated. Sensor 1 showed a highly selective colorimetric response to fluoride anions based on a deprotonation process and fluorescent response to cyanide anions (606-fold fluorescence quantum yield enhancement) based on a cyclization process. Moreover, the cyclization of phenolic Schiff bases induced by cyanide could be used as a new way to synthesize 2-substituted benzoxazoles.     

Dipyrrole carboxamide derived selective ratiometric probes for cyanide ion

Two anion probes 1 and 2 featuring the dipyrrole carboxamide moiety for anion recognition have been prepared. These structurally simple anion probes display great selectivity for the cyanide anion over other common inorganic anions in semiaqueous environment via the formation of cyanohydrin derivatives. [structure: see text]     

Colorimetric fluoride ion sensing by boron-containing pi-electron systems.

The boron-containing pi-conjugated systems, including tri(9-anthryl)borane (1) and tris[(10-dimesitylboryl)-9-anthryl]borane (2), have been investigated as a new type of fluoride chemosensor. Upon complexation of 1 with a fluoride ion, a significant color change from orange to colorless was observed and, in the UV-visible absorption spectra, the characteristic band of 1 at 470 nm disappeared and new bands around 360-400 nm assignable to pi-pi transitions of the anthryl moieties were observed. This change can be rationalized as a result of the interruption of the pi-conjugation extended through the vacant p-orbital of the boron atom by the formation of the corresponding fluoroborate. The binding constant of compound 1 with the fluoride ion was quite high [(2.8 +/- 0.3) x 10(5) M(-1)], whereas 1 only showed small binding constants with AcO- and OH- of around 10(3) M(-1) and no sensitivity to other halide ions such as Cl-, Br-, and I-, thus demonstrating its selective sensing ability to the fluoride ion. In contrast to the monoboron system 1, compound 2 having four boron atoms showed multistage changes in the absorption spectra by the stepwise complexation with fluoride ions.     

An intramolecular charge transfer fluorescent probe: synthesis and selective fluorescent sensing of Ag+

An intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino) benzamide, has been designed and synthesized. The ions-selective signaling behaviors of the probe were investigated. Upon the addition of Ag+, an overall emission enhancement of 14-fold was observed. Compound 1 displayed highly selective chelation enhanced fluorescence (CHEF) effect with Ag+ over alkali, alkali earth metal ions and some transition metal ions in aqueous methanol solutions. The prominent selective and efficient fluorescent enhancing behavior could be utilized as a new chemosensing probe for the analysis of Ag+ ion in aqueous environment.     

The sensing mechanism of fluoride anion for a novel molecule D2: Desilylation and intramolecular charge transfer

In this work, the sensing mechanism of a new fluoride chemosensor 12‐([tert‐butyldiphenylsilyl]oxy)‐8a,13a‐dihydro‐7H‐benzo[de]benzo[4,5]imidazo[2,1‐a]‐isoquinolin‐7‐one (abbreviated as D2) is investigated using density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. The theoretical electronic spectra (vertical excitation energies and fluorescence peak) reproduced previous experimental results (D. Li et al., Spectrochim. Acta A Mol. Biomol. Spectrosc. 2017 , 185, 173), which confirms the rationality of the theoretical level used in this work. The constructed potential energy curve of the desilylation process suggests that the low barrier could be responsible for the rapid response to fluoride anions. Analyses of the binding energies show that only fluoride anion can be detected by D2 chemosensor in dimethylsulfoxide (DMSO). In view of the excitation process, the strong intramolecular charge transfer (ICT) process of the S₀ → S₁ transition explains the red shift of the absorption peak of the D2 sensor with the addition of fluoride anions. This work not only presents a straightforward sensing mechanism of sensing of the fluoride anion by the D2 chemosensor but should also play an important role in the synthesis and design of fluorescent sensors in future.     

A novel fluoride selective optical chemosensor based on internal charge transfer signaling

A new internal charge transfer probe, NAPH-1, synthesized by incorporating photoemitive naphthalimide core with an acidic imidazolium ring, offers highly selective colorimetric and ratiometric ‘off-on’ signaling for targeting F⁻, while Cl⁻, Br⁻, I⁻, , SCN⁻, AcO⁻, and do not appreciably perturb the photophysical properties of the probe even at relatively higher concentrations than the F⁻. Deprotonation of the imidazolium ring, supported by the ¹H NMR and theoritical studies, seems to cause the spectral modulations.     

Colorimetric and ratiometric fluorescence sensing of fluoride ions based on competitive intra- and intermolecular proton transfer

Receptors 1 and 4 show fluoride ion selective changes in their absorbance and emission behaviours amongst F⁻, Cl⁻, Br⁻, I⁻, , CH₃COO⁻, , and anions. Fluoride ion mediated ‘ON-OFF-ON’ switching behaviour of 4 provides opportunities for ratiometric estimation of fluoride ions.     

Tunable fluorescent sensing of cysteine and homocysteine by intramolecular charge transfer

An amphiphilic oligo p-phenylene derivative (DCHO) bearing electron-donating group (–NH(CH₂)₂OH) and electron-withdrawing group (–CHO) has been synthesised and characterised. The sensing characteristics of this probe (DCHO) for cysteine (Cys) and homocysteine (Hcy) are studied in a mixture solution of DMSO–HEPES by UV–vis and fluorescence spectra. ¹H NMR, MALDI-TOF and UV–vis titration experiments proved that thiazolidine and thiazinane derivatives were formed. The highly Cys/Hcy-selective fluorescence hypsochromic shift (>110 nm) can be observed due to the switching of intramolecular charge transfer, leading to potential fabrication of ratiometric fluorescent detection of Cys/Hcy.     

Colorimetric fluoride ion sensing by polyborylated ferrocenes: structural influences on thermodynamics and kinetics

The thermodynamic factors underlying the use of ferroceneboronic esters as electrochemical or colorimetric fluoride ion sensors have been investigated through the synthesis of a range of systematically related derivatives differing in the number/nature of the boronic ester substituents and in the nature of ancillary ligands. Thus, if the shift in electrochemical potential associated with the conversion of one (or more) boronic ester group(s) to anionic boronate(s) on fluoride binding is sufficient to allow oxidation of the resulting host/guest complex by dioxygen, colorimetric sensing is possible. In practice, while monofunctional systems of the type CpFe[eta(5)-C(5)H(4)B(OR)(2)] offer selectivity in fluoride binding, electrochemical shifts in chloroform solution are insufficient to allow for a colorimetric response. Two chemical modification strategies have been shown to be successful in realizing a colorimetric sensor: (i) the use of the more strongly electron-donating Cp(*) ancillary ligand (which shifts the oxidation potential of both the free receptor and the resulting fluoride adduct cathodically by ca. -400 mV) and (ii) receptors featuring two or more binding sites and consequently a larger fluoride-induced electrochemical shift. Thus, in the case of [eta(5)-C(5)H(4)B(OR)(2)](2)Fe [(OR)(2) = OC(H)PhC(H)PhO, 2(s)], the binding of 2 equiv of fluoride gives an electrochemical shift (in chloroform) of -960 mV (cf. -530 mV for the corresponding monofunctional analogue, 1(s)). Related tris- and tetrakis-functionalized systems are also shown to be oxidized as the bis(fluoride) adducts, presumably because of fast oxidation kinetics, relative to the rate of the (electrostatically unfavorable) binding of a third equivalent of fluoride. Furthermore, the rate of sensor response (as measured by UV/vis spectroscopy) is found to be strongly enhanced by the presence of pendant (uncomplexed) three-coordinate boronic ester functions (e.g., a rate enhancement of 1-2 orders of magnitude for 3(s)/4(s) with respect to 2(s)) and/or delocalized aromatic substituents.     

Colorimetric determination of the fluoride ion based on interference with extractability

Summary The sensitivity of colorimetric determinations based on reduction of extinction by complex-forming anions can be increased by measuring the change of absorbancy in the organic phase after shaking with a suitable solvent, i. e. by determining the interference of the anion with extractability. This principle has been applied to the determination of the fluoride ion.

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Zusammenfassung Die Empfindlichkeit kolorimetrischer Bestimmungen, die auf Herabsetzung der Extinktion durch komplexbildende Anionen beruhen, kann durch Messung der Absorptionsänderung in der organischen Phase nach Schütteln mit einem geeigneten Lösungsmittel, das heißt durch Bestimmung der durch das Anion verminderten Extrahierbarkeit, erhöht werden. Dieses Prinzip wurde auf die Bestimmung von Fluoridion angewendet.

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Résumé La sensibilité des déterminations colorimétriques basée sur l'effet de diminution d'extinction dû à des anions donnant lieu à la formation de complexes peut être accrue par la variation de l'absorption dans la phase organique après agitation avec un solvant approprié, ce qui revient à déterminer l'effet d'empêchement d'extraction dû à l'anion. Ce principe a été appliqué au dosage de l'ion fluorure.

     

Selective fluorescent probes based on CN isomerization and intramolecular charge transfer (ICT) for zinc ions in aqueous solution

As the second most abundant transition-metal ion in the human body, Zn2+ plays crucial roles in many important biological processes; while in the environment, an excessive concentration of Zn2+ may reduce the soil microbial activity resulting in phytotoxic effects. Therefore, developing effective and sensitive detection method for Zn2+ has become crucially important and necessary both in life and environment science. Two new fluorescence probes, 2-((2-hydroxynaphthalen-1-yl)methyleneamino)-3-(1H-imidazol-5-yl) propanoic acid (2) and 2-hydroxy-2-((2-hydroxynaphthalen-1-yl) methyleneamino) acetic acid (3), were easily prepared by a one step reaction between 2-hydroxy-1-naphthaldehyde with histidine and serine, respectively, in ethanol. The optical properties of them were investigated by fluorescence spectra, which displayed specific and sensitive recognition to Zn2+ and especially avoided the interference of Cd2+ when they were tested against a range of physiological and environmentally relevant metal ions in aqueous solution. The responsive mechanism of the two probes to Zn2+ were involved both the CN isomerization and ICT, which were clarified by NBO charge analysis and the HOMO-LUMO energy gap calculation by using B3LYP/6-31G density functional theory.     

Colorimetric and ratiometric fluorescence sensing of fluoride: tuning selectivity in proton transfer

[reaction: see text] Phenyl-1H-anthra[1,2-d]imidazole-6,11-dione (1) and its derivatives (2 and 3) have been investigated as new colorimetric and ratiometric fluorescent chemosensors for fluoride. Acute spectral responses of 1 and 3 to fluoride in acetonitrile have been observed: an approximately 100 nm red shift in absorption and fluorescence emission and a very large ratiometric fluorescent response (Rmax/Rmin is 88 for sensor 1 and 548 for sensor 3). From the changes in the absorption, fluorescence, and 1H NMR titration spectra, proton-transfer mechanisms have been deduced. In ground states, a two-step process has been observed: first, the formation of the sensor-fluoride hydrogen-bond complex [LH...F]- and then the fluoride-induced deprotonation of the complex to form L- and FHF-. In excited states, the excited-state intermolecular proton-transfer made a contribution to the deprotonation. The selectivity for F- can be tuned by electron push-pull properties of the substituents on the phenyl para position of the sensors. Sensor 1 shows the best selectivity. The excellent selectivity of 1 for F- is attributed to the fitness in the acidity of its NH-group, which is tuned to be able to distinguish the subtle difference in the affinity of F-, CH3CO2(-), and H2PO4(-) to proton.     

Amide-nitrophenyl based colorimetric receptors for selective sensing of fluoride ions

New chromogenic receptors containing 2-nitrophenyl or 3,5-dinitrophenyl groups appended to the amide or in secondary amine positions have been synthesized and characterized. Upon addition of fluoride to two of the receptors in acetonitrile, the solution acquired a yellow colour. The third receptor showed an intense purple colour with fluoride in acetonitrile and the appearance of the purple colour can be detected by the naked eye at parts per million level. The addition of chloride, bromide and iodide to the receptors did not induce any colour. Thus the receptors can act as fluoride ion sensors even in the presence of other halide ions.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

Reaction-based sensing of fluoride ions using desilylation method for triggering excited-state intramolecular proton transfer

Naphthalene based benzothiazole (NBT) has been investigated as new colorimetric and ratiometric fluorescent chemosensor for fluoride. The selectivity of NBT has been explored based on combination of desilylation reaction and modulation of the excited-state intramolecular proton transfer (ESIPT) from the desilylation product to F^?. The method exhibited a high selectivity and a great sensitivity toward fluoride anions through ‘turn-on’ chromogenic activity and sensitivity. The structure of HNBT has been established by single-crystal XRD. Density functional theory and TDDFT calculations were performed in order to demonstrate the electronic properties of HNBT, NBT and their anion. Upon treatment with fluoride in aqueous CH₃CN solution, the TBS protective group of probe NBT was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 415 nm and an increase in a new fluorescence peak around 586 nm. An easy-to-prepare test paper, obtained by dipping the paper into the solution of NBT, was able to detect fluoride ions in practical samples. The detection limit of the probe in the determination of fluoride ions was 10.18 μM.     

A colorimetric chemosensor for fast detection of thiols based on intramolecular charge transfer

A TCNQ-based triphenylamine derivative (1) with strong intramolecular charge transfer (ICT) character was designed as a colorimetric sensor for thiols. Upon addition of thiols, the absorption spectrum of sensor 1 in aqueous solution was remarkably changed because the adduct of 1 with thiol decreased its ICT character. Sensor 1 showed highly selective and sensitive sensing ability toward thiols over other analytes. This selectivity could be easily observed by naked eyes, indicating that sensor 1 is a potential colorimetric sensor for thiols.     

Colorimetric and fluorometric dual-modal probes for cyanide detection based on the doubly activated Michael acceptor and their bioimaging applications

In this study, we synthesized CTB and CB probes based on doubly activated Michael acceptors to selectively detect cyanide (CN⁻) anions through a one-step condensation reaction of coumarinyl acrylaldehyde with the corresponding derivatives of malonyl urea (thiourea). Through the conjugated addition of CN⁻ to the β-site of the Michael acceptor, both probes displayed colorimetric and fluorometric dual-modal responses that were highly reactive and selective. CTB generates an active fluorescent response, whereas CB displays a ratiometric fluorescent response. The fluorescent signal of the probes reached its maximum given only 1 CN⁻ equivalent and the signal change was linearly proportional to CN⁻ concentrations ranging from 0 to 5 μM with the detection limits 18 and 23 nM, respectively. The reaction rate of the probes is highly dependent on the methylene acidity of malonyl urea derivatives. Thus, the response rate of CTB to CN⁻ is 1.2-fold faster than that of CB, and the response rate of CB to CN⁻ is 1.2-fold faster than that of the previously examined CM. We then verified the highly reactive nature of the β-site of the probes through density functional reactivity theory calculations. In addition, according to proof-of-concept experiments, these probes may be applied to analyze CN⁻ contaminated water and biomimetic samples. Finally, cell cytotoxicity and bioimaging studies revealed that the probes were cell-permeable and could be used to detect CN⁻ with low cytotoxicity.