The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2 Response

Substituent effects of halogens on the excited-state intermolecular proton transfer reactions

Fluorescent aromatic urea compounds undergo excited-state intermolecular proton transfer (ESPT) in the presence of acetate anions to produce an excited state of the tautomer (T*) from the excited state of the complex (N*), resulting in dual fluorescence. Herein, we performed spectroscopic measurements of anthracen-1-yl-3-phenylurea derivatives with substituents, -CF3, -F, or -Cl, at the p-position of the phenyl group in the presence of acetate to investigate the substituent effects on the ESPT reaction and the deactivation processes of N* and T*. Kinetic analysis showed that the reverse ESPT rate constant (k-PT) depended on the respective substituents, suggesting that each substituent may influence the reverse ESPT process differently. In particular, since the electron-withdrawing properties of -F are estimated by the - I and + Iπ effects, it is plausible that -F has a slight electron-donating property and influences the reverse process from T* to N* in the excited state. This study shows that it is possible to control emission by selecting specific substituents in the ESPT system.

New type of tin(IV) complex based turn-on fluorescent chemosensor for fluoride ion recognition: elucidating the effect of molecular structure on sensing property

A novel type of chemosensor based on tin(IV) complexes incorporating hydroxyquinoline derivatives has been designed and investigated for selectively detecting fluoride ions. Sn(meq)2Cl2 (meq = 2-methyl-8-quinolinol) (complex 1) exhibits a significant enhancement in luminescence upon the introduction of fluoride ions. This enhancement greatly surpasses that observed with Snq2Cl2 and Sn(dmqo)2Cl2 (q = 8-hydroxyquinnoline; dmqo = 5,7-dimethyl-8-quinolinol). Furthermore, complex 1 displays excellent sensitivity and selectivity for fluoride detection in comparison to halides and other anions. As a result, complex 1 serves as an outstanding turn-on fluorescent chemosensor, effectively sensing fluoride ions. The Benesi-Hilderbrand method and Job's plot confirmed that complex 1 associates with F- in a 1 : 2 binding stoichiometry. Also, complex 1 exhibited a large binding constant (pKb = 10.4 M-2) and a low detection limit (100 nM). To gain a deeper insight into the photophysical properties and the underlying mechanism governing the formation of the tin(IV) fluoride complex via halide exchange, we successfully synthesized partially fluorinated Sn(meq)2F0.67Cl1.33 (2) and fully fluorinated Sn(meq)2F2 (3), all of which were characterized through computational studies, thereby elucidating their photophysical properties. DFT studies reveal that converting Sn(meq)2Cl2 to Sn(meq)2F2, an endergonic process, leads to greater stability due to reducing steric hindrance about the metal center. Furthermore, the fluorinated complex significantly increases dipole moment, resulting in high affinity toward the F- ion.

Luminescent Ruthenium-Terpyridine Complexes Coupled with Stilbene-Appended Naphthalene, Anthracene, and Pyrene Motifs Demonstrate Fluoride Ion Sensing and Reversible Trans-Cis Photoisomerization

A new family of luminescent heteroleptic Ru(II)-terpyridine complexes coupled with stilbene-appended naphthalene, anthracene, and pyrene motifs is reported. Each of the complexes features moderately intense emission at room temperature having a lifetime of 16.7 ns for naphthalene and 11.4 ns for anthracene, while a substantially elevated lifetime of 8.3 μs was observed for the pyrene derivative. All the three complexes display a reversible couple in the positive potential window due to Ru2+/Ru3+ oxidation but multiple reversible and/or quasi-reversible peaks in the negative potential domain because of the reduction of the terpyridine moieties. All the complexes selectively sense F- among the studied anions via the intermediary of different noncovalent interactions. The interaction event is monitored through absorption, emission, and 1H and 19F NMR spectroscopy. Additionally, upon utilizing the stilbene motif, reversible trans-cis isomerization of the complexes has been undertaken upon alternate treatment of visible and UV light so that the complexes can act as potential photomolecular switches. We also carried out the anion sensing characterization of the cis form of the complexes. Theoretical calculation employing density functional theory is also executed for a selective complex (naphthalene derivative) to elucidate different noncovalent interactions that are operative during the complex-fluoride interplay.

Thiourea-Based Receptors for Anion Recognition and Signaling

This work reports on two thiourea-based receptors with pyridine and amine units including 1-naphthyl (MT1N) and 4-nytrophenyl (MT4N) as signaling units. For both compounds, their affinity and signaling ability toward various anions of different geometry and basicity in DMSO were studied using UV-vis, fluorescence, and 1H NMR techniques. Anion recognition studies revealed that both MT1N and MT4N have, in general, high affinities toward basic anions. In this regard, a higher acidity of the MT4N receptor was demonstrated. Furthermore, MT4N has a higher affinity for fluoride (log K1 = 5.98) than for the other anions and can effectively detect it through colorimetric changes that can be monitored by the UV-vis technique. The interaction between receptors and anions mainly involves the hydrogens of the amino and thiourea groups of the former. Complementary single-crystal X-ray diffraction studies and molecular modeling at the DFT level were also performed.

A neutral rhenium-biimidazole complex for the selective recognition of fluoride ions

The neutral rhenium(I)-biimidazole complex [Re(CO)₃(biimH)(1,4-NVP)] (1) was designed and synthesized by a one-pot reaction of Re₂(CO)₁₀, 2,2'-biimidazole (biimH₂) and 4-(1-naphthylvinyl)pyridine (1,4-NVP). The structure of 1 was characterized by various spectroscopic techniques including IR, ¹H NMR, FAB-MS, and elemental analysis and further confirmed by a single-crystal X-ray diffraction analysis. The mononuclear complex 1, a relatively simple structure with an octahedral geometry, is comprised of facial-arranged carbonyl groups, one chelated biimH monoanion, and one 1,4-NVP. Complex 1 shows the lowest energy absorption band at around 357 nm and an emission band at 408 nm in THF. The luminescent characteristics of 1 combined with the hydrogen bonding ability of the partially coordinated monoionic biimidazole ligand permits the complex to selectively recognize fluoride ions (F^-) in the presence of other halides through a dramatic luminescence enhancement. The recognition mechanism of 1 can be convincingly explained in terms of H-bond formation and proton abstraction upon the addition of F^- ions by ¹H and ¹⁹F NMR titration experiments. The electronic properties of 1 were further supported by time dependent density functional theory (TDDFT) computational studies.

Interaction of Ions in Organic and Aqueous Media with an Ion-Pair Sensor Equipped with a BODIPY Reporter: An ON1-OFF-ON2-ON3 Fluorescent Assay

Here, we present a ditopic ion-pair sensor, B1, containing the BODIPY reporter unit in its structure, which is shown to be able-thanks to the presence of two heterogeneous binding domains-to interact with anions in an enhanced manner in the presence of cations. This enables it to interact with salts even in 99% aqueous solutions, making B1 a good candidate in terms of visual salt detection in the aquatic environment. Receptor B1's ability to extract and release salt was applied in the transport of potassium chloride through a bulk liquid membrane. Working with a concentration of B1 in the organic phase and with the presence of a specific salt in an aqueous solution, an inverted transport experiment was also demonstrated. By varying the type and the amount of the anions added to B1, we were able to develop diverse optical responses, including a unique four-step ON1-OFF-ON2-ON3 output.

"Vermellogens" and the Development of CB[8]-Based Supramolecular Switches Using pH-Responsive and Non-Toxic Viologen Analogues

We present herein the "vermellogens", a new class of pH-responsive viologen analogues, which replace the direct linking between para-substituted pyridinium moieties within those by a hydrazone functional group. A series of such compounds have been efficiently synthesized in aqueous media by hydrazone exchange reactions, displaying a marked pH-responsivity. Furthermore, the parent N,N'-dimethylated "vermellogen": the "red thread", an analogue of the herbicide paraquat and used herein as a representative model of the series, showed anion-recognition abilities, non-reversible electrochemical behavior, and non-toxicity of the modified bis-pyridinium core. The host-guest chemistry for the "red thread" with the CB[7,8] macrocyclic receptors has been extensively studied experimentally and by dispersion corrected density functional theory methods, showing a parallel behavior to that previously described for the herbicide but, crucially, swapping the well-known redox reactive capabilities of the viologen-based inclusion complexes by acid-base supramolecular responsiveness.

Tritopic Bis-Urea Receptors for Anion and Ion-Pair Recognition

This work reports on the synthesis and characterization of three tritopic receptors and their binding properties toward various anions, as their tetrabutylammonium salts, and three alkali metal-acetate salts by UV-vis, fluorescence, ¹H, ⁷Li, ²³Na, and ³⁹K NMR in MeCN/dimethyl sulfoxide (DMSO) 9:1 (v/v). Molecular recognition studies showed that the receptors have good affinity for oxyanions. Furthermore, these compounds are capable of ion-pair recognition of the alkali metal-acetate salts studied through a cooperative mechanism. Additionally, molecular modeling at the density functional theory (DFT) level of some lithium and sodium acetate complexes illustrates the ion-pair binding capacity of receptors. The anion is recognized through strong hydrogen bonds of the NH- groups from the two urea sites, while the cation interacts with the oxygen atoms of the polyether spacer. This work demonstrates that these compounds are good receptors for anions and ion pairs.

Electronic state of a fluoranthene–urea compound and the kinetics of its emissive tautomer state in the presence of acetate anions

The fluorescence spectrum of 3FU–Ac around 600 nm agrees well with that of 3FU–DBU, indicating that the electronic state of tautomer has a proton-abstracted structure.

An ESIPT-ICT steered naphthylthioic-based ionic probe with dual emissive channels exhibiting CHEF and CHEQ effects

A naphthylthioic-based emissive probe (M) bearing a hydroxyl and amine group was designed and synthesized via a one-step Schiff base reaction process. The probe was characterized spectroscopically using ¹H NMR, UV-Vis and fluorescence spectrophotometers. The probe turned out to be spectroscopically and colorimetrically selective and sensitive to multiple cations and anions. Interestingly, the probe displayed characteristics of excited-state intramolecular proton transfer (ESIPT)-driven dual emissive channels; experiencing fluorescence enhancement upon the molar additions of Al³⁺ as well as the anions used, events presumably ascribed to chelation fluorescence enhancement (CHEF), hydrogen bonding and deprotonation effects. Moreover, the fluorometric titration with Hg²⁺ resulted in ratiometric spectral behaviors of M, with the disappearance of the peak at 450 nm, concomitant with the appearance of a new peak at 520 nm, distinguished by a clear isosbestic point, the same behaviors exhibited by Sn²⁺ and Ag⁺ analytes towards M. The introduction of all other cations used, resulted in fluorescence quenching, attributable to chelation enhanced fluorescence quenching (CHEQ), thereby inhibiting the ESIPT process. The experiments were all carried out in the aqueous environment medium of DMSO–H₂O (9 : 1) at ambient temperature. Theoretical density functional theory calculations were carried out to gain insight into the interaction of M with cations and anions, and their influence on the HOMO–LUMO energy gaps.

A naphthylthioic-based emissive probe (M) bearing a hydroxyl and amine group was designed and synthesized via a one-step Schiff base reaction process.

Fluorescence Recognition of Anions Using a Heteroditopic Receptor: Homogenous and Two-Phase Sensing

In contrast to monotopic receptor 3, the anthracene functionalized squaramide dual-host receptor 1 is capable of selectively extracting sulfate salts, as was evidenced unambiguously by DOSY, mass spectrometry, fluorescent and ion chromatography measurements. The receptors were investigated in terms of anion and ion pair binding using the UV-vis and 1H NMR titrations method in acetonitrile. The reference anion receptor 3, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by the presence of cations. Besides the ability to bind anions in an enhanced manner exhibited by ion pair receptors 2 and 4, changing the 1-aminoanthracene substituent resulted in their exhibiting a lower anion affinity than receptor 1. By using receptor 1 and adjusting the water content in organic phase it was possible to selectively detect sulfates both by "turn-off" and "turn-on" fluorescence, and to do so homogenously and under interfacial conditions. Such properties of receptor 1 have allowed the development of a new type of sensor capable of recognizing and extracting potassium sulfate from the aqueous medium across a phase boundary, resulting in an appropriate fluorescent response in the organic solution.

Environment-sensitive emission of anionic hydrogen-bonded urea-derivative-acetate-ion complexes and their aggregation-induced emission enhancement

Anions often quench fluorescence (FL). However, strong ionic hydrogen bonding between fluorescent dyes and anion molecules has the potential to control the electronic state of FL dyes, creating new functions via non-covalent interactions. Here, we propose an approach, utilising ionic hydrogen bonding between urea groups and anions, to control the electronic states of fluorophores and develop an aggregation-induced emission enhancement (AIEE) system. The AIEE ionic hydrogen-bonded complex (IHBC) formed between 1,8-diphenylnaphthalene (p-2Urea), with aryl urea groups at the para-positions on the peri-phenyl rings, and acetate ions exhibits high environmental sensitivities in solution phases, and the FL quantum yield (QY) in ion-pair assemblies of the IHBC and tetrabutylammonium cations is more than five times higher than that of the IHBC in solution. Our versatile and simple approach for the design of AIEE dye facilitates the future development of environment-sensitive probes and solid-state emitting materials.

Optical Anion Receptors with Urea/Thiourea Subunits on a TentaGel Support

Two simple chemosensors with urea (L1) and thiourea (L2) groups were synthesized and studied by different spectroscopic techniques. Both receptors can sense acetate (Ac^-), dihydrogen phosphate (H₂PO₄ ^-), and fluoride (F^-) anions, accompanied by changes in UV-vis and ¹H NMR spectra, and an optical response is observed as a color change of the solutions due to deprotonation and hydrogen-bonding processes. Also, L1 and L2 were supported on TentaGel resins (R1 and R2), and their fluoride-sensing properties in DMSO and water solutions were studied. Interestingly, R2 can sense fluoride ions in sample solutions of 100% water.

Sensing mechanism of fluorogenic urea with fluoride in solvent media: A new fluorescence quenching mechanism

The interaction of 1-Phenyl-3-(pyren-1-yl) urea (LH) and fluoride anion (F^-) with a unique ON¹-OFF-ON² fluorescent response has been investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The hydrogen-bonding dynamics and photophysical properties of the complex LH-F, as well as its isolated receptor LH and anion form L-H₁, have been studied in detail. We demonstrate that the intermolecular hydrogen bond (N-H…F) of the complex LH-F is greatly enhanced in the electronically excited state. The nonradiative deactivation via electron transfer and internal conversion rather than excited-state intramolecular proton transfer (ESIPT) can be facilitated by the excited state hydrogen bond strengthening. The results have been cross-validated by molecular structure, electronic spectra, frontier molecular orbitals, and infrared spectra as well as hydrogen bond binding energy. These results indicate that the current calculations completely reproduce the experimental results and provide compelling evidence for the sensing mechanism of LH for F^-.

Dynamic covalent bond constrained ureas for multimode fluorescence switching, thermally induced emission, and chemical signaling cascades

A combination of organic ureas and dynamic covalent chemistry was demonstrated for multistate switching, thermally induced fluorescence, and signaling cascades.

A colorimetric chemosensor for pyrophosphate based on mono-pyrenylurea in aqueous media

Research on pyrophosphate ions detection remains important because it plays crucial roles in various fields. A simple and new colorimetric sensor for pyrophosphate (PPi) based on mono-pyrenylurea ligand (L) has been designed and synthesized by a simple reaction of 1-pyrenemethylamine hydrochloride with p-nitrophenylisocyanate. In DMSO-15% H₂O solution and DMSO-15% HEPES (10 mM, pH = 7.2) buffer solution, L displayed a selective colorimetric response for pyrophosphate (PPi) against other anions by changing color from colorless to yellow. This recognition process was confirmed by UV-vis spectroscopy. Also, the colorimetric properties of L are attributed to the anion-induced deprotonation of the urea subunit as demonstrated by ¹H NMR titration method. Moreover, convenient test strips coated with L could be utilized to detect PPi in aqueous solution by naked-eye.

Effect of Moderate Hydrogen Bonding on Tautomer Formation via Excited-State Intermolecular Proton-Transfer Reactions in an Aromatic Urea Compound with a Steric Base

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), which forms weak hydrogen bonds despite the high basicity caused by its hindered structure, was used to investigate tautomer formation via excited-state intermolecular proton-transfer (ESPT) reactions. The kinetics of the ESPT reactions of anthracen-2-yl-3-phenylurea (2PUA) in the presence of DBU were compared to that observed for the acetate anion (Ac) using time-resolved fluorescence measurement. Based on the association constants in the ground state, the intermolecular hydrogen bond between 2PUA and DBU was less stable than the bond between 2PUA and Ac due to steric hindrance and the geometry of the hydrogen bond. In the fluorescence spectra, 2PUA-DBU displayed prominent tautomeric emission in chloroform (CHCl₃), whereas 2PUA-Ac exhibited distinct tautomeric emissions in dimethyl sulfoxide (DMSO). Kinetic analysis revealed that the rate constant of the ESPT reaction of 2PUA-DBU remarkably decreased when the proton-accepting ability of the solvent increased whereas the reaction of 2PUA-Ac was linked to the solvent polarity rather than proton-accepting ability. These results indicated that moderate hydrogen bonds due to steric hindrance were influenced by the type of solvent present, particularly if the solvents exhibited proton-accepting capabilities like DMSO. This, in turn, affected the rate constant of tautomer formation.

Theoretical study on the sensing mechanism of an ON1-OFF-ON2 type fluoride fluorescent chemosensor

A density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been used to study the sensing mechanism of an ON¹-OFF-ON² type fluoride anion fluorescent chemosensor (Bis[[7-(diethylamino)-2-oxo-2H-chromene]methyl-ene]‑carbonothioic dihydrazide (CTC). The current theoretical calculation presents a different sensing mechanism from the experimentally proposed one (Sensor and Actuators B 2016, 222, 823-828). Instead of the combination of CTC deprotonation and poorly emissive excited state tautomer or ICT mechanism, the theoretical results predict the sensing mechanism based on dissociation reaction and excited-state proton transfer (ESPT). The calculated vertical excitation energies both in the ground states and first excited states of different forms of CTC, as well as the potential-energy curves, have completely reproduced the experimental results, providing powerful evidence for our proposed CTC sensing mechanism for fluoride anion.

End-group functionalization of a conjugated azomethine with ureas for property tailoring

A conjugated azomethine end-capped with phenyl-ureas was prepared by a convergent approach. Hydrogen bonding with the ureas was confirmed and the spectroscopic properties could be enhanced with the supramolecular bonds.

A Dual Emissive Coumarin-urea Derivative with an Electron-withdrawing Substituent in the Presence of Acetate Anion

We investigated the fluorescent properties, including the excited-state intermolecular proton transfer, of urea derivatives comprising a coumarin ring, which is a widely used fluorophore. We prepared two different coumarin-urea derivatives, 6CU and 7CU, which bear a urea-based substituent at the 6 and 7 positions of a coumarin ring, respectively. In the presence of the acetate ion, 7CU showed additional tautomer fluorescence emission with respect to 6CU, indicating that tautomer formation depends on the positions of the urea-based substituent on the coumarin ring. Thus, the resonance structures of urea derivatives might play an important role in the behavior of dual fluorescence, which is an important phenomenon applicable to photochemical anion sensing. Moreover, in order to further improve the fluorescence properties of the mentioned derivatives, a CF₃ group was introduced in a phenyl ring opposite to a coumarin ring. The fluorescence quantum yield of 7CUCF₃ thus synthesized was 65 times as large as that of 7CU, an observation that renders 7CUCF₃ a suitable anion sensor candidate. The results of this study will contribute to the development of new molecular designs for highly fluorescent sensing.

Multiply aryl-substituted dipyrrolyldiketone boron complexes exhibiting anion-responsive emissive properties

Multiply aryl-substituted pyrrole-based anion-responsive π-electronic molecules were synthesized. The effects of the introduced aryl moieties on solution-state emissive behaviours were examined, depending on the solvent polarity and anion binding.

An integrated urea and halogen bond donor based receptor for superior and selective sensing of phosphates

A new RuII based bis-heteroleptic ditopic receptor, 1[PF6]2 (C44H33F12IN10OP2Ru), having integrated anion binding iodotriazole (halogen bond donor) and urea units (-NH bond donor) is employed for selective sensing of phosphates (e.g., H2PO4- and HP2O73-). 1[PF6]2 showed superiority in phosphate sensing in CH3CN as compared to its hydrogen bond donor analogue, 2[PF6]2 (C44H34F12N10OP2Ru), non-urea halogen bond analogue, 3[PF6]2 (C38H27F12IN8P2Ru) and non-urea hydrogen bond donor analogue, 4[PF6]2 (C38H28F12N8P2Ru) in terms of enhanced binding constant values, longer excited state lifetimes and lower detection limit values. 1H-NMR, Isothermal Titration Calorimetry (ITC) and photophysical studies revealed the implementation of the combined role of both the halogen bond donor iodotriazole unit and the -NH unit of the urea moiety for selective and enhanced binding of phosphates.

A novel approach to white-light emission using a single fluorescent urea derivative and fluoride

First observation of white fluorescence using a single fluorescent urea derivative and fluoride.

Novel Luminescent Ionic Adducts Based on Pyrene-1-sulfonate

The potential of pyrene-1-sulfonate to act as an emitting anion for the development of ionic liquids is explored here. Amphiphilic trimethylpropylammonium hepta(isooctyl)octasilsesquioxane and conventional imidazolium, namely, 1-vinyl-3-hexyl-, 1-vinyl-3-decyl-, and 1-methyl-3-decyl-imidazolium, featuring moderate alkyl chain length substituents, have been chosen as countercations. The new species have been synthesized via simple metathesis reactions involving pyrene-1-sulfonate sodium salt and the appropriate halide cation precursors. Their thermal behavior has been investigated by thermogravimetric and differential scanning calorimetry at different scanning rates. According to this latter technique, only the trimethylpropylammonium hepta(isooctyl)octasilsesquioxane pyrenesulfonate adduct, displaying a reversible glass transition at -4.2 °C, may be classified as an ionic liquid. All pyrene-1-sulfonate imidazolium-based ion pairs are crystalline solids with the melting point just above 100 °C that produce very complex, nonreversible, and scanning rate-dependent thermograms, very likely arising from polymorphism phenomena. Such a behavior may be attributed to the pyrene-1-sulfonate polycyclic system, which in solution, as confirmed through spectroscopic characterization, displays a general attitude in promoting supramolecular structures via cation interactions. Emission lifetime measurements on the emitting fluorophore reveal that there are at least two different active species, whereas light scattering measurements show the presence of aggregates with hydrodynamic radii depending on the medium and adduct concentration. Tests aimed at investigating the potential of these novel pyrene-1-sulfonate salts in functionalization/exfoliation of graphite flakes are also reported here.

Monitoring ADP and ATP in vivo using a fluorescent Ga(iii)-probe complex

A naphthol-based sensor (L) was designed and synthesized for the specific recognition of Ga3+ using fluorescence enhancement. An in situ generated L-Ga3+ ensemble detected ADP and ATP more selectively through a fluorescence "switch off" response, which was confirmed both in cells and in adult zebrafish.

Kinetic analysis of tautomer forms of aromatic-urea compounds with acetate ions: solvent effect of excited state intermolecular proton transfer

In this paper, we report the solvent effect of excited state intermolecular proton transfer (ESIPT) reactions of urea compounds in the presence of tetrabutylammonium acetate (TBAAc). We prepared anthracene-urea compounds (9An and 2An), a pyrene-urea compound (Py) and an anthracene-diurea compound (9,10An), which have alkylsulfonyl groups to improve their solubility in various organic solvents. We investigated the solvent effects of the ESIPT reaction using absorption, fluorescence, and 1H NMR spectroscopy along with fluorescence decay measurements in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), tetrahydrofuran (THF) and toluene. The tautomer fluorescence of 9An showed remarkable solvent dependence on the spectral red-shift compared with 2An, Py and 9,10An. As a result of the detailed spectroscopic investigations with regard to the solvent including kinetic analysis of the ESIPT for 9AnAcO-, we revealed that the energy gap between the normal and tautomer forms in the excited state depended on the hydrogen bond acceptor basicity (β), which is one of the Kamlet-Taft solvent parameters. Finally, we discovered that the tautomer structures of aromatic-urea compounds were stabilized by hydrogen bond interactions.

Fluoride-Catalyzed Deblocking: A Route to Polymeric Urethanes

We report a fluoride-catalyzed deblocking of urethanes as "blocked" isocyanates. Organic and inorganic sources of fluoride ion proved effective for deblocking urethanes and for converting polyurethanes to small molecules. Distinct from conventional deblocking chemistry involving organometallic compounds and high temperatures, the method we describe is metal-free and operates at or slightly above room temperature. The use of fluorescent blocking agents enabled visual and spectroscopic monitoring of blocking/deblocking reactions, and the selected conditions proved applicable to urethanes containing a variety of blocking groups. The method additionally enabled a one pot deblocking and polymerization with α,ω-diols. Overall, this deblocking/polymerization strategy offers a convenient and efficient solution to problems that have limited the breadth of applications of polyurethane chemistry.

Anion-induced isomerization of fluorescent semi(thio)carbazones

An anion-controlled flapping motion has been observed in a series of novel semi(thio)carbazone molecules.

Secondary inner filter effect allows extremely efficient pure white light emission by spatially separated organic fluorophores

White light emission with 70 ± 3% efficiency under dilute conditions was obtained via the secondary inner filter effect between 2-(1-pyrenyl)benzimidazole (PyBIM) and pyrelene monoimide (PMI).

Fast and Selective Two-Stage Ratiometric Fluorescent Probes for Imaging of Glutathione in Living Cells

Two fluorescent, m-nitrophenol-substituted difluoroboron dipyrromethene dyes have been designed by nucleophilic substitution reaction of 3,5-dichloro-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). Nonsymmetric and symmetric probes, that is. BODIPY 1 (with one nitrophenol group at the position 3) and BODIPY 2 (with two nitrophenol groups at the positions 3 and 5) were applied to ratiometric fluorescent glutathione detection. The detection is based on the two-step nucleophilic aromatic substitution of the nitrophenol groups of the probes by glutathione in buffer solution containing CTAB. In the first stage, probe 1 showed ratiometric fluorescent color change from green (λ_em = 530 nm) to yellow (λ_em = 561 nm) because of monosubstitution with glutathione (I_561nm/I_530nm). Addition of excess glutathione caused the second stage of ratiometric fluorescent color change from yellow to reddish orange (λ_em = 596 nm, I_596nm/I_561nm) due to disubstitution with glutathione. Therefore, different concentration ranges of glutathione (from less to excess) could be rapidly detected by the two-stage ratiometric fluorescent probe 1 in 5 min. While, probe 2 shows single-stage ratiometric fluorescent detection to GSH (from green to reddish orange, I_596nm/I_535nm). Probes 1 and 2 exhibit excellent properties with sensitive, specific colorimetric response and ratiometric fluorescent response to glutathione over other sulfur nucleophiles. Application to cellular ratiometric fluorescence imaging indicated that the probes were highly responsive to intracellular glutathione.

Fluorescent chemosensors: the past, present and future

Fluorescent chemosensors for ions and neutral analytes have been widely applied in many diverse fields such as biology, physiology, pharmacology, and environmental sciences. The field of fluorescent chemosensors has been in existence for about 150 years. In this time, a large range of fluorescent chemosensors have been established for the detection of biologically and/or environmentally important species. Despite the progress made in this field, several problems and challenges still exist. This tutorial review introduces the history and provides a general overview of the development in the research of fluorescent sensors, often referred to as chemosensors. This will be achieved by highlighting some pioneering and representative works from about 40 groups in the world that have made substantial contributions to this field. The basic principles involved in the design of chemosensors for specific analytes, problems and challenges in the field as well as possible future research directions are covered. The application of chemosensors in various established and emerging biotechnologies, is very bright.

Synthesis of Bisimidazole Derivatives for Selective Sensing of Fluoride Ion

Rapid and efficient analysis of fluoride ion is crucial to providing key information for fluoride ion hazard assessment and pollution management. In this study, we synthesized one symmetrical structure called 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (1a) and two asymmetrical structures, namely 2-(4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-1H-phenanthro(9,10-d)imidazole (1b) and 2-(4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-1H-imidazo(4,5-f)(1,10)phenanthroline (1c), which served as an efficient anion sensor for fluoride ion over a wide range of other anions (Cl-, Br-, I-, NO₃-, ClO₄-, HSO₄-, BF₄-, and PF₆-) owing to imidazole group in the main backbone. The absorption intensity of compound 1a at λmax 358 nm slightly decreased; however, a new band at λmax 414 nm appeared upon the addition of fluoride ion, while no evident change occurred upon the addition of eight other anions. The photoluminescence intensity of compound 1a at λmax 426 nm was nearly quenched and fluorescence emission spectra were broadened when fluoride ion was added into dimethyl sulfoxide (DMSO) solution of compound 1a. Compared with the optical behaviors of the DMSO solution of compound 1a in the presence of Bu₄N⁺F-, compounds 1b and 1c exhibited considerable sensitivity to fluoride ion due to the increase in coplanarity. Furthermore, compared with the fluorescence emission behaviors of the DMSO solutions of compounds 1a and 1b in the presence of Bu₄N⁺F-, compound 1c exhibited the most significant sensitivity to fluoride ion due to the charge transfer enhancement. Consequently, the detection limits of compounds 1a-1c increased from 5.47 × 10-6 M to 4.21 × 10-6 M to 9.12 × 10-7 M. Furthermore, the largest red shift (75 nm) of the DMSO solution compound 1c in the presence of fluoride ion can be observed. Our results suggest that the increase in coplanarity and the introduction of electron-withdrawing groups to the imidazole backbone can improve the performance in detecting fluoride ion.

Fluorescent triazolium for sensing fluoride anions in semi-aqueous solution

A new triazolium-based receptor for fluorescent sensing of inorganic fluoride anions in semi-aqueous solution and the solid state was reported.