Fluorescence Properties of Simple N-Substituted Aldimines with a B−N Interaction and Their Fluorescence Quenching by a Cyanide Ion

Development of a novel chromophore reaction-based fluorescent probe for biogenic amines detection

Biogenic amines (BAs) are important biomarkers to monitor meat spoilage. However, the design of efficient BA fluorescent probes with distinct colorimetric and ratiometric fluorescent dual-channels is still a critical challenge because of similar chemical properties and basicity between BAs and other amines. Herein, pyrrolopyrrole cyanine (PPCy-1) is reported to display distinctly high reactivity toward BAs through an ultrasensitive irreversible chromophore reaction for the first time. The reaction mechanism is ascribed to synergistic aza-Michael addition and B-N detachment, followed by hydrolysis to produce low-conjugated diketopyrrolopyrrole and heteroaromatic acetonitrile compounds. As a result, colorimetric and ratiometric fluorescent dual-channel (Δλ_ab = 188 nm and Δλ_em = 151 nm) signals and a limit of detection up to 62.1 nM level for BA solution are acquired. In addition, the colorimetric detection of volatile amine vapor using the PPCy-1-loaded filter paper, showing a color change from green to yellow, is feasible. A simple and cost-effective fluorescence "turn on" method using the filter paper or the CAD-40 resin loaded with PPCy-1 to detect TVB (total volatile bases) originating from shrimp spoilage is further demonstrated.

Iridium-Catalyzed Site-Selective Borylation of 8-Arylquinolines

We report a convenient method for the highly site-selective borylation of 8-arylquinoline. The reaction proceeds smoothly in the presence of a catalytic amount of [Ir(OMe)(cod)]2 and 2-phenylpyridine derived ligand using bis(pinacolato)diborane as the borylating agent. The reactions occur with high selectivity with many functional groups, providing a series of borylated 8-aryl quinolines with good to excellent yield and excellent selectivity. The borylated compounds formed in this method can be transformed into various important synthons by using known transformations.

Discovery of Functional Luminescence Properties Based on Flexible and Bendable Boron-Fused Azomethine/Azobenzene Complexes with O,N,O-Type Tridentate Ligands

Azomethine (C=N) and azo (N=N) scaffolds are a part of structural units in poly(p-phenylene azomethine) (PAM) and poly(p-phenylene azo) (PAZ), respectively. Poly(p-phenylene vinylene) (PPV) is known to be one of luminescent π-conjugated polymers, meanwhile PAM and PAZ, which are the aza-substituted PPV analogues, are regarded as weak or no emissive materials. However, by the boron complexation, intense emission can be induced. Furthermore, environment-sensitivity and stimuli-responsivity were also observed. In this review, we demonstrate unique and versatile luminescent properties based on "flexible and bendable" π-conjugated systems composed of the boron-fused azomethine and azobenzene complexes (BAm and BAz) with the O,N,O-type tridentate ligands. The "flexible and bendable" luminophores showed intriguing optical behaviors, such as thermosalient effect, aggregation-induced emission (AIE) and crystallized-induced emission (CIE). Moreover, highly efficient emissions both in solution and film states were observed from the polymers. We illustrate the results and mechanisms on these luminescent properties from the series of our recent studies with BAm and BAz complexes and polymers.

Facile strategy for obtaining luminescent polymorphs based on the chirality of a boron-fused azomethine complex

A chloro-substituted boron-fused azomethine complex (BAmCl) having a stereogenic boron center was synthesized for obtaining a luminescent chiral crystal. We succeeded in isolating the (R)- and (S)-enantiomers of BAmCl and preparing the homochiral polymorphic crystal, while we obtained the racemic crystal with rac-BAmCl. Single crystal X-ray diffraction analyses suggest that a variety of intermolecular interaction patterns and intrinsic flexibility of the molecular framework should play a significant role in stabilizing the homochiral crystal. We found the difference in molecular arrangements between the racemic and the homochiral crystals, and we observed distinctly different emission colors. In particular, we observed heat-initiated homogeneous racemization without the need for a solvent or catalyst in the molten state of the homochiral crystal (R)-BAmCl. Our results mean that chiral resolution of a flexible fused-skeleton having a stereogenic boron center can be a platform for creating luminescent polymorphic materials.

A novel and efficient chromophore reaction based on a lactam-fused aza-BODIPY for polyamine detection

Polyamines (such as spermine, spermidine) play important roles in biomedical and food field. The elevated polyamines have been proposed to serve as target analytes for monitoring meat spoilage. Because of structural similarity and low concentration of polyamines in real samples, it is exceedingly challenging to design and develop sensitive probes for visual detection of polyamines. To address this issue, a highly efficient probe was reported based on a newly developed chromophore reaction between lactam-fused aza-BODIPY (abbreviation: LAB) and polyamines by virtue of unique multiple amino groups character of polyamines. This chromophore reaction includes a kinetic-controllable reaction of a B-N bond cleavage by polyamines followed by a fast hydrolysis reaction to yield much smaller conjugated molecules. With 130 nm hypsochromic shift of the absorption peak and up to 99% fluorescence quenching within 1 min, LAB can be used as a highly sensitive fluorescent probe for detection of polyamines solution and monitoring fish spoilage with synchronous colorimetric and fluorescent changes.

The Design Strategy for an Aggregation- and Crystallization-Induced Emission-Active Molecule Based on the Introduction of Skeletal Distortion by Boron Complexation with a Tridentate Ligand

We describe here a new design strategy for obtaining boron complexes with aggregation- and crystallization-induced emission (AIE and CIE, respectively) properties based on the introduction of skeletal distortion. According to our recent results, despite the fact that an almost planar structure and robust conjugation were obtained, the boron azomethine complex provided a slight emission in solution and an enhanced emission in aggregation and crystal. Quantum calculation results propose that unexpected emission annihilation in solution could be caused through intramolecular bending in the excited state. Herein, to realize this unique molecular motion and obtain AIE and CIE molecules, the phenyl quinoline-based boron complexes BPhQ and BPhQm with distorted and planar structures were designed and synthesized, respectively. BPhQm showed emission in solution and aggregation-caused quenching (ACQ, BPhQm: ΦF,sol. = 0.21, ΦF,agg. = 0.072, ΦF,cryst. = 0.051), while BPhQ exhibited a typical AIE and CIE (BPhQ: ΦF,sol. = 0.008, ΦF,agg. = 0.014, ΦF,cryst. = 0.017). The optical data suggest that a large degree of molecular motion should occur in BPhQ after photo-excitation because of the intrinsic skeletal distortion. Furthermore, single-crystal X-ray diffraction data indicate that the distorted π-conjugated system plays a positive role in presenting solid-state emission by inhibiting consecutive π–π interactions. We demonstrate in this paper that the introduction of the distorted structure by boron complexation should be a new strategy for realizing AIE and CIE properties.

Real-time monitoring of newly acidified organelles during autophagy enabled by reaction-based BODIPY dyes

Real-time monitoring of newly acidified organelles during autophagy in living cells is highly desirable for a better understanding of intracellular degradative processes. Herein, we describe a reaction-based boron dipyrromethene (BODIPY) dye containing strongly electron-withdrawing diethyl 2-cyanoacrylate groups at the α-positions. The probe exhibits intense red fluorescence in acidic organelles or the acidified cytosol while exhibiting negligible fluorescence in other regions of the cell. The underlying mechanism is a nucleophilic reaction at the central meso-carbon of the indacene core, resulting in the loss of π-conjugation entailed by dramatic spectroscopic changes of more than 200 nm between its colorless, non-fluorescent leuco-BODIPY form and its red and brightly emitting form. The reversible transformation between red fluorescent BODIPY and leuco-BODIPY along with negligible cytotoxicity qualifies such dyes for rapid and direct intracellular lysosome imaging and cytosolic acidosis detection simultaneously without any washing step, enabling the real-time monitoring of newly acidified organelles during autophagy.

Emissive Azobenzenes Delivered on a Silver Coordination Polymer

Azobenzene has become a ubiquitous component of functional molecules and polymeric materials because of the light-induced trans → cis isomerization of the diazene group. In contrast, there are very few applications utilizing azobenzene luminescence, since the excitation energy typically dissipates via nonradiative pathways. Inspired by our earlier studies with 2,2'-bis[ N,N'-(2-pyridyl)methyl]diaminoazobenzene (AzoAM oP) and related compounds, we investigated a series of five aminoazobenzene derivatives and their corresponding silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission under ambient conditions, form silver coordination polymers that are luminescent at room temperature. AzoAE pP (2,2'-bis[ N,N'-(4-pyridyl)ethyl]diaminoazobenzene) assembles into a three-dimensional coordination polymer (AgAAE pP) that undergoes a reversible loss of emission upon the addition of metal-coordinating analytes such as pyridine. The switching behavior is consistent with the disassembly and reassembly of the coordination polymer driven by displacement of the aminoazobenzene ligands by coordinating analytes.

Conjugated Polyimine Dynamers as Phase-Sensitive Membrane Probes

In this report, dynamic polyimines are introduced as multifunctional sensors of lipid bilayer phases. Under mildly acidic conditions, self-condensation of push-pull amino formyl fluorenes into polyimines occurs in solid- or liquid-ordered phases but not in liquid-disordered phases of vesicular membranes. The obtained conjugated polymers are characterized by a progressive red shift of the absorption maxima, the appearance of exciton-coupled circular dichroism (CD) bands, and fluorescence quenching. These characteristics allow multiple modes of detection of membrane phases, which are known to change under membrane tension.

Fluorescence light-up detection of cyanide in water based on cyclization reaction followed by ESIPT and AIEE

Schiff base 1 (2,4-di-tert-butyl-6-((2-hydroxyphenyl-imino)-methyl)phenol) containing two hydroxyl groups could undergo an oxidative cyclization reaction and then generate hydroxyphenylbenzoxazole (HBO) 2 when CN^- was present as a catalyst. The multistep cyclization reaction was proved by spectroscopy, ¹H NMR, ¹³C NMR and mass spectra. C[double bond, length as m-dash]N isomerization is the predominant decay process of the excited states, so sensor 1 is weakly emissive in solution at ambient temperature. When 1 reacts with CN^-, the emission is remarkably enhanced, where 1 is converted to 2. The cyclization product HBO 2 displays bright green luminescence in micellar due to the ESIPT (excited-state intramolecular proton transfer) as well as AIEE (aggregation-induced emission enhancement) effect. The detection limit is 5.92 × 10^-7 M, lower than the WHO guideline of CN^- in drinking water (1.9 μM). The selective and competitive experiments reveal that sensor 1 shows high sensing selectivity and sensitivity for CN^- over other anions. Test papers containing absorbed 1 were prepared and applied for practical application of cyanide detection.

Ultrasensitive reversible chromophore reaction of BODIPY functions as high ratio double turn on probe

Chromophore reactions with changes to conjugation degree, especially those between the conjugated and unconjugated state, will bring a large spectral variation. To realize such a process, a meso-naked BODIPY (MNBOD) with two electron-withdrawing groups around the core is designed and synthesized. The resulting system is extremely sensitive to bases. The red, highly fluorescent solution readily becomes colorless and non-fluorescent after base addition; however, the color and fluorescence can be totally and instantly restored by addition of acid or formaldehyde. Analyses show that two identical MNBODs are connected by a C–C single bond (sp³) at the meso-position through a radical reaction that results in an unconjugated, colorless dimer complexed with bases. When the bases are consumed, the dimer immediately dissociates into the red, highly fluorescent, conjugated MNBOD monomer. With 260 nm spectral change and over 120,000 turn-on ratio, this chromophore-reaction can be utilized as a sensitive reaction-based dual-signal probe.

BODIPY dyes, though widely explored, have not been pursued as chromophore reaction based chemical probes. Here, the authors synthesize a meso-naked BODIPY core flanked with two electron-withdrawing groups, which undergoes a reversible change in conjugated structure in the presence of base and functions as a dual signal and ultrahigh turn-on ratio chemical probe.

2,4 and 2,5-bis(benzooxazol-2′-yl)hydroquinone (DHBO) and their borate complexes: synthesis and optical properties

2,4 and 2,5-bis(benzooxazol-2′-yl)hydroquinone (DHBO) scaffolds show fluorescence stemming from an ESIPT process but also act as chelates toward boron fragments.

Detection of cyanide by a novel probe with a V-shaped structure based on aggregation of the probe adduct

A novel V-shaped probe is reported with extremely rapid response, high selectivity and very low detection limit to cyanide anions in aqueous solution based on the intensely emissive aggregates of the probe adduct.

A coumarin-indole based colorimetric and "turn on" fluorescent probe for cyanide

A novel coumarin-indole based chemodosimeter with a simple structure was designed and prepared via a condensation reaction in high yield. The probe exhibited very high selectivity towards cyanide on both fluorescence and UV-vis spectra, which allowed it to quantitatively detect and imaging cyanide ions in organic-aqueous solution by either fluorescence enhancement or colorimetric changes. Confirmed by (1)H NMR and HRMS spectra, the detection mechanism was proved to be related with the Michael addition reaction induced by cyanide ions, which blocked the intramolecular charge transfer (ICT) of the probe. Moreover, the probe was able to be utilized efficiently in a wide pH range (7.5-10) with negligible interference from other anions and a low detection limit of 0.51μM. Application in 5 kinds of natural water source and accurate detection of cyanide in tap water solvent system also indicated the high practical significance of the probe.

A novel strategy for chromogenic chemosensors highly selective toward cyanide based on its reaction with 4-(2,4-dinitrobenzylideneamino)benzenes or 2,4-dinitrostilbenes

N-(2,4-dinitrobenzylidene)-4-methoxyaniline (1), 4-(N,N-dimethylamine)-N-(2,4-dinitrobenzylidene)aniline (2), 2,4-dinitro-4'-methoxystilbene (3), and 2,4-dinitro-4'-(dimethylamino)stilbene (4) were synthesized and studied in dimethyl sulfoxide in a novel strategy as anionic chromogenic chemosensors. The color of the solutions of these compounds changed only in the presence of cyanide. The kinetic studies were performed with compounds 1-3 in an excess of cyanide. Higher second-order rate constant values were obtained for the compounds containing a methoxy group in relation to the compounds with a dimethylamino substituent, since the methoxy group donates electronic density to the 2,4-dinitrophenyl electron-accepting group less easily compared with the dimethylamino group. Stilbenes generally have greater structural rigidity than imines, facilitating the action of the substituents through the mesomeric effect. The data obtained indicate that the anion acts as a nucleophile, being responsible for CN bond breaking. The CC bridge is not broken in the stilbene dyes, but cyanide performs a nucleophilic attack on the 2,4-dinitrophenyl group.

Pyrene and imidazole functionalized luminescent bimetallic Ru(ii) terpyridine complexes as efficient optical chemosensors for cyanide in aqueous, organic and solid media

Luminescent bimetallic Ru(ii)-terpyridine complexes based on the pyrenyl-bis-phenylimidazole spacer as highly efficient sensors for cyanide in aqueous, organic and solid media are reported.

A selective colorimetric chemosensor with an electron-withdrawing group for multi-analytes CN− and F−

A colorimetric chemosensor with an electron-withdrawing group (–NO₂) 1 for the detection of CN⁻ and F⁻ was developed.

An imidazolyl-pyreno-imidazole conjugate as a cyanide sensor and a set–reset memorized sequential logic device

A pyrenyl-bisimidazole system can act as an efficient cyanide sensor and a ‘set–reset’ memory device.

Pyrene-biimidazole based Ru(ii) and Os(ii) complexes as highly efficient probes for the visible and near-infrared detection of cyanide in aqueous media

Pyrenyl-biimidazole based Ru(ii) and Os(ii) complexes are used as highly efficient cyanide sensors in aqueous media.

A diaminomaleonitrile based selective colorimetric chemosensor for copper(ii) and fluoride ions

A chemosensor showed colorimetric sensing for copper(ii) and fluoride by changing color from yellow to colorless and to orange.

A new fluorescent “turn-on” chemodosimeter for cyanide based on dual reversible and irreversible deprotonation of NH and CH groups

A new chemodosimeter for cyanide ions based on dual deprotonation of NH and CH groups was developed for the first time, with high selectivity, dramatic red-shifted absorption (≈263 nm) and fluorescent “turn-on” detection.

Chromogenic chemodosimeter for highly selective detection of cyanide in water and blood plasma based on Si-O cleavage in the micellar system

A novel silylated imine was designed to act efficiently in a chemodosimeter approach for the selective detection of cyanide in an aqueous micellar CTABr solution. This simple system allows the detection of cyanide, with high sensitivity and specific selectivity, in water and in human blood plasma.

A simple colorimetric chemosensor bearing a carboxylic acid group with high selectivity for CN-

A new simple 'naked eye' chemosensor 1 (sodium (E)-2-((2-(3-hydroxy-2-naphthoyl)hydrazono)methyl)benzoate) has been synthesized for detection of CN- in a mixture of DMF/H2O (9:1). The sensor 1 comprises of a naphthoic hydrazide as efficient hydrogen bonding donor group and a benzoic acid as the moiety with the water solubility. The receptor 1 showed high selectivity toward cyanide ions in a 1:1 stoichiometric manner, which induces a fast color change from colorless to yellow for CN- over other anions. Therefore, receptor 1 could be useful for cyanide detection in aqueous environment, displaying a high distinguishable selectivity from hydrogen bonded anions and being clearly visible to the naked eye.

Novel salicylaldehyde derivatives as fluorescence turn-on sensors for cyanide ion

A novel series of fluorescent probes containing diphenylacetylene as fluorogenic units and salicylaldehyde as a CN(-) receptor were successfully synthesized by using sonogashira coupling. In the presence of Triton-X 100 non-ionic surfactant, the fluorophores exhibited fluorescence "turn-on" in response to CN(-) in aqueous media with the detection limit of 1.6 μM. The turn-on signal is the result of the conversion of the aldehyde to cyanohydrin via the internal acid catalyzed addition of cyanide. Using paper sensor strips, a naked eye detection of cyanide is possible down to 5 nmol.

A colorimetric chemosensor based on a Schiff base for highly selective sensing of cyanide in aqueous solution: the influence of solvents

A new highly selective colorimetric chemosensor 1 shows exclusive response toward cyanide by a color change in aqueous solution.

Colorimetric and luminescent bifunctional Ru(ii) complexes for rapid and highly sensitive recognition of cyanide

Four novel ruthenium(ii) complexes have been characterized for the colorimetric and luminescent bi-functional sensing of cyanide ions. The structure of one complex is also determined by single crystal X-ray diffraction.