3-Ethoxysalicylaldimine-based symmetrical azine-linked luminogen exhibiting ESIPT and bright orange colour AIE behaviour with live cell bioimaging application

Herein, we describe the design and development of a new cell-permeable aggregation-induced emission (AIE) active 3-ethoxysalicylaldimine-based symmetrical azine molecule HDBE. The synthesized compound underwent comprehensive investigation of different spectroscopic methods, like NMR, mass and single crystal X-ray diffraction analysis. The fluorophore HDBE exhibited the bright orange colour AIE behaviour in THF-H2O mixture. The drastic enhancement of emission was achieved upon adding the water to the THF solution of HDBE, with a concentration of 90%. Along with the dynamic light scattering (DLS) and quantum yield measurements, the formation of aggregates was also verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Further, HDBE demonstrated excited state intramolecular proton transfer (ESIPT) characteristics in different polarity of solvents, which was corroborated by absorption, emission and lifetime spectroscopical investigations. The detailed scrutiny of X-ray structure of HDBE displayed the two strong intramolecular hydrogen bonding interactions, while solid-state fluorescent spectra showed dual emission that corresponds to enol and keto form confirming the ESIPT feature. Further, the synthesized AIE molecule was non-toxic and cell-permeable, making it easy to label as a biomarker in live HeLa cells via fluorescent bioimaging. These studies offer a quick and easy way to develop both AIE and ESIPT-coupled molecules for live cell bioimaging applications.

Modulation of excited state intramolecular proton transfer and intramolecular charge transfer pathways of symmetrical azines through micellar medium

The photophysical studies of fluorescent probes in micellar medium can give a better insight about their interaction with biological membranes. This study attempts to access the photophysical properties of the dual emitting azine based probe diethylamino salicylidene azine dimer (DEASAD) in micellar media. DEASAD showed dual charge transfer emission due to the presence of open enol (480 nm) and closed enol (510 nm) forms in polar protic solvents. Upon increasing the concentration of ionic surfactants, there is a significant increase in the emission intensity of both the enol forms of DEASAD until premicellar concentration. After micellization, occurrence of a new anomalous keto form emission through excited state intramolecular proton transfer (ESIPT) was observed around 530 nm in ionic micelles and its intensity changes depend on the micellar surface charge. The emission studies revealed the position and interaction of DEASAD with the charge of micellar stern layer as confirmed through interaction of metal ion with the probe and control molecules with and without ESIPT and ICT moieties. In contrast, the new anomalous longer wavelength keto form of DEASAD emission was absent in neutral micelles like Triton X-100.

Using Cu2+ to regulate the emission feature of near-infrared fluorescent sensor with AIE: To detect ascorbic acid in food samples and its application in bioimaging

Conventional ascorbic acid (AA) detection methods such as chromatography, capillary electrophoresis, colorimetry, electrochemical detection, and enzymatic analysis require expensive equipment and complicated operation. Simple, rapid, and accurate AA detection is essential to inspect food quality, diagnose diseases, and assess immunity in humans. In this study, the first near-infrared fluorescence sensor DBHM with aggregation-induced emission was developed to detect AA under the involvement of Cu2+. The DBHM + Cu2+ sensor showed high sensitivity to AA with a limit of detection of 2.37 μM. The AA detection mechanism was investigated by optical studies, 1H NMR titration, high-resolution mass spectrometry, and infrared spectroscopy. AA was detected qualitatively and quantitatively by the DBHM + Cu2+ sensor in beverages, fruits, and Vitamin C tablets using a dual-mode (fluorescence and smartphone app) sensing platform. The new sensing system also showed low toxicity and excellent bioimaging in HeLa cells, C. elegans, and mice. This sensor could advance AA detection technology in the food industry and has potential bioimaging applications.

An ESIPT solvatochromic fluorescent and colorimetric probe for sensitive and selective detection of copper ions in environmental samples and cell lines

Copper is one of the most important transition metals in our body for various physiological functions. An imbalance in the homeostasis of copper in our body can lead to various neurological disorders such as Alzheimer's disease, Parkinson's, and Menkes disease. As a result, there is a need for the detection of excess copper present in the environment and the human system. In this work, we have designed a quinoline-based fluorescent/colorimetric probe (QHS) for rapid and selective detection of copper ions via quenching of fluorescence/color change from yellow to peach which is visible to the naked eye. The probe displayed high selectivity towards copper(II), i.e., Cu(II) in the presence of different metal analytes in water samples. The sensing mechanism of the probe was confirmed by NMR, HRMS, IR spectroscopy, and SEM. The detection limit of Cu(II) ions was found to be 0.493 μM which is lower than the tolerable limit of copper (20 μM) in drinking water as per the EPA. The probe was also utilized for the visualization of Cu(II) in cell lines. The probe was also demonstrated for its application in real-time detection of aqueous samples using portable paper strips.

A Simple Schiff Base Probe for Quintuplicate-Metal Analytes with Four Emission-Wavelength Responses

A versatile mono-Schiff compound consisting of o-aminobenzene-hydroxyjulolidine (ABJ-MS) has been easily synthesized using a one-step reaction. ABJ-MS displays four diverse fluorescence responses to the addition of Zn2+/Al3+/Fe3+/Ag+, with the maximum fluorescence emission at 530 nm undergoing a hypsochromic shift to 502/490/440/430 nm, synchronously with the discriminating fluorescence enhancement being 10.6/22.8/2.6/7.1-fold, respectively. However, the addition of Cu2+ into ABJ-MS leads to an opposite behavior, namely, fluorescence quenching. Meanwhile, ABJ-MS also displays distinct absorption changes after adding these five metal ions due to different binding affinities between them and ABJ-MS, which gives ABJ-MS quite a versatile detecting nature for Cu2+/Zn2+/Al3+/Fe3+/Ag+. Moreover, ABJ-MS can mimic a series of versatile AND/OR/INH-consisting logic circuits on the basis of the Cu2+/Zn2+/Al3+/Fe3+/Ag+-mediated diverse optical responses. These will endow the smart ABJ-MS molecule and potential applications in the multi-analysis chemosensory and molecular logic material fields.

Heteroaryl-Substituted Bis-Anils: Aggregation-Induced Emission (AIE) Derivatives with Tunable ESIPT Emission Color and pH Sensitivity

The two-step synthesis, structural, and photophysical properties of a series of heteroaryl-substituted bis-anil derivatives presenting aggregation-induced emission (AIE) coupled with an excited-state intramolecular proton transfer (ESIPT) process is described. The fluorescence color of the aggregates can be fine tuned by changing the electronic nature of the peripheral substitution, leading to a wide range of emission wavelengths (from green to the near infra-red). Moreover, upon introduction of strong electron-withdrawing groups such as cyano (CN), a competition between ESIPT and deprotonation is observed leading to the emission of the anionic species at low water percentage. This observation led to the synthesis of an additional mixed AIE fluorophore, functionalized by methoxy groups on one side and cyano groups on the other side. Upon addition of water, this dye displays first anionic emission, followed by typical AIE/ESIPT red fluorescence upon formation of the aggregates. TD-DFT calculations on selected AIE dyes were performed to rationalize the nature of the emissive transitions in these derivatives.

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.

Crystal Structure Determination of 4-[(Di-p-tolyl-amino)-benzylidene]-(5-pyridin-4-yl-[1,3,4]thiadiazol-2-yl)-imine along with Selected Properties of Imine in Neutral and Protonated Form with Camforosulphonic Acid: Theoretical and Experimental Studies

The crystal structure was determined for the first time for 4-[(di-p-tolyl-amino)benzylidene]-(5-pyridin-4-yl-[1,3,4]thiadiazol-2-yl)-imine (trans-PPL9) by X-ray diffraction. The imine crystallized in the monoclinic P21/n space group with a = 18.9567(7) Å, b = 6.18597(17) Å, c = 22.5897(7) Å, and β = 114.009(4)°. Intermolecular interactions in the PPL9 crystal were only weak C-H⋯N hydrogen bonds investigated using the Hirshfeld surface. The electronic and geometric structure of the imine were investigated by the density functional theory and the time-dependent density-functional theory. The properties of the imine in neutral and protonated form with camforosulphonic acid (CSA) were investigated using cyclic voltammetry, UV-vis and 1H NMR spectroscopy. Theoretical and experimental studies showed that for the 1:1 molar ratio the protonation occured on nitrogen in pyridine in the PPL9 structure, as an effect of Brönsted acid-base interactions. Thermographic camera was used to defined defects in constructed simple devices with ITO/PPL9 (or PPL9:CSA)/Ag/ITO architecture. In conclusion, a thermally stable imine was synthesized in crystalline form and by CSA doping, a modification of absorption spectra together with reduction of overheating process was observed, suggesting its potential application in optoelectronics.

Aggregation-induced Emission Properties of Triphenylamine Chalcone Compounds

Two triphenylamine chalcone derivatives 1 and 2 were synthesized through the Vilsmeier-Haack reaction and Claisen-Schmidt condensation reaction. Through ultraviolet absorption spectroscopy and fluorescence emission spectroscopy experiments, it was confirmed that these two compounds exhibited good aggregation-induced emission (AIE) behavior in ethanol/water mixtures. The solvent effect test showed with the increase of the orientation polarizability of the solvent, the Stokes shift in the solvent of compound 1 and compound 2 shows a linear change trend. Through solid state fluorescence test and universal density function theory (DFT), the existence of π-π stacking interaction in the solid state of the compound has been studied, resulting in weak fluorescence emission. pH has no effect on the fluorescence intensity of the aggregate state of excited state intramolecular proton transfer (ESIPT) molecules in an acidic environment, but greatly weakens its fluorescence intensity in an alkaline environment. Cyclic voltammetry (CV) test shows that compound 1 was more prone to oxidation reaction than compound 2. The results of thermal stability test show that the thermal stability of compound 1 was better than that of compound 2, indicating that triphenylamine chalcone derivatives can improve the thermal stability of compounds by increasing the number of branches.

Photophysical property change of N-(5-bromo-salicylidene)-3-aminoethylpyridine monohydrated crystals via dehydration phase transition

The crystals of N-salicylideneaniline (SA) and SA derivatives are classic functional materials that exhibit reversible colour changes (photochromism) and/or excited-state intramolecular proton transfer (ESIPT) fluorescence emission under ultraviolet (UV) light irradiation. In this study, a novel SA derivative was synthesised with an extended alkyl chain, N-(5-bromo-salicylidene)-3-aminoethylpyridine (5Br-SAEP). The photophysical properties of 5Br-SAEP were characterised in the crystalline state. The monohydrated crystal (1H) of 5Br-SAEP was dehydrated to form the anhydrous crystal (1A) at a relative humidity of less than 76%. The photochromic activity was switched by the dehydration phase transition from the non-photochromic 1H to the photochromic 1A. The quantum yield of fluorescence decreased significantly from 8% in 1H to 3% in 1A. The in situ change of photophysical properties occurred due to the change in the crystal structure. This indicated the potential of the solvated crystals of the SAEP derivatives for applications in novel switching or smart materials.

Tuning the mechanical flexibility of organic molecular crystals by polymorphism for flexible optical waveguides

The ability to selectively tune the optical and the mechanical properties of organic molecular crystals offers a promising approach towards developing flexible optical devices.

Recent advances in excited state intramolecular proton transfer mechanism-based solid state fluorescent materials and stimuli-responsive fluorescence switching

Substitutional change and controlling intra and intermolecular interactions of ESIPT molecules resulted in realizing multifunctional fluorescence properties.

A new 7-diethylamino-4-hydroxycoumarin based reversible colorimetric/fluorometric probe for sequential detection of Al3+/PPi and its potential use in biodetection and bioimaging applications

A new 7-diethylamino-4-hydroxycoumarin appended acylhydrazone probe was prepared and utilized for the sequential detection of Al³⁺/PPi in a reversible off–on–off emissive manner. The various practical applications of the probe were established.

C3-symmetric triaminoguanidine based colorimetric and fluorometric chemosensor: Sequential detection of Zn2+/PPi, its RGB performance for detection of Zn2+ ion and construction of IMPLICATION logic gate

In this work, new ethyl(E)-2-cyano-3-(1H-pyrrol-2-yl)acrylate appended C₃-symmetric star-shape triaminoguanidine based Schiff base (LH₃) was designed and synthesized from simple synthons. New probe, LH₃ was completely analyzed by ¹H NMR, ¹³C NMR and mass spectrum. In the present probe LH₃, effective π-conjugated ethyl(E)-2-cyano-acrylate unit was introduced on the periphery of the pyrrole-triaminoquanidine conjugates by using carefully chosen building units. The probe LH₃ shows high selectivity and sensitivity towards Zn²⁺ ion via colorimetric and fluorometric changes. The yellowish orange color of LH₃ solution turned to wine red color upon addition of Zn²⁺ solution, along with red shifted absorption maxima from 450 nm to 550 nm, this indicates the formation of LH₃-Zn²⁺ species. Job's plot and mass spectrum analysis confirms the formation of 1:3 stoichiometric complex between the LH₃ and Zn²⁺ ions. Further this ensemble shows selective detection towards PPi anion over the other anions based on displacement metal ion approach. Hence, reversible colorimetric/emission response of LH₃ towards Zn²⁺ and PPi ions via "on-off-on" manner could allow the construction of IMPLICATION logic gate functions. The practical efficacy of the probe LH₃ was established by utilization of the probe for the detection of Zn²⁺ ions in real water sample analysis. Further, the significant noticeable colorimetric changes of the probe LH₃ upon addition of Zn²⁺ ion have been successfully integrated with a smartphone app RGB color value to construct a real-time analysis of Zn²⁺ ions.

Solvent-assisted planar structure of a stilbene-based salicylhydrazone compound: crystal structure, solvent- and aggregation-induced emission, and switchable luminescence colouration

A novel stilbene-based salicylhydrazone compound {systematic name: (E)-4,4'-(ethene-1,2-diyl)bis[(N'E)-N'-(2-hydroxybenzylidene)benzohydrazide] dimethyl sulfoxide disolvate, C₃₀H₂₄N₄O₄·2C₂H₆OS or L·2DMSO} was synthesized and characterized by single-crystal X-ray diffraction, powder X-ray diffraction and luminescence spectroscopy. The title compound crystallizes in the monoclinic space group P2₁/c, with half a symmetry-independent L molecule and one dimethyl sulfoxide (DMSO) solvent molecule in the asymmetric unit. The L molecule adopts an almost planar structure, with a small dihedral angle between the planes of the stilbene and salicylhydrazone groups. There are multiple π-π stacking interactions between adjacent L molecules. The DMSO solvent molecules act as proton donors and acceptors, forming hydrogen bonds of various strengths with the L molecules. In addition, the geometry optimization of a single molecule of L and its luminescence properties either in solution, as a solvated solid or as a desolvated solid were studied. The compound shows an aggregation-induced emission (AIE) effect and exhibits switchable luminescence colouration in the solid state by the simple removal or re-addition of the DMSO solvent.

D-π-A azine based AIEgen with solvent dependent response towards a nerve agent

We developed a D-π-A based unsymmetrical azine molecule 4-((E)-((E)-(4-(dipropylamino)benzylidene)hydrazono)methyl)benzonitrile [DPBN] and studied its optical and aggregation induced emission properties. The DPBN molecule shows good aggregation induced emission (AIE) behaviour with 1157-fold fluorescence enhancement in the aggregated state. In addition to that, both colorimetric as well as fluorometric sensing studies revealed that DPBN selectively detects diethylchlorophosphate (DCP), a potent nerve agent. Interestingly, DPBN shows solvent dependent optical output in the presence of DCPvia two different mechanisms. In the monomer state, it shows red shifted fluorescence enhancement along with color change from colorless to orange color via the formation of a new intramolecular charge transfer state in pure tetrahydrofuran (THF). In the aggregated state, DPBN shows blue shifted emission with fluorescence enhancement in THF-water mixture by protonation at the amine nitrogen centre. Thus, DPBN can be used as a diagnostic measure to selectively detect nerve agents like DCP. This study also paves the way for further development of molecular probes for nerve agents that would represent immense implications in various fields of chemistry and biology.

Simple C3-symmetric triaminoguanidine-triphenylamine conjugate as an efficient colorimetric sensor for Cu(II) and fluorescent sensor for Fe(III) ions

The design and construction of novel C₃-symmetric triaminoguanidine-triphenylamine conjugate (L) has been demonstrated and it displays positive solvatochromic behaviour with an increase in solvent polarity. The probe L acts as a selective colorimetric sensor for Cu²⁺ ions over other metal ions. Further, it shows high selective and sensitive detection of Fe³⁺ ions through turn-on fluorescence response. Moreover, the detection limits for Cu²⁺ and Fe³⁺ ions were found to be within the allowable range of the World Health Organisation (30 μM). The real-time application of the probe was showed by paper strip experiments as well as detection of Fe³⁺ ions in pharmaceutical tablets.

Azine based AIEgens with multi-stimuli response towards picric acid

Selective detection of picric acid using AIEgens via fluorescence enhancement and quenching in the monomer and aggregated from respectively.

A potential naphthyl-thiazole-based organic dye and a ditopic chromogenic probe for CN− and Fe3+ with molecular logic functions

Dye sensitizers are entities designed primarily to serve the function of harvesting light photons in the functional wavelength, which is centered on charge transfer mechanisms.