Photophysics and Biological Applications of the Environment-Sensitive Fluorophore 6-N,N-Dimethylamino-2,3-naphthalimide

Push-Pull Fluorescent Dyes with Trifluoroacetyl Acceptor for High-Fidelity Sensing of Polarity and Heterogeneity of Lipid Droplets

Imaging and sensing of lipid droplets (LDs) attracted significant attention due to growing evidence for their important role in cell life. Solvatochromic dyes are promising tools to probe LDs' local polarity, but this analysis is biased by their non-negligible emission from intracellular membranes and capacity to emit from both the apolar core and polar interface of LDs. Here, we developed two push-pull solvatochromic dyes based on naphthalene and fluorene cores bearing an exceptionally strong electron acceptor, the trifluoroacetyl group. The latter was found to boost the optical properties of the dyes by shifting their absorption and emission to red and increasing their extinction coefficient, photostability, and sensitivity to solvent polarity (solvatochromism). In contrast to classical solvatochromic dyes, such as parent aldehydes and reference Nile Red, the new dyes exhibited strong fluorescence quenching by millimolar water concentrations in organic solvents. In live cells, the trifluoroacetyl dyes exhibited high specificity to LDs, whereas the parent aldehydes and Nile Red showed a detectable backgrounds from intracellular membranes. Experiments in model lipid membranes and nanoemulsion droplets confirmed the high selectivity of new probes to LDs in contrast to classical solvatochromic dyes. Moreover, the new probes were found to be selective to the LDs oil core, where they can sense lipid unsaturation and chain length. Their ratiometric imaging in cells revealed strong heterogeneity in polarity within LDs, which covered the range of polarities of unsaturated triglyceride oils, whereas Nile Red failed to properly estimate the local polarity of LDs. Finally, the probes revealed that LDs core polarity can be altered by fatty acid diets, which correlates with their chain length and unsaturation.

Synthesis of Unsymmetrically Condensed Benzo- and Thienotriazologermoles

Germoles and siloles unsymmetrically condensed with heteroaromatic units are attracting much interest. In this study, compounds containing a triazologermole core unit condensed with a benzene or thiophene ring were prepared. Thienotriazologermole was subjected to bromination to obtain the bromide, which underwent transformation via the palladium-catalyzed Stille coupling reaction to form triphenylamine-substituted thienotriazolegermole, with an effective extension of conjugation. The electronic states and properties of these triazologermole derivatives are discussed on the basis of optical and electrochemical measurements and density functional theory calculations. Triphenylamine-substituted thienotriazolegermole showed clear solvatochromic properties in photoluminescence measurements, suggesting that intramolecular charge transfer occurs at the photo-excited state. This clearly indicates that the triazologermole unit is useful as an acceptor of donor-acceptor compounds. The potential application of triphenylamine-substituted thienotriazolegermole as a sensing material was also explored.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Intracellular Physical Properties with Small Organic Fluorescent Probes: Recent Advances and Future Perspectives

The intracellular physical parameters i. e., polarity, viscosity, fluidity, tension, potential, and temperature of a live cell are the hallmark of cellular health and have garnered immense interest over the past decade. In this context, small molecule organic fluorophores exhibit prominent useful properties including easy functionalizability, environmental sensitivity, biocompatibility, and fast yet efficient cellular uptakability which has made them a popular tool to understand intra-cellular micro-environmental properties. Throughout this discussion, we have outlined the basic design strategies of small molecules for specific organelle targeting and quantification of physical properties. The values of these parameters are indicative of cellular homeostasis and subtle alteration may be considered as the onset of disease. We believe this comprehensive review will facilitate the development of potential future probes for superior insight into the physical parameters that are yet to be quantified.

Synthesis and Photophysical Properties of a New Push–Pull Pyrene Dye with Green-to-Far-red Emission and its Application to Human Cellular and Skin Tissue Imaging

Herein, we discuss a new pyrene-based push–pull dye (PC) and our investigation of its photophysical properties and applicability to biological applications. The newly synthesized dye exhibits highly polarity-sensitive fluorescence over...

Effect of Protonation on Optical and Electrochemical Properties of Thiophene-Phenylene-Based Schiff Bases with Alkoxy Side Groups

Three polyazomethines and their corresponding model compounds were protonated with trifluoroacetic acid, and its effect on their optical (UV–vis absorption and photoluminescence) properties and electrochemical behavior has been studied, in the context of the presence and elongation of alkoxy side groups. Moreover, the effect of environment dielectric constants (i.e., polarity of the solvent) was considered on the doping process. It has been proven that the presence of alkoxy side groups is necessary for protonation to occur, while unsubstituted compounds undergo hydrolysis to constitutive units. Acid doping of imines consisting of alkoxy side chains has resulted in a distinct bathochromic shift (>200 nm) of the low-energy absorption band. Even the length of alkyl chains has not affected the position of shifted bands; it has been observed that azomethines with smaller, methoxy side groups undergo the protonation process much faster than their octyloxy-substituted analogues, due to the absence of steric hindrance. The electrochemical studies of these alkoxy-substituted imines have indicated a better p-type behavior after protonation induced by the capability of the protonated form to easily oxidize in acetonitrile and to generate the native molecules. The environmental polarity has also had impact on the doping process, which took place only in low-polar media.

Visualizing the Dynamic Metalation State of New Delhi Metallo-β-lactamase-1 in Bacteria Using a Reversible Fluorescent Probe

New Delhi metallo-β-lactamase (NDM) grants resistance to a broad spectrum of β-lactam antibiotics, including last-resort carbapenems, and is emerging as a global antibiotic resistance threat. Limited zinc availability adversely impacts the ability of NDM-1 to provide resistance, but a number of clinical variants have emerged that are more resistant to zinc scarcity (e.g., NDM-15). To provide a novel tool to better study metal ion sequestration in host-pathogen interactions, we describe the development of a fluorescent probe that reports on the dynamic metalation state of NDM within Escherichia coli. The thiol-containing probe selectively coordinates the dizinc metal cluster of NDM and results in a 17-fold increase in fluorescence intensity. Reversible binding enables competition and time-dependent studies that reveal fluorescence changes used to detect enzyme localization, substrate and inhibitor engagement, and changes to metalation state through the imaging of live E. coli using confocal microscopy. NDM-1 is shown to be susceptible to demetalation by intracellular and extracellular metal chelators in a live-cell model of zinc dyshomeostasis, whereas the NDM-15 metalation state is shown to be more resistant to zinc flux. The development of this reversible turn-on fluorescent probe for the metalation state of NDM provides a new tool for monitoring the impact of metal ion sequestration by host defense mechanisms and for detecting inhibitor-target engagement during the development of therapeutics to counter this resistance determinant.

Practical access to fluorescent 2,3-naphthalimide derivatives via didehydro-Diels-Alder reaction

A practical and efficient approach for the synthesis of fluorescent 2,3-naphthalimide derivatives has been developed from readily available starting materials via an intramolecular didehydro-Diels-Alder reaction, which proceeded well under room temperature, exhibiting a wide substrate scope and good functional group tolerance. The practicability of this methodology has been verified by one-step synthesis of the environmentally sensitive fluorophore 6-DMN on a gram scale with a shorter time, fewer steps and less waste disposal, and without the utilization of toxic transition metals. The present experimental and computational studies support the crucial role of the propiolimide moiety in the transformation.

Distinctive tunable photophysical properties of versatile environmentally-sensitive tribranched cyanopyridine fluorophores

In the present work, twenty-four environmentally-sensitive cyanopyridine fluorophores bearing pyrene and/ or fluorene with different para-substituted-phenyl moieties that have been previously designed and synthesized by us are studied in depth for their photophysical properties. Initially, the optical performances of the compounds were investigated by employing UV-visible and fluorescence spectroscopic tools in various aprotic and protic solvents. All the compounds exhibited absorption bands between 310 and 452 nm, and emission bands between 454 and 633 nm. High sensitivity emission spectra with solvents of different polarities were recorded and studied. The fluorescence quantum yield (ϕ_f) increased in solvents of low polarity and decreased on increasing the polarity of solvents. On the other hand, in case of strong electron donating (-NMe₂) and strong electron attracting (-CN) substitution, a pronounced increase in Stokes shifts (up to 252 nm, 14250 cm^-1) were recorded. Lippert-Mataga and Reichardts correlations, applied for estimating the variation in dipole moments (Δμ), suggested that the emissive state of designed fluorescence 3-cyanopyridine derivatives is of strong ICT character. The aprotic and protic solvents gave a linear plot for the Stokes shifts in a Lippert-Mataga plot, which appeared as two distinct domains in E_T(30) scales indicating the presence of hydrogen bondings. It was observed that for compounds 5b - 8b, with (-NMe₂) group on the skeleton of phenyl ring, the Lippert-Mataga and Reichardt-Dimroth's plots deviated from linearity signifying that 5b - 8b molecules were involved in specific interaction with protic solvents.

Spectroscopic and electrochemical properties of thiophene-phenylene based Shiff-bases with alkoxy side groups, towards photovoltaic applications

The influence of presence and elongation of alkoxy side chains in the π-conjugated Schiff-bases has been considered on the basis of UV-Vis absorption and photoluminescence spectra of model compounds and polymers solutions in chloroform and binary solvents of different polarity. The results of these investigations have been supported by electrochemical data. It has been demonstrated that introduction of electron donating methoxy side groups decreases the energy gap, however the elongation of alkyl chains only slightly affects the electronic structure of model compounds. In the case of polymers, such octyloxy side chains improves the solubility, enabling formation of longer polymer chains, with the enhanced effective π-conjugation length and narrower energy gap, however the intensity of emission band clearly decreased. Positive solvatochromism has been observed in both absorbance and photoluminescence spectra for all investigated compounds. As the concluding task, bulk-heterojunction (BHJ) photovoltaic (PV) structures, consisting of polyazomethines blended with the fullerene derivative, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been prepared and tested in the context of potential application in solar cells. All investigated polymers have shown the photovoltaic effect, but the best power conversion efficiency and other PV parameters have been obtained for polyazomthine with octyloxy side chains.

Solvatochromic and pH-Sensitive Fluorescent Membrane Probes for Imaging of Live Cells

Membrane trafficking is essential for all cells, and visualizing it is particularly useful for studying neuronal functions. Here we report the synthesis, characterization, and application of several membrane- and pH-sensitive probes suitable for live-cell fluorescence imaging. These probes are based on a 1,8-naphthalimide fluorophore scaffold. They exhibit a solvatochromic effect, and one of them, ND6, shows a substantial fluorescence difference between pH 6 and 7. The solvatochromic effect and pH-sensitivity of those probes are explained using quantum chemical calculations, and molecular dynamics simulation confirms their integration and interaction with membrane lipids. For live-cell fluorescence imaging, we tested those probes in a cancer cell line (MCF7), cancer spheroids (MDA-MB-468), and cultured hippocampal neurons. Confocal imaging showed an excellent signal-to-noise ratio from 400:1 to about 1300:1 for cell membrane labeling. We applied ND6 during stimulation to label nerve terminals via dye uptake during evoked synaptic vesicle turnover. By ND6 imaging, we revealed cholesterol's multifaced role in replenishing synaptic vesicle pools. Our results demonstrate these fluorescent probes' great potential in studying membrane dynamic and synaptic functions in neurons and other secretory cells and tissues.

Array-Based Discriminative Optical Biosensors for Identifying Multiple Proteins in Aqueous Solution and Biofluids

Identification of proteins is an important issue both in medical research and in clinical practice as a large number of proteins are closely related to various diseases. Optical sensor arrays with recognition ability have been flourished to apply for distinguishing multiple chemically or structurally similar analytes and analyzing unknown or mixed samples. This review gives an overview of the recent development of array-based discriminative optical biosensors for recognizing proteins and their applications in real samples. Based on the number of sensor elements and the complexity of constructing array-based discriminative systems, these biosensors can be divided into three categories, which include multi-element-based sensor arrays, environment-sensitive sensor arrays and multi-wavelength-based single sensing systems. For each strategy, the construction of sensing platform and detection mechanism are particularly introduced. Meanwhile, the differences and connections between different strategies were discussed. An understanding of these aspects may help to facilitate the development of novel discriminative biosensors and expand their application prospects.

Femto- to Millisecond Time-Resolved Photodynamics of a Double-Functionalized Push-Pull Organic Linker: Potential Candidate for Optoelectronically Active MOFs

The design of improved organic linkers for the further engineering of smarter metal–organic framework (MOF) materials has become a paramount task for a wide number of material scientists. In this report, a luminescent double-functionalized push–pull (electron donor–acceptor) archetype organic molecule, dimethyl 4-amino-8-cyanonaphthalene-2,6-dicarboxylate (Me₂CANADC), has been synthesized and characterized. The optical steady-state properties of Me₂CANADC are strongly influenced by the surrounding environment as a direct consequence of its strong charge transfer (CT) character. The relaxation from its first electronically excited singlet state follows a double pathway: (1) on one side deactivating from its local excited (LE) state in the sub-picosecond or picosecond time domain, and (2) on the other side undergoing an ultrafast intramolecular charge transfer (ICT) reaction that is slowing down in viscous solvents. The deactivation to the ground state of these species with CT character is the origin of the Me₂CANADC luminescence, and they present solvent-dependent lifetime values ranging from 8 to 18 ns. The slow photodynamics of Me₂CANADC unveils the coexistence of a non-emissive triplet excited state and the formation of a long-lived charge separated state (2 µs). These observations highlight the promising optical properties of Me₂CANADC linker, opening a window for the design of new functional MOFs with huge potential to be applied in the fields of luminescent sensing and optoelectronics.

Fluorescent amino acids as versatile building blocks for chemical biology

Fluorophores have transformed the way we study biological systems, enabling non-invasive studies in cells and intact organisms, which increase our understanding of complex processes at the molecular level. Fluorescent amino acids have become an essential chemical tool because they can be used to construct fluorescent macromolecules, such as peptides and proteins, without disrupting their native biomolecular properties. Fluorescent and fluorogenic amino acids with unique photophysical properties have been designed for tracking protein-protein interactions in situ or imaging nanoscopic events in real time with high spatial resolution. In this Review, we discuss advances in the design and synthesis of fluorescent amino acids and how they have contributed to the field of chemical biology in the past 10 years. Important areas of research that we review include novel methodologies to synthesize building blocks with tunable spectral properties, their integration into peptide and protein scaffolds using site-specific genetic encoding and bioorthogonal approaches, and their application to design novel artificial proteins, as well as to investigate biological processes in cells by means of optical imaging.

4-Aminophthalimide Amino Acids as Small and Environment-Sensitive Fluorescent Probes for Transmembrane Peptides

Fluorescence probing of transmembrane (TM) peptides is needed to complement state-of-the art methods-mainly oriented circular dichroism and solid-state NMR spectroscopy-and to allow imaging in living cells. Three new amino acids incorporating the solvatofluorescent 4-aminophthalimide in their side chains were synthesized in order to examine the local polarity in the α-helical TM fragment of the human epidermal growth factor receptor. It was possible to distinguish their locations, either in the hydrophobic core of the lipid bilayer or at the membrane surface, by fluorescence readout, including blue shift and increased quantum yield. An important feature is the small size of the 4-aminophthalimide chromophore. It makes one of the new amino acids approximately isosteric to tryptophan, typically used as a very small fluorescent amino acid in peptides and proteins. In contrast to the only weakly fluorescent indole system in tryptophan, the 4-aminophthalimide moiety produces a significantly more informative fluorescence readout and is selectively excited outside the biopolymer absorption range.

Visualizing TRPA1 in the Plasma Membrane for Rapidly Screening Optical Control Agonists via a Photochromic Ligand Based Fluorescent Probe

Fluorescent probes have been used as effective methods for profiling proteins in biological systems because of their high selectivity, sensitivity, and temporal-spatial resolution. A specific fluorescent probe for understanding the function of the transient receptor potential ankyrin 1 (TRPA1) channel that is closely related with various diseases like persistent pain, respiratory, and chronic itch syndromes, however, is still lacking. Here, we report a "turn-on" fluorescent probe (A1CA) for visualizing TRPA1 channels in the plasma membrane of live cells based on a photochromic ligand derived from 4-(phenylazo)benzenamine. Evaluating the specificity and sensitivity of A1CA by electrophysiology and confocal imaging showed that the A1CA probe displays higher affinity and selectivity to TRPA1 channel versus all other ion channels including TRPV1, TRPV3, Nav1.4, Nav1.5, and hERG. Based on the supporting evidence, A1CA has great potential as a molecular imaging probe for high-throughput screening of novel TRPA1 agonists.

R. M, Gurusamy Thangavelu SA, Krishnamoorthy A. Tunable yellow–green emitting cyclotriphosphazene appended phenothiazine hydrazone hybrid material: synthesis, characterisation, photophysical and electrochemical studies

A novel tunable yellow–green emitting inorganic–organic luminescent hybrid molecule, CTP-PTZ, an assembly of six units of the PTZ hydrazone Schiff base on the periphery of inorganic heterocycle cyclotriphosphazene, is reported.

Development of light-emitting liquid-crystalline polymers with a pentafluorinated bistolane-based luminophore

Light-emitting liquid-crystalline polymers showing PL in the pristine solid state can control their PL color from blue to light-blue via a thermal phase transition to LC phases, which originates from a dynamic change of aggregated structures.

Revealing the dynamics and energetics of interaction of a cationic biological photosensitizer within a bile salt aggregate

The present investigation reports a detailed characterization of the interaction of a cationic photosensitizer, phenosafranin (PSF) with sodium deoxycholate (NaDC) bile salt aggregates based on spectroscopic and calorimetric techniques. Our explicit spectroscopic results not only establish the occurrence of PSF-NaDC binding interaction, but also reveal marked lowering of micropolarity at the interaction site (E_T(30) = 55.97 kcal mol^-1 in the presence of NaDC as compared to E_T(30) = 63.1 kcal mol^-1 in bulk aqueous buffer). A thorough mathematical analysis of the fluorescence depolarization results based on the two-step and wobbling in cone model yields critical insight into the complex rotational relaxation dynamics of the bound drug. The impartation of motional restriction on the PSF molecules within the bile salt aggregates is evidenced from enhancement of average rotational correlation time from <τ_r> = 136 ps in aqueous buffer to 1.11 ns with added NaDC (8.0 mM). This is further supported from a high value of the generalized order parameter (S = 0.81) as well as the diffusion coefficient (D_w = 1.40 × 10¹² s^-1). Furthermore, our extensive calorimetric investigation unveils the complicated thermodynamics of the interaction process in terms of predominant entropic contribution over the enthalpic part in the lower temperature regime (TΔS = 18.84 ± 1.13 kJ mol^-1, ΔH = -5.82 ± 0.35 kJ mol^-1 at 288 K) with subsequent reversal of the relative contributions with increasing temperature (TΔS = 7.54 ± 0.39 kJ mol^-1, ΔH = - 17.09 ± 0.90 kJ mol^-1 at 318 K). The instrumental role of the hydrophobic effect underlying the PSF-NaDC interaction is characterized by a negative heat capacity change (ΔC_p = -364 J mol^-1 K^-1). An intriguing thermodynamic feature in terms of enthalpy-entropy compensation (with increasing temperature ΔG remains almost constant while ΔH and TΔS vary significantly) aptly corroborates the aforesaid argument and establishes an appreciable hydrophobic contribution to the overall binding energies.

Ladder-Type Dye with Large Transition Dipole Moment for Solvatochromism and Microphase Visualization

A large transition dipole moment is usually pursued by strategies of twisted intramolecular charge transfer (TICT) or planar intramolecular charge transfer (PICT) to obtain obvious Stokes shifts and dramatic color changes with tuning of polarities. However, both strategies have their drawbacks and suffer from fluorescence quenching in solid states. Herein, a ladder-type molecule ISOAA-H with an intramolecular hydrogen bond is designed, which undergoes intramolecular charge transfer and proton shift to harvest a large transition dipole moment under light irradiation. Thanks to its out-of-plane side chains, the intermolecular π-π stacking of backbones is prohibited and solid emission is generated. ISOAA-H exhibits outstanding solvatochromic behavior with polarity changes of solvents or polymer matrixes and is successfully used to detect the microphase separation of polymer blends. These results indicate that a strategy combining the advantages of TICT and PICT is established for environment-sensitive dyes used in both solution and solid state.

Synthesis and characterization of bent fluorine-containing donor-π-acceptor molecules as intense luminophores with large Stokes shifts

Herein, we prepared novel bent fluorine-containing donor-π-acceptor (D-π-A) molecules from commercially available octafluorocyclopentene using a facile two-step procedure, revealing that the above molecules absorb UV-light and exhibit yellow photoluminescence (PL) with high PL efficiencies (ΦPL) in solution. The corresponding Stokes shifts exceeded 10 000 cm-1, and the maximum PL wavelength (λPL) strongly depended on solvent polarity or intermolecular interactions in the solid state. On the basis of a Lippert-Mataga plot, PL was confidently assigned to radiative relaxation from an intramolecular charge-transfer excited state. Moreover, the synthesized luminophores showed intense PL even in the crystalline state and exhibited alkoxy chain length-dependent PL behavior (e.g., high ΦPL, λPL = 486-540 nm).

Novel V- and Y-Shaped Light-Emitting Liquid Crystals with Pentafluorinated Bistolane-Based Luminophores

Herein, we describe the synthesis of novel light-emitting liquid-crystalline (LC) compounds bearing pentafluorinated bistolane-based luminophores with a V- or a Y-shaped molecular geometry and the evaluation of their LC and photophysical characteristics. The V- or Y-shaped compounds exhibited a unique LC phase and showed photoluminescence (PL) behavior under various circumstances, such as in dilute solution or in the solid state. Notably, PL characteristics were observed even under high-temperature conditions with a crystal (Cr) to LC phase transition, although the PL efficiency (Φ PL) was gradually reduced because of thermal molecular motion. Interestingly, Φ PL was found to be completely recovered through the LC → Cr phase transition during the cooling process; the PL characteristics of the V- or Y-shaped compounds were sensitively changed by external thermal stress, giving these compounds the ability to act as thermoresponsive PL sensing materials.

A three-dimensional ratiometric sensing strategy on unimolecular fluorescence-thermally activated delayed fluorescence dual emission

Visualized sensing through fluorescence signals is a powerful method for chemical and physical detection. However, the utilization of fluorescent molecular probes still suffers from lack of precise signal self-calibration in practical use. Here we show that fluorescence and thermally activated delayed fluorescence can be simultaneously produced at the single-molecular level. The thermally activated delayed fluorescence serves as a sensing signal with its wavelength and lifetime both altered correlating to polarity, whereas the fluorescence always remains unchanged as an internal reference. Upon the establishment of a three-dimensional working curve upon the ratiometric wavelength and photoluminescence lifetime vs. polarity, disturbance factors during a relevant sensing process can be largely minimized by such a multiple self-calibration. This strategy was further applied into a precise detection of the microenvironmental polarity variation in complex phospholipid systems, towards providing new insights for convenient and accurate diagnosis of membrane lesions.

New photoluminescent strategies are required to realize high-performance ratiometric sensors for bio-sensing applications. Here, the authors demonstrate a fluorescence-thermally activated delayed fluorescence dual emission strategy for ratiometric sensors with high precision.

Deciphering micro-polarity inside the endoplasmic reticulum using a two-photon active solvatofluorochromic probe

A new class of two-photon active and solvatofluorochromic dyes for the determination of ER polarity is reported. The fluorescent colour spans almost the entire visible spectrum. One of the derivatives is rationally designed for specific ER targeting. Finally, the fluorescence spectral scanning technique has been utilised to determine the micro-polarity inside the ER which is found to be much lower than that of water.

Weak Donor-/Strong Acceptor-Linked Anthracenyl π-Conjugates as Solvato(fluoro)chromophore and AEEgens: Contrast between Nitro and Cyano Functionality

Steady development on photophysical behaviors for a variety of organic fluorophores inspired us to generate anthracene-based fluorescent molecules with a strong acceptor and a significantly weak donor through a π-spacer. Such molecules are found to have substantial twisted conformational orientations in the solid state and enhanced apolar character because of the attachment of tolyl or mesityl group with an anthracenyl core. Upon exposure to a variety of solvents, strong solvatochromism was noticed for 4-nitro compound (84 nm red shift) in contrast to the cyano analogue (18 nm red shift). Both these probes were highly emissive in apolar solvents while nitro-analogue, in particular, could discriminate the solvents of E _T(30) (a measure of microscopic solvent polarity) ranging from 31 to 37. Thus, 4-nitro compounds can be successfully employed to detect and differentiate the apolar solvents. On the contrary, the 2-nitro analogue is almost nonemissive for the same range of solvents perhaps because of favorable excited-state intramolecular proton-transfer process. The fundamental understanding of solvatochromic properties through the formation of twisted intramolecular charge-transfer (TICT) state is experimentally analyzed by synthesizing and studying the π-conjugates linked to only benzene in place of nitro or cyanobenzene, which exhibits no solvatochromism and that helped finding the possible emission, originated from the locally excited state. Moreover, the molecular structures for these compounds are determined by the single-crystal X-ray diffraction studies to examine the change in emission properties with molecular packing and alignment in the aggregated state. The measurement of dihedral angles between the substituents and anthracenyl core was helpful in finding the possible extent of electronic conjugations within the system to decipher both solvatochromism and aggregation enhanced emission (AEE)-behavior. The cyano analogue exhibited prominent AEE-behavior, whereas nitro analogues showed the aggregation-caused quenching effect. The reason behind such dissimilarity in solvatochromism and AEE-behavior between cyano- and nitro-linked anthracenyl π-conjugates are also addressed through experimental outcomes.

Quinone-Derived π-Extended Phenazines as New Fluorogenic Probes for Live-Cell Imaging of Lipid Droplets

We describe a new synthetic methodology for the preparation of fluorescent π-extended phenazines from the naturally-occurring naphthoquinone lapachol. These novel structures represent the first fluorogenic probes based on the phenazine scaffold for imaging of lipid droplets in live cells. Systematic characterization and analysis of the compounds in vitro and in cells led to the identification of key structural features responsible for the fluorescent behavior of quinone-derived π-extended phenazines. Furthermore, live-cell imaging experiments identified one compound (P1) as a marker for intracellular lipid droplets with minimal background and enhanced performance over the lipophilic tracker Nile Red.

Three-Component Activation/Alkynylation/Cyclocondensation (AACC) Synthesis of Enhanced Emission Solvatochromic 3-Ethynylquinoxalines

2-Substituted 3-ethynylquinoxaline chromophores can be readily synthesized by a consecutive activation-alkynylation-cyclocondensation (AACC) one-pot sequence in a three-component manner. In comparison with the previously published four-component glyoxylation starting from electron-rich π-nucleophiles, the direct activation of (hetero)aryl glyoxylic acids allows the introduction of substituents that cannot be directly accessed by glyoxylation. By introducing N,N-dimethylaniline as a strong donor in the 2-position, the emission solvatochromicity of 3-ethynylquinoxalines can be considerably enhanced to cover the spectral range from blue-green to deep red-orange with a single chromophore in a relatively narrow polarity window. The diversity-oriented nature of the synthetic multicomponent reaction concept enables comprehensive investigations of structure-property relationships by Hammett correlations and Lippert-Mataga analysis, as well as the elucidation of the electronic structure of the emission solvatochromic π-conjugated donor-acceptor systems by DFT and time-dependent DFT calculations with the PBEh1PBE functional for a better reproduction of the dominant charge-transfer character of the longest wavelength absorption band.

A new probe for detecting zinc-bound carbonic anhydrase in cell lysates and cells

We report the synthesis and application of a small molecule probe for carbonic anhydrase (CA) to track holo-CA in cell lysates and live-cell models of zinc dyshomeostasis. The probe displays a 12-fold increase in fluorescence upon binding to bovine CA and also responds to human CA isoforms.

Fluorescence behaviour of 2-, 3- and 4-amino-1,8-naphthalimides: effects of the substitution positions of the amino functionality on the photophysical properties

2-Amino-1,8-naphthalimide showed blue fluorescence independent of the microenvironment of the solvents whereas the corresponding 3- and 4-amino derivatives displayed marked positive solvatofluorochromism.

Design and synthesis of solution processable green fluorescent D–π–A dyads for OLED applications

Synthesized solution processable green fluorescent donor–acceptor dyads and their investigated photophysical, electrochemical, and morphological properties for OLED applications.

One-pot syntheses of blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones by T3P® activation of 3-arylpropiolic acids

In situ activation of 3-arylpropiolic acids with T3P^® (n-propylphosphonic acid anhydride) initiates a domino reaction furnishing 4-arylnaphtho[2,3-c]furan-1,3-diones in excellent yields. Upon employing these anhydrides as reactive intermediates blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones are formed by consecutive pseudo three-component syntheses in a one-pot fashion. The Stokes shifts correlate excellently with the Hammett-Taft σ_R parameter indicating an extended degree of resonance stabilization in the vibrationally relaxed excited singlet state.

Chameleonic Dye Adapts to Various Environments Shining on Macrocycles or Peptide and Polysaccharide Aggregates

This work describes latent fluorescence particles (LFPs) based on a new environmentally sensitive carbazole compound aggregated in water and their use as sensors for probing various cavitands and the different stages of aggregating systems. Cyclodextrins (CDs), cucurbit[n]urils (CB[n], n = 6, 7, 8), and a resorcinarene capsule were used to study the dynamic nature of the LFPs. The fluorescence was dramatically enhanced by a proposed disaggregation-induced emission enhancement (DIEE) mechanism with specific features for CB[n]. Then, the aggregated states of the dipeptides Leu-Leu, Phe-Phe, and Fmoc-Leu-Leu (vesicles, crystals, fibers) were studied by fluorescence spectroscopy and confocal fluorescence microscopy thanks to the adaptive and emissive behavior of the LFPs, allowing us to study an interesting polymorphism phenomenon. The LFPs have then been used in the sensing of the aggregation of the polysaccharide alginate, for which distinct fluorescence turn-on is detected upon stepwise biopolymer assembly, and for amylose detection. The carbazole particles not only adapt to various environments but also display multicolor fluorescent signals. They can be used for the fast probing of the aggregation propensity of newly prepared molecules or biologically relevant compounds or to accelerate the discovery of new macrocycles or of self-assembling peptides in water.