Solvatochromic and Fluorogenic Dyes as Environment-Sensitive Probes: Design and Biological Applications

Simple cyclic chalcone dye with multiple optical functions: Piezochromism and lysosomes staining

Both artificially synthesized and naturally occurring cyclic chalcones have been widely studied for their excellent biological activities. However, research on its photophysical properties is still limited. In the present study, we designed and synthesized a small molecule fluorescent dye based on the ICT effect, using dimethylamino as the electron-donating group and carbonyl as the electron withdrawing group, and investigated its photophysical properties in depth. Although YB is a simple small molecule, it exhibits significant piezochromic properties. The fluorescence of YB can change from green to yellow through grinding. After solvent fumigation, the fluorescence reverts to green. Furthermore, YB was used successfully in the lysosomal targeting. This study expands the research on the photophysical properties of cyclic chalcone and give richness to application of cyclic chalcone compounds.

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.

Core-Alkynylated Fluorescent Flippers: Altered Ultrafast Photophysics to Track Thick Membranes

Fluorescent flippers have been introduced as small-molecule probes to image membrane tension in living systems. This study describes the design, synthesis, spectroscopic and imaging properties of flippers that are elongated by one and two alkynes inserted between the push and the pull dithienothiophene domains. The resulting mechanophores combine characteristics of flippers, reporting on physical compression in the ground state, and molecular rotors, reporting on torsional motion in the excited state, to take their photophysics to new level of sophistication. Intensity ratios in broadened excitation bands from differently twisted conformers of core-alkynylated flippers thus report on mechanical compression. Lifetime boosts from ultrafast excited-state planarization and lifetime drops from competitive intersystem crossing into triplet states report on viscosity. In standard lipid bilayer membranes, core-alkynylated flippers are too long for one leaflet and tilt or extend into disordered interleaflet space, which preserves rotor-like torsional disorder and thus weak, blue-shifted fluorescence. Flipper-like planarization occurs only in highly ordered membranes of matching leaflet thickness, where they light up and selectively report on these thick membranes with red-shifted, sharpened excitation maxima, high intensity and long lifetime.

D-π-A type fluorescent dyes: Effect of π-bridge units on optical and G4 DNA binding properties

Fluorescent solvatochromic dyes that are sensitive to the nature of local microenvironmental, have been explored as probes in applications ranging from the imaging biomolecules to understanding of basic biomolecule functions. To expand the scope of fluorescent solvatochromic dyes for G-quadruplex (G4) DNA structures, and to illustrate the relationship between structure and properties, three newly designed D-π-A type fluorescent dyes were synthesized by introducing diarylimidazole to carbazole skeleton linked to benzene, furan or thiophene π-conjugated bridge and connected with pyridinium acceptor, respectively. Their structural characteristics, optical properties, and G4 DNA binding properties were discussed in detail. In general, the incorporation of furan and thiophene as π-conjugated bridges leads the better conjugation and molecular coplanarity with more efficient intramolecular charge transfer (ICT) effect compared with benzene bridge. The fluorescence intensities induced upon interaction were found that TP-6 with thiophene π-conjugated bridge had the strongest response toward G4 DNAs. In addition, the application of this dye as a fluorescent agent for living cell imaging was also demonstrated.

Regulation of Absorption and Emission in a Protein/Fluorophore Complex

Human cellular retinol binding protein II (hCRBPII) was used as a protein engineering platform to rationally regulate absorptive and emissive properties of a covalently bound fluorogenic dye. We demonstrate the binding of a thio-dapoxyl analog via formation of a protonated imine between an active site lysine residue and the chromophore's aldehyde. Rational manipulation of the electrostatics of the binding pocket results in a 204 nm shift in absorption and a 131 nm shift in emission. The protein is readily expressed in mammalian systems and binds with exogenously delivered fluorophore as demonstrated by live-cell imaging experiments.

Fluorometric Analysis of Carrier-Protein-Dependent Biosynthesis through a Conformationally Sensitive Solvatochromic Pantetheinamide Probe

Carrier proteins (CPs) play a fundamental role in the biosynthesis of fatty acids, polyketides, and non-ribosomal peptides, encompassing many medicinally and pharmacologically relevant compounds. Current approaches to analyze novel carrier-protein-dependent synthetic pathways are hampered by a lack of activity-based assays for natural product biosynthesis. To fill this gap, we turned to 3-methoxychromones, highly solvatochromic fluorescent molecules whose emission intensity and wavelength are heavily dependent on their immediate molecular environment. We have developed a solvatochromic carrier-protein-targeting probe which is able to selectively fluoresce when bound to a target carrier protein. Additionally, the probe displays distinct responses upon CP binding in carrier-protein-dependent synthases. This discerning approach demonstrates the design of solvatochromic fluorophores with the ability to identify biosynthetically active CP-enzyme interactions.

Discovery of GPR84 Fluorogenic Probes Based on a Novel Antagonist for GPR84 Bioimaging

GPR84 is a promising therapeutic target and biomarker for a range of diseases. In this study, we reported the discovery of BINOL phosphate (BINOP) derivatives as GPR84 antagonists. By investigating the structure-activity relationship, we identified 15S as a novel GPR84 antagonist. 15S exhibits low nanomolar potency and high selectivity for GPR84, while its enantiomer 15R is less active. Next, we rationally designed and synthesized a series of GPR84 fluorogenic probes by conjugating Nile red and compound 15S. The leading hybrid, probe F8, not only retained GPR84 activity but also exhibited low nonspecific binding and a turn-on fluorescent signal in an apolar environment. F8 enabled visualization and detection of GPR84 in GPR84-overexpressing HEK293 cells and lipopolysaccharide-stimulated neutrophils. Furthermore, we demonstrated that F8 can detect upregulated GPR84 protein levels in mice models of inflammatory bowel disease and acute lung injury. Thus, compound F8 represents a promising tool for studying GPR84 functions.

Fluorescence Lifetime Imaging of Lipid Heterogeneity in the Inner Mitochondrial Membrane with a Super-photostable Environment-Sensitive Probe

The inner mitochondrial membrane (IMM) undergoes dynamic morphological changes, which are crucial for the maintenance of mitochondrial functions as well as cell survival. As the dynamics of the membrane are governed by its lipid components, a fluorescent probe that can sense spatiotemporal alterations in the lipid properties of the IMM over long periods of time is required to understand mitochondrial physiological functions in detail. Herein, we report a red-emissive IMM-labeling reagent with excellent photostability and sensitivity to its environment, which enables the visualization of the IMM ultrastructure using super-resolution microscopy as well as of the lipid heterogeneity based on the fluorescence lifetime at the single mitochondrion level. Combining the probe and fluorescence lifetime imaging microscopy (FLIM) showed that peroxidation of unsaturated lipids in the IMM by reactive oxygen species caused an increase in the membrane order, which took place prior to mitochondrial swelling.

Targeted mitochondrial fluorescence probe with large stokes shift for detecting viscosity changes in vivo and in ferroptosis process

We created four fluorescent sensors in our work to determine the viscosity of mitochondria. Following screening, the probe Mito-3 was chosen because in contrast to the other three probes, it had a greater fluorescence enhancement, large Stokes shift (113 nm) and had a particular response to viscosity that was unaffected by polarity or biological species. As the viscosity increased from PBS to 90 % glycerol, the fluorescence intensity of probe at 586 nm increased 17-fold. Mito-3 has strong biocompatibility and is able to track changes in cell viscosity in response to nystatin and monensin stimulation. Furthermore, the probe has been successfully applied to detect changes in viscosity caused by nystatin and monensin in zebrafish. Above all, the probe can be applied to the increase in mitochondrial viscosity that accompanies the ferroptosis process. Mito-3 has the potential to help further study the relationship between viscosity and ferroptosis.

Temperature-Modulated Evolution of Surface Structures Induces Significant Enhancement of Two-Photon Fluorescent Emission from a Dye Molecule

Fluorescence is an essential property of molecules and materials that plays a pivotal role across various areas such as lighting, sensing, imaging, and other applications. For instance, temperature-sensitive fluorescence emission is widely utilized for chemo-/biosensing but usually decreases the intensity upon the increase in temperature. In this study, we observed a temperature-induced enhancement of up to ∼150 times in two-photon fluorescence (TPF) emission from a dye molecule, 4-(4-diethylaminostyry)-1-methylpyridinium iodide (D289), as it interacted with binary complex vesicles composed of two commonly applied surfactants: sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB). By employing second harmonic generation (SHG) and TPF techniques, we clearly revealed the temperature-dependent kinetic behavior of D289 on the surface of the vesicles and utilized it to interpret the origin of the significant TPF enhancement. Additionally, we also demonstrated a similar heating-induced enhancement of the TPF emission from D289 on the membrane of phospholipid vesicles, indicating the potential application of TPF in temperature sensing in the biology systems. The embedding of D289 in the tightly packed alkane chains was identified as the key factor in enhancing the TPF emission from D289. This finding may provide valuable information for synthesizing fluorescence materials with a high optical yield.

Structural insight into piezo-solvatochromism of Reichardt's dye

To date, accurate modelling of the solvation process is challenging, often over-simplifying the solvent-solute interactions. The interplay between the molecular arrangement associated with the solvation process and crystal nucleation has been investigated by analysis of the piezo-solvatochromic behaviour of Reichardt's dye, ET(1), in methanol, ethanol and acetone under high pressure. High-pressure single-crystal X-ray diffraction and UV-Vis spectroscopy reveal the impact of solute-solvent interactions on the optical properties of ET(1). The study underscores the intricate relationship between solvent properties, molecular conformation and crystal packing. The connection between liquid and solid phases emphasizes the capabilities of high-pressure methods for expanding the field of crystal engineering. The high-pressure environment allowed the determination of the crystal structures reported here that are built from organic molecules fourfold solvated with ethanol or methanol: ET(1)·4CH3OH and ET(1)·4C2H5OH·H2O. The observed piezo-solvatochromic effects highlight the potential of ET(1) in nonlinear optoelectronics and expand the application of solvatochromic chemical indicators to pressure sensors.

Effects and avoidance of photoconversion-induced artifacts in confocal and STED microscopy

Fluorescence microscopy is limited by photoconversion due to continuous illumination, which results in not only photobleaching but also conversion of fluorescent molecules into species of different spectral properties through photoblueing. Here, we determined different fluorescence parameters of photoconverted products for various fluorophores under standard confocal and stimulated emission depletion (STED) microscopy conditions. We observed changes in both fluorescence spectra and lifetimes that can cause artifacts in quantitative measurements, which can be avoided by using exchangeable dyes.

Alared: Solvatochromic and Fluorogenic Red Amino Acid for Ratiometric Live-Cell Imaging of Bioactive Peptides

To fill the need for environmentally sensitive fluorescent unnatural amino acids able to operate in the red region of the spectrum, we have designed and synthesized Alared, a red solvatochromic and fluorogenic amino acid derived from the Nile Red chromophore. The new unnatural amino acid can be easily integrated into bioactive peptides using classical solid-phase peptide synthesis. The fluorescence quantum yield and the emission maximum of Alared-labeled peptides vary in a broad range depending on the peptide's environment, making Alared a powerful reporter of biomolecular interactions. Due to its red-shifted absorption and emission spectra, Alared-labeled peptides could be followed in living cells with minimal interference from cellular autofluorescence. Using ratiometric fluorescence microscopy, we were able to track the fate of the Alared-labeled peptide agonists of the apelin G protein-coupled receptor upon receptor activation and internalization. Due to its color-shifting environmentally sensitive emission, Alared allowed for distinguishing the fractions of peptides that are specifically bound to the receptor or unspecifically bound to different cellular membranes.

Nonpolar selective emission (NPSE) of carbonyl-bridged rhodols

Polarity-responsive luminophores (PRLs), whose emission properties change in response to the polarity of the surrounding environment, are used for the fluorescence sensing of intracellular environments and various chemical compounds. Herein, we propose a concept called nonpolar selective emission (NPSE) for the development of a new PRL family. Unlike the conventional emission of PRLs, the NPSE luminophore can switch to a completely non-emissive state upon a slight increase in solvent polarity. The NPSE concept offers a new means of distinguishing between nonpolar and low-polarity environments. Moreover, the NPSE property is little affected by the viscosity of the surrounding medium. We demonstrate that NPSE dyes can be used as emission sensors for molecular gases. Furthermore, we discovered the potential use of NPSE dyes as a time-dependent security ink triggered by the volatilization of polar molecules.

Fluorescence lifetime imaging microscopy of flexible and rigid dyes probes the biophysical properties of synthetic and biological membranes

Sensing of the biophysical properties of membranes using molecular reporters has recently regained widespread attention. This was elicited by the development of new probes of exquisite optical properties and increased performance, combined with developments in fluorescence detection. Here, we report on fluorescence lifetime imaging of various rigid and flexible fluorescent dyes to probe the biophysical properties of synthetic and biological membranes at steady state as well as upon the action of external membrane-modifying agents. We tested the solvatochromic dyes Nile red and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (NBD), the viscosity sensor Bodipy C12, the flipper dye FliptR, as well as the dyes 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO), Bodipy C16, lissamine-rhodamine, and Atto647, which are dyes with no previous reported environmental sensitivity. The performance of the fluorescent probes, many of which are commercially available, was benchmarked with well-known environmental reporters, with Nile red and Bodipy C12 being specific reporters of medium hydration and viscosity, respectively. We show that some widely used ordinary dyes with no previous report of sensing capabilities can exhibit competing performance compared to highly sensitive commercially available or custom-based solvatochromic dyes, molecular rotors, or flipper in a wide range of biophysics experiments. Compared to other methods, fluorescence lifetime imaging is a minimally invasive and nondestructive method with optical resolution. It enables biophysical mapping at steady state or assessment of the changes induced by membrane-active molecules at subcellular level in both synthetic and biological membranes when intensity measurements fail to do so. The results have important consequences for the specific choice of the sensor and take into consideration factors such as probe sensitivity, response to environmental changes, ease and speed of data analysis, and the probe's intracellular distribution, as well as potential side effects induced by labeling and imaging.

Chemical Control of Fluorescence Lifetime towards Multiplexing Imaging

Fluorescence lifetime imaging has been a powerful tool for biomedical research. Recently, fluorescence lifetime-based multiplexing imaging has expanded imaging channels by using probes that harbor the same spectral channels and distinct excited state lifetime. While it is desirable to control the excited state lifetime of any given fluorescent probes, the rational control of fluorescence lifetimes remains a challenge. Herein, we chose boron dipyrromethene (BODIPY) as a model system and provided chemical strategies to regulate the fluorescence lifetime of its derivatives with varying spectral features. We find electronegativity of structural substituents at the 8' and 5' positions is important to control the lifetime for the green-emitting and red-emitting BODIPY scaffolds. Mechanistically, such influences are exerted via the photo-induced electron transfer and the intramolecular charge transfer processes for the 8' and 5' positions of BODIPY, respectively. Based on these principles, we have generated a group of BODIPY probes that enable imaging experiments to separate multiple targets using fluorescence lifetime as a signal. In addition to BODIPY, we envision modulation of electronegativity of chemical substituents could serve as a feasible strategy to achieve rational control of fluorescence lifetime for a variety of small molecule fluorophores.

A ratiometric fluorescent probe revealing the abnormality of acetylated tau by visualizing polarity in Alzheimer's disease

Abnormal neuronal polarity leads to early deficits in Alzheimer's disease (AD) by affecting the function of axons. Precise and rapid evaluation of polarity changes is very important for the early prevention and diagnosis of AD. However, due to the limitations of existing detection methods, the mechanism related to how neuronal polarity changes in AD is unclear. Herein, we reported a ratiometric fluorescent probe characterized by neutral molecule to disclose the polarity changes in nerve cells and the brain of APP/PS1 mice. Cy7-K showed a sensitive and selective ratiometric fluorescence response to polarity. Remarkably, unlike conventional intramolecular charge transfer fluorescent probes, the fluorescence quantum yield of Cy7-K in highly polar solvents is higher than that in low polar solvents due to the transition of neutral quinones to aromatic zwitterions. Using the ratiometric fluorescence imaging, we found that beta-amyloid protein (Aβ) inhibits the expression of histone deacetylase 6, thereby increasing the amount of acetylated Tau protein (AC-Tau) and ultimately enhancing cell polarity. There was a high correlation between polarity and AC-Tau. Furthermore, Cy7-K penetrated the blood-brain barrier to image the polarity of different brain regions and confirmed that APP/PS1 mice had higher polarity than Wild-type mice. The probe Cy7-K will be a promising tool for assessing the progression of AD development by monitoring polarity.

Charge-regulated fluorescent anchors enable high-fidelity tracking of plasma membrane dynamics during biological events

Many biological processes generally require long-term visualization tools for time-scale dynamic changes of the plasma membrane, but there is still a lack of design rules for such imaging tools based on small-molecule fluorescent probes. Herein, we revealed the key regulatory roles of charge number and species of fluorescent dyes in the anchoring ability of the plasma membrane and found that the introduction of multi-charged units and appropriate charge species is often required for fluorescent dyes with strong plasma membrane anchoring ability by systematically investigating the structure-function relationship of cyanostyrylpyridium (CSP) dyes with different charge numbers and species and their imaging performance for the plasma membrane. The CSP-DBO dye constructed exhibits strong plasma membrane anchoring ability in staining the plasma membrane of cells, in addition to many other advantages such as excellent biocompatibility and general universality of cell types. Such a fluorescent anchor has been successfully used to monitor chemically induced plasma membrane damage and dynamically track various cellular biological events such as cell fusion and cytokinesis over a long period of time by continuously monitoring the dynamic morphological changes of the plasma membrane, providing a valuable precise visualization tool to study the physiological response to chemical stimuli and reveal the structural morphological changes and functions of the plasma membrane during these important biological events from a dynamic perspective. Furthermore, CSP-DBO exhibits excellent biocompatibility and imaging capability in vivo such as labelling the plasma membrane in vivo and monitoring the metabolic process of lipofuscin as an aging indicator.

Single-Molecule Spectroscopy and Super-Resolution Mapping of Physicochemical Parameters in Living Cells

By superlocalizing the positions of millions of single molecules over many camera frames, a class of super-resolution fluorescence microscopy methods known as single-molecule localization microscopy (SMLM) has revolutionized how we understand subcellular structures over the past decade. In this review, we highlight emerging studies that transcend the outstanding structural (shape) information offered by SMLM to extract and map physicochemical parameters in living mammalian cells at single-molecule and super-resolution levels. By encoding/decoding high-dimensional information-such as emission and excitation spectra, motion, polarization, fluorescence lifetime, and beyond-for every molecule, and mass accumulating these measurements for millions of molecules, such multidimensional and multifunctional super-resolution approaches open new windows into intracellular architectures and dynamics, as well as their underlying biophysical rules, far beyond the diffraction limit.

The Influence of the Alkylamino Group on the Solvatochromic Behavior of 5-(4-substituted-arylidene)-1,3-dimethylpyrimidine-2,4,6-triones: Synthesis, Spectroscopic and Computational Studies

Advances in electronics and medical diagnostics have made organic dyes extremely popular as key functional materials. From a practical viewpoint, it is necessary to assess the spectroscopic and physicochemical properties of newly designed dyes. In this context, the condensation of 1,3-dimethylbarbituric acid with electron-rich alkylaminobenzaldehyde derivatives has been described, resulting in a series of merocyanine-type dyes. These dyes exhibit intense blue-light absorption but weak fluorescence. An electron-donating alkylamino group at position C4 is responsible for the solvatochromic behavior of the dyes since the lone electron pair of the nitrogen atom is variably delocalized toward the barbituric ring, which exhibits electron-withdrawing properties. This was elucidated, taking into account the different geometry of the amino group. The intramolecular charge transfer in the molecules is responsible for the relatively high redshift in absorption and fluorescence spectra. Additionally, an increase in solvent polarity moves the absorption and fluorescence to lower energy regions. The observed solvatochromism is discussed in terms of the four-parameter Catalán solvent polarity scale. The differences in the behavior of the dyes were quantified with the aid of time-dependent density functional theory calculations. The obtained results made it possible to find regularities linking the basic spectroscopic properties of the compounds with their chemical structure. This is important in the targeted search for new, practically important dyes.

Readily Accessible and Brightly Fluorogenic BODIPY/NBD-Tetrazines via SNAr Reactions

We describe SNAr reactions of some commercial amino-tetrazines and halo-dyes, which give efficiently quenched BODIPY/NBD-tetrazines (ΦFl < 0.01) in high yields and, importantly, with high purities affordable via simple silica gel chromatography only. The dyes exhibit large Stokes shifts, moderate environmental sensitivity, and emission enhancements (up to 193-fold) upon Tz ligation with BCN─a strained dienophile. They successfully serve as labels for HSA protein premodified with BCN, resulting in bright blue-green emission upon ligation.

Fluorescent Solvatochromic Probes for Long-Term Imaging of Lipid Order in Living Cells

High-resolution spatio-temporal monitoring of the cell membrane lipid order provides visual insights into the complex and sophisticated systems that control cellular physiological functions. Solvatochromic fluorescent probes are highly promising noninvasive visualization tools for identifying the ordering of the microenvironment of plasma membrane microdomains. However, conventional probes, although capable of structural analysis, lack the necessary long-term photostability required for live imaging at the cellular level. Here, an ultra-high-light-resistant solvatochromic fluorescence probe, 2-N,N-diethylamino-7-(4-methoxycarbonylphenyl)-9,9-dimethylfluorene (FπCM) is reported, which enables live lipid order imaging of cell division. This probe and its derivatives exhibit sufficient fluorescence wavelengths, brightness, polarity responsiveness, low phototoxicity, and remarkable photostability under physiological conditions compared to conventional solvatochromic probes. Therefore, these probes have the potential to overcome the limitations of fluorescence microscopy, particularly those associated with photobleaching. FπCM probes can serve as valuable tools for elucidating mechanisms of cellular processes at the bio-membrane level.

Investigating the Photophysical Aspects of a Naphthalene-Based Excited-State Proton Transfer Dye 1-(1H-Benzo[d]imidazol-2-yl)naphthalen-2-ol: pH-Dependent Modulation of Photodynamics

The steady state and time-resolved photophysical behavior of a proton transfer dye 1-(1H-benzo[d]imidazol-2-yl)naphthalen-2-ol (H-BINO) was investigated. The excited state intramolecular proton transfer (ESIPT) reaction in H-BINO is predominant in nonpolar solutions (toluene and DCM) with a lifetime of ∼1.0 ns. However, in polar media (DMF and MeOH), the excited state photodynamics is characterized by a complex equilibrium of emission from the locally excited state (0.1-2.3 ns), the phototautomer (0.5-1.2 ns), and the anionic emission (2.1-5.4 ns). In the solid state, emission from the various aggregated states dictates the photobehavior. Interestingly, the photodynamics in aqueous solution changes starkly as a function of pH with the anionic (2.1 ns) and phototautomeric (0.5-1.0 ns) emissions guiding the photodynamics as the pH of the medium increases. Optimized structural parameters at the proton donor and acceptor sites for the enol and keto forms and the calculated potential energy curve along the proton transfer coordinate at the density functional theory (DFT) level with the B3LYP/6-311G++(d,p) theory support a favorable and barrierless ESIPT process. The current results will surely boost the ongoing research on small molecule emissive materials.

Deciphering the distinct biocontrol activities of lipopeptides fengycin and surfactin through their differential impact on lipid membranes

Lipopeptides produced by beneficial bacilli present promising alternatives to chemical pesticides for plant biocontrol purposes. Our research explores the distinct plant biocontrol activities of lipopeptides surfactin (SRF) and fengycin (FGC) by examining their interactions with lipid membranes. Our study shows that FGC exhibits a direct antagonistic activity against Botrytis cinerea and no marked immune-eliciting activity in Arabidopsis thaliana while SRF only demonstrates an ability to stimulate plant immunity. It also reveals that SRF and FGC exhibit diverse effects on membrane integrity and lipid packing. SRF primarily influences membrane physical state without significant membrane permeabilization, while FGC permeabilizes membranes without significantly affecting lipid packing. From our results, we can suggest that the direct antagonistic activity of lipopeptides is linked to their capacity to permeabilize lipid membrane while the stimulation of plant immunity is more likely the result of their ability to alter the mechanical properties of the membrane. Our work also explores how membrane lipid composition modulates the activities of SRF and FGC. Sterols negatively impact both lipopeptides' activities while sphingolipids mitigate the effects on membrane lipid packing but enhance membrane leakage. In conclusion, our findings emphasize the importance of considering both membrane lipid packing and leakage mechanisms in predicting the biological effects of lipopeptides. It also sheds light on the intricate interplay between the membrane composition and the effectiveness of the lipopeptides, providing insights for targeted biocontrol agent design.

Design and Screening of Fluorescent Probes Based upon Hemicyanine Dyes for Monitoring Mitochondrial Viscosity in Living Cells

Viscosity, at the subcellular level, plays a crucial role as a physicochemical factor affecting microenvironment homeostasis. Abnormal changes in mitochondrial viscosity often lead to various diseases in the organism. Based on the twisted intramolecular charge transfer mechanism, four hemicyanine dye fluorescent probes (HT-SA, HT-SA-S, HT-Bzh, and HT-NA) were designed and synthesized for viscosity response. The single bond between the nitrogen-containing heterocycle and the carbon-carbon double in the structure of the probe bond served as the viscosity response site. Finally, the probe HT-Bzh was screened as the optimal mitochondrial viscosity probe according to its responsiveness, targeting, and interference resistance. The fluorescence intensity of the probe HT-Bzh increased 22-fold when the viscosity was increased from 13.75 to 811.2 cP. In summary, all four viscosity probes we have developed can be used in different applications depending on the external environment, providing a valuable reference for the design of potential tools to address viscosity monitoring in biological systems.

NRhFluors: Quantitative Revealing the Interaction between Protein Homeostasis and Mitochondria Dysfunction via Fluorescence Lifetime Imaging

Degenerative diseases are closely related to the changes of protein conformation beyond the steady state. The development of feasible tools for quantitative detection of changes in the cellular environment is crucial for investigating the process of protein conformational variations. Here, we have developed a near-infrared AIE probe based on the rhodamine fluorophore, which exhibits dual responses of fluorescence intensity and lifetime to local viscosity changes. Notably, computational analysis reveals that NRhFluors fluorescence activation is due to inhibition of the RACI mechanism in viscous environment. In the chemical regulation of rhodamine fluorophores, we found that variations of electron density distribution can effectively regulate CI states and achieve fluorescence sensitivity of NRhFluors. In addition, combined with the AggTag method, the lifetime of probe A9-Halo exhibits a positive correlation with viscosity changes. This analytical capacity allows us to quantitatively monitor protein conformational changes using fluorescence lifetime imaging (FLIM) and demonstrate that mitochondrial dysfunction leads to reduced protein expression in HEK293 cells. In summary, this work developed a set of near-infrared AIE probes activated by the RACI mechanism, which can quantitatively detect cell viscosity and protein aggregation formation, providing a versatile tool for exploring disease-related biological processes and therapeutic approaches.

Aromatic Alcohol-Based pH-Sensitive Chromophore with a Unique Near-Infrared Dual-Band Solvatochromic Property and Its Application as a Ratiometric Fluorescent Sensor for G-Quadruplexes

Solvatochromes have gained great attention because of their unique roles in monitoring biomolecular location, interaction, and dynamics. Particularly, solvatochromes presenting both red-shifting excitation and dual-band switchable emission are in great demand yet significantly difficult to come true. In this article, we disclose an aromatic alcohol-based pH-sensitive chromophore NIR-HBT that not only presents red-shifting excitation and solvent-dependent dual-band emission but also shows high photostability and excellent brightness. To the best of our knowledge, this is the first solvatochrome to simultaneously display these optical properties. Especially, in contrast to the reported dual-band solvatochromes whose solvatochromism is achieved by affecting their excited state behaviors, the solvatochromism of NIR-HBT is realized by modulating its ground state proton dissociation, which is a new solvatochromic mechanism that has not been reported. Furthermore, based on the dual-band solvatochromism of NIR-HBT and its intrinsic binding ability to GQs, near-infrared ratiometric detection of GQs is achieved. These results indicate that NIR-HBT is an attractive solvatochrome that can be used to develop near-infrared ratiometric biosensors for biological research. More broadly, the discovered solvatochromic mechanism can also open new horizons for exploring the solvatochrome.

Relative Humidity-Dependent Phase Transitions in Submicron Respiratory Aerosols

Respiratory viruses, such as influenza and severe acute respiratory syndrome coronavirus 2, represent a substantial public health burden and are largely transmitted through respiratory droplets and aerosols. Environmental factors such as relative humidity (RH) and temperature impact virus transmission rates, and a precise mechanistic understanding of the connection between these environmental factors and virus transmission would improve efforts to mitigate respiratory disease transmission. Previous studies on supermicrometer particles observed RH-dependent phase transitions and linked particle phase state to virus viability. Phase transitions in atmospheric aerosols are dependent on size in the submicrometer range, and actual respiratory particles are expelled over a large size range, including submicrometer aerosols that can transmit diseases over long distances. Here, we directly investigated the phase transitions of submicrometer model respiratory aerosols. A probe molecule, Nile red, was added to particle systems including multiple mucin/salt mixtures, a growth medium, and simulated lung fluid. For each system, the polarity-dependent fluorescence emission was measured following RH conditioning. Notably, the fluorescence measurements of mucin/NaCl and Dulbecco's modified Eagle's medium particles indicated that liquid-liquid phase separation (LLPS) also occurs in submicron particles, suggesting that LLPS can also impact the viability of viruses in submicron particles and thus affect aerosol virus transmission. Furthermore, the utility of fluorescence-based measurements to study submicrometer respiratory particle physicochemical properties in situ is demonstrated.

Dual-state emission, mechanofluorochromism, and lipid droplet imaging of asymmetric D-π-A-D'-type triads

Dual-state emission (DSE) is an emerging phenomenon wherein organic luminescent molecules display bright emissions in both molecularly isolated and packed states, addressing the challenge associated with the traditional paradigm of dyes with mono-state emission. This study presents the design and synthesis of two unsymmetrical triads, TPCA and TPCT, featuring a D-π-A-D' electronic structure by integrating phenothiazines, triphenylamines, and cyanostilbene. Photophysical assessments reveal that both molecules serve as robust DSEgens, exhibiting strong emissions in both solution and solid phases. TPCA displays ΦTHF 53.2% and Φsolids 43.2%, while TPCT exhibits ΦTHF 49.6% and Φsolids 37.5%. However, due to differences in molecular conformation and packing, they diverge in solid-state emission wavelengths and mechanofluorochromic behavior. In the solid state, TPCA emits strong red fluorescence, contrasting with TPCT, which emits orange fluorescence. Furthermore, TPCA demonstrates significant mechanofluorochromism (MFC), shifting from yellow to yellow-red upon mechanical grinding, while TPCT exhibits negligible MFC owing to conformational distinctions. As robust and low-toxic bioimaging agents, both TPCA and TPCT prove highly effective for lipid-droplet imaging studies. This research contributes valuable insights to the evolving field of DSE materials, elucidating the promising applications and mechanisms governing their versatile emission behaviors.

Studies About the Effect of Halogenated Solvents on the Fluorescence Properties of 9-Aryl-Substituted Isoquinolinium Derivatives - A Case Study

It was demonstrated that 9-aryl-substituted isoquinolinium derivatives have significantly increased fluorescence quantum yields in halogenated solvents, mostly pronounced in chloroalkanes, which appears to be specific for this type of solvents. Further analysis with selected halogenated solvents revealed that the type and number of halogen substituents and the dielectric constant of the solvent have a distinct impact on the emission quantum yield. The solvent effect is explained by a solvation of the charge shift (CS) state by attractive halogen-π interactions (halogen bond), which impedes the torsional relaxation of the excited state.

New Charged Cholinesterase Inhibitors: Design, Synthesis, and Characterization

Triazoles and triazolium salts are very common subunits in the structures of various drugs. Medicaments with a characteristic 1,2,3-triazole core are also being developed to treat neurodegenerative disorders associated with cholinesterase enzyme activity. Several naphtho- and thienobenzo-triazoles from our previous research emerged as being particularly promising in that sense. For this reason, in this research, new naphtho- and thienobenzo-triazoles 23-34, as well as 1,2,3-triazolium salts 44-51, were synthesized and tested. Triazolium salts 44-46 showed excellent activity while salts 47 and 49 showed very good inhibition toward both butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) enzymes. In contrast, neutral photoproducts were shown to be selective towards BChE but with very good inhibition potential as molecules 24-27. The representative of newly prepared compounds, 45 and 50, were stable in aqueous solution and revealed intriguing fluorimetric properties, characterized by a strong Stokes shift of >160 nm. Despite their condensed polycyclic structure shaped similarly to well-known DNA-intercalator ethidium bromide, the studied compounds did not show any interaction with ds-DNA, likely due to the unfavorable steric hindrance of substituents. However, the studied dyes bind proteins, particularly showing very diverse inhibition properties toward AChE and BChE. In contrast, neutral photoproducts were shown to be selective towards a certain enzyme but with moderate inhibition potential. The molecular docking of the best-performing candidates to cholinesterases' active sites identified cation-π interactions as the most responsible for the stability of the enzyme-ligand complexes. As genotoxicity studies are crucial when developing new active substances and finished drug forms, in silico studies for all the compounds synthesized have been performed.

Design of Bright Chemogenetic Reporters Based on the Combined Engineering of Fluorogenic Molecular Rotors and of the HaloTag Protein

The combination of fluorogenic probes (fluorogens) and self-labeling protein tags represent a promising tool for imaging biological processes with high specificity but it requires the adequation between the fluorogen and its target to ensure a good activation of its fluorescence. In this work, we report a strategy to develop molecular rotors that specifically target HaloTag with a strong enhancement of their fluorescence. The divergent design facilitates the diversification of the structures to tune the photophysical and cellular properties. Four bright fluorogens with emissions ranging from green to red were identified and applied in wash-free live cell imaging experiments with good contrast and selectivity. A HaloTag mutant adapted from previous literature reports was also tested and shown to further improve the brightness and reaction rate of the most promising fluorogen of the series both in vitro and in cells. This work opens new possibilities to develop bright chemogenetic reporters with diverse photophysical and biological properties by exploring a potentially large chemical space of simple dipolar fluorophores in combination with protein engineering.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Resolving the Nanoscale Structure of β-Sheet Peptide Self-Assemblies Using Single-Molecule Orientation-Localization Microscopy

Synthetic peptides that self-assemble into cross-β fibrils are versatile building blocks for engineered biomaterials due to their modularity and biocompatibility, but their structural and morphological similarities to amyloid species have been a long-standing concern for their translation. Further, their polymorphs are difficult to characterize by using spectroscopic and imaging techniques that rely on ensemble averaging to achieve high resolution. Here, we utilize Nile red (NR), an amyloidophilic fluorogenic probe, and single-molecule orientation-localization microscopy (SMOLM) to characterize fibrils formed by the designed amphipathic enantiomers KFE8L and KFE8D and the pathological amyloid-beta peptide Aβ42. Importantly, NR SMOLM reveals the helical (bilayer) ribbon structure of both KFE8 and Aβ42 and quantifies the precise tilt of the fibrils' inner and outer backbones in relevant buffer conditions without the need for covalent labeling or sequence mutations. SMOLM also distinguishes polymorphic branched and curved morphologies of KFE8, whose backbones exhibit much more heterogeneity than those of typical straight fibrils. Thus, SMOLM is a powerful tool to interrogate the structural differences and polymorphism between engineered and pathological cross-β-rich fibrils.

Live-cell biosensors based on the fluorescence lifetime of environment-sensing dyes

In this work, we examine the use of environment-sensitive fluorescent dyes in fluorescence lifetime imaging microscopy (FLIM) biosensors. We screened merocyanine dyes to find an optimal combination of environment-induced lifetime changes, photostability, and brightness at wavelengths suitable for live-cell imaging. FLIM was used to monitor a biosensor reporting conformational changes of endogenous Cdc42 in living cells. The ability to quantify activity using phasor analysis of a single fluorophore (e.g., rather than ratio imaging) eliminated potential artifacts. We leveraged these properties to determine specific concentrations of activated Cdc42 across the cell.

Non-destructive real-time monitoring and investigation of the self-assembly process using fluorescent probes

Self-assembly has been considered as a strategy to construct superstructures with specific functions, which has been widely used in many different fields, such as bionics, catalysis, and pharmacology. A detailed and in-depth analysis of the self-assembly mechanism is beneficial for directionally and accurately regulating the self-assembly process of substances. Fluorescent probes exhibit unique advantages of sensitivity, non-destructiveness, and real-time self-assembly tracking, compared with traditional methods. In this work, the design principle of fluorescent probes with different functions and their applications for the detection of thermodynamic and kinetic parameters during the self-assembly process were systematically reviewed. Their efficiency, limitations and advantages are also discussed. Furthermore, the promising perspectives of fluorescent probes for investigating the self-assembly process are also discussed and suggested.

Target-activated multicolor fluorescent dyes for 3D imaging of plasma membranes and tracking of apoptosis

Real-time tracking of dynamic changes in the three-dimensional morphology of the cell plasma membrane is of great importance for a deeper understanding of physiological processes related to the cell plasma membrane. However, there is a lack of imaging dyes that can specifically be used for a long term labelling of plasma membranes, especially for plant cells. Here, we have used molecular engineering strategies to develop a series of target-activated multicolour fluorescent dyes that can be used for long-term and three-dimensional imaging of plant cell plasma membranes. By combining different electron acceptors and donors, four molecular backbones with different emission colours from green to NIR have been obtained. In the designed styrene-based dyes, referred to as the SD dyes, several functional groups were introduced into the backbones to achieve the properties of target-activated fluorescence, rapid and wash-free staining, high plasma membrane targeting ability and long-term imaging function. Using onion epidermal cells as a platform, these dye molecules can provide high-quality imaging of the plasma membrane for up to 6 hours, providing a powerful tool for long-term monitoring of plasma membrane-related biological events. Calcium-mediated apoptosis of plant cells has been tracked for the first time by monitoring the morphological changes of the plasma membrane in real time using SD dyes. These dyes also exhibit excellent 3D imaging performance of the plasma membrane and were further used to track in real time the 3D morphological changes of the plasma membrane during plasmolysis of plant cells, providing a powerful imaging tool for three-dimensional (3D) biology. This work provides a set of multi-colour dye tools for long-term and three-dimensional imaging of plant cell plasma membranes, and also provides molecular design principles for guiding the transmembrane transport of small molecules.

A multi-chromic boron trifluoride-pyridyl Lewis adduct

A boron trifluoride-pyridyl Lewis adduct is reported, which exhibits various types of chromism and high solid-state photoluminescence quantum yields, as well as excitation-dependent emission in the mechanically ground form. The facile synthetic approach offers a simple and potentially versatile strategy for inducing chromism in pyridyl ligands with donor moieties. We envisage this approach as having a dual benefit: simplicity and extensive applicability.

Quantifying Fluorescence Lifetime Responsiveness of Environment-Sensitive Probes for Membrane Fluidity Measurements

The structural diversity of different lipid species within the membrane defines its biophysical properties such as membrane fluidity, phase transition, curvature, charge distribution, and tension. Environment-sensitive probes, which change their spectral properties in response to their surrounding milieu, have greatly contributed to our understanding of such biophysical properties. To realize the full potential of these probes and avoid misinterpretation of their spectral responses, a detailed investigation of their fluorescence characteristics in different environments is necessary. Here, we examined the fluorescence lifetime of two newly developed membrane order probes, NR12S and NR12A, in response to alterations in their environments such as the degree of lipid saturation, cholesterol content, double bond position and configuration, and phospholipid headgroup. As a comparison, we investigated the lifetime sensitivity of the membrane tension probe Flipper in these environments. Applying fluorescence lifetime imaging microscopy (FLIM) in both model membranes and biological membranes, all probes distinguished membrane phases by lifetime but exhibited different lifetime sensitivities to varying membrane biophysical properties (e.g., cholesterol). While the lifetime of Flipper is particularly sensitive to the membrane cholesterol content, the NR12S and NR12A lifetimes are moderately sensitive to both the cholesterol content and lipid acyl chains. Moreover, all of the probes exhibit longer lifetimes at longer emission wavelengths in membranes of any complexity. This emission wavelength dependency results in varying lifetime resolutions at different spectral regions, which are highly relevant for FLIM data acquisition. Our data provide valuable insights on how to perform FLIM with these probes and highlight both their potential and limitations.

Visual detection of water content in liquor with near-infrared fluorescence sensor assisted by smartphone

Water content was an essential indicator in organic solvents, and it was necessary to develop a facile, cheap and readily available tool for the real-time, specifical and sensitive detection of water content. In this work, two novel D-π-A type near-infrared fluorescence sensors (DCM-1 and DCM-2) were designed and synthesized for the detection of trace water in organic solvents. DCM-1 and DCM-2 with solvent-dependent effects and large Stokes shift (>120 nm) showed good linear "intensity-to-content" relationships in four commonly-used organic solvents, and accomplished the ultra-fast and high-accuracy detection of the trace water in organic solvents. More importantly, a portable, fast, and accurate smartphone-assisted visual assay was designed for visual quantitative detection of the water content in organic solvents with a detection limit as low as 1.028 % v/v (e.g. in ethanol) and a wide detection range (0-60 % v/v). The smartphone-based visual assay was further applied to estimate the water content in disinfection alcohol and commercial liquor, which furnished a new strategy and broad prospects to achieve the accurate onsite detection of water content.

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.

Molecular Rotors: Fluorescent Sensors for Microviscosity and Conformation of Biomolecules

The viscosity and crowding of biological environment are considered vital for the correct cellular function, and alterations in these parameters are known to underly a number of pathologies including diabetes, malaria, cancer and neurodegenerative diseases, to name a few. Over the last decades, fluorescent molecular probes termed molecular rotors proved extremely useful for exploring viscosity, crowding, and underlying molecular interactions in biologically relevant settings. In this review, we will discuss the basic principles underpinning the functionality of these probes and will review advances in their use as sensors for lipid order, protein crowding and conformation, temperature and non-canonical nucleic acid structures in live cells and other relevant biological settings.

Trifluoroethylamine-substituted solvatochromic fluorophores exhibit polarity-insensitive high brightness

In this study, we have uncovered that trifluoroethylamine-substituted solvatochromic fluorophores maintain consistently high and stable fluorescence intensity in diverse polar environments, including highly polar and protic solvents. The 1,8-naphthalimide derivatives serve as a buffering fluorogenic indicator for lipid droplet morphology during the fusion process and ratiometric probe for microenvironment polarity based on Halo-tag technology.

Fluorescent Probes and Quenchers in Studies of Protein Folding and Protein-Lipid Interactions

Fluorescence spectroscopy provides numerous methodological tools for structural and functional studies of biological macromolecules and their complexes. All fluorescence-based approaches require either existence of an intrinsic probe or an introduction of an extrinsic one. Moreover, studies of complex systems often require an additional introduction of a specific quencher molecule acting in combination with a fluorophore to provide structural or thermodynamic information. Here, we review the fundamentals and summarize the latest progress in applications of different classes of fluorescent probes and their specific quenchers, aimed at studies of protein folding and protein-membrane interactions. Specifically, we discuss various environment-sensitive dyes, FRET probes, probes for short-distance measurements, and several probe-quencher pairs for studies of membrane penetration of proteins and peptides. The goals of this review are: (a) to familiarize the readership with the general concept that complex biological systems often require both a probe and a quencher to decipher mechanistic details of functioning and (b) to provide example of the immediate applications of the described methods.

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.

Unravelling photoisomerization dynamics in a metastable-state photoacid

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Polymerization-Mediated Through-Space Charge Transfer: An Emerging Strategy for Light-Emitting Materials

Through-space charge transfer (TSCT) has attracted increasing attention owing to its great potential in designing efficient light-emitting molecules and polymers. Complementary to through-bond charge transfer and Förster resonance energy transfer, TSCT offers an alternative approach for the manipulation of molecular fluorescence. Recently, the synergy between TSCT and polymer systems through polymerization-mediated charge transfer has fostered the advancements of innovative light-emitting functional materials featuring thermally activated delayed fluorescence and/or aggregation-induced emission. This perspective highlights the significant progress in tailoring emission properties through structural engineering of donor and acceptor groups within polymeric systems, leveraging the TSCT mechanism. This strategy has transcended the limitations of traditional charge transfer systems with its tolerance to extended donor-acceptor distance, paving the way for novel applications beyond organic light-emitting diodes. The discussion concludes with a forward-looking analysis of potential future research trajectories in the field of polymerization-mediated charge transfer for developing next-generation light-emitting materials.

Effect of hydrophilicity-imparting substituents on exciton delocalization in squaraine dye aggregates covalently templated to DNA Holliday junctions

Molecular aggregates exhibit emergent properties, including the collective sharing of electronic excitation energy known as exciton delocalization, that can be leveraged in applications such as quantum computing, optical information processing, and light harvesting. In a previous study, we found unexpectedly large excitonic interactions (quantified by the excitonic hopping parameter Jm,n) in DNA-templated aggregates of squaraine (SQ) dyes with hydrophilic-imparting sulfo and butylsulfo substituents. Here, we characterize DNA Holliday junction (DNA-HJ) templated aggregates of an expanded set of SQs and evaluate their optical properties in the context of structural heterogeneity. Specifically, we characterized the orientation of and Jm,n between dyes in dimer aggregates of non-chlorinated and chlorinated SQs. Three new chlorinated SQs that feature a varying number of butylsulfo substituents were synthesized and attached to a DNA-HJ via a covalent linker to form adjacent and transverse dimers. Various characteristics of the dye, including its hydrophilicity (in terms of log Po/w) and surface area, and of the substituents, including their local bulkiness and electron withdrawing capacity, were quantified computationally. The orientation of and Jm,n between the dyes were estimated using a model based on Kühn-Renger-May theory to fit the absorption and circular dichroism spectra. The results suggested that adjacent dimer aggregates of all the non-chlorinated and of the most hydrophilic chlorinated SQ dyes exhibit heterogeneity; that is, they form a mixture of dimers subpopulations. A key finding of this work is that dyes with a higher hydrophilicity (lower log Po/w) formed dimers with smaller Jm,n and large center-to-center dye distance (Rm,n). Also, the results revealed that the position of the dye in the DNA-HJ template, that is, adjacent or transverse, impacted Jm,n. Lastly, we found that Jm,n between symmetrically substituted dyes was reduced by increasing the local bulkiness of the substituent. This work provides insights into how to maintain strong excitonic coupling and identifies challenges associated with heterogeneity, which will help to improve control of these dye aggregates and move forward their potential application as quantum information systems.

Measuring Solvent Exchange in Silica Nanoparticles with Rotor-Based Fluorophore

Measuring the diffusivity of molecules is the first step toward understanding their dependence and controlling diffusion, but the challenge increases with the decrease of molecular size, particularly for non-fluorescent and non-reactive molecules such as solvents. Here, the capability to monitor the solvent exchange process within the micropores of silica with millisecond time resolution is demonstrated, by simply embedding a rotor-based fluorophore (thioflavin T) in colloidal silica nanoparticles. Basically, the silica provides an extreme case of viscous microenvironment, which is affected by the polarity of the solvents. The fluorescence intensity traces can be well fitted to the Fickian diffusion model, allowing analytical solution of the diffusion process, and revealing the diffusion coefficients. The validation experiments, involving the water-to-ethanol and ethanol-to-water solvent exchange, the comparison of different drying conditions, and the variation in the degree of cross-linking in silica, confirmed the effectiveness and sensitivity of this method for characterizing diffusion in silica micropores. This work focuses on the method development of measuring diffusivity and the high temporal resolution in tracking solvent exchange dynamics over a short distance (within 165 nm) opens enormous possibilities for further studies.

Engineered fluorogenic HaloTag ligands for turn-on labelling in live cells

Recent years have seen dramatic improvements in the design of organic fluorophores based on limiting non-radiative decay pathways. We sought to extend this understanding to benzothiadiazoles that have been used as turn-on fluorescent substrates for the self-labeling protein HaloTag. When conjugated to HaloTag, the benzothiadiazoles reside in a narrow tunnel that precludes twisted internal charge transfer, which allowed us to explore steric and electronic effects on other non-radiative decay pathways. By minimizing both non-radiative decay and nonspecific interactions with cellular components, we produced improved turn-on dyes with 136-fold increase in fluorescence over background in cells.