A Photoexcitation‐Induced Twisted Intramolecular Charge Shuttle

Charge transfer and separation are important processes governing numerous chemical reactions. Fundamental understanding of these processes and the underlying mechanisms is critical for photochemistry. Herein, we report the discovery of a new charge‐transfer and separation process, namely the twisted intramolecular charge shuttle (TICS). In TICS systems, the donor and acceptor moieties dynamically switch roles in the excited state because of an approximately 90° intramolecular rotation. TICS systems thus exhibit charge shuttling. TICSs exist in several chemical families of fluorophores (such as coumarin, BODIPY, and oxygen/carbon/silicon–rhodamine), and could be utilized to construct functional fluorescent probes (i.e., viscosity‐ or biomolecule‐sensing probes). The discovery of the TICS process expands the current perspectives of charge‐transfer processes and will inspire future applications.     

A General Strategy to Enhance Donor‐Acceptor Molecules Using Solvent‐Excluding Substituents

While organic donor‐acceptor (D‐A) molecules are widely employed in multiple areas, the application of more D‐A molecules could be limited because of an inherent polarity sensitivity that inhibits photochemical processes. Presented here is a facile chemical modification to attenuate solvent‐dependent mechanisms of excited‐state quenching through addition of a β‐carbonyl‐based polar substituent. The results reveal a mechanism wherein the β‐carbonyl substituent creates a structural buffer between the donor and the surrounding solvent. Through computational and experimental analyses, it is demonstrated that the β‐carbonyl simultaneously attenuates two distinct solvent‐dependent quenching mechanisms. Using the β‐carbonyl substituent, improvements in the photophysical properties of commonly used D‐A fluorophores and their enhanced performance in biological imaging are shown.     

Non‐Conjugated Polymer Dots with Crosslink‐Enhanced Emission in the Absence of Fluorophore Units

A new type of fluorescent material is presented, which is called non‐conjugated polymer dots (NCPDs). The NCPDs only possess sub‐fluorophores (which are groups such as C?O, C?N, N?O) instead of typical conjugated fluorophore groups, and thus these materials should not have strong photoluminescence (PL) in the usual sense. Nevertheless, the PL of these sub‐fluorophores can be enhanced by chemical crosslinking or physical immobilization of polymer chains, which is named the crosslink‐enhanced emission (CEE) effect. The significant advances achieved by us and other groups on both experimental and theoretical aspects are discussed, and the covalent‐bond CEE, rigidity‐aggregated CEE, or supramolecular CEE in NCPDs is elaborated. Moreover, synthetic strategies, unique optical properties, and the promise of NCPDs in bio‐related fields, such as bioimaging and drug delivery, are systematically discussed.     

Dual‐Polarization Imaging of a Dual‐Fluorophore Ion Sensor: A Single‐Molecule Study

The fluorescence emission of the dual‐fluorophore Ca²⁺ ion sensor molecule, calcium‐green 2 (CG‐2), has been characterized using dual‐polarization imaging at the single‐molecule level. By comparing the fluorescence intensity of individual CG‐2 molecules in two mutually orthogonal polarization image channels, information about the relative orientation of the two constituent fluorophores in the molecule is obtained. Experimental results from polarization measurements are compared with those predicted from a geometric model based on coupled‐fluorophores that are randomly distributed in space. The results confirm previous optical spectroscopy‐based predictions of the orientation of CG‐2′s fluorophores, and the general applications of this dual‐polarization imaging approach for characterizing the optical properties of molecules containing multiple fluorophores is discussed.     

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water

This Minireview covers the latest developments of chemosensors based on transition‐metal receptors and organic fluorophores with specific binding sites for the luminescent detection and recognition of iodide in aqueous media and real samples. In all selected examples within the last decade (made‐post 2010), the iodide sensing and recognition is probed by monitoring real‐time changes of the fluorescence or phosphorescence properties of the chemosensors. This review highlights effective strategies to iodide sensing from a structural approach where the iodide recognition/sensing process, through supramolecular interactions as coordination bonds, hydrogen bonds, halogen bonds and electrostatic interactions, is transduced into an optical change easily measurable. The selective iodide sensing is an active field of research with global interest due to the importance of iodide in biological, medicinal, industrial, environmental and chemical processes.     

SCOTfluors: Small,Conjugatable, Orthogonal,and Tunable Fluorophores for In Vivo Imaging of Cell Metabolism

The transport and trafficking of metabolites are critical for the correct functioning of live cells. However, in situ metabolic imaging studies are hampered by the lack of fluorescent chemical structures that allow direct monitoring of small metabolites under physiological conditions with high spatial and temporal resolution. Herein, we describe SCOTfluors as novel small‐sized multi‐colored fluorophores for real‐time tracking of essential metabolites in live cells and in vivo and for the acquisition of metabolic profiles from human cancer cells of variable origin.     

SCOTfluors: Small,Conjugatable, Orthogonal,and Tunable Fluorophores for In Vivo Imaging of Cell Metabolism

The transport and trafficking of metabolites are critical for the correct functioning of live cells. However, in situ metabolic imaging studies are hampered by the lack of fluorescent chemical structures that allow direct monitoring of small metabolites under physiological conditions with high spatial and temporal resolution. Herein, we describe SCOTfluors as novel small‐sized multi‐colored fluorophores for real‐time tracking of essential metabolites in live cells and in vivo and for the acquisition of metabolic profiles from human cancer cells of variable origin.     

DCDHF Fluorophores for Single‐Molecule Imaging in Cells

There is a persistent need for small‐molecule fluorescent labels optimized for single‐molecule imaging in the cellular environment. Application of these labels comes with a set of strict requirements: strong absorption, efficient and stable emission, water solubility and membrane permeability, low background emission, and red‐shifted absorption to avoid cell autofluorescence. We have designed and characterized several fluorophores, termed “DCDHF” fluorophores, for use in live‐cell imaging based on the push–pull design: an amine donor group and a 2‐dicyanomethylene‐3‐cyano‐2,5‐dihydrofuran (DCDHF) acceptor group, separated by a π‐rich conjugated network. In general, the DCDHF fluorophores are comparatively photostable, sensitive to local environment, and their chemistries and photophysics are tunable to optimize absorption wavelength, membrane affinity, and solubility. Especially valuable are fluorophores with sophisticated photophysics for applications requiring additional facets of control, such as photoactivation. For example, we have reengineered a red‐emitting DCDHF fluorophore so that it is dark until photoactivated with a short burst of low‐intensity violet light. This molecule and its relatives provide a new class of bright photoactivatable small‐molecule fluorophores, which are needed for super‐resolution imaging schemes that require active control (here turning‐on) of single‐molecule emission.     

Chemoselective Alteration of Fluorophore Scaffolds as a Strategy for the Development of Ratiometric Chemodosimeters

Ratiometric sensors generally couple binding events or chemical reactions at a distal site to changes in the fluorescence of a core fluorophore scaffold. However, such approaches are often hindered by spectral overlap of the product and reactant species. We provide a strategy to design ratiometric sensors that display dramatic spectral shifts by leveraging the chemoselective reactivity of novel functional groups inserted within fluorophore scaffolds. As a proof‐of‐principle, fluorophores containing a borinate ( RF₆₂₀ ) or silanediol ( SiOH2R ) functionality at the bridging position of the xanthene ring system are developed as endogenous H₂O₂ sensors. Both these fluorophores display far‐red to near‐infrared excitation and emission prior to reaction. Upon oxidation by H₂O₂ both sensors are chemically converted to tetramethylrhodamine, producing significant (≥66 nm) blue‐shifts in excitation and emission maxima. This work provides a new concept for the development of ratiometric probes.     

Photoactivatable Aggregation‐Induced Emission Fluorophores with Multiple‐Color Fluorescence and Wavelength‐Selective Activation

Photoactivatable (caged) fluorophores are widely used in chemistry, materials, and biology. However, the development of such molecules exhibiting photoactivable solid‐state fluorescence is still challenging due to the aggregation‐caused quenching (ACQ) effect of most fluorophores in their aggregate or solid states. In this work, we developed caged salicylaldehyde hydrazone derivatives, which are of aggregation‐induced emission (AIE) characteristics upon light irradiation, as efficient photoactivatable solid‐state fluorophores. These compounds displayed multiple‐color emissions and ratiometric (photochromic) fluorescence switches upon wavelength‐selective photoactivation, and were successfully applied for photopatterning and photoactivatable cell imaging in a multiple‐color and stepwise manner.     

Single‐Molecule Photoactivation FRET: A General and Easy‐To‐Implement Approach To Break the Concentration Barrier

Single‐molecule fluorescence resonance energy transfer (sm‐FRET) has become a widely used tool to reveal dynamic processes and molecule mechanisms hidden under ensemble measurements. However, the upper limit of fluorescent species used in sm‐FRET is still orders of magnitude lower than the association affinity of many biological processes under physiological conditions. Herein, we introduce single‐molecule photoactivation FRET (sm‐PAFRET), a general approach to break the concentration barrier by using photoactivatable fluorophores as donors. We demonstrate sm‐PAFRET by capturing transient FRET states and revealing new reaction pathways during translation using μm fluorophore labeled species, which is 2–3 orders of magnitude higher than commonly used in sm‐FRET measurements. sm‐PAFRET serves as an easy‐to‐implement tool to lift the concentration barrier and discover new molecular dynamic processes and mechanisms under physiological concentrations.     

Amplification of a FRET Probe by Lipid–Water Partition for the Detection of Acid Sphingomyelinase in Live Cells

Real‐time monitoring of acid sphingomyelinase (ASM) activity is crucial for investigating its role in lipid‐mediated signaling processes. In this study, we synthesized fluorescent phosphosphingolipids capable of FRET by phosphorodichloridate chemistry. These sphingomyelin analogues are substrates for recombinant human ASM and can be used to monitor ASM activity by fluorescence spectroscopy. Incubation with cell lysates from wild‐type and knock‐out mice further confirmed probe cleavage to be exclusive to ASM. We also systematically exploited the environmental sensitivity of the fluorophores to achieve significant increases in responsiveness. This concept may be transferred to other lipid probes in the future. The ASM activity in live cells was imaged by two‐photon‐excitation microscopy.     

Giant Biodegradable Poly(ethylene glycol)‐block‐Poly(ε‐caprolactone) Polymersomes by Electroformation

Here, the formation of giant enzyme‐degradable polymersomes using the electroformation method is reported. Poly(ethylene glycol)‐block‐poly(ε‐caprolactone) polymersomes have been shown previously to be attractive candidates for the detection of bacterial proteases and protease mediated release of encapsulated reporter dyes and antimicrobials. To maximize the efficiency, the maximization of block copolymer (BCP) vesicle size without compromising their properties is of prime importance. Thus, the physical‐chemical properties of the BCP necessary to self‐assemble into polymeric vesicles by electroformation are first identified. Subsequently, the morphology of the self‐assembled structures is extensively characterized by different microscopy techniques. The vesicular structures are visualized for giant polymersomes by confocal laser scanning microscopy upon incorporation of reporter dyes during the self‐assembly process. Using time correlated single photon counting and by analyzing the fluorescence decay curves, the nanoenvironment of the encapsulated fluorophores is unveiled. Using this approach, the hollow core structure of the polymersomes is confirmed. Finally, the encapsulation of different dyes added during the electroformation process is studied. The results underline the potential of this approach for obtaining microcapsules for subsequent triggered release of signaling fluorophores or antimicrobially active cargo molecules that can be used for bacterial infection diagnostics and/or treatment.     

Highly Efficient Far Red/Near‐Infrared Solid Fluorophores: Aggregation‐Induced Emission,Intramolecular Charge Transfer,Twisted Molecular Conformation,and Bioimaging Applications

The development of organic fluorophores with efficient solid‐state emissions or aggregated‐state emissions in the red to near‐infrared region is still challenging. Reported herein are fluorophores having aggregation‐induced emission ranging from the orange to far red/near‐infrared (FR/NIR) region. The bioimaging performance of the designed fluorophore is shown to have potential as FR/NIR fluorescent probes for biological applications.     

Highly Efficient Far Red/Near‐Infrared Solid Fluorophores: Aggregation‐Induced Emission,Intramolecular Charge Transfer,Twisted Molecular Conformation,and Bioimaging Applications

The development of organic fluorophores with efficient solid‐state emissions or aggregated‐state emissions in the red to near‐infrared region is still challenging. Reported herein are fluorophores having aggregation‐induced emission ranging from the orange to far red/near‐infrared (FR/NIR) region. The bioimaging performance of the designed fluorophore is shown to have potential as FR/NIR fluorescent probes for biological applications.     

Excited Fluorophores Enhance the Opening of Vesicles at Electrode Surfaces in Vesicle Electrochemical Cytometry

Electrochemical cytometry is a method developed recently to determine the content of an individual cell vesicle. The mechanism of vesicle rupture at the electrode surface involves the formation of a pore at the interface between a vesicle and the electrode through electroporation, which leads to the release and oxidation of the vesicle's chemical cargo. We have manipulated the membrane properties using excited fluorophores conjugated to lipids, which appears to make the membrane more susceptible to electroporation. We propose that by having excited fluorophores in close contact with the membrane, membrane lipids (and perhaps proteins) are oxidized upon production of reactive oxygen species, which then leads to changes in membrane properties and the formation of water defects. This is supported by experiments in which the fluorophores were placed on the lipid tail instead of the headgroup, which leads to a more rapid onset of vesicle opening. Additionally, application of DMSO to the vesicles, which increases the membrane area per lipid, and decreasing the membrane thickness result in the same enhancement in vesicle opening, which confirms the mechanism of vesicle opening with excited fluorophores in the membrane. Light‐induced manipulation of membrane vesicle pore opening might be an attractive means of controlling cell activity and exocytosis. Additionally, our data confirm that in experiments in which cells or vesicle membranes are labeled for fluorescence monitoring, the properties of the excited membrane change substantially.     

Multicolour Fluorescent Memory Based on the Interaction of Hydroxy Terphenyls with Fluoride Anions

Memory operations based on variation of a molecule’s properties are important because they may lead to device miniaturization to the molecular scale or increasingly complex information processing protocols beyond the binary level. Molecular memory also introduces possibilities related to information‐storage security where chemical information (or reagents) might be used as an encryption key, in this case, acidic/basic reagents. Chemical memory that possesses both volatile and non‐volatile functionality requires reversible conversion between at least two chemically different stable or quasi‐stable states. Here we have developed the phenol–phenoxide equilibrium of phenol fluorophores as a data storage element, which can be used to write or modulate data using chemical reagents. The properties of this system allow data to be stored and erased either in non‐volatile or volatile modes. We also demonstrate non‐binary switching of states made possible by preparation of  a composite containing the molecular memory elements.     

Fluorescence and Sensing Applications of Graphene Oxide and Graphene Quantum Dots: A Review

Graphene oxide and graphene quantum dots are attractive fluorophores that are inexpensive, nontoxic, photostable, water‐soluble, biocompatible, and environmentally friendly. They find extensive applications in fluorescent biosensors and chemosensors, in which they serve as either fluorophores or quenchers. As fluorophores, they display tunable photoluminescence emission and the “giant red‐edge effect”. As quenchers, they exhibit a remarkable quenching efficiency through either electron transfer or Förster resonance energy transfer (FRET) process. In this review, the origin of fluorescence and the mechanism of excitation wavelength‐dependent fluorescence of graphene oxide and graphene quantum dots are discussed. Sensor design strategies based on graphene oxide and graphene quantum dots are presented. The applications of these sensors in health care, the environment, agriculture, and food safety are highlighted.     

Oligothiophenes Nano‐organized on a Cyclotetrasiloxane Scaffold as a Model of a Silica‐Bound Monolayer: Evidence for Intramolecular Excimer Formation

Excimer formation in a new class of terthiophene‐based fluorophores covalently bonded to a cyclotetrasiloxane scaffold has been demonstrated and the photophysical process ruling it has been investigated in detail and modeled theoretically. In contrast to the conventional systems in which long‐living fluorophores such as pyrene are linked in the same molecule, an excimer is formed only when two terthiophene‐based branches nano‐organized on the same cyclotetrasiloxane scaffold are close enough together when excitation takes place. In such a case, excimer formation is extremely efficient, and the new bound excited states are quite stable.     

A Highly Photostable Near‐Infrared Labeling Agent Based on a Phospha‐rhodamine for Long‐Term and Deep Imaging

Various fluorescence microscopy techniques require bright NIR‐emitting fluorophores with high chemical and photostability. Now, the significant performance improvement of phosphorus‐substituted rhodamine dyes (PORs) upon substitution at the 9‐position with a 2,6‐dimethoxyphenyl group is reported. The thus obtained dye PREX 710 was used to stain mitochondria in living cells, which allowed long‐term and three‐color imaging in the vis‐NIR range. Moreover, the high fluorescence longevity of PREX 710 allows tracking a dye‐labeled biomolecule by single‐molecule microscopy under physiological conditions. Deep imaging of blood vessels in mice brain has also been achieved using the bright NIR‐emitting PREX 710‐dextran conjugate.