Aggregation‐Induced Emission‐Active 1,4‐Dihydropyridine‐Based Dual‐Phase Fluorescent Sensor with Multiple Functions

A N‐2‐phenylethyl‐substituted 1,4‐dihydropyridine derivative (NDHP) containing 5,5‐dimethylcyclohexane‐1,3‐dione and naphthylethylene was designed and synthesized. NDHP acts as a multifunctional fluorescent sensor in dual phases. The crystal structure analysis confirms that the NDHP molecules have highly twisted conformations. The twisted conformation results in aggregation‐induced emission properties and solid‐state emission, by restricting the intramolecular free rotation in the aggregated or solid state. In the solid state, NDHP exhibits reversible mechanochromic properties as a result of the transition between the amorphous and crystalline states. NDHP also exhibits a rare phenomenon of acid‐fumed solid‐state emission enhancement owing to the change in packing mode from a zigzag arrangement to J‐aggregation. The solid‐state stimuli‐responsive fluorescence switching is applied to realize a rewritable optical recording media and a multiple output combinational logic system. In solution, NDHP shows a selective fluorescence response for environmentally harmful Hg²⁺, with a limit of detection of 2.7 nm . This results from the “turn‐on” responsive behavior owing to the Hg²⁺‐triggered aggregation of the NDHP molecules. NDHP is also used in the imaging of intracellular Hg²⁺ in HeLa cells. These findings provide a feasible and attractive route for developing multifunctional fluorescent sensors for use in dual phases.     

5,12‐Diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxalines: Solid‐State Fluorescence,AIE Properties,and Orbital Switching by Substituent Effect

The compound 5,12‐diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxaline 1 a and its derivatives were prepared, and their solid‐ and solution‐state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self‐quenching. Phenyl‐ or alkoxy‐substituted derivatives 1 b – d show solid‐state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy‐substituted derivatives were hardly changed compared to 1 a and 1 b , although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO‐1 and HOMO‐2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure.     

Organelle‐Specific Activity‐Based Protein Profiling in Living Cells

A multimodal activity‐based probe for targeting acidic organelles was developed to measure subcellular native enzymatic activity in cells by fluorescence microscopy and mass spectrometry. A cathepsin‐reactive warhead conjugated to a weakly basic amine and a clickable alkyne, for subsequent appendage of a fluorophore or biotin reporter tag, accumulated in lysosomes as observed by structured illumination microscopy (SIM) in J774 mouse macrophage cells. Analysis of in vivo labeled J774 cells by mass spectrometry showed that the probe was very selective for cathepsins B and Z, two lysosomal cysteine proteases. Analysis of starvation‐induced autophagy, a catabolic pathway involving lysosomes, showed a large increase in the number of tagged proteins and an increase in cathepsin activity. The organelle‐targeting of activity‐based probes holds great promise for the characterization of enzyme activities in the myriad diseases linked to specific subcellular locations, particularly the lysosome.     

Stimuli‐Responsive Naphthalene Diimide as Invisible Ink: A Rewritable Fluorescent Platform for Anti‐Counterfeiting

Herein, we report an approach to combat counterfeiting and storage of valuable information based on the solid‐state fluorescence switching behavior of isoniazid functionalized naphthalene diimide (ISO_NDI) in response to an external stimuli (i. e., HCl vapor). The unique feature of ISO_NDI is further utilized to develop an invisible ink (ISO_NDI‐PVA) with commercial polymer polyvinyl alcohol (PVA). A solid‐state fluorescence recovery was observed while loading with HCl vapors. This exclusive property of the material could be applied directly as a security ink for confidential data storage purpose. Based on above strategy, we successfully realized the rewritable application by using ISO_NDI‐PVA ink and confirm its practical efficacy on various substrates by creating different patterns. The solid‐state fluorescence switching behavior of ISO_NDI‐PVA ink exhibited reversible on/off signal for multiple cycles under the influence of HCl/NH₃ vapors. Mechanistic investigation supports a clear participation of intermolecular charge transfer (ICT) phenomenon in the solid‐state fluorescence switching property. The ease of fabricating the ink with invisible to visible characteristics in response to HCl vapors provides new opportunities for exploring the application of ISO_NDI‐PVA as invisible ink for targeted security applications.     

Polymorph‐Dependent Solid‐State Fluorescence and Selective Metal‐Ion‐Sensor Properties of 2‐(2‐Hydroxyphenyl)‐4(3H)‐quinazolinone

2‐(2‐Hydroxy‐phenyl)‐4(3H)‐quinazolinone (HPQ), an organic fluorescent material that exhibits fluorescence by the excited‐state intramolecular proton‐transfer (ESIPT) mechanism, forms two different polymorphs in tetrahydrofuran. The conformational twist between the phenyl and quinazolinone rings of HPQ leads to different molecular packing in the solid state, giving structures that show solid‐state fluorescence at 497 and 511 nm. HPQ also shows intense fluorescence in dimethyl formamide (DMF) solution and selectively detects Zn²⁺ and Cd²⁺ ions at micromolar concentrations in DMF. Importantly, HPQ not only detects Zn²⁺ and Cd²⁺ ions selectively, but it also distinguishes between the metal ions with a fluorescence λ_max that is blue‐shifted from 497 to 420 and 426 nm for Zn²⁺ and Cd²⁺ ions, respectively. Hence, tunable solid‐state fluorescence and selective metal‐ion‐sensor properties were demonstrated in a single organic material.     

Colorimetric and Fluorescent Chemosensors for the Detection of 2,4,6‐Trinitrophenol and Investigation of their Co‐Crystal Structures

A full account of our studies of 2,4,6‐trinitrophenol (TNP) sensing is provided. A series of chemosensors 2 , 3 , 4 , 5 with a variety of aromatic chromophores for specific recognition of TNP has been designed and then realized through the fluorescence “on/off” mechanism. These chemosensors demonstrated highly selective, sensitive, and fluorescent quenching of TNP with remarkable visual changes through the intramolecular charge‐transfer (ICT) process. Their host–guest interactions were investigated by ¹H NMR spectroscopic titrations and their corresponding co‐crystal structures, which showed that the 1:1 host–guest complexes were formed by multiple hydrogen‐bond interactions in solution or in the solid state. The origins of the significant affinity demonstrated during the fluorescence recognition process were further disclosed through DFT calculations of corresponding compounds.     

Metal‐Free Facile Synthesis of Multisubstituted 1‐Aminoisoquinoline Derivatives with Dual‐State Emissions

Although isoquinoline is a good traditional fluorescent structural unit, most of its derivatives emit fluorescence in solution and a few of them can emit solid‐state fluorescence as well. Herein, a series of multisubstituted 1‐aminoisoquinoline derivatives were synthesized by a simple reaction of a readily available 4H‐pyran derivative and secondary amines. The reaction had advantages of metal‐free, mild conditions, simple operation, and good yields, which was realized by a ring‐opening and sequential ring‐closing mechanism. These 1‐aminoisoquinoline derivatives were found to exhibit interesting dual‐state emissions. In the solution, they emitted strong blue fluorescence at about 458 nm. In the solid state, they emitted solid‐state blue fluorescence at 444–468 nm with high fluorescence quantum yields of 40.3–98.1%. Crystal structural analyses indicated that solid‐state emissions of these compounds originated from twisted molecular conformations and the resultant loose stacking arrangements. Furthermore, their solid‐state fluorescence wavelengths were demonstrated to depend on molecular conformations rather than stacking arrangements. The discovery of these 1‐aminoisoquinolines with multiple reaction sites provides new possibilities for the development of solid‐state fluorescent materials based on the traditional isoquinoline skeleton.     

Tautomeric Switching and Metal‐Cation Sensing of Ligand‐Equipped 4‐Hydroxy‐/4‐oxo‐1,4‐dihydroquinolines

Novel 4‐hydroxyquinoline (4HQ) based tautomeric switches are reported. 4HQs equipped with coordinative side arms (8‐arylimino and 3‐piperidin‐1‐ylmethyl groups) were synthesized to access O‐ or N‐selective chelation of Zn²⁺ and Cd²⁺ ions by 4HQ. In the case of the monodentate arylimino group, O chelation of metal ions induces concomitant switching of phenol tautomer to the keto form in nonpolar or aprotic media. This change is accompanied by selective and highly sensitive fluorometric sensing of Zn²⁺ ions. In the case of the bidentate 8‐(quinolin‐8‐ylimino)methyl side arm, NMR studies in CD₃OD indicated that both Cd²⁺ and Zn²⁺ ions afford N chelation for 4HQ, coexisting with tautomeric switching from quinolin‐4(1H)‐one to quinolin‐4‐olate. In corroboration, UV/Vis‐monitored metal‐ion titrations in toluene and methanol implied similar structural changes. Additionally, fluorescence measurements indicated that the metal‐triggered tautomeric switching is associated with compound signaling properties. The results are supported by DFT calculations at the B3LYP 6‐31G* level. Several X‐ray structures of metal‐free and metal‐chelating 4HQ are presented to support the solution studies.     

Modulating Photo‐ and Electroluminescence in a Stimuli‐Responsive π‐Conjugated Donor–Acceptor Molecular System

Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli‐responsive electroluminescence (EL) has not yet been explored for color‐tunable emitters in organic light‐emitting diodes (OLEDs). Here a stimuli‐responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X‐ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited‐state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli‐responsive fluorescence behavior, two‐color emissive OLEDs were produced using the same fluorophore in different solid phases.     

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.     

Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence Switching of Tetraphenylethene‐Dithienylethene‐Perylenemonoimide Triad with Donor–Bridge–Acceptor Structure

Photochromic 1,2‐dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor–photochromic bridge–acceptor tetraphenylethene‐dithienylethene‐perylenemonoimide (TPE‐DTE‐PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE‐DTE‐PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE‐DTE‐PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.     

Supramolecular Host‐Inhibited Excited‐State Proton Transfer and Fluorescence Switching of the Anti‐Cancer Drug,Topotecan

The effect of cucurbit[7]uril (CB[7]) nano‐caging on the photophysical properties, particularly excited‐state proton transfer (ESPT) reaction, of an eminent anti‐cancer drug, topotecan (TPT), is demonstrated through steady‐state and time‐resolved fluorescence measurements. TPT in water (pH 6) exists exclusively as the cationic form (C) in the ground state. However, the drug emission mainly comes from the excited‐state zwitterionic form (Z*) of TPT, and is attributed to water‐assisted ESPT between the 10‐hydroxyl group and water, which leads to the transformation of C* to Z* of TPT. In the presence of CB[7], it is found that selective encapsulation of the C form of TPT results in the formation of a 1:1 inclusion complex (CB[7]:TPT), and the ESPT process is inhibited by this encapsulation process. As a result, C* becomes the dominant emitting species in the presence of CB[7] rather than Z*, and fluorescence switching takes place from green to blue. Time‐resolved studies also support the existence of CB[7]‐encapsulated cationic species as the major emitting species in the presence of the macrocyclic host. Semi‐empirical quantum chemical calculations are employed to gain insight into the molecular picture of orientation of TPT in the inclusion complex. It is clearly seen from the optimised structure of 1:1 CB[7]:TPT inclusion complex that both 10‐hydroxyl and 9‐dimethylaminomethylene groups of TPT lie partly inside the cavity, and thereby inhibit the excited‐state transformation of C* to Z* by the ESPT process. Finally, controlled release of the drug is achieved by means of fluorescence switching by introducing NaCl, which is rich in cells, as an external stimulus.     

All‐Solid‐State Ion Selective and All‐Solid‐State Reference Electrodes

This paper is aiming to give a brief overview of recent research in the field of all‐solid‐state, internal solution free, ion‐selective electrodes and reference electrodes, employing conducting polymers or nano‐/microstructures as solid contacts beneath the polymeric, ion‐selective or reference membranes. The emphasis is on papers published in the last five years (after 2006). According to the papers published, poly(3‐octylthiophene) conducting polymer transducers offer highly reliable sensors for various applications, involving demanding analytical approaches and miniature sensors. On the other hand, the search for alternative materials continues: the sensors obtained by placing nano‐/microstructures (conducting polymers but also other materials, like, e.g., carbon nanotubes) underneath the receptor membrane are intensively tested. The recent years have also shown how useful the application of advanced instrumental methods is for the investigation of processes occurring within all‐solid‐state ion‐selective electrodes.     

How a Small Structural Difference Can Turn Optical Properties of π‐Extended Coumarins Upside Down: The Role of Non‐Innocent Saturated Rings

The fluorescence properties of two new families of heterocycles possessing either a seven‐ or five‐membered ring attached at the core molecule are entirely different in solution and in the solid state. Crystallization has the effect of inhibiting non‐radiative excited‐state deactivation pathways, operative in solution for the seven‐membered ring compounds, thus leading to significant fluorescence efficiency in the solid state, with quantum yields ranging from 0.10 to 0.36. Conversely, the five‐membered ring derivatives, which display notable emission properties in solution, are almost non‐emissive in the crystalline state, characterized by a long‐range π‐stacked arrangement. When embedded in polymeric films, both series show fluorescence features similar to the solution case, with remarkable fluorescence quantum yields ranging from 0.09 to 0.41. According to quantum chemical calculations, 3H‐chromeno[3,4‐c]pyridine‐4,5‐diones show the specific mechanism of fluorescence quenching. The derivatives bearing the seven‐membered ring undergo, in solution, a significant structural deformation in the excited state, resulting in a large decrease of the energy gap between S₁ and S₀ and hence to a substantial contribution of the internal conversion in the relaxation process. The fluorescence quenching of the five‐membered ring derivatives is in turn related to the intermolecular interaction between adjacent molecules prevailing to a greater extent in the crystal lattice.     

Voltammetric Properties of All‐solid State Ion‐selective Electrodes with Multiwalled Carbon Nanotubes‐poly(3‐octylthiophene‐2,5‐diyl) Nanocomposite Transducer

Voltammetric response of an all‐solid‐state ion‐selective electrode was studied on example of potassium‐selective sensor with poly(vinyl chloride) based membrane and nanocomposite transducer containing poly(3‐octylthiophene‐2,5‐diyl) and multiwalled carbon nanotubes. Factors limiting the rate of the electrochemical process and the response were discussed. The challenge in voltammetric applications of ion‐selective electrodes is thickness of the plastic membrane. It was found that although a relatively thick ion‐selective membrane was applied, as typically used in potentiometric studies, the position of the reduction peak, corresponding to potassium ions incorporation, was dependent on ions concentration in a Nernstian manner. This opens possibility of deviation from the paradigm of ultrathin membranes in voltammetric applications, thus potentially extending the sensors lifetime. The high resistance of the membrane did not affect the voltammetric characteristics, because the resistance was independent of ions concentration in solution. On the other hand, high resistance results in charge trapping effect in the solid contact material, leading to advantageous retention of the oxidized‐conducting state of the solid contact, independently of the applied electrode potential.     

ortho‐Phenylene‐Bridged Cyclic Oligopyrroles: Conformational Flexibilities and Optical Properties

ortho‐Phenylene‐bridged cyclic trimeric oligopyrrole C3 and hexameric oligopyrrole C6 were synthesized by Suzuki–Miyaura coupling reactions. The twisted structures of C3 and C6 were unambiguously revealed by X‐ray diffraction analysis. The optical properties of these cyclic oligopyrroles were compared with linear oligopyrrole L3 and cyclic tetramer C4 . The cyclic oligopyrroles exhibited large Stokes shifts and blue fluorescence with high quantum yields in solution and in the solid state. In addition, selective N‐methylation and N‐tolylation of C3 were used to tune the optical and electrochemical properties by changing the molecular twists and conformational flexibilities. Throughout these studies, the structure–property relationship of these cyclic strained oligopyrroles has been illustrated as an interesting molecular motif for novel cyclic π‐conjugated systems.     

Near‐Infrared Fluorescent Probe with New Recognition Moiety for Specific Detection of Tyrosinase Activity: Design,Synthesis, and Application in Living Cells and Zebrafish

Fluorescence imaging of tyrosinase (a cancer biomarker) in living organisms is of great importance for biological studies. However, selective detection of tyrosinase remains a great challenge because current fluorescent probes that contain the 4‐hydroxyphenyl moiety show similar fluorescence responses to both tyrosinase and some reactive oxygen species (ROS), thereby suffering from ROS interference. Herein, a new tyrosinase‐recognition 3‐hydroxybenzyloxy moiety, which exhibits distinct fluorescence responses for tyrosinase and ROS, is proposed. Using the recognition moiety, we develop a near‐infrared fluorescence probe for tyrosinase activity, which effectively eliminates the interference from ROS. The high specificity of the probe was demonstrated by imaging and detecting endogenous tyrosinase activity in live cells and zebrafish and further validated by an enzyme‐linked immunosorbent assay. The probe is expected to be useful for the accurate detection of tyrosinase in complex biosystems.     

3‐Formyl‐BODIPY Phenylhydrazone as a Chromo‐Fluorogenic Probe for Selective Detection of NO2 (g)

A new colorimetric and fluorogenic probe, based on a 3‐formyl boron dipyrromethene (BODIPY) phenylhydrazone, for the sensitive and selective detection NO₂ (g) has been prepared. The probe in solution experiences a remarkable hypsochromic shift of its absorption and fluorescence emission bands in the presence gaseous NO₂ (g), leading to limits of detection of few ppb. The probe also works in the solid phase, adsorbed on filter paper strips, or chemically immobilized on the surface of silica nanoparticles, with limits of detection to the naked eye of about 0.5 ppm.     

Near‐Infrared Fluorescent Probe with New Recognition Moiety for Specific Detection of Tyrosinase Activity: Design,Synthesis, and Application in Living Cells and Zebrafish

Fluorescence imaging of tyrosinase (a cancer biomarker) in living organisms is of great importance for biological studies. However, selective detection of tyrosinase remains a great challenge because current fluorescent probes that contain the 4‐hydroxyphenyl moiety show similar fluorescence responses to both tyrosinase and some reactive oxygen species (ROS), thereby suffering from ROS interference. Herein, a new tyrosinase‐recognition 3‐hydroxybenzyloxy moiety, which exhibits distinct fluorescence responses for tyrosinase and ROS, is proposed. Using the recognition moiety, we develop a near‐infrared fluorescence probe for tyrosinase activity, which effectively eliminates the interference from ROS. The high specificity of the probe was demonstrated by imaging and detecting endogenous tyrosinase activity in live cells and zebrafish and further validated by an enzyme‐linked immunosorbent assay. The probe is expected to be useful for the accurate detection of tyrosinase in complex biosystems.     

A Lysosome‐Compatible Near‐Infrared Fluorescent Probe for Targeted Monitoring of Nitric Oxide

The requirement for nitric oxide (NO) of lysosomes has motivated the development of a sophisticated fluorescent probe to monitor the distribution of this important biomolecule at the subcellular level in living cells. A near‐infrared (NIR) fluorescent Si‐rhodamine (SiRB)‐NO probe was designed based on the NO‐induced ring‐opening process of Si‐rhodamine. The probe exhibits fast chromogenic and fluorogenic responses, and high sensitivity and selectivity toward trace amounts of NO. Significantly, the spirolactam in Si‐rhodamine exhibits very good tolerance to H⁺, which in turn brings extremely low background fluorescence not only in the physiological environment but also under acidic conditions. The stability of the highly fluorescent product in acidic solution provides persistent fluorescence emission for long‐term imaging experiments. To achieve targeted imaging with improved spatial resolution and sensitivity, an efficient lysosome‐targeting moiety was conjugated to a SiRB‐NO probe, affording a tailored lysosome‐targeting NIR fluorescent Lyso‐SiRB‐NO probe. Inheriting the key advantages of its parent SiRB‐NO probe, Lyso‐SiRB‐NO is a functional probe that is suited for monitoring lysosomal NO with excellent lysosome compatibility. Imaging experiments demonstrated the monitoring of both exogenous and endogenous NO in real time by using the Lyso‐SiRB‐NO probe.