Copper Catalyst-Promoted Regioselective Multicomponent Cascade Cyclization of 3-Aza-1,5-enynes with Sulfur Dioxide and Cycloketone Oxime Esters to Access Cyanoalkylsulfonylated 1,2-Dihydropyridines.

Radical cascade cyclization via the cracking of alkenyl C-H has emerged as an attractive and remarkable tool for the rapid construction of ring frameworks with endocyclic double bonds. We developed a cascade reaction of 3-aza-1,5-enynes with sulfur dioxide and cycloketone oxime esters to access cyanoalkylsulfonylated 1,2-dihydropyridines, which can be easily converted to pyridine derivatives. This protocol involves radical addition to the C≡C bond and 6-endo cyclization and features high regioselectivity and a broad substrate scope.

Exploring the Photophysics of a Zn2+ Fluorescence Sensor and Its Sensing Mechanism.

Sensor X is a turn-on sensor, which is applied in the fluorescence detection of Zn2+ ions. Its photophysical process is comprehensively investigated to clarify its weak fluorescence. With the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT), the potential energy surfaces (PES) of X on both ground and first excited states are studied. Excited-state intramolecular proton transfer (EPT) processes as well as molecule twisting motion are observed, which induces several minima on the excited-state PES. Transition states as well as rate constants for these dynamic processes are obtained to evaluate their occurrences. The twisting motion of the sensor is an ultrafast process, which is initiated by a specific EPT process and leads to a nonemissive twisted intramolecular charge transfer (TICT) state. The fluorescence of the sensor is barely observable because of the easily attainable TICT state on the excited PES. This mechanism is trustworthy and intrinsically different from the previously proposed mechanism. After clarifying the photophysical process of the sensor, the Zn2+ sensing mechanism is uncovered. Also, the selectivity against Cd2+ and Hg2+ is fully discussed.

Theoretical Investigations on the Detecting Mechanism of a Typical 2,4,6-Trinitrophenol Fluorescence Sensor and Its Design Strategy.

Fluorescence sensors based on small organic molecules are drawing increasing attention. In this contribution, the underlying detection mechanism of a typical fluorescence sensor for 2,4,6-trinitrophenol (TNP) based on fluorescence quenching is comprehensively investigated. The TNP molecule is proved to plant an intermolecular electron transfer state (dark state) below the bright state. Strong π-π interaction is observed between the sensor and TNP, which provides considerable orbital overlaps between the sensor and analyte. Electron transfer from the sensor to analyte is facilitated by such a strong interaction, which quenches the sensor's fluorescence. The design strategy for such TNP sensors is proposed based on the detection mechanism, and a series of new sensors is designed, which is likely to have better sensitivity than the original sensor.

Role of the Weak Interactions during the 2,4,6-Trinitrophenol Detecting Process of a Fluorescein-Based Sensor.

Achieving fast and precise fluorescence sensing of 2,4,6-trinitrophenol (TNP) is of fundamental importance for homeland security and environment protection. Weak interactions between the sensor and an analyte always play a critical role, which is capable of affecting the photophysics of the sensor. This study performs a thorough investigation on the effects of the weak interaction between TNP and a typical fluorescein-based sensor. The photophysics of the sensor before and after interacting with TNP is fully discussed by analyzing the potential energy surface (PES) of the sensor and rate constants of the excited-state dynamic processes. TNP is found to affect the PES greatly, which plants an intermolecular electron transfer state (dark state) below the bright state. The π-π interaction is proved to induce considerable orbital overlaps between the analyte and the sensor, which facilitates the electron transfer process and generates the dark state.

A sensitive and rapid "off-on" fluorescent probe for the detection of esterase and its application in evaluating cell status and discrimination of living cells and dead cells.

The discrimination of living and dead cells shows great importance in the development of biology, pathology, medicine, and pharmacology research. Herein, we synthesized a simple benzothiazole-based probe, EP, which was characterized via1H NMR (hydrogen nuclear magnetic resonance) spectroscopy, 13C NMR (carbon nuclear magnetic resonance) spectroscopy and HRMS (high-resolution mass spectroscopy). The fluorescence changes in response to esterase were characterized via fluorescence spectroscopy. EP exhibited a 70-fold fluorescence enhancement in the presence of esterase and possessed a very low limit of detection (4.73 × 10-5 U mL-1). EP also showed high selectivity to esterase compared to other biological species. Bright fluorescence appeared in living cells, which was activated by esterase when incubated with EP. In paraformaldehyde or H2O2 pretreated cells, the fluorescence became very weak since esterase became inactive in these cells. In summary, the EP probe can monitor esterase activity both in vitro and in living cells and can be used to evaluate the health status of cells and discriminate living and dead cells effectively.

Theoretical Investigations on the Excited-State Dynamics of an Al3+ Fluorescence Sensor.

Twisted internal charge transfer (TICT) states are of fundamental importance during the photo-physical processes of dyes and sensors. In this contribution, excited-state dynamics of an Al3+ fluorescence sensor 1-{[(2-hydroxyphenyl)-imino]methyl}naphthalen-2-ol based on the turn-on signal is clarified. Two different dark TICT states are observed by exploring the excited-state potential energy surface. With the twist of the C2-N bond, the two dark states can be reached facilely, which induce the experimentally observed weak fluorescence of the sensor. The sensing mechanism is then uncovered by investigating the electronic coupling between the sensor and analyte. Al3+ is proved to form strong coordination bonds with the sensor, which restricts the motion of the C2-N bond. Consequently, the TICT states are eliminated, which generate the turn-on signal. This sensing mechanism is trustworthy and intrinsically different from the previously proposed one, which would shed some light on the design of turn-on sensors.

A terbium(III)-functionalized zinc(II)-organic framework for fluorometric determination of phosphate.

A terbium(III)-functionalized zinc(II)-organic framework (Tb-MOF-Zn) is shown to be a viable fluorescent probe for phosphate. The organic ligands 4,4',4″-[((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(oxy)]tribenzoic acid (H3L3) contains multiple carboxyl groups that can react with zinc(II) to yield tubular MOF-Zn. The MOF-Zn was further functionalized with Tb(III) to produce a lanthanide composite of type Tb-MOF-Zn which displays strong fluorescence with excitation/emission maxima at 285/544 nm. Fluorescence is quenched by phosphate because of the specific interaction with Tb(III) in Tb-MOF-Zn. The concentration of Tb-MOF-Zn, reaction time and pH value of the solution were optimized. Fluorescence drops linearly in the 0.01 to 200.0 µM phosphate concentration range, and the detection limit is 4.0 nM. The fluorescent probe was also used to prepare a microdot array on a glass slide for visual detection of phosphate under illumination with UV light. Graphical abstractA terbium(III) functionalized zinc(II)-organic framework was synthesized and used as fluorescent probe for determination of phosphate ions.

Electron transfer and intersystem crossing triggered fluorescence quenching detection of mercury ions.

Via a specific fluorescence quenching response, organic compound L serves as an excellent probe for Hg2+ in aqueous solution. The underlying detection mechanism is proposed based on the analyses of orbital interactions between probe L and Hg2+, which is new and intrinsically different from the previously proposed one. By investigating the excitation process and the excited state deactivation process of the organometallic compound formed by L and Hg2+, two non-emissive channels, namely intermolecular electron transfer and intersystem crossing, are observed. The co-effect of the two channels leads to the significant fluorescence quenching of L. Hopefully, the newly proposed mechanism can inspire experimentalists to encapsulate the two channels into one probe to achieve accurate detection of metal ions.

A GSH Fluorescent Probe with a Large Stokes Shift and Its Application in Living Cells.

Intracellular GSH is the most abundant non-protein biothiol and acts as a central antioxidant to defend against aging toxins and radicals. Meanwhile abnormal level of intracellular GSH concentration is directly related to some diseases. In this case, detecting intracellular GSH rapidly and sensitively is of great significance. We synthesize a simple fluorescent probe (named GP) which can discriminate GSH from Cys (cysteine) or Hcy (homocysteine) and presents a 50-fold fluorescence increasing. The response time of GP to GSH was only 5 min and the product GO (the product of GP after reacting with GSH) after reacting with GSH possesses a larger Stokes shift for 135 nm than that in reported work. Probe GP can detect intracellular effectively and shows obvious yellow fluorescence. Briefly, probe GP can detect intracellular GSH rapidly and effectively both in vitro and in living cells.

Three-dimensional nitrogen-doped graphene nanoribbons aerogel as a highly efficient catalyst for the oxygen reduction reaction.

A highly conductive, ultralight, neat and versatile nitrogen-doped GNRs aerogel has been fabricated by a new hydrothermal method for the first time. The newly developed aerogel shows a very promising performance when used as a novel ORR catalyst in both alkaline and acidic solutions.

A Programmed DNA Marker Based on Bis(4-ethynyl-1,8-naphthalimide) and Three-Methane-Bridged Thiazole Orange.

Two large conjugated naphthalimide derivatives with or without three-methane-bridged thiazole orange (TO3; i.e., compounds 1 a and 2 a, respectively) were designed and synthesized. The fluorescence of the naphthalimide group in compound 1 a at λ=532 nm initially decreased and that for the TO3 group at λ=655 nm increased sequentially upon adding Salmon testes (St) DNA. In contrast, without the TO3 group, the fluorescence intensity of compound 2 a monotonously decreased in response to the addition of DNA. The non-monotonic change in the fluorescence for compound 1 a could be divided into two linear sections with two different wavelengths in the range of 0

White light emission from a two-component hybrid gel via an energy transfer process.

A two-component light-harvesting organogel containing a naphthalimide-based gelator (1) as a donor and a phosphorescent Ir(III) complex [Ir(bt)2(acac)] (bt = 2-phenylbenzothiazole and acac = acetylacetone) (Ir) as an acceptor was used to produce white-light-emitting organogels. The addition of complex Ir to the gel 1 had a certain effect on the self-assembly behaviour of molecule 1, but did not affect the gelation ability, mechanical strength and structure surface wettability of the gel. The optical properties of the two-component gel 1-Ir could be tuned via high intermolecular energy transfer efficiency between 1 and complex Ir, which was confirmed by geometry optimizations and harmonic vibrational analyses. The white-light-emitting organogel was obtained with the molar ratio of complex Ir in the range of 0.3-1.0. In particular, the gel 1-Ir with the addition of 0.5 equivalent of Ir could emit white-light with the Commission Internationale de L' eclairage (CIE) coordinates of 0.33 and 0.31 under the excitation of 374 nm light.

Large red-shifted fluorescent emission via intermolecular π-π stacking in 4-ethynyl-1,8-naphthalimide-based supramolecular assemblies.

Two low molecular weight gelators containing 4-ethynyl-1,8-naphthalimide groups with large conjugated structure via different length of alkyl chains were synthesized and fully characterized. The gelation properties, structural character, and fluorescence of the gels were investigated via methods of scanning electron microscopy, X-ray diffraction, and spectral studies. The gelators have high fluorescence quantum yields in both solution and solid state. Interestingly, the wavelength of the fluorescent emission in the reversible sol-gel transition process of the gels has a large red-shift of 80 nm in DMF, which is extremely sparse for 1,8-naphthalimide derivatives in the literature. The intermolecular π-π stacking between naphthalimide is suggested to be the main driving force for the gel formation and fluorescent variation by means of temperature-dependent (1)H NMR study and theoretical calculation.

Solvent induced helical aggregation in the self-assembly of cholesterol tailed platinum complexes.

Three alkynylplatinum(ii) bipyridyl complexes in which two cholesterol groups are combined with a bipyridyl group via alkyl chains and amido bonds were designed and synthesized. The complexes have different lengths of ethylene glycol chains at the para-position of 1-phenylethyne. All three complexes can self-assemble to gel networks in DMSO, while only the morphology of 1a without an ether chain shows a well-defined right-handed helical structure in layer packing mode. However, 1c with long ethylene glycol chains forms perfect regular left-handed helical structures in aqueous ethanol solution while the volume percentage of water is less than 5% (v/v). As the ratio of water increases, the chirality changes from a left-handed helix to a right-handed helix and the packing mode alters from a monolayer structure to a hexagonal structure. As the ratio of water further increases to greater than 50% (v/v), the structure of the assembly finally transforms into bilayer vesicles. The process of the morphology transition is traced by circular dichroism spectra, powder X-ray diffraction, SEM and TEM images. The result indicates that a polar solvent (water) acts as a trigger to change the self-assembly of the chiral structures of the complex due to the strong hydrophobic interaction between cholesterol groups and the balance of the hydrophobicity and hydrophilicity of the solvent environment.

(-)-Menthol based thixotropic hydrogel and its application as a universal antibacterial carrier.

A kind of novel hydrogelator based on (-)-menthol, a traditional cooling compound, tailed by an amino acid derivate through an alkyl chain, has been designed and synthesized. The hydrogelator containing an l-lysine can form a stable hydrogel with thixotropic character in a large pH range. An interesting feature is that the viscoelastic character of the hydrogel can be enhanced by mechanical force. The mechanism of the self-assembly process was investigated by means of IR, SEM, AFM and X-ray diffraction. The formation of three dimensional multiporous networks through acid base interactions and strong double hydrogen bonding between amino acids is proposed to be the driving force for the construction of the stable hydrogel. As a result, the hydrogelator can further gelate aqueous solutions of some confirmed antibacterial agents such as Zn(2+) and a series of water soluble organic antibiotic medicines like lincomycin, amoxicillin, etc., in such a unique way that the concentration of the antibacterial agents loaded into the hydrogel can be tuned to a large extent. The antimicrobial susceptibility of the hydrogels loaded with Zn(2+) or lincomycin is much more effective than that of the corresponding aqueous solution tested by the Oxford cup method. Furthermore, the hydrogelator is completely innoxious to living cells by measurement of MTT assay. Thus, the hydrogel can be developed as a universal carrier for antibacterial agents and may also be widely used in the fields of cell culture, tissue engineering, or drug delivery systems.

Fluorescence turn-on detection of Sn2+ in live eukaryotic and prokaryotic cells.

Sn(2+) is usually added to toothpaste to prevent dental plaque and oral disease. However, studies of its physiological role and bacteriostatic mechanism are restricted by the lack of versatile Sn(2+) detection methods applicable to live cells, including Streptococcus mutans. Here we report two Sn(2+) fluorescent probes containing a rhodamine B derivative as a fluorophore, linked via the amide moiety to N,N-bis(2-hydroxyethyl)ethylenediamine (R1) and tert-butyl carbazate group (R2), respectively. These probes can selectively chelate Sn(2+) and show marked fluorescence enhancement due to the ring open reaction of rhodamine induced by Sn(2+) chelation. The probes have high sensitivity and selectivity for Sn(2+) in the presence of various relevant metal ions. Particularly, both R1 and R2 can target lysosomes, and R2 can probe Sn concentrations in lysosomes with rather acidic microenvironment. Furthermore, these two probes have low toxicity and can be used as imaging probes for monitoring Sn(2+) not only in live KB cells (eukaryotic) but also in Streptococcus mutans cells (prokaryotic), which is a useful tool to study the physiological function of Sn(2+) in biological systems.

In Situ Molecular Engineering Strategy to Construct Hierarchical MoS2 Double-Layer Nanotubes for Ultralong Lifespan "Rocking-Chair" Aqueous Zinc-Ion Batteries.

Rechargeable aqueous zinc ion batteries (AZIBs) have gained considerable attention owing to their low cost and high safety, but dendrite growth, low plating/stripping efficiency, surface passivation, and self-erosion of the Zn metal anode are hindering their application. Herein, a one-step in situ molecular engineering strategy for the simultaneous construction of hierarchical MoS2 double-layer nanotubes (MoS2-DLTs) with expanded layer-spacing, oxygen doping, structural defects, and an abundant 1T-phase is proposed, which are designed as an intercalation-type anode for "rocking-chair" AZIBs, avoiding the Zn anode issues and therefore displaying a long cycling life. Benefiting from the structural optimization and molecular engineering, the Zn2+ diffusion efficiency and interface reaction kinetics of MoS2-DLTs are enhanced. When coupled with a homemade ZnMn2O4 cathode, the assembled MoS2-DLTs//ZnMn2O4 full battery exhibited impressive cycling stability with a capacity retention of 86.6% over 10 000 cycles under 1 A g-1anode, outperforming most of the reported "rocking-chair" AZIBs. The Zn2+/H+ cointercalation mechanism of MoS2-DLTs is investigated by synchrotron in situ powder X-ray diffraction and multiple ex situ characterizations. This research demonstrates the feasibility of MoS2 for Zn-storage anodes that can be used to construct reliable aqueous full batteries.

From G-quartets to G-ribbon gel by concentration and sonication control.

Two guanosine analogues have been designed and synthesized by connecting one (1) or three adamantane branches (2). The compound containing a single adamantane branch formed G-quartets in acetonitrile solution, and was then transformed into a G-ribbon gel at concentrations higher than the critical gelation concentration. In contrast, the compound with three adamantane branches precipitated after a heating-cooling process. By means of circular dichroism and UV/visible spectra, NMR, SEM, and structural studies, the mechanism of the formation of the G-quartets and G-ribbon gel, as well as the difference in the self-assembly modes of the two compounds, have been fully elucidated. Compound 1 firstly self-assembled into G-quartets in solutions in the concentration range 5.0 × 10(-4) to 1.0 × 10(-2) M, and these G-quartets were transformed into a G-ribbon on further increasing the concentration. Gelation occurred when the G-ribbon self-assembled into a hexagonal columnar structure with the help of intermolecular hydrogen-bonding and hydrophobic interactions. This gel was sensitive to sonication and underwent a morphology change from a columnar structure to a flower-like structure composed of flakes. In contrast, due to steric hindrance, compound 2 only assembled into a spherical structure based on hydrophobic interactions.

Reconsider the fluorescence properties of 5-ASA and its sensing mechanism for iodide ion.

5-ASA shows multiple fluorescence peaks at 408 nm, 480 nm and 500 nm in methanol solution, which can serve as ratiometric sensor for I-. However, fluorescence mechanism of 5-ASA remains controversial and I- detecting mechanism is unclear. With the aid of density functional theory and time-dependent density functional theory, this paper has carried out detailed investigations on both aspects at the molecular level. The origins of all the fluorescence peaks of 5-ASA have been assigned and a new fluorescence mechanism is proposed, which is convincing and intrinsically different from previously proposed mechanisms. Based on the new fluorescence mechanism, the I- detecting mechanism is fully addressed. Two I- detecting mechanisms have been proposed and are likely to coexist which lead to the sensitive detection of I-.

Photoisomerization-induced morphology and transparency transition in an azobenzene based two-component organogel system.

A two-component gel containing long chain alkylated gallic acid (GA) and photochromic phenazopyridine (PAP) was prepared. The gel was thoroughly characterized by UV-visible and IR spectra, SEM and POM images, XRD diffraction and dynamic oscillatory measurements. The structure and transparency of the two-component gel can be reversibly changed by alternative UV light irradiation and warming in the palm of the hand. This kind of soft material has potential application in upscale surface functional materials.