Intramolecular Electron Transfer Research Articles

Hemicyanine-Based Highly Water-Soluble Probe for Extracellular Nitroreductase.

Nitroreductase (NTR) has long been a target of interest for its important role involved in the nitro compounds metabolism. Various probes have been reported for NTR analysis, but rarely able to distinguish the extracellular NTR from intracellular ones. Herein we reported a new NTR sensor, HCyS-NO2, which was a hemicyanine molecule with one nitro and two sulfo groups attached. The nitro group acted as the reporting group to respond NTR reduction. Direct linkage of nitro group into the hemicyanine π conjugate system facilitated the intramolecular electron transfer (IET) process and thus quenched the fluorescence of hemicyanine core. Upon reduction with NTR, the nitro group was rapidly converted into the hydroxylamino and then the amino group, eliminating IET process and thus restoring the fluorescence. The sulfo groups installed significantly increased the hydrophilicity of the molecule, and introduced negative charges at physiological pH, preventing the diffusion into bacteria. Both gram-negative and gram-positive bacteria were able to turn on the fluorescence of HCyS-NO2, without detectable diffusion into cells, providing a useful tool to probe the extracellular reduction process.

Total synthesis of Comfreyn A and structural analogues via two photochemical key steps.

In this work the influence of o-fluorine substituents on the photo-dehydro-Diels-Alder (PDDA) reaction was investigated and the findings of this study were applied to the total synthesis of natural products. The reactant molecules consisted of two alkyl arylpropiolates, connected by a suberic acid tether and bearing fluorine substituents in each of the o-positions. While quantum chemical calculations suggested that a fluorine substituent prevents an attack of the adjacent carbon atom in the second C-C bond forming step of the PDDA reaction, this attack took place nevertheless. The subsequent fluoride elimination, assisted by protic solvents or trialkylsilanes, resulted in an "Umpolung" of the 4-position of the cycloallene intermediate enabling the introduction of nucleophiles at this position. The nucleophilic replacement of the second fluorine substituent could also be triggered photochemically. After removal of the tether, the two arene moieties stand nearly perpendicular to each other and a selective excitation of the naphthalene moiety was possible. This led to an intramolecular photoinduced electron transfer (PET) followed by a nucleophilic replacement of the fluoride according to a SR+N1Ar* mechanism. The formed phenolic hydroxyl group underwent spontaneous lactonization with the adjacent ester group. Based on these results, the first total synthesis of the lignan Comfreyn A and some structural analogues were developed.

Preparation and Third‐Order Nonlinear Optical Properties of Novel Axial Fullerenol‐Substituted Phthalocyanines

AbstractThis research presents the optical limiting properties of a novel axially linked D‐A type compound, LaPc‐C60(OH)n, where lanthanide phthalocyanine serves as the donor and fullerenol as the acceptor. The incorporation of fullerenol expands the π‐conjugated system, decreases the aggregation of the phthalocyanine, and significantly increases the solubility of the compound. The establishment of this D‐A system promotes intramolecular electron transfer, effectively improving both the non‐linear optical (NLO) response and optical limiting properties. In addition, homogeneous poly(methyl methacrylate) (PMMA) composite films (LaPc‐C60(OH)n/PMMA) are prepared using a simple solution casting method. Compared to fullerenol or lanthanum phthalocyanine individually, LaPc‐C60(OH)n exhibited a superior NLO response both in solution and in the solid composite film. In particular, LaPc‐C60(OH)n/PMMA demonstrated an increase in the non‐linear absorption coefficient (2050 cm GW−1) and a larger third‐order magnetization (1.84 × 10−8 esu). In addition to exhibiting an excellent NLO response at 532 nm, LaPc‐C60(OH)n also shows significant NLO enhancement at 1064 nm, extending the limiting range into the near‐infrared region. This behavior is attributed to different nonlinear absorption mechanisms and the synergistic effect that arise from the photo‐induced electron transfer process between the fullerenol and LaPc.

Charge and Spin Transfer Dynamics in a Weakly Coupled Porphyrin Dimer.

The dynamics of electron and spin transfer in the radical cation and photogenerated triplet states of a tetramethylbiphenyl-linked zinc-porphyrin dimer were investigated, so as to test the relevant parameters for the design of a single-molecule spin valve and the creation of a novel platform for the photogeneration of high-multiplicity spin states. We used a combination of multiple techniques, including variable-temperature continuous wave EPR, pulsed proton electron-nuclear double resonance (ENDOR), transient EPR, and optical spectroscopy. The conclusions are further supported by density functional theory (DFT) calculations and comparison to reference compounds. The low-temperature cw-EPR and room-temperature near-IR spectra of the dimer monocation demonstrate that the radical cation is spatially localized on one side of the dimer at any point in time, not coherently delocalized over both porphyrin units. The EPR spectra at 298 K reveal rapid hopping of the radical spin density between both sites of the dimer via reversible intramolecular electron transfer. The hyperfine interactions are modulated by electron transfer and can be quantified using ENDOR spectroscopy. This allowed simulation of the variable-temperature cw-EPR spectra with a two-site exchange model and provided information on the temperature-dependence of the electron transfer rate. The electron transfer rates range from about 10.0 MHz at 200 K to about 53.9 MHz at 298 K. The activation enthalpies Δ‡H of the electron transfer were determined as Δ‡H = 9.55 kJ mol-1 and Δ‡H = 5.67 kJ mol-1 in a 1:1:1 solvent mixture of CD2Cl2/toluene-d8/THF-d8 and in 2-methyltetrahydrofuran, respectively, consistent with a Robin-Day class II mixed valence compound. These results indicate that the interporphyrin electronic coupling in a tetramethylbiphenyl-linked porphyrin dimer is suitable for the backbone of a single-molecule spin valve. Investigation of the spin density distribution of the photogenerated triplet state of the Zn-porphyrin dimer reveals localization of the triplet spin density on a nanosecond time scale on one-half of the dimer at 20 K in 2-methyltetrahydrofuran and at 250 K in a polyvinylcarbazole film. This establishes the porphyrin dimer as a molecular platform for the formation of a localized, photogenerated triplet state on one porphyrin unit that is coupled to a second redox-active, ground-state porphyrin unit, which can be explored for the formation of high-multiplicity spin states.

Theoretical investigation on synthesis of fused [1,2,3]-triazoloheteroarene via DBU-catalyzed intramolecular azo annulation and 3,4,5-trisubstituted [1,2,4]-triazole via I2‑catalyzed cycloaddition

The mechanism is investigated for DBU-catalyzed intramolecular azo annulation of N-tosylhydrazone and I2‑catalyzed cycloaddition of phenyl tosyl hydrazone with N-phenylbenzamidine. The former contains rate-limiting α-elimination of tosylate facilitated by DBU affording amphiphilic nitrene and intramolecular, 5-endo-trig cyclization forming [1,2,3]-triazoloheteroarene. This method is also applicable for other nonpyridine heteroarene based on enhanced activity of DBU. The latter is composed of eight steps including activation of hydrazone mediated by I2, nucleophilic addition of amidine, intramolecular electron transfer via two times of proton transfer, intramolecular nucleophilic attack followed by third proton transfer achieving ring closure and final reductive elimination of HI, TsNH2 yielding 3,4,5-trisubstituted [1,2,4]-triazole. The rate-limiting step is intramolecular electron transfer under I2 catalysis. The positive solvation effect is suggested by decreased absolute and activation energies in solution compared with in gas. These results are supported by Multiwfn analysis on FMO composition of specific TSs, and MBO value of vital bonding, breaking.

Method of splitting polarization coordinates for description of ultrafast multistage electron transfer in a non-debye medium

A method for constructing the space of medium states in reactions of ultrafast multistage intramolecular electron transfer in media with several relaxation times is developed. The method uses the splitting of polarization coordinates into relaxation components, and is a generalization of two previously developed approaches used (1) to describe multistage reactions, and (2) to consider multicomponent relaxation. Within the suggested generalized scheme, a model of charge transfer in a three-center molecular system in the environment with a two-component relaxation function is considered, an algorithm for constructing the diabatic free energy surfaces of electronic states is described, a system of equations for the evolution of the distribution functions is written. The results of the general model are shown to reproduce well-known solutions in particular cases.

A switch and a failsafe: KatG’s mechanism for preservation of catalase activity using a conformationally dynamic Arg and an active-site Trp

Many novel structural features impart a robust catalase activity to KatG that is absent from all other members of its superfamily. The conformationally dynamic “Arg switch” and oxidizable “proximal Trp” have both figured prominently in investigations of KatG structure and mechanism, but the full scope of their contributions to catalysis remains unclear. The switch (R418) appears to regulate active-site intramolecular electron transfer. The proximal Trp (W321) is a conspicuous site of radical formation, but W321•+ does not appear to participate directly in the KatG catalase cycle. To explore the extent to which these residues may cooperate in KatG’s catalase activity, we generated R418N and W321F/R418N KatG and compared their catalytic and spectroscopic properties to wt KatG. R418N KatG showed pH-independent susceptibility to H2O2-based inactivation, whereas wt KatG only showed this response under conditions where the Arg switch would be oriented away from the active site (i.e., low pH). Peroxidatic electron donors (PxEDs) prevented inactivation of wt and R418N KatG regardless of pH; however, protection of R418N KatG activity by this mechanism produced at least ten-fold greater extents of PxED oxidation. Elimination of the proximal Trp in addition to the Arg switch (i.e., W321F/R418N KatG) resulted in a near complete inability to sustain H2O2 degradation. Remarkably, W321F KatG showed resistance to H2O2-dependent inactivation indistinguishable from wt at pH 7 (i.e., when the Arg switch is oriented toward the active site) but sensitivity to H2O2-dependent inactivation indistinguishable from W321F/R418N KatG at pH 5 (i.e., when the Arg switch is oriented away from the active site). These data suggest loss of the Arg switch (either by mutagenesis or conformationally due to environmental pH) results in a KatG that is substantially compromised in the sustained degradation of H2O2. This can be overcome provided that KatG retains its ability to utilize the proximal Trp as a site of protein-based oxidation and has a PxED available to repair protein oxidation events. However, if both the Arg switch and the proximal Trp are absent, rapid H2O2-dependent inactivation is observed, and PxEDs are unable to effectively intervene to preserve KatG’s catalase activity.

Responsive β-Diketonate-europium(III) Complex-Based Probe for Time-Gated Luminescence Detection and Imaging of Hydrogen Sulfide In Vitro and In Vivo.

As a crucial biological gasotransmitter, hydrogen sulfide (H2S) plays important roles in many pathological and physiological processes. Highly selective and sensitive detection of H2S is significant for the precise diagnosis and evaluation of diverse diseases. Nevertheless, challenges remain in view of the interference of autofluorescence in organisms and the stronger reactivity of H2S itself. Herein, we report the design and synthesis of a novel H2S-responsive β-diketonate-europium(III) complex-based probe, [Eu(DNB-Npketo)3(terpy)], for background-free time-gated luminescence (TGL) detection and imaging of H2S in autofluorescence-rich biological samples. The probe, consisting of a 2,4-dinitrobenzenesulfonyl (DNB) group coupled to a β-diketonate-europium(III) complex, shows almost no luminescence owing to the existence of intramolecular photoinduced electron transfer. The cleavage of the DNB group by a H2S-triggered reaction results in the recovery of the long-lived luminescence of the Eu3+ complex, allowing the detection of H2S in complicated biological samples to be performed in TGL mode. The probe showed a fast response, high specificity, and high sensitivity toward H2S, which enabled it to be successfully used for the quantitative TGL detection of H2S in tissue homogenates of mouse organs. Additionally, the low cytotoxicity of the probe allowed it to be further used for the TGL imaging of H2S in living cells and mice under different stimuli. All of the results suggested the potential of the probe for the investigation and diagnosis of H2S-related diseases.

Tuning fluorescence of fexofenadine by switching OFF intramolecular photoinduced electron transfer: Application to development of one-step green and high throughput microwell spectrofluorimetric assay for analysis of tablets and plasma.

Fexofenadine (FEX) is a non-sedating antihistamine commonly used for the treatment of allergic conditions such as seasonal rhinitis and chronic idiopathic urticaria. This study describes the tuning "ON" the intrinsic fluorescence of FEX by switching "OFF" its intramolecular photoinduced electron transfer (PET) through the protonation of the piperidinyl nitrogen atom using sulfuric acid. The resulting fluorescence was utilized as a basis for the development of a highly sensitive microwell spectrofluorimetric assay (MW-SFA) for the one-step determination of FEX in pharmaceutical tablets and plasma. The linear range of the assay was 10-500 ng ml-1, and its limit of quantitation was 25.9ng ml-1. The proposed MW-SFA was successfully applied to analyze FEX in pharmaceutical tablets and plasma samples, demonstrating good accuracy and precision. The greenness of the assay was confirmed using three metric assessment tools. In conclusion, the MW-SFA is a straightforward, single-step analysis that requires no experimental adjustments. It offers high sensitivity, efficient sample processing, and environmental sustainability. This assay is highly recommended for pharmaceutical quality control and clinical lab use, particularly for measuring FEX levels.

Acridinedione-phthalimide conjugates: Intramolecular electron transfer and singlet oxygen generation studies for optical and photodynamic therapy applications

We reported herein the synthesis, characterization of hybrid conjugates composed of phthalimide (Phth) and acridine-1,8-diones (Acr) for optical and medical applications. For the synthetic procedure, a three-step synthetic strategy has been utilized. The optical properties of the examined 1,8-acridinedione–phthalimide connected molecules (AcrPhth 1–5) have been examined utilizing various spectroscopic techniques, e.g., steady-state absorption and fluorescence, and time-correlated single photon counting. The steady-state absorption studies showed that AcrPhth 1–5 absorbs the light in the UV and visible region. The fluorescence studies of AcrPhth 1–5 exhibited significant fluorescence quenching compared to the acridinedione control compounds (Acr 1–5) suggesting the occurrence of electron-transfer reactions from the electron donating acridinedione moiety (Acr) to the electron accepting phthalimide moiety (Phth). The rate and efficiency of the electron-transfer reactions were determined from the fluorescence lifetime measurements indicating the fast electron-transfer processes of the covalently connected AcrPhth 1–5 conjugates. Computational studies supported the intramolecular electron-transfer reaction of AcrPhth conjugates using ab initio B3LYP/6-311G methods. In the optimized structures, the HOMO was found to be entirely located on the Acr entity, while the LUMO was found to be entirely on the Phth entity. Further, the synthesized compounds were tested as photosensitizers for generating the singlet oxygen species, which is a key factor in the photodynamic therapy (PDT) applications. The nanosecond laser flash measurements enable us to detect the triplet-excited states of examined Acr and AcrPhth conjugates, determining the triplet quantum yields, and direct detecting the singlet oxygen in an accurate way. From this observation, the singlet quantum yields were found to be in the range of 0.12–0.27 (for Acr 1–5) and 0.07–0.19 (for AcrPhth 1–5 conjugates). The molecular docking studies revealed that compound AcrPhth 2 exhibited high binding affinity with for key genes (p53, TOP2B, p38, and EGFR) suggesting its potential as a targeted anticancer therapy.Graphical abstractEfficient intramolecular electron-transfer reaction for the light harvesting conjugates! In addition, the conjugates can be used as good photosensitizers for PDT

Replacement of Tyrosines by Unnatural Amino Acid Aminophenylalanine Leads to Metal-Mediated Aniline Free Radical Formation in a Copper Amine Oxidase.

Copper amine oxidases (CAOs) catalyze the oxidative deamination of primary amines to aldehyde, ammonia, and hydrogen peroxide as products and are widely distributed in bacteria, plants, and eukaryotes. These enzymes initiate the single turnover, post-translational conversion of an active site tyrosine to the redox cofactor 2,4,5-trihydroxyphenylalanine quinone (TPQ), subsequently employing TPQ to catalyze steady-state amine oxidation. The mechanisms of TPQ biogenesis and steady-state amine oxidation have been studied extensively, with consensus mechanisms proposed for both reactions. One unresolved issue has been whether the Cu2+ center must undergo formal reduction to Cu1+ in the course of the reaction. Herein, we investigate the properties of the active site of a yeast (Hansenula polymorpha) amine oxidase (HPAO) that has undergone site-specific insertion of a para-aminophenylalanine (pAF) into the position of either the precursor tyrosine to TPQ (Y405) or the two strictly conserved neighboring tyrosines (Y305 and Y407). While our original intention was to interrogate cofactor biogenesis using a precursor unnatural amino acid (UAA) of altered redox potential and pKa, we instead observe an unanticipated reaction assigned to an intramolecular electron transfer from pAF to the active site copper ion. We establish the generality of the observed active site chemistry using exogenously added, aniline-containing substrates under conditions that prevent side chain amine oxidation. The results support previous proposals that the activation of the TPQ precursor occurs in the absence of a formal valence change at the active site copper site. The described reaction of pAFs with the active site redox Cu2+ center of HPAO provides a prototype for either the engineering of the enzymatic oxidation of exogenous anilines or the insertion of site-specific free radical probes within proteins.

Thio and Seleno-Psoralens as Efficient Triplet Harvesting Photosensitizers for Photodynamic Therapy.

The Psoralen (Pso) molecule finds extensive applications in photo-chemotherapy, courtesy of its triplet state forming ability. Sulfur and selenium replacement of exocyclic carbonyl oxygen of organic chromophores foster efficient triplet harvesting with near unity triplet quantum yield. These triplet-forming photosensitizers are useful in Photodynamic Therapy (PDT) applications for selective apoptosis of cancer cells. In this work, we have critically assessed the effect of the sulfur and selenium substitution at the exocyclic carbonyl (TPso and SePso, respectively) and endocyclic oxygen positions of Psoralen. It resulted in a significant redshifted absorption spectrum to access the PDT therapeutic window with increased oscillator strength. The reduction in singlet-triplet energy gap and enhancement in the spin-orbit coupling values increase the number of intersystem crossing (ISC) pathways to the triplet manifold, which shortens the ISC lifetime from 10-5 s for Pso to 10-8 s for TPso and 10-9 s for SePso. The intramolecular photo-induced electron transfer process, a competitive pathway to ISC, is also considerably curbed by exocyclic functionalizations. In addition, a maximum of 115 GM of two-photon absorption (2PA) with IR absorption (660-1050 nm) confirms that the Psoralen skeleton can be effectively tweaked via single chalcogen atom replacement to design a suitable PDT photosensitizer.

Photodecarboxylative Ring Annulation of α‐ and β‐Functionalized Phthaloyl‐GABA Derivatives: Bioactive Pyrroloisoindolinones with High Quantum Efficiency

AbstractThe triplet‐sensitized (by the solvent acetone) as well as the direct (λex=300–320 nm) photochemical decarboxylation of N‐phthaloylated γ‐aminobutyric acid (GABA) derivatives are versatile and high‐yielding routes to benzopyrrolizidines via intramolecular electron transfer initiated decarboxylation followed by radical coupling. The ß‐mono‐ and ß,ß'‐disubstituted N‐phthaloyl GABA derivatives 7 a–7 g, respectively, were applied as substrates. Decarboxylative photocyclization yielded hydroxy benzopyrrolizidines 8 a–8 g in high chemical yields and with moderate diastereoselectivities from the ß‐monosubstituted substrates. The analogous α‐substituted GABA derivatives 11 a–11 c were also applied as potential substrates for memory of chirality effects. The reaction quantum yields of the photodecarboxylation reactions for the parent GABA derivative 13 and for the new substrates 7 h and 11 a were determined by the quantum yield determination system (QYDS) and showed a remarkable concentration dependency indicating aggregation at higher substrate concentrations. Inhibition studies on the atherogenic human serine hydrolase cholesterol esterase showed derivatives 8 a and 8 d to exhibit a hyperbolic mode of inhibition with moderate IC50 values of about 60–80 μM.

Exploring the Cooperation of the Redox Non-Innocent Ligand and Di-Cobalt Center for the Water Oxidation Reaction Catalyzed by a Binuclear Complex.

Water oxidation is a crucial reaction in the artificial photosynthesis system. In the present work, density functional calculations were employed to decipher the mechanism of water oxidation catalyzed by a binuclear cobalt complex, which was disclosed to be a homogeneous water oxidation catalyst in pH=7 phosphate buffer. The calculations showed that the catalytic cycle starts from the CoIII,III-OH2 species. Then, a proton-coupled electron transfer followed by a one-electron transfer process leads to the generation of the formal CoIV,IV-OH intermediate. The subsequent PCET produces the active species, namely the formal CoIV,V=O intermediate (4). The oxidation processes mainly occur on the ligand moiety, including the coordinated water moiety, implying a redox non-innocent behavior. Two cobalt centers keep their oxidation states and provide one catalytic center for water activation during the oxidation process. 4 triggers the O-O bond formation via the water nucleophilic attack pathway, in which the phosphate buffer ion functions as the proton acceptor. The O-O bond formation is the rate-limiting step with a calculated total barrier of 17.7 kcal/mol. The last electron oxidation process coupled with an intramolecular electron transfer results in the generation of O2.

The critical role of a conserved lysine residue in periplasmic nitrate reductase catalyzed reactions.

Periplasmic nitrate reductase NapA from Campylobacter jejuni (C. jejuni) contains a molybdenum cofactor (Moco) and a 4Fe-4S cluster and catalyzes the reduction of nitrate to nitrite. The reducing equivalent required for the catalysis is transferred from NapC → NapB → NapA. The electron transfer from NapB to NapA occurs through the 4Fe-4S cluster in NapA. C. jejuni NapA has a conserved lysine (K79) between the Mo-cofactor and the 4Fe-4S cluster. K79 forms H-bonding interactions with the 4Fe-4S cluster and connects the latter with the Moco via an H-bonding network. Thus, it is conceivable that K79 could play an important role in the intramolecular electron transfer and the catalytic activity of NapA. In the present study, we show that the mutation of K79 to Ala leads to an almost complete loss of activity, suggesting its role in catalytic activity. The inhibition of C. jejuni NapA by cyanide, thiocyanate, and azide has also been investigated. The inhibition studies indicate that cyanide inhibits NapA in a non-competitive manner, while thiocyanate and azide inhibit NapA in an uncompetitive manner. Neither inhibition mechanism involves direct binding of the inhibitor to the Mo-center. These results have been discussed in the context of the loss of catalytic activity of NapA K79A variant and a possible anion binding site in NapA has been proposed.

Fine-tuning Fluorescence of Amlodipine Adopting on Blocking of Photoinduced Electron Transfer (PET): Application in Human Plasma.

Assessing medication adherence through the determination of antihypertensive drugs in biological matrices holds significant importance. Amlodipine (AP), a potent antihypertensive medication extensively prescribed for hypertensive patients, is particularly noteworthy in this context. This article aims to introduce a rapid, simple, improved sensitivity, and reproducibility in detecting AP in its pure form, tablet formulation, and spiked human plasma than the other reported methods. The proposed method utilizes a fluorescence approach, relying on the inhibition of the intramolecular photoinduced electron transfer (PET) effect of the lone pair of the N-atom in the primary amino moiety of AP. This inhibition is achieved by acidifying the surrounding medium using 0.2M acetic acid. By blocking PET, the target AP drug is sensitively detected, at [Formula: see text] 423nm over a concentration range 25-500 ng mL- 1 showcasing an exceptionally low quantitation limit of 1.41 ng mL- 1. Notably, this innovative technique was successfully applied to detect AP in its solid dosage form and spiked human plasma. Remarkably, matrix interference was found to be insignificant, underscoring the robustness and applicability of the established approach. The combination of speed, sensitivity, and reproducibility makes this method particularly suitable for assessing medication adherence in patients prescribed AP for hypertension.

Viologen‐based host–guest supramolecule with tunable intramolecular/intermolecular electron transfer chromism and dynamic fluorescence

AbstractViologen, as a type of strong electron acceptor, is prone to undergo electron transfer (ET) and change color under external stimuli. However, due to the easy aggregation of viologen molecules, they usually suffer from poor fluorescence emission in the condensed phase. Herein, a new viologen derivative of VioCl2·2Cl (12+·2Cl) was designed and synthesized, in which the fluorescence was enhanced by introducing Me‐β‐CD to weaken the interactions between viologen molecules. Then a viologen‐based host–guest supramolecule of 12+@Me‐β‐CD was obtained by electrostatic interactions. Photo‐/chemo‐responded guest 12+ supplies 12+@Me‐β‐CD, a green and dark purple caused by intramolecular and intermolecular ET. Furthermore, 12+@Me‐β‐CD displays an additional thermal responsive purple color. The triple chromic behaviors all exhibit excellent reversibility and cycling stability. As expected, 12+@Me‐β‐CD exhibits strong photoluminescence (PL) in solid–liquid dual states, presenting an improved quantum yield (Φ) from 12+ (Φs = 0.37%, Φl = 16.74%) to 12+@Me‐β‐CD (Φs = 10.45%, Φl = 25.86%), and the fluorescence intensity can be dynamically modulated by light, heat, and acid/base vapors. The multi‐responsive chromism and tunable fluorescence of 12+@Me‐β‐CD allow for potential applications in information security and smart windows.

Synthesis of chlorophyll–quinone conjugates by Diels–Alder reaction and their fluorescence quenching by intramolecular electron transfer

Methyl pyropheophorbides-a possessing a 1,4-naphthoquinon-5-yl or 9,10-anthraquinon-1-yl group at the 3-position were prepared by the Diels–Alder reaction of 3-(1,3-butadienyl)chlorin with benzoquinone or naphthoquinone and successive dehydrogenative oxidation. The synthetic chlorophyll-a derivatives directly bonded with the quinone were atropisomers around the C3–C31 bond, which were readily separated by HPLC. Since the quinone moiety was nearly perpendicular to the chlorin π-system, the distance and orientation between the chlorin and quinone chromophores were fixed in a molecule. The visible absorption spectra of the synthetic chlorin–quinone conjugates were independent of the quinone part and the axial chirality. Their fluorescence emissions were almost completely quenched by intramolecular electron transfer from the photoexcited chlorin to the quinone, which resembled the charge-separating process in photosynthetic reaction centers.

Rapid sequential detection of Al3+ and glyphosate using an “Off-On-Off” fluorescent probe based on salicylate modified layered double hydroxides

A fluorescent probe based on salicylate modified layered double hydroxide (LDH-SA) is presented, enabling the swift sequential detection of Al3+, fosetyl-Al and glyphosate in aqueous environment. The probe was synthesized using a simple co-precipitation procedure, and its properties and synthesis conditions were thoroughly characterized and optimized. A unique “off-on-off” fluorescent response was observed when the probe sequentially interacted with Al3+ and glyphosate, and the detection method based on this phenomenon was established. The limits of detection for Al3+ and glyphosate were determined as 0.03 μmol/L and 0.03 mg/L, respectively, with rapid detection periods of one minute and four minutes. The LDH-SA/Al3+ complex requires Al3+ to generate a chelation-gathered fluorescence effect, which is the mechanism by which it quenches LDH-SA. This is possible due to the inhibition of excited-state intramolecular proton transfer and photoinduced electron transfer processes within LDH-SA after incorporating Al3+. Upon interaction with glyphosate, competitive complexation between glyphosate and Al3+ is initiated, which leads to a recovery of the fluorescence spectrum of LDH-SA and demonstrating the “off-on-off” behavior. An “INHIBIT” logic gate system was devised utilizing the response, indicating potential applications in fluorescence-based devices. Such a rapid, sequential detection capacity is impressive. It attests to the utility of LDH-SA as a probe for Al3+ or glyphosate, and suggests promise for applications in pollutant analysis or environmental monitoring applications.

Novel electrocatalytic capacitive deionization with catalytic electrodes for selective phosphonate degradation: Performance and mechanism

Phosphonate is becoming a global interest and concern owing to its environment risk and potential value. Degradation of phosphonate into phosphate followed by the recovery is regarded as a promising strategy to control phosphonate pollution, relieve phosphorus crisis, and promote phosphorus cycle. Given these objectives, an anion-membrane-coated-electrode (A-MCE) doped with Fe-Co based carbon catalyst and cation-membrane-coated-electrode (C-MCE) doped with carbon-based catalyst were prepared as catalytic electrodes, and a novel electrocatalytic capacitive deionization (E-CDI) was developed. During charging process, phosphonate was enriched around A-MCE surface based on electrostatic attraction, ligand exchange, and hydrogen bond. Meanwhile, Fe2+ and Co2+ were self-oxidized into Fe3+ and Co3+, forming a complex with enriched phosphonate and enabling an intramolecular electron transfer process for phosphonate degradation. Additionally, benefiting from the stable dissolved oxygen and high oxygen reduction reaction activity of C-MCE, hydrogen peroxide accumulated in E-CDI (158 μM) and thus hydroxyl radicals (·OH) were generated by activation. E-CDI provided an ideal platform for the effective reaction between ·OH and phosphonate, avoiding the loss of ·OH and triggering selective degradation of most phosphonate. After charging for 70 min, approximately 89.9% of phosphonate was degraded into phosphate, and phosphate was subsequently adsorbed by A-MCE. Results also showed that phosphonate degradation was highly dependent on solution pH and voltage, and was insignificantly affected by electrolyte concentration. Compared to traditional advanced oxidation processes, E-CDI exhibited a higher degradation efficiency, lower cost, and less sensitive to co-existed ions in treating simulated wastewaters. Self-enhanced and selective degradation of phosphonate, and in-situ phosphate adsorption were simultaneously achieved for the first time by a E-CDI system, showing high promise in treating organic-containing saline wastewaters.