Charge-transfer Complexes Research Articles

Red-Light-Induced Ligand-to-Metal Charge Transfer Catalysis by Tuning the Axial Coordination of Cobyrinate.

Photoinduced ligand-to-metal charge transfer (LMCT) is a powerful technique for the formation of reactive radical species via homolytic cleavage of the metal-ligand bond. Here, we present that the excited state LMCT of a cobyrinate complex can be accessed by tuning its axial coordination with thiolates as ligands. We demonstrate the photoreduction of cobalt via the excited state Co-S bond homolytic cleavage, as guided by the DFT calculations, which signify the relevance of thiolate axial ligands facilitating the LMCT reactivity. By exploiting this excited state LMCT of cobyrinate, we developed a catalyst-controlled activator regeneration mechanism to catalyze an efficient atom transfer radical polymerization (ATRP) under low-energy light irradiation. Tuning the coordination sphere of cobyrinate provides further control over the electronic properties of the complex while also accessing photothermal conversion in mediating ATRP catalysis.

CeCl3-Catalyzed C-H Alkylation of N-Sulfonyl Ketimines with Alkanes and Ether via Photoinduced Ligand-to-Metal Charge Transfer.

A cerium-catalyzed C-H alkylation of N-sulfonyl ketimines with low-cost and readily available alkanes as alkyl sources was developed. This transformation proceeded through the synergy of photoinitiated ligand-to-metal charge transfer (LMCT) using a chlorine radical as an HAT reagent and air as a green oxidant. A series of alkylated N-sulfonyl ketimines were synthesized with moderate to good yields in a highly atom-economic manner under chemical oxidant-free conditions.

Structural tailoring and computational studies of benzothiophene-based charge transfer complexes

Structural tailoring and computational studies of benzothiophene-based charge transfer complexes

Molecular Probe for Specific Recognition of TKX-50: 'Luminescence-ON' Response and its Integration to a Smart Device for Surveillance.

In response to the growing concerns about the unauthorized use of advanced secondary explosives such as TKX-50 against non-combatant targets, there is an urgent need for effective detection methods or techniques to ensure efficient security screening, homeland security, and public safety. Herein, a new polymeric receptor (IV) derived from functionalized tetraphenylethylene moiety (TPE) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) moietiesfor the efficient detection of TKX-50 through a 'switch ON' luminescence response upon specific binding to the explosive, is reported. The observed 'luminescence ON' response is rationalized based on a charge transfer complex formation between TKX-50 and the polymeric receptor IV (Ka = 1.7 × 104 m-1). This is validated by the steady and excited-state luminescence studies, along with detailed computational studies. The authors' presumptions are further validated with adequate control studies using an appropriate monomeric derivative (III) of TPE. Moreover, this 'luminescence ON' response can be integrated into a smart and user-friendly Internet of Things (IoT)-based prototype device. This device can effectively convert optical responses into digital output to develop an optical device for real-time detection of TKX-50 in solution. This lightweight, portable device is ideally suited for remote surveillance and monitoring of TKX-50; such examples are rare in contemporary literature.

Study of the Photophysical Properties and the DNA Binding of Enantiopure [Cr(TMP)2(dppn)]3+ Complex.

The preparation, electrochemistry and photophysical properties of a heteroleptic chromium(III) polypyridyl complex rac-[Cr(TMP)2(dppn)]3+ (1) containing two 3,4,7,8-tetramethyl-1,10-phenanthroline (TMP) ligands and the π-extended benzodipyrido[3,2-a:2',3'-c]phenazine (dppn) ligand are reported. The visible absorption spectrum of 1 reveals distinct bands between 320 and 420 nm characteristic of dppn-based ligand-centered transitions, with 1 found to be nonemissive in aqueous solution but weakly luminescent in aerated acetonitrile solution. Transient visible absorption (TrA) spectroscopy reveals that 400 nm excitation of 1 leads to initial population of a ligand-to-metal charge transfer (LMCT) state which evolves within tens of ps to form a dppn-localized intraligand (3IL) state which persists for longer than 7 ns and efficiently sensitizes singlet oxygen. Chiral resolution and DNA binding of the lambda and delta enantiomers of 1 to four different DNA systems is reported. In all cases the lambda enantiomer shows greater affinity for DNA and in particular AT-rich DNA. Thermal denaturation reveals that the lambda enantiomer stabilizes the DNA more. There is also a greater stabilization of the AT-containing DNA sequences compared to GC DNA.

Biodegradable Janus sonozyme with continuous reactive oxygen species regulation for treating infected critical-sized bone defects

Critical-sized bone defects are usually accompanied by bacterial infection leading to inflammation and bone nonunion. However, existing biodegradable materials lack long-term therapeutical effect because of their gradual degradation. Here, a degradable material with continuous ROS modulation is proposed, defined as a sonozyme due to its functions as a sonosensitizer and a nanoenzyme. Before degradation, the sonozyme can exert an effective sonodynamic antimicrobial effect through the dual active sites of MnN4 and Cu2O8. Furthermore, it can promote anti-inflammation by superoxide dismutase- and catalase-like activities. Following degradation, quercetin-metal chelation exhibits a sustaining antioxidant effect through ligand-metal charge transfer, while the released ions and quercetin also have great self-antimicrobial, osteogenic, and angiogenic effects. A rat model of infected cranial defects demonstrates the sonozyme can rapidly eliminate bacteria and promote bone regeneration. This work presents a promising approach to engineer biodegradable materials with long-time effects for infectious bone defects.

Investigation on the effect of complex formation on prominent high static/dynamic first and second hyperpolarizability of Co(II) complex including 4-chloro-Pyridine-2-Carboxylic acid

Nonlinear optical (NLO) materials with promising first- and second-order hyperpolarizabilities play a crucial role in the fields of electronics and optoelectronics. For the metal-organic coordination complexes, the logical selection of an appropriate ligand and metal ion is vital in designing such materials to enhance their NLO response. In this study, a novel Co(II) complex, including 4-Chloro-pyridine-2-carboxylic acid ligand, was synthesized. The crystal structure and spectroscopic properties were determined by XRD, FT-IR, and UV–Vis spectroscopies. B3LYP and CAM-B3LYP level calculations demonstrated that there is a strong electronic absorption band at 356 nm, originating from the metal-ligand charge transfer (MLCT) transfer in the UV–Vis spectrum. Both XRD and FT-IR data demonstrated that the Co(II) complex has a distorted octahedral coordination geometry. The calculations of static and frequency-dependent α, β, and γat frequencies of ω = 0.0856252 a.u (λ = 532 nm) and w = 0.0428126 au. (λ = 1064 nm) for 4Clpca and Co(II) complex have been also carried out using B3LYP and CAM-B3LYP levels. The dc-Kerr effect γ(-ω; ω,0,0) and SHG γ(-2ω; ω,ω,0) parameters for Co(II) complex calculated as 329.59 × 10−36 and 433.93 × 10−36 esu are found as dramatically higher those for single pca ligand (6.0267 × 10−36 and 163.70 × 10−36 esu). So, a significant increase was detected in the parameters of frequency-dependent α (-w; 0), β(-w; w,0), and γ (-w; w,0,0) with complex formation.

Redox Reactions of [RuIII(pic)3] (pic‐= picolinate) with Sulfite in Aqueous Solution: Kinetic, and Mechanistic Studies

The reaction of [RuIII(pic)3] (pic‐ = picolinate) and sulfite (SO32‐) resulting in the formation of [RuII(pic)3]‐ and sulfate (SO42‐) has been studied spectrophotometrically and kinetically in aqueous solution. The rate of the reaction studied by following the appearance of the MLCT (metal to ligand charge transfer) band of the [RuII(pic)3)]‐ complex at 466 nm was found to be first‐order in both the complex and sulfite concentrations. The values of the observed rate constant depend on the pH, since it controls the speciation of oxoanions of sulfur(IV)‐species viz. bisulfite (HSO3‐) and sulfite (SO32‐) in the pH range 3.8 to 8.3. The product of the reaction, sulfate was ascertained by LDI‐mass spectral analysis of the post‐reaction solution. The activation parameters (DH¹ =56 ± 3kJ mol‐1 and DS¹ = ‐179 ± 9 J mol‐1 K‐1) corresponding to the reduction of [RuIII(pic)3] with SO32‐ supports the operation of an outer‐sphere electron transfer mechanism. The experimentally observed electron‐transfer rate constant (k = 5.60 M‐1s‐1 at 25 oC) is found to be in good agreement with that of calculated (k12 = 4.75 M‐1s‐1) using Marcus cross‐reaction relationship for outer‐sphere electron‐transfer reactions. A mechanism consistent with the spectral and kinetic data is proposed for the aforesaid reaction.

A theoretical comprehension of photophysical processes in Cu2+ sensing by 1,7-di(2-pyridyl)bispyrazolo[3,4-b:4′,3′-e]pyridines

Due to its simplicity and sensitivity, metal ion sensing by fluorescent probes has a high biological and ecological impact, and several preliminary applications for Cu2+ have been found. However, the poor understanding of photophysical phenomena by which probes work has led to the growth of unhelpful literature. In this way, 4-aryl-1,7-di(pyridin-2-yl)bispyrazolo[3,4-b:4′,3′-e]pyridines Py2BP2a-c were studied as tridentate ligands in developing the probe Py2BP2a (Ar = Ph, LODCu2+ = 26 nM); thus, this previous work is completed herein by DFT/TD-DFT studies to understand the sensing process. The basal and first excited state of Py2BP2a-c (Ar: Ph, 4-An, 4-Py) and the parent l,4,7-triphenylbispyrazolo[3,4-b:3′,4′-e]pyridine Ph3BP2 were optimised. Results suggest that the probes' fluorescence is due to a twisted intramolecular charge transfer (TICT) and ICT processes around the 4-aryl and 1,7-dipyridin-2-yl groups; likewise, the fluorescence turn-off in the presence of Cu2+ by probe Py2BP2b is due to a photoinduced electron transfer (PET) process, favouring a ligand-to-metal charge transfer (LMCT). These findings enhance our understanding of the sensing process and open new possibilities for its applications in various fields.

Synthesis and luminescence properties of Ca2AlNbO6:Fe3+ phosphor for plant growth lighting

With the development of plant growth lighting, more and more attention has been paid to red and far red luminescent materials. In this work, we synthesize successfully a non-rare earth and non-toxic Ca2AlNbO6:Fe3+ phosphor in air and investigate the crystal structure, morphology, and luminescence properties of Ca2AlNbO6:Fe3+ by using powder X-Ray Diffraction, the field emission scanning electron microscopy, and the Edinburgh steady-state FS5 spectrometer, respectively. The far-red emission with peak at 714 nm of Ca2AlNbO6:Fe3+ is due to the 4T1(4G) → 6A1(6S) transition of Fe3+ ion. Photoluminescence excitation (PLE) spectrum of Ca2AlNbO6:Fe3+ includes four PLE peaks, which are assigned to the O2− → Fe3+ charge transfer band (CTB) (309 nm), the 6A1(6S) → 4T1(4P) (364 nm), 6A1(6S) → 4E(4D) (395 nm), and 6A1(6S) → 4T2(4G) (532 nm) transitions of Fe3+, respectively. We further study the concentration/temperature dependent emission spectra and time-resolved emission spectra. The optimal Fe3+ doping concentration (0.6mol%) and the only luminous center (Fe3+) in Ca2AlNbO6:Fe3+ are confirmed. The decay curves of Ca2AlNbO6:xFe3+ (0.2 mol% ≤ x ≤ 1.2 mol%) are measured and their lifetimes decrease from 463.24 to 385.29 μs＀ The emission spectra under different temperatures prove that Ca2AlNbO6:Fe3+ has a good heat stability. We explain the luminous mechanism and concentration/thermal quenching mechanisms, and further research the application prospect of Ca2AlNbO6:Fe3+ in plant growth lighting.

Carrier-free porphyrin-based supramolecular complexes for near-infrared photothermal-enhanced wound healing

Inspired by the assembly of biological macromolecules, supramolecular chemistry has been used to develop various biofunctional mateirals. For example, charge-transfer complexes (CTCs) are recently engineered for photothermal conversion in the near-infrared (NIR) region. Most CTCs rely on surfactants to enhance their dispersity and stability in water, but the potential toxicity associated with surfactants may limit further development of CTCs in biomedical field. Herein, we constructed carrier-free CTCs (TAPP/PDI-G NPs) by selecting 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) as the electron donor and perylenediimide derivative (PDI-G) as the acceptor. TAPP/PDI-G NPs exhibit a NIR absorption peak at 758 nm, significantly longer than that of individual TAPP or PDI-G. Moreover, upon irradiation with an 808 nm laser, TAPP/PDI-G NPs exert exceptional photothermal performance and display efficient antibacterial properties both in vitro and in vivo. The proposed strategy highlights the great potential of supramolecular assembly in constructing desirable materials.

Design of Clickable Aromatic Donor-Acceptor Pairs for Easy Installation onto Polymers.

New intrinsically unsymmetric aromatic donors (D) and acceptors (A) were designed to simplify the incorporation of a single reactive group. Naphthalene diimide (NDI) was desymmetrized by replacing one of the imide units with two nitro groups to yield 3,6-dinitronaphthalene monoimide (NMI(NO2)2); likewise, 3,6-dimethoxycarbazole (CBZ(OMe)2) was designed as a 3-ring aromatic donor. Different derivatives of naphthalene monoimide (NMI) and carbazole (CBZ) were prepared, and charge-transfer complex formation between them was studied using NMR and UV-visible titrations; the estimated association constants (Ka) were compared with the common pair, NDI, and 1,5-dialkoxy naphthalene (DAN). To rationalize the variation of Ka values, the electrostatic potential maps and the energies of highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of different derivatives were computed by using density functional theory. Some of the new D-A pairs outperform the DAN-NDI pair; as expected, the best pair was NMI(NO2)2 and CBZ(OMe)2. The Ka for this pair was 50 M-1, which is slightly higher than that of NDI-DAN but with the advantage that a single reactive handle, like azide, can be easily incorporated and used to click them onto supramolecular motifs or polymers. To illustrate this feature, a low Tg copolymer of n-butyl acrylate and propargyl acrylate (10 mol %) was clicked with the azido-derivatives of the new donors and acceptors; the influence of the charge-transfer interaction on the mechanical properties of blends of these copolymers carrying D and A units was examined using nanoindentation studies. Additionally, the propargyl acrylate copolymers were also coclicked with both D and A units, and their properties were examined as a function of the density of interacting sites. In summary, this study illustrates an alternate design strategy for aromatic donors and acceptors that are easier to synthesize, scale-up, and derivatize; importantly, they can be readily installed onto different motifs for the efficient design of supramolecular materials and polymers.

Utilisation of charge transfer complexation reactions for the quantification of antifungal drug: Voriconazol in pure and dosage forms

Utilisation of charge transfer complexation reactions for the quantification of antifungal drug: Voriconazol in pure and dosage forms

Fabrication and characterization of PVDF/PMMA nanocomposite membranes doped with graphene oxide (GO) for separation of CO2/CH4 from flue gas

This work focuses on developing polyvinylidene fluoride/polymethyl methacrylate (PVDF/PMMA) polymer blend nanocomposites incorporating varying concentrations of graphene oxide (GO) nanoparticles fabricated via casting solution processes. XRD and FT-IR confirmed GO-induced structural modifications in PVDF/PMMA blend. The addition of GO into PMMA/PVDF blend decreased and weakened the intensity of XRD peaks, suggesting that adding GO gradually reduces the crystallinity of the films. FT-IR verified miscibility and complex formation between the pristine polymer blend and the GO-filled nanocomposite. The incorporation of GO caused shifts in the optical absorption edge to lower wavelengths, causing a decrease in the optical bandgap energy (Eg). The real and imaginary parts of the dielectric permittivity (ε′ and ε′′), electric modulus (M′, M″), and AC conductivity (σac) have been measured from 0.1 Hz to 6 MHz. Both ε′ and ε′′ declined with rising frequency. The addition of different concentrations of GO generated charge transfer complexes in the polymer nanocomposites. SEM images show good homogeny with random dispersion of GO inside the polymer blend. The thermoluminescence (TL) glow curves for the nanocomposite samples irradiated 0.5, 1.0, 1.5, and 2 Gy doses show peak cantered at 207 °C. The peak position remained unchanged with increasing irradiation dosage. The dose-dependent linear growth in maximum peak height indicates radiation detection and monitoring capability.

A TEMPO-N3 Complex Enables the Electrochemical C-H Azidation of N-Heterocycles through the Cleavage of Alkoxyamines.

A TEMPO-N3 charge-transfer complex enables the electrochemical C-H azidation of various N-heterocycles. The TEMPO+ ion, generated from TEMPO, assists in producing N3• by forming a TEMPO-N3 complex with N3-. The formation of this complex is supported by UV-vis absorption spectra, cyclic voltammetry studies, and ESI-HRMS studies. The reaction likely proceeds by forming a highly labile azidooxygenation adduct, which undergoes oxidative alkoxyamine mesolytic cleavage. Subsequent deprotonation of the resulting carbocation exclusively produces the azidation product. It is important to note that substituted olefins generally yield azidooxygenation or diazidation as the final product. Our study demonstrates that N-heterocycles deliver a selective monoazidation product, possibly due to steric reasons. ESI-HRMS studies provide evidence for forming azidooxygenation and alkoxyamine radical cation adducts. The regio- and chemoselectivity of this azidation reaction using the TEMPO-N3 complex have been discussed.

Lewis Acidic Ionic-Liquid-Catalyzed Radical-Cascade Alkylation/Cyclization of N-Alkyl-N-methacryloyl Benzamides with Alkanes via Visible-Light-Induced Ligand-to-Metal Charge Transfer: Access to Alkylated Isoquinoline-1,3(2H,4H)-diones.

With the flourishing progress of ligand-to-metal charge transfer (LMCT) photocatalysis, various metals were developed as catalysts to activate abundant alkane feedstocks for the synthesis of functionalized organic compounds. However, to the best of our knowledge, most of the LMCT catalysts are difficult to recover and reuse for the next cycles. Herein, we report a reusable Lewis acidic ionic liquid (LAIL)-catalyzed radical-cascade alkylation/cyclization of N-alkyl-N-methacryloyl benzamides with unactivated alkanes for the synthesis of alkylated isoquinoline-1,3-(2H,4H)-diketones. The protocol features mild reaction conditions, high atom utilization efficiency, scale-up synthesis, simple operation, and recycling of catalysts.

The Emissive and Electrochemical Properties of Hypervalent Pyridine-Dipyrrolide Bismuth Complexes.

We present a series of six hypervalent bismuth complexes Bi(R1PDPR2)X bearing ligands characterized by the pyridine-2,6-bis(pyrrolide) (PDP) structural motif. While bismuth holds considerable potential for facilitating efficient intersystem crossing (ISC), reports on phosphorescent molecular bismuth complexes are still scarce and mostly based on systems that exhibit inter- or intraligand charge transfer character of their optical excitations. Herein, the UV/vis absorptive, luminescent, and electrochemical properties of complexes Bi(R1PDPR2)X are explored, where the substituents R1 and R2, as well as the halide ligand X are varied. These compounds are characterized by an intense HOMO→LUMO transition of mixed ligand-to-metal charge transfer (LMCT) and interligand charge transfer (LL'CT) character, as shown by time-dependent density functional theory (TD-DFT) calculations. At 77 K in a 2-MeTHF matrix, these compounds exhibit red, long-lived phosphorescence with lifetimes ranging from 479 to 14 μs. Cyclic voltammetry measurements and TD-DFT calculations show that the substituents influence HOMO and LUMO energies to almost equal extent, resulting in nearly constant emission wavelengths throughout this series. Single-crystal X-ray diffraction studies of four of the six complexes exemplify the inherent Lewis acidity of the coordinated Bi3+ ion, in spite of its hypervalency.

Regulation of electron density in Pt nanoparticles via bimetallic metal-organic frameworks for enhancing photothermal catalysis of toluene decomposition

Volatile Organic Compounds (VOCs) are omnipresent in the sphere of human industrial, harboring latent adverse consequences for health and the ecological system. The photothermal catalytic oxidation of VOCs is an advanced integrated technology that harnesses the combined effects of light and heat energy to enhance the efficiency of VOCs degradation. Herein, a bimetallic Metal-Organic Framework (MOF) was synthesized with the incorporation of Ce into the UiO-66-NH2(Zr) (i.e., UNH(Zr)), UiO-66-NH2(Zr2Ce) (i.e., UNH(Z2C)), which was achieved with Ce atom substituting for a portion of Zr atom within the Zr-oxo clusters. Pt nanoparticles (NPs) are integrated with MOFs to form composites using the dual-solvent method. Ce-oxo fulfills a bifunctional role: it not only facilitates the enhancement of the ligand-to-metal charge transfer (LMCT), but also establishes interaction with Pt NPs. Ce-oxo mediates an enhancement of electron density on Pt NPs. This phenomenon enhances the adsorption and activation of oxygen, significantly boosting the photocatalytic performance for toluene degradation, as demonstrated by a reduction of 30 ℃ for complete mineralization of toluene as compared to that of Pt@UiO-66-NH2(Zr) (i.e., PUNH(Zr)). This study potentially offers new insights into the relationship between electron transfer effects in bimetallic MOF-based catalysts and their efficient catalytic performance for VOCs degradation.

Chiral Self-Assembly of a Pyrene-Appended Glutamylalanine Dipeptide and Its Charge Transfer Complex: Fabrication of Magneto-Responsive Hydrogels and Human Cell Imaging.

The formation of a robust, self-healing hydrogel of a novel pyrene-appended dipeptide, Py-E-A (L-Glutamic acid short as E; L-Alanine short as A) is demonstrated. Detailed studies suggest that nanoscopic fibers with a length of several micrometers have formed by chiral self-organization of Py-E-A gelators. Additionally, live human PBMCs imaging is shown using the Py-E-A fluorophore. Interestingly, electron-rich Py-E-A couples with electron-deficient NDI-β-A (β-Alanine short as β-A) by charge transfer (CT) complexation and forms stable deep violet-colored CT super-hydrogel. X-ray diffraction, DFT, and 2D ROESY NMR studies suggest lamellar packing of both Py-E-A and the alternating CT stack in its hydrogel matrixes. Supramolecular chirality of the Py-E-A donor can be altered by adding an achiral acceptor NDI-β-A. Notably, the fibers of the CT hydrogel are found to be even thinner than the Py-E-A fibers, which, in turn, makes the CT hydrogel more tolerant to the applied strain. Further, the self-healing and injectable properties of the hydrogels are shown. Finally, the magneto-responsive behavior of the Py-E-A and CT hydrogels loaded with spin-canted Cu-ferrite (Cu0.6Zn0.4Fe2O4) nanoparticles (NPs) is demonstrated. The presence of magnetic NPs within the hydrogels has changed the fibrous morphology to rod-like nanoclusters.

Low background dual-ligand Cu-MOF nanoprobe for plant tissue imaging and fast screening as well as sensitive detection of glyphosate in environmental samples

The monitoring of glyphosate residue in environmental samples is critically important due to its high environmental risk. Here, we reported a low background dual-ligand and fast response copper-based metal organic framework (Cu-MOF) nanoprobe for imaging glyphosate in plant tissue, rapid screening of glyphosate-contaminated samples, and sensitive detection of glyphosate in environmental samples. The Cu-MOF nanoprobe was prepared with 2-Aminoisophthalic Acid (AIA) and trimesic acid (H3BTC) as ligands, and Cu2+ as a metal node. Thanking to both ligand-to-metal charge transfer (LMCT) and photoinduced electron transfer (PET) effects, the fluorescence of ligand AIA could be fully quenched in Cu-AIA/BTC probe. Upon the addition of glyphosate, it competed with the ligands in Cu-AIA/BTC probe, causing the collapse of MOF structure and the release of ligand AIA with obvious fluorescence recovery. This nanoprobe exhibited a desirable linear response for glyphosate in the concentration range of 0.1–80 μM, with a low detection limit of 33 nM, much lower than the maximum contaminant level (4.1 μM) set by the U.S. Environmental Protection Agency (EPA). Furthermore, it was also successfully applied for plant tissue imaging, fast screening of glyphosate-contaminated samples and monitoring of the degradation of glyphosate on tea leaves and in soil, indicating the broad application prospect of the nanoprobe.