Donor Electron Research Articles

A novel fluorescent probe with a large Stokes shift targeting lysosomes and lipid droplets and its application to cellular imaging

Lipid droplets and lysosomes are crucial organelles that play significant roles in cellular metabolism. Therefore, studying both lipid droplets and lysosomes simultaneously holds great value. A new fluorescent probe BTD-Lyso was designed based on the principle of intramolecular charge transfer (ICT). The probe was formed with a typical D-π-A structure, and it used benzothiadiazole as the electronic acceptor and morpholine as the electronic donor. The experimental results showed that BTD-Lyso was able to detect the change of polarity in the microenvironment. Furthermore, the probe showed a large Stokes shift (300 nm), excellent selectivity, polarity sensitivity, and photostability. In addition, it could be successfully utilized in cellular imaging due to its low cytotoxicity and could target lipid droplets and lysosomes well.

Intramolecular Charge Transfer Inhibition Strategy toward a Desired Solvatochromic Fluorescent Platform: Visualization of Duple Organelles and Detection of Carbon Dioxide.

Solvatochromic fluorescent probes are crucial molecular tools to investigate and aggregate proteins' fold, visualize fine structures in biomembranes, and label different organelles in dual emission colors. However, solvatochromic fluorogens often displayed a weak emission at high polarity, hindering their bioimaging applications. To resolve this problem, herein, we propose an intramolecular charge transfer (ICT) inhibition strategy. The probe was designed with a single electronic donor and two acceptors in order to split and inhibit the ICT procedure. As a result, the probe displayed an intense emission at both low and high polarities and showed a large emission shift (84 nm) upon polarity change. Using the probe, we successfully imaged lipid droplets and the endoplasmic reticulum in different fluorescence colors. Moreover, the different degrees of lipid accumulation by oleic acid, stearic acid, and cholesterol (oleic acid > stearic acid > cholesterol) have been revealed. The lipid accumulation induced by the three lipids could be rapidly consumed under lipid-less conditions, and the lipids with stearic acid were the most difficult to be consumed. The biological results could facilitate the understanding and treatment of lipid accumulation and obesity. Furthermore, utilizing the polarity increase of diethylamine after the reaction with CO2, the ratiometric detection of CO2 has been achieved for the first time with the probe.

Unconventional exchange bias and enhanced spin pumping efficiency due to diluted magnetic oxide at the Co/ZnO interface

Exchange bias (EB) is normally created by the interfacial exchange coupling at a ferromagnetic/antiferromagnetic (FM/AFM) interface. FM/AFM interfaces have also been proved to perform enhanced spin angular momentum transfer efficiency in spin pumping (SP), compared with typical FM/nonmagnetic interfaces. Here, we report an unexpected EB and enhanced SP between a ferromagnet and semiconductor. Considerable EB has been observed in Co films grown on ZnO single crystal due to the interface antiferromagnetism of the Zn1−xCoxO (x depends on the Co solubility limit in ZnO) layer. Moreover, SP measurements demonstrate a giant spin pumping efficiency at the Co/ZnO interface with a bump (spin mixing conductance Geff↑↓= 28 nm−2) around the blocking temperature TB ∼ 75 K. The enhanced SP is further confirmed by inverse spin Hall effect measurements and the spin Hall angle θISHE of Zn1−xCoxO is estimated to be 0.011. The bound magnetic polarons with s–d exchange interaction between donor electrons and magnetic cation ions in Zn1−xCoxO play a key role in the formation of antiferromagnetism with giant Geff↑↓. Our work provides a new insight into spin physics at FM/semiconducting interfaces.

Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits

Scalable quantum processors require high-fidelity universal quantum logic operations in a manufacturable physical platform. Donors in silicon provide atomic size, excellent quantum coherence and compatibility with standard semiconductor processing, but no entanglement between donor-bound electron spins has been demonstrated to date. Here we present the experimental demonstration and tomography of universal one- and two-qubit gates in a system of two weakly exchange-coupled electrons, bound to single phosphorus donors introduced in silicon by ion implantation. We observe that the exchange interaction has no effect on the qubit coherence. We quantify the fidelity of the quantum operations using gate set tomography (GST), and we use the universal gate set to create entangled Bell states of the electrons spins, with fidelity 91.3 ± 3.0%, and concurrence 0.87 ± 0.05. These results form the necessary basis for scaling up donor-based quantum computers.

Impact of Donor and Acceptor Modification on TADF and Roll‐Off Behaviors in Solution Processed OLED

AbstractUp‐conversion of triplet into singlet exciton in the emitting layer is believed to be one of the ways that thermally activated delayed fluorescent (TADF) materials may employ to reduce triplet exciton density hence preventing device quenching by triplet excitons. Yet, two donor‐acceptor type molecules; 5‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐12‐(3‐(triphenylsilyl) phenyl)‐5,12‐dihydroindolo[3,2‐a]carbazole (SiPhCzTrz) and 5‐phenyl‐12‐(4‐(4‐phenyl‐6‐(3‐(triphenylsilyl) phenyl)‐1,3,5‐triazin‐2‐yl)pheny‐l)‐5,12‐dihydroindolo [3,2‐a]carbazole (SiTrzPhCz), which exhibited different TADF properties depending on the relative positions of their electron donor unit (PhCz) and electron acceptor unit (Trz), show opposite behaviors. These materials are used as sensitizer in phosphorescent solution‐processed organic light emitting diodes (s‐OLEDs) showing moderately high current efficiencies of 19.3 and 20 cd/A, respectively. SiTrzPhCz exhibits stronger TADF properties compared to SiPhCzTrz; however, these stronger TADF characteristics lead to a more pronounced efficiency roll‐off, mainly due to the longer residence time of excitons in SiTrzPhCz, leading to exciton quenching. In contrast, due to their twisted structures, the efficiency roll‐off is efficiently suppressed, particularly for SiTrzPhCz, when both materials are used as hosts. Their twisted structures promote aggregate‐induced emission and prevent aggregation‐caused quenching. Further analysis of exciton dynamics reveals faster decay rate for both singlet and triplet densities in SiPhCzTrz compared to SiTrzPhCz, indicated by its higher fast prompt emission, kFRET and knrT.

Natural variation in photosynthetic electron transport of wheat flag leaves in response to dark-induced senescence

Early leaf senescence affects photosynthetic efficiency and limits growth during the late production stage of winter wheat (Triticum aestivum). Natural variation in photosystem response to senescence represents a valuable resource for improving the aging traits of flag leaves. To explore the natural variation of different phases of photosynthetic electron transport in modern wheat cultivars during senescence, we exposed the flag leaves of 32 wheat cultivars to dark conditions to induce senescence process, and simultaneously measured prompt fluorescence and modulated 820 nm reflection. The results showed that the chlorophyll content, activity of PSII donor side, PSI and electron transfer between PSII and PSI were all decreased during dark-induced senescence, but they showed different sensitivity to dark-induced senescence. Furthermore, natural variation in photosynthetic parameters among the 32 wheat cultivars were also observed and showed by variation coefficient of the different parameters. We observed that PSII and PSI activity showed less sensitivity to dark-induced senescence than electron transfer between them, while PSII and PSI activity exhibit greater natural variation than electron transport between PSII and PSI. It suggests that Cytb6f might degrade faster and have less variation than PSII and PSI during dark-induced senescence.

Study on the role of d electronic donor in Ni-rich layered oxide cathode materials for Li-ion battery

The further development of electrochemical devices and electric vehicles requires advanced Li-ion battery with higher energy density and longer lifetimes. Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes are receiving increased attention due to its high specific capacity. Nevertheless, the role of d electrons in Ni-rich cathodes has not been uniformly reported, which hinders the design of high-performance Ni-rich cathodes. To address this issue, the role of d electronic donor in Ni-rich layered oxide cathode materials was investigated by first principles calculations and explored the mechanism for enhancing the electrochemical performance in this work. The results show that the d electronic donor V and Mo can elevate the intercalation potential, improve the stability, and enhance the diffusion rate of Li-ions in Ni-rich cathode. This theoretical study complements the mechanism for improving the electrochemical performance of Ni-rich cathode, and provides a theoretical basis for the design of high-performance Li-ion battery.

The impact of a ZnO space layer on enhancing the efficiency of P3HT: PCBM blend-based organic solar cells

The performance of Organic Solar Cells (OSCs) based on a composite photoactive of poly(3-hexylthiophene) (P3HT) and 6,6-phenylC61-butyric acid methyl ester (PCBM) as the donor and acceptor electrons, respectively, is examined in this paper through a simulation study. Optical Zinc oxide (ZnO) spacers between the active layer and the Aluminum (Al) cathode contact layer, serving as a hole transport layer (HTL), are compared to the usual configuration without a ZnO spacer in the second device construction. With and without the ZnO optical spacer, we calculate the electric field intensities and generation rates (G) for a range of incident wavelengths. Under AM 1.5 G at 100 mW cm−2 illumination and in the dark, the short-circuit current density (Jsc) is improved by up to 9.47% after adding the ZnO layer as an optical spacer. In addition, there is a gain of up to 11.42% in external quantum efficiency (EQE). Furthermore, the ZnO spacer layer device has a markedly higher incident photon-to-electron conversion efficiency (IPCE), going from 77% in the control device (without ZnO) to 90% with ZnO spacer layer. The numerical simulation results based on the finite element method (FEM) are consistent with the published experimental data.

Physiological and transcriptomic analysis reveals the toxicological mechanisms of polystyrene micro- and nano-plastics in Chlamydomonas reinhardtii

With the accumulation of plastic waste in the environment, the toxicity of micro- and nano-plastics (MNPs) to microalgae has attracted increasing attention. However, the underlying toxic mechanisms of MNPs remain to be elucidated. In this study, we synthesized micro- and nano-scale of polystyrene MNPs (PS MNPs) to investigate their toxicity and toxic mechanisms in Chlamydomonas reinhardtii. We found that PS MNPs significantly inhibit the production of photosynthetic pigments and increase soluble protein content. The detailed analysis of results shows that both materials affect photosynthetic efficiency by damaging the donor side, reaction center, and electron transfer of photosystem II. Moreover, compared to PS MPs, PS NPs have a greater negative impact on algal cells. Analyzing the transcriptome of cells suggests that the most sensitive metabolic pathways in response to PS MNPs involve oxidative phosphorylation, biosynthesis of secondary metabolites, and photosynthesis. Especially, genes related to photosynthesis and oxidative phosphorylation showed significant changes in expression after exposure to PS MNPs. This study provided molecular-level insights into the toxic mechanisms of PS MNPs on microalgae.

Charge transfer interaction dynamics between 6-Aminoindole and picric acid: Synthesis, spectrophotometric, characterization, computational, DNA binding, and anticancer properties

The charge transfer complex (CT) chemistry is studied between electron donor 6-Aminoindole (AMI) and electron acceptor Picric acid (PA) at 25 °C, in an acetonitrile medium by using UV–Visible absorption spectroscopy. The molecular ratio of the CT complex was determined to be 1:1 by using jobs variation and spectrophotometric titration processes. To assess the stability constant (KCT), and molar extinction coefficient (ƐCT) by using the Benesi-Hildebrand equation. These outcomes reveal that the synthesized CT complex is highly stable. The thermodynamic study is carried out in different temperatures (20-45 °C), and these results indicate increases the stability with increasing temperature. The CT complex structure was analyzed by FT-IR, 1H NMR, and 13C NMR. The surface morphology and elemental composition were identified by the SEM-EDX technique. The crystalline nature was analyzed by powdered XRD investigation. The CT complex has been screened for DNA binding and invitro cytotoxicity. The DNA binding constant value (2.3 × 105 M−1) and in vitro cytotoxicity test results on HeLa, suggest that it could be used as a potential medicinal drug in the future. Finally, the geometry optimizations were carried out by the DFT with the wB97XD method and 6-31+G(d,p) basis set. Here we investigated different parameters like bond parameters, NBO analysis, FMO analysis, DOS, PDOS, and computed electronic spectra. The computational analysis provided additional support for the experimental CT complex formation.

Synthesis, Spectral and Electrochemical Properties, and Theoretical Studies of Highly Functionalized Fused Triazole-Isoindoline Derivatives as a Donor-Acceptor System

Triazole and propargyl group functionalities are frequently utilized in the construction of efficient blue-emitting materials, serving as acceptors and donors, respectively. Leveraging the synergistic effect of donor-acceptor coupling between these moieties, three new compounds, namely CB-But-I2, Ph-But-I2, and DP-But-I2, were synthesized. Given the advantageous strategy of combining electron donors and acceptors to optimize carrier injection and transport, CB-But-I2, Ph-But-I2, and DP-But-I2 demonstrate remarkable capabilities in both hole and electron transportation. A metal-free synthesis approach was employed to produce highly functionalized 1,2,3-triazole cores incorporating electronic donor and acceptor functionalities. This synthesis method involved the “Click reaction” ring formation between acetylene and azide compounds of 2-(4-bromobutyl) isoindoline-1,3-dione, catalyzed by copper sulfate pentahydrate and assisted by ascorbic acid as a base. The process boasts several advantages, including mild reaction conditions, accessibility of precursors, metal-free synthesis, straightforward setup, and the generation of various substituted regioselective triazole compounds in satisfactory yields. All synthesized compounds were found to possess thermal stability and exhibited fluorescence within the 292–355 nm range. Moreover, a significant positive solvato-chromic behavior was noted across different solvents with varying polarities, and the theoretical values were compared against experimental data for validation.

Boosting photoelectron transfer by Fermi and doping levels regulation in carbon nitride towards efficient solar-driven hydrogen production

The exploration of efficient photosystems with fast carrier dynamics is an important pursuit for solar conversion into hydrogen energy, while tremendous challenges remain since the intrinsic relationship between the band structure and the charge transfer behavior in semiconductors is still elusive. Towards this, P doped graphitic carbon nitride (P-g-C3N4) is fabricated as a prototype photocatalyst with P existing in P − N = C state. Through tuning the content of P to modulate the electronic donor concentration, the Fermi level of g-C3N4 is unidirectionally regulated from below to above the doping level induced by P. And it is revealed that the relative position of Fermi level vs. doping level plays a crucial role in determining the charge migration dynamics, where only a doping state near the Fermi level can act as the most efficient electron trap to prolong carrier lifetime. Accordingly, the precisely tailored P-g-C3N4 achieves a superior apparent quantum efficiency (AQE) of 5.7 % at 420 nm in overall water splitting that is among the highest levels of current photosystems. Our findings offer an innovative strategy for band structure regulation toward efficient photocatalysis.

Sensitizing TiO2 nanoparticles with Ru(II) polypyridyl complexes containing anchoring nitrile groups for applications in solar cells

In this work, five dyes-based on Ru(II) complexes with 2,2́- bipyridyl ligands substituted at the 4,4́-positions with a different number of nitrile anchoring groups and distinct −X electronic donor units were tested as TiO2 electrode sensitizers in solar cells (SCs). All dyes were PF6- salts: three complexes of formulae [Ru(4,4́-(X)2-bpy)2(Mebpy-CN)]2+ with one nitrile group and X = − CH3, –OCH, −N(CH3)2; and two of formulae [Ru(4,4′-(X)2-bpy)(Mebpy-CN)2]2+, with two nitrile units and X = –OCH3, −N(CH3)2. In all cases, bpy = 2,2′-bipyridine and Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile. After characterizing the electrodes morphologically and optically, they were employed in DSSCs (dye-sensitized SCs) using Pt/FTO as counter-electrode (CE) and a KI/I2 in CH3CN as liquid electrolyte. Current density–voltage (J-V) and impedance spectroscopy (IS) measurements highlighted that the SC with the highest efficiency (η) is the one that contains − N(CH3)2 as donor units and two nitrile groups as anchoring entities. It could be concluded that higher electronic delocalization that occurs between the bpys substituted with strong electronic donor groups and the metal orbitals impose excellent properties to improve efficiency, which further improves if two anchoring groups are employed.

Mechanism research of type I reactive oxygen species conversion based on molecular and aggregate levels for tumor photodynamic therapy

AbstractType I photosensitizers (PSs) with the ability to generate reactive oxygen species (ROS) containing superoxide anion and hydroxyl radical have promising application potential for treating hypoxia tumors, but the deep mechanism of type II ROS converts to the type I ROS in the PSs is still unclear, it is urgent to reveal influencing factors about inducing type I ROS generation. Herein, six PSs with aggregation‐induced emission properties, which were fabricated with the same electronic acceptor but different electronic donors and “π‐bridge”, have been successfully prepared to explore the influencing mechanism of generating superoxide anion and hydroxyl radical from organic PSs. Experimental results discovered two factors containing molecular structure and aggregated environment could decide the ROS efficiency and types of PSs. On the level of designing molecular structure, we discovered that “π‐bridge” with a lower energy level of the lowest triplet state could be beneficial for triggering the production of superoxide anion, and electronic donor of triphenylamine was an important factor in producing hydroxyl radical than another donor of dimethylamine. On the level of designing aggregates of PS‐based polymeric nanoparticles, bovine serum albumin could improve largely the generation efficiency of superoxide anion. Due to the satisfactory ROS efficiency and better biocompatibility, synthetic PSs showed excellent photodynamic therapy outcomes in vitro/vivo.

Tandem Four-Component Reaction to Access Fused Polycycles Exhibiting Aggregation-Enhanced Through-Space Charge Transfer Emission.

Rapid construction of new fluorescence emitters is essential in advancing synthetic luminescent materials. This study illustrated a piperidine-promoted reaction of chiral dialdehyde with benzoylacetonitrile and malonitrile, leading to the formation of the 6/6/7 fused cyclic product in good yield. The proposed reaction mechanism involves a dual condensation/cyclization process, achieving the formation of up to six bonds for fused polycycles. The single crystal structure analysis revealed that the fused cyclic skeleton contains face-to-face naphthyl and cyanoalkenyl motifs, which act as the electronic donor and acceptor, respectively, potentially resulting in through-space charge transfer (TSCT) emission. While the TSCT emissions were weak in solution, a notable increase in luminescence intensity was observed upon aggregation, indicating bright fluorescent light. A series of theoretical analyses further supported the possibility of spatial electronic communication based on frontier molecular orbitals, the distance of charge transfer, and reduced density gradient analysis. This work not only provides guidance for the one-step synthesis of complex polycycles, but also offers valuable insights into the design of aggregation-enhanced TSCT emission materials.

Rational design and investigation of nonlinear optical response properties of pyrrolopyrrole aza-BODIPY-based novel push-pull chromophores.

Intramolecular charge transfer (ICT)-based chromophores are highly sought after for designing near-infrared (NIR) absorbing and emitting dyes as well as for designing materials for nonlinear optical (NLO) applications. The properties of these 'push-pull' molecules can easily be modified by varying the electronic donor (D) and acceptor (A) groups as well as the π-conjugation linker. This study presents a methodical approach and employs quantum chemical analysis to explore the relationship between the structural features, electro-optical properties, and the NLO characteristics of molecules with D-π-A framework. The one- and two-photon absorption (2PA), linear polarizability (α), and first hyperpolarizability (β) of some novel chromophores, consisting of a dimeric aza-Boron Dipyrromethene (aza-BODIPY) analogue, called, pyrrolopyrrole aza-BODIPY (PPAB), serving as the acceptor, have been investigated. The electronic donors used in this study are triphenylamine (TPA) and diphenylamine (DPA), and they are conjugated to the acceptor via thienyl or phenylene π-linkers. Additionally, the Hyper-Rayleigh Scattering (βHRS), which enables direct estimation of the second-order NLO properties, is calculated for the studied chromophores with 1064 nm excitation in acetonitrile. The β value shows a significant increase with increasing solvent polarity, indicating that the ICT plays a crucial role in shaping the NLO response of the studied molecules. The enhancement of the 2PA cross-section of the investigated molecules can also be achieved by modulating the combinations of donors and linkers. The results of our study indicate that the novel D-π-A molecules designed in this work demonstrate considerably higher hyperpolarizability values than the standard p-nitroaniline, making them promising candidates for future NLO applications.

Diphenylene Iodonium as a Prominent Halogen Bond Donor: The Case of Human Monoamine Oxidase B.

Herein we have investigated the formation and interplay of several noncovalent interactions (NCIs) involved in the inhibition of human monoamine oxidase B (MAO B). Concretely, an inspection of the Protein Data Bank (PDB) revealed the formation of a halogen bond (HlgB) between a diphenylene iodonium (DPI) inhibitor and a water molecule present in the active site, in addition to a noncovalent network of interactions (e. g. lone pair-π, hydrogen bonding, OH-π, CH-π and π-stacking interactions) with surrounding protein residues. Several theoretical models were built to understand the strength and directionality features of the HlgB in addition to the interplay with other NCIs present in the active site of the enzyme. Besides, a computational study was carried out using DPI as HlgB donor and several electron rich molecules (CO, H2O, CH2O, HCN, pyridine, OCN-, SCN-, Cl- and Br-) as HlgB acceptors. The results were analyzed using several state-of-the-art computational tools. We expect that our results will be useful for those scientists working in the fields of rational drug design, chemical biology as well as supramolecular chemistry.

How do we leverage data through replication and warehousing to meet blood collection and transfusion service needs.

With the widespread adoption of Blood Establishment Computer Systems and other Blood Collection and Transfusion Service (BCTS) clinical information systems (CIS), electronic blood donor, product, and patient data are now routinely required for clinical, regulatory, operational, and quality needs. That data are often not readily accessible for such secondary use within CIS databases, particularly for applications with significant data availability requirements such as machine learning and artificial intelligence. Data replication provides one avenue by which CIS data can be made more readily available. Members of the AABB's Information Systems Committee along with institutional information technology colleagues provided a multi-institutional viewpoint on data replication through the lens of BCTS specific use cases. Case studies of informatics offerings leveraging such technologies were also elicited. Six distinct use cases describe the potential role of data replication including the creation of data warehouses for frontline laboratory staff. Specific BCTS examples for each use case are presented to highlight the value of data replication, including visualization of critical inventory (O red blood cells, HLA-compatible platelets) and utilization analytics for patient blood management. Two case studies describe the approach to implement such technologies to (1) optimize staffing via laboratory workload reporting and (2) improve access to blood via antigen-negative blood product location services. Data replication and warehousing can empower BCTS analytic offerings not otherwise natively available through one's CIS to improve patient care and laboratory operations.

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads.

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads. The dyads utilized a heteroleptic Cu(I) bis-diimine architecture with a copper(I) bis-phenanthroline chromophore donor and anthraquinone electron acceptor. We characterized the structures of the complexes using x-ray crystallography and density functional theory calculations and the photophysical properties via resonance Raman and optical transient absorption spectroscopy. The calculations and resonance Raman spectroscopy revealed that excitation of the Cu(I) metal-to-ligand charge-transfer (MLCT) transition transfers the electron to a delocalized ligand orbital. The optical transient absorption spectroscopy demonstrated that each dyad formed the oxidized copper-reduced anthraquinone charge-separated state. Unlike most Cu(I) bis-phenanthroline complexes where increasingly bulky substituents on the phenanthroline ligands lead to longer MLCT excited-state lifetimes, here, we observe a decrease in the long-lived charge-separated state lifetime with increasing steric bulk. The charge-separated state lifetimes were best explained in the context of electron-transfer theory rather than with the energy gap law, which is typical for MLCT excited states, despite the complete conjugation between the phenanthroline and anthraquinone moieties.

Modeling Complex Ligands for High Oxidation State Catalysis: Titanium Hydroamination with Unsymmetrical Ligands.

A method for modeling high oxidation state catalysts is used on precatalysts with unsymmetrical and symmetrical bidentate ligands to get a more detailed understanding of how changes to ancillary ligands affect the hydroamination of alkynes catalyzed by titanium. To model the electronic donor ability, the ligand donor parameter (LDP) was used, and to model the steric effects, percent buried volume (% Vbur) was employed. For the modeling study, 7 previously unpublished unsymmetrical Ti(XX')(NMe2)2 precatalysts were prepared, where XX' is a chelating ligand with pyrrolyl/indolyl linkages. The rates of these unsymmetrical and 10 previously reported symmetrical precatalysts were used with the model kobs = a + b(LDP)1 + c(LDP)2 + d(% Vbur)1 + e(% Vbur)2, where a-e were found through least-squares refinement. The model suggests that (1) the two attachment points of the bidentate ligand XX' are in different environments on the metal (e.g., axial and equatorial in a trigonal bipyramidal or square pyramidal structure), (2) the position of the unsymmetrical ligand on the metal is determined by the electronics of the ligand rather than the sterics, and (3) that one side of the chelating ligand's electronics strongly influences the rate, while the other side's sterics more strongly influences the rate. From these studies, we were able to generate catalysts fitting to this model with rate constants larger than the fastest symmetrical catalyst tested.