Cr Centers Research Articles

Transition-Metal-Related Quantum Emitters in Wurtzite AlN and GaN.

Transition-metal centers exhibit a paramagnetic ground state in wide-bandgap semiconductors and are promising for nanophotonics and quantum information processing. Specifically, there is a growing interest in discovering prominent paramagnetic spin defects that can be manipulated using optical methods. Here, we investigate the electronic structure and magneto-optical properties of Cr and Mn substitutional centers in wurtzite AlN and GaN. We use state-of-the-art hybrid density functional theory calculations to determine level structure, stability, optical signatures, and magnetic properties of these centers. The excitation energies are calculated using the constrained occupation approach and rigorously verified with the complete active space configuration interaction approach. Our simulations of the photoluminescence spectra indicate that in AlN and in GaN are responsible for the observed narrow quantum emission near 1.2 eV. We compute the zero-field splitting (ZFS) parameters and outline an optical spin polarization protocol for and . Our results demonstrate that these centers are promising candidates for spin qubits.

Isolation and Diverse Reactivity of an Unsymmetrical 1,2-Bis(silylene)-Stabilized Pentacarbonyl Chromium(0) Species.

The construction of the unsymmetrical 1,2-bis(silylene) pentacarbonyl chromium(0) complex 1 was achieved through the reaction of chlorosilylene with half an equivalent of K2Cr(CO)5. X-ray diffraction analysis of 1 confirms the formation of the Si-Si bond and the coordination of one of the silicon atoms to the Cr center. Density functional theory (DFT) calculations disclose that highest occupied molecular orbital (HOMO) mainly corresponds to the lone pair of electrons on the silicon atom and the σ-bonding interaction between two Si atoms. Based on its unique electronic structure, its diverse reactivity toward the transition metal compounds and small molecules was investigated in detail. The reactions of 1 with Fe2(CO)9 or CuCl yielded the 1,2-bis(silylene)-stabilized heterobimetallic complex 2 or oxidized product 3, respectively. Additionally, treatments of 1 with selenium, CO2, or Me3SiN3 led to the formation of the corresponding selenium-, oxo-, and nitrogen-bridged complexes 4-7. All compounds were characterized by multinuclear NMR spectroscopy and X-ray crystallography.

Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy.

C-H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C-H σ-bond binds to the metal. Due to the weak nature of metal-alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal-alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C-H bond activation reactions.

The synthesis of amide bond by a simple one-step method to connect ionic liquid and MIL-101-NH2 firmly for efficient CO2 cycloaddition

Finding robust combination modes between ionic liquids and MOFs is still a challenge. In this work, using a simple one step-synthesis method, the ionic liquid was grafted onto MOF without any additives, and the binding mode of the two was characterized by means of XPS, FT-IR, TGA, SEM etc. It was found that the carboxyl group in the ionic liquid bonded with the amino group in MOF to form an amide bond. Thus, a novel stable composite material MIL-101-NH2-1-carboxypropyl-3-methylimidazolium bromine (1-C-3-M@MIL-101-NH2) was synthesized. This composite employed as a catalyst for CO2 cycloaddition demonstrated excellent catalytic activity, achieving a target product PC (Propylene carbonate) yield of 91.7 % under the conditions of 80 °C, 1.0 MPa, and a reaction time of 4 h, without the addition of any co-catalysts or solvents. The good catalytic activity was benefit from the synergistic effect of Lewis acidic Cr centers and nucleophilic Br-, as well as the high specific surface area and good CO2 affinity of 1-C-3-M@MIL-101-NH2. In addition, owing to this robust bonding configuration, the catalyst can be recycled up to five times without losing activity. It is a potential catalyst for CO2 capture and conversion.

Training and attitudes concerning cardiac rehabilitation in Portugal: A national survey of physician members of the Portuguese Society of Cardiology

Introduction and ObjectivesCardiac rehabilitation (CR) is a central component in the management of cardiovascular disease. While its potential benefits have been extensively explored and confirmed, its implementation is still suboptimal, due to various possible barriers. This study aimed to assess training and attitudes concerning CR among physicians in a Portuguese setting. MethodsAn online questionnaire structured in three parts (participant characteristics, training and attitudes concerning CR, and a brief general knowledge assessment) was developed and sent to members of the Portuguese Society of Cardiology. The study population encompassed physicians with a medical specialty or residents from the third year onward of a specialty program. ResultsA total of 97 individuals (57.7% male, 61.9% aged ≤50 years) presented valid answers. CR was available at the workplace of 54.6% of participants. Most of them considered that the time allocated to CR training during residency was inadequate, and thought that more time was needed for this purpose. Most had not dedicated (or intended to dedicate) time for CR training, with lack of time being the most frequently attributed reason. In terms of referral, a substantial proportion of subjects did not refer patients, with lack of CR centers and human resources being the most frequent reasons. ConclusionsThis survey provides contemporary data on CR training and attitudes, highlighting areas of potential improvement, such as time allocated to training in this area. These results could provide a useful pragmatic framework for optimization of training and awareness in this pivotal field of cardiovascular medicine.

Creating Order in Ultrastable Phosphonate Metal-Organic Frameworks via Isolable Hydrogen-Bonded Intermediates.

The stability presented by trivalent metal-organic frameworks (MOFs) makes them an attractive class of materials. With phosphonate-based ligands, crystallization is a challenge, as there are significantly more binding motifs that can be adopted due to the extra oxygen tether compared to carboxylate counterparts and the self-assembly processes are less reversible. Despite this, we have reported charge-assisted hydrogen-bonded metal-organic frameworks (HMOFs) consisting of [Cr(H2O)6]3+ and phosphonate ligands, which were crystallographically characterized. We sought to use these HMOFs as a crystalline intermediate to synthesize ordered Cr(III)-phosphonate MOFs. This can be done by dehydrating the HMOF to remove the aquo ligands around the Cr(III) center, forcing metal-phosphonate coordination. Herein, a new porous HMOF, H-CALF-50, is synthesized and then dehydrated to yield the MOF CALF-50. CALF-50 is ordered, although it is not single crystalline. It does, however, have exceptional stability, maintaining crystallinity and surface area after boiling in water for 3 weeks and soaking in 14.5 M H3PO4 for 24 h and 9 M HCl for 72 h. Computational methods are used to study the HMOF to MOF transformation and give insight into the nature of the structure and the degree of heterogeneity.

Direct Determination of the Rate of Intersystem Crossing in a Near-IR Luminescent Cr(III) Triazolyl Complex.

A detailed understanding of the dynamics of photoinduced processes occurring in the electronic excited state is essential in informing the rational design of photoactive transition-metal complexes. Here, the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter is directly determined through the use of ultrafast broadband fluorescence upconversion spectroscopy (FLUPS). In this contribution, we combine 1,2,3-triazole-based ligands with a Cr(III) center and report the solution-stable complex [Cr(btmp)2]3+ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (13+), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%) in fluid solution. The excited-state properties of 13+ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-to-picosecond FLUPS. Although TA spectroscopy allows us to observe the evolution of phosphorescent excited states within the doublet manifold, more significantly and for the first time for a complex of Cr(III), we utilize FLUPS to capture the short-lived fluorescence from initially populated quartet excited states immediately prior to the intersystem crossing process. The decay of fluorescence from the low-lying 4MC state therefore allows us to assign a value of (823 fs)-1 to the rate of intersystem crossing. Importantly, the sensitivity of FLUPS to only luminescent states allows us to disentangle the rate of intersystem crossing from other closely associated excited-state events, something which has not been possible in the spectroscopic studies previously reported for luminescent Cr(III) systems.

Cardiac rehabilitation designed for obese patients with atrial fibrillation: OPTICARE XL results on burden, frequency and severity of Atrial Fibrillation

Abstract Funding Acknowledgements Type of funding sources: None. Background Atrial Fibrillation (AF) is associated with substantial morbidity, mortality and low quality of life. Obesity is an important risk factor for AF and weight reduction is associated with a decrease in AF burden. Previous studies showed that conventional CR programs lead to insufficient weight reduction in obese patients. The OPTImal CArdiac REhabilitation XL (OPTICARE XL) study was an open label randomized controlled trial (RCT), evaluating effects of a CR program specifically designed for weight reduction in patients with obesity and non-valvular AF. Objective We hypothesize that this novel tailor-made CR program (OPTICARE XL) will lead to reduced AF burden, as compared to conventional CR. Design and setting: Multi-center RCT, conducted between February 2017 and January 2019 in 2 large Dutch CR centers. Patients were randomized to the OPTICARE XL CR intervention-program or a conventional CR-program. Methods The primary endpoint was AF symptom burden, assessed by the Atrial Fibrillation Severity Scale (AFSS). Secondary endpoints were AF symptom number, AF symptom frequency and AF symptom severity. Data were collected at baseline (prior to CR, T0), at the end of the intervention and conventional program (T1) and 1.5 year after inclusion (T2). Results Of 68 included patients, 28 were randomized to conventional CR and 40 to OPTICARE XL CR. Patients were predominantly male (60%), had a mean age of 62.4 ± 10.4 years and a mean BMI of 36.1 ± 4.4 kg/m2. With regards to AF burden, no significant within and between-group differences were observed between T0 and T1, T1 and T2, and T0 and T2 (OPTICARE XL: T0 14.6 ± 4.4, T1 14.1 ± 5.9, T2 14.3 ± 6.9; conventional CR: T0 16.4 ± 6.5, T1 15.3 ± 6.3, T2 15.9 ± 7.6; Figure 1). There was a significant difference (p=0.02) in the AF symptom number at T2 in favor of the intervention group (Figure 2). AF symptom frequency decreased in the intervention group from T0 to T2 (31.5 to 25.8; p=0.027) and in conventional CR from T0 to T1 (32.2 to 28.2; p=0.010). However, for both AF symptom frequency and symptom severity, no significant between-group differences were observed. Conclusion Compared to conventional CR, OPTICARE XL CR did not result in a reduced AF burden. However, it did reduce the AF symptom number at long-term follow-up. Novel CR programs such as OPTICARE XL may have added value in the treatment of obese patients with AF.

Chromium-Based Complexes Bearing Aminophosphine and Phosphine-Imine-Pyrryl Ligands for Selective Ethylene Tri/Tetramerization.

A series of Cr-based complexes 6-10 bearing aminophosphine (P,N) ligands Ph2P-L-NH2 [L = CH2CH2 (1), L = CH2CH2CH2 (2), and L = C6H4CH2 (3)] and phosphine-imine-pyrryl (P,N,N) ligands 2-(Ph2P-L-N=CH)C4H3NH [L = CH2CH2CH2 (4) and L = C6H4CH2 (5)] were prepared, and their catalytic properties were examined for ethylene tri/tetramerization. X-ray crystallographic analysis of complex 8 indicated the κ2-P,N bidentate coordination mode at the Cr(III) center and the distorted octahedral geometry of monomeric P,N-CrCl3. Upon activation by methylaluminoxane (MAO), complexes 7-8 bearing P,N (PC3N backbone) ligands 2-3 showed good catalytic reactivity for ethylene tri/tetramerization. On the other hand, complex 6 bearing the P,N (PC2N backbone) ligand 1 was found active for non-selective ethylene oligomerization, while complexes 9-10 bearing P,N,N ligands 4-5 only produced polymerization products. In particular, the high catalytic activity of 458.2 kg/(g·Cr·h), excellent selectivity of 90.9% (1-hexene and 1-octene combined), and extremely low PE content of 0.1% were obtained with complex 7 in toluene at 45 °C and 45 bar. These results suggest that rational control of P,N and P,N,N ligand backbones, including a carbon spacer and rigidity of a carbon bridge, can lead to the high-performance catalyst for the ethylene tri/tetramerization process.

Effects of diethylaluminum ethoxide pre‐reduction and chromium loading over bistriphenylsilyl chromate/silica for ethylene polymerization

AbstractBistriphenylsilyl chromate (BC)/silica (S‐2 catalyst) is a commercially used high‐density polyethylene (HDPE) catalyst with great importance. In this work, the catalytic performance of this catalyst was tried to be improved by increasing the loading amount of Cr and pre‐reducing with diethylaluminum ethoxide (DEALE). It is found that, when the Cr loading amount was doubled, the productivity in gPE · g−1Cat became almost doubled as well, indicating the maintenance of overall efficiency for Cr centres. Meanwhile, pre‐reduction of BC/silica with a small amount of DEALE (the molar ratio of DEALE/Cr < 2.4) was adequate to promote its productivity greatly, while large dose of DEALE led to the activity decay fast, which might originate from the over‐reduction of the Cr centre, as well as pore blockages caused by the added DEALE. Moreover, it is found that both the increased Cr loading and DEALE pre‐reduction offered an alternative way to reduce the weight‐average molecular weight (MW) and molecular weight distribution (MWD) of the polyethylene products mainly by shaving the high MW shoulder.

Comparisons of bpy and phen Ligand Backbones in Cr-Mediated (Co-)Electrocatalytic CO2 Reduction

Due to the rise in atmospheric carbon dioxide (CO2) concentrations, there is a need for the development of new strategies to enhance the selectivity and activity of the electrocatalytic conversion of CO2 to value-added products. The incorporation of redox mediators (RMs) as cocatalysts to enhance the transfer of redox equivalents during catalysis has been gaining more attention in recent years across a variety of small molecule transformations. We have shown that using Cr-centered complexes with sulfone-based RMs leads to an enhancement of CO2 reduction electrocatalysis under protic conditions via an inner-sphere mechanism. In these cocatalytic systems, an oxygen atom of the reduced RM binds to the Cr center to form a key intermediate stabilized by pancake bonding between the reduced aromatic components of the catalyst ligand backbone and the RM. This interaction facilitates the transfer of an electron and accesses a more kinetically favorable reaction pathway. Here, we show that expanding the aromatic character of the ligand backbone of the catalyst as well as the RM can cause a greater enhancement of coelectrocatalytic activity. These results suggest that further activity improvements can be achieved by focusing on the kinetic and thermodynamic parameters which control association between the catalyst and RM.

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity.

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)5] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2e- reduction governed by reduction potential compression/inversion. Treatment of bis(η6-naphthalene)chromium(0) with six equiv. of 2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene (6) or [(OC)5Cr(η1-2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (11) afforded homoleptic Cr(0) complexes 13 and 14 with a 12e- (per molecule) ligand-based reduction capacity at mild E1/2 of -1.29 V and -1.15 V vs. Cp2Fe0/+, respectively. The overall reversible redox capacity varies from 15e- for the mononuclear complex 13 to 21e- for the heptanuclear complex 14. The latter "nanocomplex" has a diameter of ca. 5 nm and features seven Cr(0) centers interlinked with six 2,2'-diisocyano-6,6'-biazulenic bridges. The X-ray structure of [(OC)5Cr(2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (7) indicated a 43.5° interplanar angle between the two azulenic moieties. Self-assembly of 11 on a Au(111) substrate afforded an organometallic monolayer film of 11 featuring approximately upright orientation of the 2,2'-diisocyano-6,6'-biazulenic linkers, as evidenced by ellipsometric measurements and the RAIR signature of the C4v-symmetric [(-NC)Cr(CO)5] infrared reporter within 11. Remarkably, comparing the FTIR spectrum of 11 in solution with the RAIR spectrum of 11 adsorbed on Au(111) suggested electronic coupling at a ca. 2 nm distance between the Cr(0) and Au atoms linked by the 2,2'-diisocyano-6,6'-biazulene bridge.

Hydroperoxyl-mediated C-H bond activation on Cr single atom catalyst: An alternative to the Fenton mechanism

Direct conversion of methane to methanol is an attractive alternative to the indirect, energy-intensive syngas production and conversion route. Herein, we present a comprehensive density functional theory (DFT) study of converting methane directly into methanol over the Cr single atom catalyst (SAC) on anatase TiO2 (101) surface. Cr species stabilize in the form of CrO with H2O2 as oxidant, on which methane reacts with the deprotonated H2O2 via an Eley–Rideal mechanism. The formation of a three-centered transition-state structure is observed in the rate-determining CH bond activation step with an activation barrier of 0.44 eV. The H2O2 not only serves as the oxidant for the formation of the most stable and reactive CrO species, but also enters the coordination sphere of the Cr center and acts as a highly effective H acceptor, which significantly lowers the activation barrier for CH bond cleavage. Over different Cr-O pairs, methane CH bond activation energy is found to linearly correlate with the energy difference between the d-bond center (εd) of Cr and p-band center of O (εp). The presence of water changes the relative stability of the reaction intermediates but does not alter the reaction pathways. This study demonstrated the ability of Cr SACs for catalyzing direct conversion of methane to methanol under mild conditions and would stimulate further interests in designing active Cr catalysts for methane conversion.

Ultra-broadband of up to 200 nm near-infrared phosphors based on one-site occupation strategy for multipurpose applications in light-emitting diodes

Portable near-infrared (NIR) lighting source stimulates great efforts to exploit NIR phosphor-converted LEDs (pc-LEDs) that push the rapid development of broadband NIR-emitting phosphors. Although Cr3+-activated NIR phosphors designed by multisites occupation strategy show significant potential in emission bandwidth, poor spectral stability caused by different thermal quenching behaviors is a brake on practical application due to different local structure around Cr centers. One-site-based NIR phosphors seem to be an effective method to solve aforementioned problem. Herein, we report a type of new ultra-broadband one-site-based NIR-emitting phosphors NaSc1-xP2O7:xCr3+ (NSP:xCr3+) with emission at 910 nm and full width at half maximum (FWHM) of up to ∼200 nm. Their FWHM value exceeds all one-site-based Cr3+-activated phosphors and some multiple-sites ones. Structural analysis and low-temperature spectra demonstrate that such impressive broadband NIR emission originates from Cr3+ occupying at single octahedral Sc site. At 425 K, integrated emission intensity of NSP:0.04Cr3+ phosphor maintains 53.8% of room temperature. A NIR pc-LED prototype is fabricated by using NSP:0.04Cr3+ phosphor with a blue LED chip, and its multipurpose applications in non-destructive detection, night vision and analysis of foodstuff are demonstrated. These findings indicate that broadband NSP:0.04Cr3+ NIR phosphor enabled by one-site occupation strategy has potential prospect for multipurpose NIR pc-LEDs applications.

Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization.

A range of novel N-substituted diphosphinoamine (PNP) ligands Ph2PN(R)PPh2 [R = F2CHCH2 (1); R = Me2CHCH2 (2); R = Me2CHCH2CH2 (3)] have been synthesized via one-step salt elimination reaction. The ligand-coordinated chromium carbonyls [Ph2PN(R)PPh2]Cr(CO)4 (4–6) were further synthesized, and X-ray crystallography analysis of complex 6 revealed the κ2-P,P bidentate binding mode of Cr center and the molecular structure of PNP ligand 3. Then the catalytic ethylene oligomerization behaviors of PNP ligands 1–3 bridging chromium chloride complexes {[Ph2PN(R)PPh2]CrCl2(μ-Cl)}2 (7–9) were further discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F2CHCH2 group can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 with the electron-donating Me2CHCH2 and Me2CHCH2CH2 groups can significantly enhance the selective ethylene tri/tetramerization. The good catalytic activity of 198.3 kg/(g Cr·h), the selectivity toward 1-hexene and 1-octene of 76.4%, and the low PE content of 0.2% were simultaneously achieved with the Al/Cr molar ratio of 600 using the complex 8/MMAO system at 45 °C and 45 bar. These excellent results were mainly attributed to the fact that the β-branching of bridging ligand 2 increased the steric bulk of the N-moiety for complex 8.

Impact of a home-based cardiac rehabilitation program in heart failure during the COVID-19 pandemic

Funding AcknowledgementsType of funding sources: Other. Main funding source(s): This work was financially supported by the project POCI-01-0145-FEDER-030011, funded by FEDER, through COMPETE2020-POCI, and by national funds, through FCT/MCTES (PTDC/MEC-CAR/30011/2017). CIAFEL, UnIC and UMIB are supported by national funds through Fundação para a Ciência e Tecnologia, I.P. [(UIDB/00617/2020), (UIDB/00051/2020 and UIDP/00051/2020), and (UIDB/00215/2020 and UIDP/00215/2020), respectively]. CS received an individual grant from CAPES [BEX 0554/14-6].IntroductionCardiac rehabilitation (CR) is an evidence-based recommended treatment of heart failure (HF) patients. During the COVID-19 pandemic, the shutdown of CR centers was necessary to limit the infection risk among high-risk patients. The integration of a home-based CR (HBCR) program in CR units can help to improve the delivery of care and improve cardiovascular outcomes of HF patients.PurposeTo assess the effectiveness of an HBCR program in HF patients.MethodsThis is a sub­study of the EXercise InTervention in Heart Failure trial (EXIT-HF), which include forty-nine HF patients (preserved and reduced ejection fraction). The HBCR program consisted in 12-week combined exercise program (60%-80% of peak oxygen consumption (VO2 peak)), 2 training sessions per week, for a total of 24 sessions. Patients performed 4 supervised training sessions and the remaining sessions at home. All patients performed a cardiopulmonary exercise test (VO2 peak), the 6-minute-walking test (6MWT), collected blood analysis (plasma NT-proBNP), and answered the Minnesota Living with Heart Failure Questionnaire.ResultsForty-two patients (86%) complete at least 80% of prescribed training sessions (age: 61.1±12; FEVE: 37.1±10.8). The HBCR program improve VO2 peak from 18.3 to 20.1ml/kg/min (+1.8 ml/kg/min; 95%IC:1.4 to 2.4; p<0.001) and the walked distance at the 6MWT from 462 to 512 meters (+49 meters; 95%IC: 38 to 60; p<0.001). In addition, overall quality of life was improved (-13 points; 95%IC:-7.8 to -18.5; p<0.001), as well physical (-6.3 points; 95%IC:-3.5 to -9; p<0.001) and emotional dimension of quality of life (-2.8points ; 95%IC: -0.9 to -4.7; p=0.06). No significant change was found in NT-proBNP levels (820±1220 vs 674±903; p=0.285).ConclusionsOur results showed that HBCR is feasible and can improve functional capacity and quality of life in HF patients.

The amplitude of small eye movements can be accurately estimated with video-based eye trackers

Estimating the gaze direction with a digital video-based pupil and corneal reflection (P-CR) eye tracker is challenging partly since a video camera is limited in terms of spatial and temporal resolution, and because the captured eye images contain noise. Through computer simulation, we evaluated the localization accuracy of pupil-, and CR centers in the eye image for small eye rotations (≪ 1 deg). Results highlight how inaccuracies in center localization are related to 1) how many pixels the pupil and CR span in the eye camera image, 2) the method to compute the center of the pupil and CRs, and 3) the level of image noise. Our results provide a possible explanation to why the amplitude of small saccades may not be accurately estimated by many currently used video-based eye trackers. We conclude that eye movements with arbitrarily small amplitudes can be accurately estimated using the P-CR eye-tracking principle given that the level of image noise is low and the pupil and CR span enough pixels in the eye camera, or if localization of the CR is based on the intensity values in the eye image instead of a binary representation.

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers.

A set of three Cr-dimer compounds, Cr2Q2(en)4X2 (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr3+ centers, which are moderate in Cr2Se2(en)4X2 and stronger in Cr2S2(en)4Cl2. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr2Se8(en)4, has a single diatomic Se bridge connecting the two Cr3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Hydrogen bond donor functionalized poly(ionic liquids)@MIL-101 for the CO2 capture and improving the catalytic CO2 conversion with epoxide

Hydrogen bond donor functionalized poly(ionic liquids) were incorporated into MIL-101 framework for synthesis of composites PIL-R@MIL-101 (R = COOH, OH, NH2). The effect of different hydrogen bond donors on CO2-epoxide cycloaddition reaction was discussed through XPS, NH3-TPD characterizations. Thereinto, PIL-COOH@MIL-101 showed efficient catalytic performance in the cycloaddition reaction of CO2 and epichlorohydrin, 92.7% substrate conversion was obtained under 70 °C, 1.0 MPa, cocatalyst- and solvent-free conditions for 2.5 h. The improved activity was benefited from the synergistic effects of Lewis acidic Cr centers, carboxyl group, and nucleophilic Br-, together with the high surface area (1178 m2/g), and good CO2 affinity. In addition, PIL-COOH@MIL-101 also showed facile recovery method by easy centrifugation and good recycling stability. Considering the good CO2 affinity, excellent catalytic performance in CO2 cycloaddition reaction, better thermal and recycling stabilities, the facilely prepared functionalized poly(ionic liquid)@MIL-101 composite would exhibit promising application prospect in CO2 capture and conversion.

The Decisive Role of Spin States and Spin Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Frameworks.

The coordinatively unsaturated chromium(II)-based Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2 /H2 . Using periodic density functional theory (DFT) calculations, we attempted to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas-bound MOFs [(Cr4 (X)4 Cl)3 (BTT)8 ]3- (X=O2 , N2 , and H2 ), we unequivocally established the role of spin states and spin coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The binding of O2 to the MOF follows an electron-transfer mechanism resulting in a CrIII superoxo species (O2 .- ) with a very strong antiferromagnetic coupling between the two centers, whereas N2 /H2 are found to weakly interact with the metal center and hence only slightly perturb the associated coupling constants. Although the gas-bound and unbound MOFs have an S=0 ground state (GS), the nature of spin the configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin state, with a significant energy margin (47.6 kJ mol-1 ). This study offers a hitherto unknown strategy of using spin state/spin couplings to control gas adsorption selectivity in MOFs.