Semiconductor Band Structure, Symmetry, and Molecular Interface Hybridization for the Chemist.

Merging molecular bonding concepts with semiconductor- and materials-based concepts of band structure is challenging due to the mutually exclusive historical development and notations used in those respective fields: symmetry adapted linear combinations (SALCs) and Mulliken terms for molecules, versus k space and Bloch sums for materials. This lack of commonality brings the issue of hybridization (aka electronic coupling) between molecules and materials to the forefront in many aspects of modern chemical research─including nanocrystal properties, solar energy conversion, and molecular computing. It is thus critical to establish a holistic approach to hybridizing orbital (molecule) and plane-wave (semiconductor/material) systems to better describe symmetry-based molecule|material bonding and the corresponding symmetry-adapted molecular orbital (MO) diagrams. Such a unified approach would enable the construction of testable hypotheses about the role of symmetry and electronic structure in determining the extent of electronic coupling between molecular orbitals and semiconductor band structure. This Perspective provides an analysis and compendium of "translations" between the physics and chemistry language of group theory. In this vein, this approach describes the symmetries─and corresponding point groups─that occur in k space along the available descent in symmetry pathways (k space vectors). As a result, chemists may arrive at a more intuitive understanding of the band symmetries of semiconductors, as well as insights into the corresponding algebraic formulations. This analysis can ultimately generate MO diagrams for hybrid molecule|material systems. Lastly, an Outlook provides some context to the application of this analysis to modern problems at the interface of molecular and materials chemistry.

Electrochemical Impedance of Well-Passivated Semiconductors Reveals Bandgaps, Fermi Levels, and Interfacial Density of States.

For well-passivated semiconductor materials, the density of states (DOS) at the band edge determines the concentration of electrons (or holes) available to participate in photo/electrochemical redox and chemical reactions. Electrochemical impedance enables the characterization of photo-electrode DOS in a functional, in situ, electrochemical environment. However, the in situ electrochemical approach remains underutilized for band structure characterization of inorganic semiconductors. In this work, we demonstrate that the DOS of the well-passivated, highly ordered semiconductors silicon and germanium is directly probed by electrochemical impedance spectroscopy (EIS). More specifically, EIS measurements of the chemical capacitance in contact with electrolyte enable direct analysis of the DOS properties. From the capacitance-potential plot, the following parameters can be extracted: Fermi level, valence band maximum, conduction band minimum, and a quantitative value of the number of states at each potential. This study aims to establish the groundwork for future EIS investigations of electronically modified semiconductor interfaces with covalently bound organic molecules, organometallic catalysts, or more complex biorelated functionalizations.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films.

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

Synthesis and Conformational Dynamics of Selenanthrene (Oxides): Establishing an Energetic Flexibility Index for Scaffolds.

The use of new dynamic scaffolds for constructing inorganic and organometallic complexes with enhanced reactivities is an important new research direction. Toward this fundamental aim, an improved synthesis of the dynamic scaffold selenanthrene, along with its monoxide, trans-dioxide and the previously unknown trioxide, is reported. A discussion of the potential reaction mechanism for selenanthrene is provided, and all products were characterized using 1H, 13C, and 77Se nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray crystallography. The dynamic ring inversion processes (i.e., "butterfly motion") for selenanthrene and its oxides were investigated using variable-temperature 1H NMR and density functional theory calculations. The findings suggest that selenanthrene possesses a roughly equal barrier to inversion as its sulfur analogue, thianthrene. However, selenanthrene oxides evidently possess larger inversion barriers as compared to their sulfur analogues due to the enhanced electrostatic intramolecular interactions inherent between the highly polar selenium-oxygen bond and adjacent C-H moieties. Finally, we propose a quantitative "flexibility index" in deg/(kcal/mol) for various tricyclic scaffolds to provide researchers with a comparative scale of dynamic motion across many different systems.

Synthesis and Magnetic Properties of Antimony-Ligated Co(II) Complexes: Stibines versus Phosphines.

Herein, we test the hypothesis that neutral, heavy-atom stibine donors can increase the extent of spin-orbit coupling on light, 3d transition metal. To this end, we developed a novel synthetic route toward coordinating a paramagnetic 3d metal ion─cobalt(II)─with neutral stibine ligands. Such complexes have not been reported in the literature due to the weak σ donor strength of stibines and the hard-soft mismatch between a 3d metal and a 5p ligand─which herein has been overcome using alkylated Sb donors. Magnetometry of [(SbiPr2Ph)2Co(I)2] (1) reveals that the stibine complex 1 exhibits a higher magnitude D value (D = |24.96| cm-1) than the spectroscopically derived value for the corresponding phosphine complex 3 (D = -13.13 cm-1), indicative of large zero-field splitting. CASSCF/NEVPT2 calculations corroborate the experimental D values for 1 and 3, predicting D = -31.9 and -8.9 cm-1, respectively. A re-examination of magnetic parameters across the entire series [(ER3)2Co(X)2] (E = P → Sb; X = Cl → I) reveals that (i) increasingly heavy pnictogens lead to an increased X-Co-X bond angle, which is correlated with larger magnitude D values, and (ii) for a given X-Co-X bond angle, the D value is always higher in the presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory calculations for 1 (stibine complex) and 3 (phosphine complex) reveal no substantial differences in spin-orbit coupling (ζ = 479.2, 480.2 cm-1) or Racah parameter (B = 947.5, 943.9 cm-1), an indicator of covalency. Thus, some "heavy atom effect" on the D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.

Tuning p-Si(111) Photovoltage via Molecule|Semiconductor Electronic Coupling.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure-function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO2; anthracene, Anth; and nitroanthracene, AnthNO2) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (Voc up to 0.43 V vs MV2+) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER Voc using pSi(111)-R|TiO2|Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function.

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H2 oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H2O/H2 binding site. In [Fe]-hydrogenase, heterolytic H2 activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H2 cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H4MPT+, thus forming methylene-H4MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Quantifying Throw Counts and Intensities Throughout a Season in Youth Baseball Players: A Pilot Study.

Overuse injuries in youth baseball players due to throwing are at an all-time high. Traditional methods of tracking player throwing load only count in-game pitches and therefore leave many throws unaccounted for. Miniature wearable inertial sensors can be used to capture motion data outside of the lab in a field setting. The objective of this study was to develop a protocol and algorithms to detect throws and classify throw intensity in youth baseball athletes using a single, upper arm-mounted inertial sensor. Eleven participants from a youth baseball team were recruited to participate in the study. Each participant was given an inertial measurement unit (IMU) and was instructed to wear the sensor during any baseball activity for the duration of a summer season of baseball. A throw identification algorithm was developed using data from a controlled data collection trial. In this report, we present the throw identification algorithm used to identify over 17,000 throws during the 2-month duration of the study. Data from a second controlled experiment were used to build a support vector machine model to classify throw intensity. Using this classification algorithm, throws from all participants were classified as being "low," "medium," or "high" intensity. The results demonstrate that there is value in using sensors to count every throw an athlete makes when assessing throwing load, not just in-game pitches.

Single-Step Sulfur Insertions into Iron Carbide Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco Analogues.

The one-step syntheses, X-ray structures, and spectroscopic characterization of synthetic iron clusters, bearing either inorganic sulfides or thiolate with interstitial carbide motifs, are reported. Treatment of iron carbide carbonyl clusters [Fen (µn -C)(CO)m ]x (n=5,6; m=15,16; x=0,-2) with electrophilic sulfur sources (S2 Cl2 , S8 ) results in the formation of several µ4 -S dimers of clusters, and moreover, iron-sulfide-(sulfocarbide) clusters. The core sulfocarbide unit {C-S}4- serves as a structural model for a proposed intermediate in the radical S-adenosyl-L-methionine biogenesis of the M-cluster. Furthermore, the electrophilic sulfur strategy has been extended to provide the first ever thiolato-iron-carbide complex: an analogous reaction with toluylsulfenyl chloride affords the cluster [Fe5 (µ5 -C)(SC7 H7 )(CO)13 ]- . The strategy described herein provides a breakthrough towards developing syntheses of biomimetic iron-sulfur-carbide clusters like FeMoco.

Non-Catalytic Benefits of Ni(II) Binding to an Si(111)-PNP Construct for Photoelectrochemical Hydrogen Evolution Reaction: Metal Ion Induced Flat Band Potential Modulation.

We report here the remarkable and non-catalytic beneficial effects of a Ni(II) ion binding to a Si|PNP type surface as a result of significant thermodynamic band bending induced by ligand attachment and Ni(II) binding. We unambiguously deconvolute the thermodynamic flat band potentials (VFB) from the kinetic onset potentials (Von) by synthesizing a specialized bis-PNP macrochelate that enables one-step Ni(II) binding to a p-Si(111) substrate. XPS analysis and rigorous control experiments confirm covalent attachment of the designed ligand and its resulting Ni(II) complex. Illuminated J-V measurements under catalytic conditions show that the Si|BisPNP-Ni substrate exhibits the most positive onset potential for the hydrogen evolution reaction (HER) (-0.55 V vs Fc/Fc+) compared to other substrates herein. Thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive VFB value (-0.02 V vs Fc/Fc+) by a wide margin. Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between Von and equivalent circuit element parameters commonly associated with HER. Overall, the resulting paradigm comprises a system wherein the extent of band bending induced by metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst) in semiconductor|catalyst hybrids for PEC-HER.

Effect of Aortic Valve Disease on 3D Hemodynamics in Patients With Aortic Dilation and Trileaflet Aortic Valve Morphology.

BACKGROUND: The effect of different expressions of aortic valve disease on 3D aortic hemodynamics is unclear. PURPOSE: To investigate changes in aortic hemodynamics in patients with dilated ascending aorta (AAo) but different severity of aortic valve stenosis (AS) and/or regurgitation (AR). STUDY TYPE: Retrospective. POPULATION: A total of 111 subjects (86 patients with AAo diameter ≥ 40 mm and 25 healthy controls, all with trileaflet aortic valve [TAV]). Patients were further stratified by TAV dysfunction: n = 9 with combined moderate or severe AS and AR (ASR, 56 ± 13 years), n = 14 with moderate or severe AS (AS, 64 ± 14 years), n = 33 with moderate or severe AR (AR, 62 ± 14 years), n = 30 with neither AS nor AR (no AS/AR, 63 ± 9 years). FIELD STRENGTH/SEQUENCE: 4D flow MRI on 1.5/3T systems for the in vivo analysis of aortic blood flow dynamics. ASSESSMENT: Data analysis included grading of 3D AAo vortex/helix flow and AAo flow eccentricity as well as quantification of systolic peak velocities and wall shear stress (WSS). STATISTICAL TESTS: Continuous variables were compared by one-way analysis of variance or Kruskal-Wallis, followed by a pairwise Tukey or Dunn test if there was a significant difference. RESULTS: All patients demonstrated markedly elevated vortex and helix flow compared with controls (P < 0.05). Peak velocities were significantly elevated in ASR, AS, and AR patients compared with controls (P < 0.05). Increased flow eccentricity was observed in entire AAo for AR, at the mid and distal AAo for ASR and AS, and at the proximal AAo for no AS/AR. Compared with controls, WSS in the AAo was significantly elevated in ASR and AS patients (P < 0.05) and reduced in no AS/AR patients (P < 0.05). DATA CONCLUSION: The presence of TAV dysfunction is associated with aberrant hemodynamics and altered WSS, which may play a role in the development of aortopathy. LEVEL OF EVIDENCE: 3 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2020;51:481-491.

Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H2 Activation and Transfer Hydrogenation.

A set of bioinspired carbamoyl CNP pincer complexes are reported that are relevant to [Fe]-hydrogenase (Hmd). The dicarbonyl species [(CNHNNHPR2)Fe(CO)2I] [R = Ph, 1; R = iPr, 2] undergoes ligand deprotonation, resulting in the dearomatized complexes of formulas [(CNHNN=PR2)Fe(CO)2] (5 and 6). The crystal structure and 1H{31P} NMR spectroscopy of the iodide-bound dearomatized species [Na(18-crown-6)][(CNHNN=PPh2)Fe(CO)2I] (7) showed that the deprotonated moiety was the phosphoramine N(H) linkage. Separately, the monocarbonyl complexes [(CNHNNHPR2)Fe(CO)(MeCN)2](BF4) (8 and 9) synthesized, as well as deprotonated and dearomatized in similar fashion. Reactivity studies revealed that the parent dicarbonyl complexes require more forceful conditions for H2 activation, compared with the monocarbonyl complexes. The ligand backbone was not found to participate in H2 activation and H2 → hydride transfer to an organic substrate was not observed in either case. Density functional theory calculations revealed that the higher reactivity of the monocarbonyl complex in H2 splitting could be attributed to its higher affinity for H2. This behavior is attributed to two key points related to the requisite dπ(Fe) → σ*(H2) back-bonding interaction in a conventional M-H2 Kubas interaction: (i) generally, the weaker π donor capacity of the dicarbonyls, and (ii) specifically, the detrimental effect of a strongly π acidic CO ligand (versus weakly π acidic MeCN ligand) trans to the H2 activation site. The higher reactivity of the monocarbonyl complex is also evidenced by the catalytic transfer hydrogenation by monocarbonyl 8, whereas dicarbonyl 1 was ineffective. Overall, the results suggest that Nature uses the dicarbonyl motif in [Fe]-hydrogenase to diminish the interaction between the Fe center and dihydrogen, thereby preventing premature H2 activation prior to substrate (H4MPT+) binding and any resulting nonspecific hydride transfer reactivity.

CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the trans-Acyl Site in H2 Heterolysis.

We report syntheses and H2 activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNSPre) or phosphine (CNPPre) donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C-H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the -PPh2 donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H2 activation (5 atm D2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H2 activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.

Isolation and X-ray Structure of a Dialkylbismuth(III) Iodo "Nanosquare": Breaking the Polymeric Mold of R2BiX.

We report the synthesis, X-ray structure, and solution behavior of a "nanosquare" formed by four repeating (iPr2BiI) units. The title complex [iPr2BiI]4 (1) formed spontaneously as the major product from the reaction of iPr3Bi with MI2 (M = Mn, Co, Fe). Complex 1 was then more directly synthesized via bromination of iPr3Bi to form (iPr2BiBr), followed by iodide substitution. The X-ray structure of 1 exhibited acute Bi-I-Bi bonds (∼85-95°) that enabled formation of the hollow square motif. Synthetic control experiments with R2BiX (R = iPr, Bn; X = I, Br) revealed that the identities of both the halide and bismuth R group are determinants for square formation: the bromo-substituted [iPr2BiBr]n (2) and benzyl-substituted [Bn2BiI]n (3) variants formed only polymers as determined by X-ray crystallography. Density functional theory calculations revealed nearly pure p-orbital-based bonding molecular orbitals on both iodide and bismuth, facilitating the ∼90°/180° bonding motifs.

Spectroscopic X-ray and Mössbauer Characterization of M6 and M5 Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation.

The present study employs a suite of spectroscopic techniques to evaluate the electronic and bonding characteristics of the interstitial carbide in a set of iron-carbonyl-carbide clusters, one of which is substituted with a molybdenum atom. The M6C and M5C clusters are the dianions (Et4N)2[Fe6(µ6-C)(µ2-CO)2(CO)14] (1), [K(benzo-18-crown-6)]2[Fe5(µ5-C)(µ2-CO)1(CO)13] (2), and [K(benzo-18-crown-6)]2[Fe5Mo(µ6-C)(µ2-CO)2(CO)15] (3). Because 1 and 2 have the same overall cluster charge (2-) but different numbers of iron sites (1: 6 sites → 2: 5 sites), the metal atoms of 2 are formally oxidized compared to those in 1. Despite this, Mössbauer studies indicate that the iron sites in 2 possess significantly greater electron density (lower spectroscopic oxidation state) compared with those in 1. Iron K-edge X-ray absorption and valence-to-core X-ray emission spectroscopy measurements, paired with density functional theory spectral calculations, revealed the presence of significant metal-to-metal and carbide 2p-based character in the filled valence and low-lying unfilled electronic manifolds. In all of the above experiments, the presence of the molybdenum atom in 3 (Fe5Mo) results in somewhat unremarkable spectroscopic properties that are essentially a "hybrid" of 1 (Fe6) and 2 (Fe5). The overall electronic portrait that emerges illustrates that the central inorganic carbide ligand is essential for distributing charge and maximizing electronic communication throughout the cluster. It is evident that the carbide coordination environment is quite flexible and adaptive: it can drastically modify the covalency of individual Fe-C bonds based on local structural changes and redox manipulation of the clusters. In light of these findings, our data and calculations suggest a potential role for the central carbon atom in FeMoco, which likely performs a similar function in order to maintain cluster integrity through multiple redox and ligand binding events.

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry.

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR3)4Cu4(I)4] (1-4, 8), where SbR3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, iPr, tBu, Ph); triphenyl, iPr2Ph, and Me2Ph stibines resulted in the formation of dimers of type [(SbR3)4(Cu)2(I)2] (5-7). While similar luminescent copper-halide cubes have been studied, the corresponding "heavy-atom" congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu-Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(SbiPr3)4Cu4(I)4] (1, in I23; λem = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)-versus only molecular symmetry-is confirmed by the counterexample of the molecularly symmetric tBu-substituted cuboid [(SbtBu3)4Cu4(I)4] (3, λem = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the SbtBu3 ligand should red-shift emission beyond that of the SbiPr3-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S0) and excited triplet state (T1) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu4 intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T1, thereby inhibiting low-energy NIR emission.

4-D flow magnetic-resonance-imaging-derived energetic biomarkers are abnormal in children with repaired tetralogy of Fallot and associated with disease severity.

BACKGROUND: Cardiac MRI plays a central role in monitoring children with repaired tetralogy of Fallot (TOF) for long-term complications. Current risk assessment is based on volumetric and functional parameters that measure late expression of underlying physiological changes. Emerging 4-D flow MRI techniques promise new insights. OBJECTIVE: To assess whether 4-D flow MRI-derived measures of blood kinetic energy (1) differentiate children and young adults with TOF from controls and (2) are associated with disease severity. MATERIALS AND METHODS: Pediatric patients post TOF repair (n=21) and controls (n=24) underwent 4-D flow MRI for assessment of time-resolved 3-D blood flow. Data analysis included 3-D segmentation of the right ventricle (RV) and pulmonary artery (PA), with calculation of peak systolic and diastolic kinetic energy (KE) maps. Total KERV and KEPA were determined from the sum of the KE of all voxels within the respective time-resolved segmentations. RESULTS: KEPA was increased in children post TOF vs. controls across the cardiac cycle, with median 12.5 (interquartile range [IQR] 10.3) mJ/m2 vs. 8.2 (4.3) mJ/m2, P<0.01 in systole; and 2.3 (2.7) mJ/m2 vs. 1.4 (0.9) mJ/m2, P<0.01 in diastole. Diastolic KEPA correlated with systolic KEPA (R2 0.41, P<0.01) and with pulmonary regurgitation fraction (R2 0.65, P<0.01). Diastolic KERV showed similar relationships, denoting increasing KE with higher cardiac outputs and increased right heart volume loading. Diastolic KERV and KEPA increased with RV end-diastolic volume in a non-linear relationship (R2 0.33, P<0.01 and R2 0.50, P<0.01 respectively), with an inflection point near 120 mL/m2. CONCLUSION: Four-dimensional flow-derived KE is abnormal in pediatric patients post TOF repair compared to controls and has a direct, non-linear relationship with traditional measures of disease progression. Future longitudinal studies are needed to evaluate utility for early outcome prediction in TOF.

4-D flow MRI aortic 3-D hemodynamics and wall shear stress remain stable over short-term follow-up in pediatric and young adult patients with bicuspid aortic valve.

BACKGROUND: Children with bicuspid aortic valve (BAV) are at risk for serious complications including aortic valve stenosis and aortic rupture. Most studies investigating biomarkers predictive of BAV complications are focused on adults. OBJECTIVE: To investigate whether hemodynamic parameters change over time in children and young adults with BAV by comparing baseline and follow-up four-dimensional (4-D) flow MRI examinations. MATERIALS AND METHODS: We retrospectively included 19 children and young adults with BAV who had serial 4-D flow MRI exams (mean difference in scan dates 1.8±1.0 [range, 0.6-3.4 years]). We compared aortic peak blood flow velocity, three-dimensional (3-D) wall shear stress, aortic root and ascending aortic (AAo) z-scores between baseline and follow-up exams. We generated systolic streamlines for all patients and visually compared their baseline and follow-up exams. RESULTS: The only significant difference between baseline and follow-up exams occurred in AAo z-scores (3.12±2.62 vs. 3.59±2.76, P<0.05) indicating growth of the AAo out of proportion to somatic growth. There were no significant changes in either peak velocity or 3-D wall shear stress between baseline and follow-up exams. Ascending aortic peak velocity at baseline correlated with annual change in AAo z-score (r=0.58, P=0.009). Visual assessment revealed abnormal blood flow patterns, which were unique to each patient and remained stable between baseline and follow-up exams. CONCLUSION: In our pediatric and young adult BAV cohort, hemodynamic markers and systolic blood flow patterns remained stable over short-term follow-up despite significant AAo growth, suggesting minimal acute disease progression. Baseline AAo peak velocity was a predictor of AAo dilation and might help in determining pediatric patients with BAV who are at risk of increased AAo growth.

Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling.

The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)- R|TiO2 photoanodes where - R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)- R|TiO2 interface and occurs by two processes- thermionic emission and/or intraband tunneling-where the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach.

Distribution of blood flow velocity in the normal aorta: Effect of age and gender.

PURPOSE: To apply flow distribution analysis in the entire aorta across a wide age range from pediatric to adult subjects. MATERIAL AND METHODS: In all, 98 healthy subjects (age 9-78 years, 41 women) underwent 4D flow MRI at 1.5T and 3T for the assessment of 3D blood flow in the thoracic aorta. Subjects were categorized into age groups: group 1 (n = 9, 5 women): 9-15 years; group 2 (n = 13, 8 women): 16-20 years; group 3 (n = 27, 14 women): 21-39 years; group 4 (n = 40, 11 women): 40-59 years; group 5 (n = 9, 3 women): >60 years. Data analysis included the 3D segmentation of the aorta, aortic valve peak velocity, mid-ascending aortic diameter, and calculation of flow velocity distribution descriptors (mean, median, standard deviation, incidence of velocities >1 m/s, skewness, and kurtosis of aortic velocity magnitude). Ascending aortic diameter was normalized by body surface area. RESULTS: Age was significantly associated with normalized aortic diameter (R = 0.73, P < 0.001), skewness (R = 0.76, P < 0.001), and kurtosis (R = 0.74, P < 0.001), all adjusted by heart rate. Aortic peak velocity and velocity distribution descriptors, adjusted by heart rate, were significantly different between age groups (P < 0.001, analysis of covariance). Skewness and kurtosis significantly increased (P < 0.001) during adulthood (>40 years) as compared with childhood (<21 years). Men and women revealed significant differences (P ≤ 0.05) for peak velocity, incidence, mean, median, standard deviation, and skewness, all adjusted by heart rate. CONCLUSION: Aortic hemodynamics significantly change with age and gender, indicating the importance of age- and gender-matched control cohorts for the assessment of the impact of cardiovascular disease on aortic blood flow. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2018;47:487-498.