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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Carbon Dioxide Insertion into Rhenium Hydrides as a Probe for the Impact of Solvent on Linear Free Energy Relationships between Thermodynamic and Kinetic Hydricity

The kinetics of CO 2 insertion into electronically different Re( R bpy)(CO) 3 H ( R bpy = 4,4'-R-2,2'- bipyridine; R = OMe, t Bu, Me, H, Br, COOMe, CF 3 ) complexes to form Re( R bpy)(CO) 3 {OC(H)O} compounds were determined in acetone, dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), and 3-methoxypropionitrile (3-MPN) and compared with previous data in acetonitrile (MeCN). The rates of CO 2 insertion for any one complex of the type Re(Rbpy)(CO) 3 H in different solvents correlates with the Dimroth-Reichardt (E T (30)) solvent parameter. Hammett plots in each solvent indicate that insertion reactions are faster for bpy ligands with electron-donating groups. There is, however, no correlation between the slope of the Hammett plot in different solvents and any common solvent parameter. Similarly, the enthalpies and entropies of activation and kinetic isotope effects associated with CO 2 insertion into Re(bpy)(CO) 3 H in different solvents do not correlate with any common solvent parameters. Theoretical calculations were used to determine the relative thermodynamic hydricities of Re( R bpy)(CO) 3 H type complexes in MeCN, acetone, DMF, and DMSO and in each solvent complexes with more electron-donating substituents on the bpy ligand are stronger hydride donors. Linear Free Energy Relationships (LFERs) between calculated thermodynamic and experimental kinetic hydricity, as measured through CO 2 insertion reactions, were observed in MeCN, acetone, DMF, and DMSO. Furthermore, the slopes of the LFERs correlate with the dielectric constant of the solvent. Overall, this work provides fundamental information about the thermodynamics and kinetics of hydride transfer reactions in different solvents, which is valuable for catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Understanding Mass Transport in Copper Electrolyte-Based Dye-Sensitized Solar Cells

Copper redox shuttles, particularly [Cu(tmby) 2 ] 2+/1+ (tmby = bis(4,4',6,6'-tetramethyl-2,2'-bipyridine)), proved to be among the best electrolytes for dye-sensitized solar cells (DSCs), realizing higher power conversion efficiencies both under full sun and indoor illumination than conventional iodide/triiodide and cobalt electrolytes. Even though [Cu(tmby) 2 ] 2+/1+ renders a relatively higher performance, this metal complex is bulky and is limited by mass transport. Since the regeneration of the dye ground state by Cu I and the reaction of Cu II at the counter electrode are comparatively faster processes, the efficiency of DSC involving Cu I /Cu II electrolytes under relatively high light intensities is largely governed by the diffusion of Cu I /Cu II species. Understanding mass transport in these solar cells will enable further improvements in the performance of such copper-based DSCs. Here in the present study, the role of illumination intensity on the photogenerated current and its relationship to mass transport is evaluated using the best cosensitized dye (D35:XY1) and copper electrolyte ([Cu(tmby) 2 ] 2+/1+ ) combination.

14 SOLAR ENERGY↗
DOE OSTI2022

An Efficient Copper-Based Redox Shuttle Bearing a Hexadentate Polypyridyl Ligand for DSCs under Low-Light Conditions

A Cu complex featuring a hexadentate ligand was synthesized and evaluated as a redox shuttle in dye-sensitized solar cell (DSC) devices, which exhibited excellent performance under low-light conditions. Cu-based redox shuttles (RSs) have been shown to perform remarkably well under low-light conditions; however, most of the known Cu-based RSs employ bidentate pyridyl ligands and often require bulky flanking groups adjacent to the nitrogen donors of these ligands to prevent distortion and binding of exogenous Lewis bases such as 4- tert -butylpyridine (TBP) that are added to enhance cell performance. Without the bulky substituents, the bidentate ligands are susceptible to ligand exchange with TBP. In this context, we have developed a Cu-based RS with a preorganized multidentate ligand designed to facilitate efficient electron transfer kinetics and high stability via the chelate effect. The Cu system, [Cu(bpyPY4)] 2+/+ , reported here is supported by the hexadentate polypyridyl ligand bpyPY4 (6,6'-bis(1,1-di(pyridine-2-yl)ethyl)-2,2'-bipyridine) and examined as a RS in DSCs. From X-ray crystallography and variable-temperature 1 H NMR studies, bpyPY4 provides a dynamic coordination environment around the metal center. Cyclic voltammetry and UV–visible and NMR spectroscopy indicate that noncoordinated pyridyl donors block binding of TBP to copper. DSC devices using [Cu(bpyPY4)] 2+/+ as the redox electrolyte gave a power conversion efficiency (PCE) value of 4.9% under 1 sun illumination (100 mW/cm 2 ). Strikingly, the device performance increased to 11.11% when irradiated with 2400 lux (0.5 mW/cm 2 ) via a fluorescent lamp light source and improved further to 15.2% PCE at 13500 lux (2.10 mW/cm 2 ). In conclusion, the Cu redox shuttle is an intriguing candidate for implementation with narrow band gap sensitizers with low oxidation potentials, which are important for high photocurrent DSC devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Plasma-Initiated Graft Polymerization of Acrylic Acid onto Fluorine-Doped Tin Oxide as a Platform for Immobilization of Water-Oxidation Catalysts

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO 3 H 2 ) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru–Cat1 = [Ru(tpy) (bpy) (H 2 O)] 2+ (tpy = 2,2':6'2"–terpyridine and bpy = 2,2;-bipyridine) and Ru–Cat2 = [Ru(Mebimpy) (bpy) (H 2 O)] 2+ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru–C3 = [Ru(bpy) 3 ] 2+ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru–Cat1 and Ru–Cat2 onto PAA|FTO displayed pH-dependent (RuIII/RuII) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru–Cat1 and Ru–Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru–C3 upon electrochemical oxidation. Furthermore, we attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru–Cat1 and Ru–Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Multifunctional two-dimensional metal-organic frameworks for radionuclide sequestration and detection

Two lanthanide containing porous coordination polymers, [Ln2(bpdc)6(phen)2] · nH2O (1) and [Ln2(bpdc)6(terpy)2] · 3H2O (2) (Ln = either Pr, Nd or Sm-Dy); bpdc: 2,2'-bipyridine-5,5'-dicarboxylic acid, phen: 1,10-phenanthroline, and terpy: 2,2':6',2''-terpyridine, have been hydrothermally synthesized and structurally characterized by powder and single crystal X-ray diffraction. The crystallographic analyses reveal compounds 1 and 2 feature Ln3+ containing dimeric nodes that form a porous two-dimensional (2D) and non-porous three-dimensional (3D) framework, respectively. Each material is stable in aqueous media between pH 3-10 and exhibits modest thermal stability to ~400ºC. Notably, a portion of the phen and bpdc ligands in 1 can be removed thermally, without compromising the crystal structure, causing the surface area and pore volume to increase. The optical properties of 1 and 2 with Gd3+, Sm3+, Tb3+, and Eu3+ were explored in the solid-state using absorbance, fluorescence, and lifetime spectroscopies. The analyses reveal a complex blend of metal and ligand emission in the materials containing Sm3+ and Tb3+, while those featuring Eu3+ are dominated by intense metal-based emission. Compound 1 with Eu3+ displayed promise for the capture and detection of the uranyl cation, (UO2)2+, from aqueous media. In short, uranyl capture was observed at pH 4 and the adsorption thereof was detectable via vibrational and fluorescence spectroscopies and colorimetrically as the off-white color of 1 turned yellow with uptake. Finally, both 1 and 2 with Eu3+ produce bright red emission upon irradiation with Cu ka X-ray radiation (8.04 keV) and are candidate materials for applications in solid-state scintillation.

Surbella, Robert G.↗
DOE OSTI2021

Influence of Surface and Structural Variations in Donor–Acceptor–Donor Sensitizers on Photoelectrocatalytic Water Splitting

Conjugated organic chromophores composed of linked donor (D) and acceptor (A) moieties have attracted considerable attention for photoelectrochemical applications. In this work, we compare the optoelectronic properties and photoelectrochemical performance of two D–A–D structural isomers with thiophene-X-carboxylic acid (X denotes 3 and 2 positions) derivatives and 2,1,3-benzothiadiazole as the D and A moieties, respectively. 5,5′-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-3-carboxylic acid), BTD1, and 5,5′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-2-carboxylic acid), BTD2, were employed in the study to understand how structural isomers affect surface attachments within chromophore–catalyst assemblies and their influence on charge-transfer dynamics. Crystal structures revealed that varying the position of the −COOH anchoring group causes the molecules to either contort out of a plane (BTD1) or adopt a near-perfect planar conformation (BTD2). BTD1 and BTD2 were co-loaded with either a water oxidation catalyst, [Ru(2,6-bis(1-methylbenzimidazol-2-yl)pyridine)-(4,4′-((HO) 2 OPCH 2 )2-2,2′-bipyridine)(OH 2 )] 2 , RuCt 2+ , or proton reduction catalyst [Ni(P 2 Ph N 2 C 6 H 4 CH 2 PO 3 H 2 ) 2 ] 2+ , NiCt 2+ , on oxide electrodes to facilitate photodriven water splitting reactions. Emission quenching measurements indicate that both BTD1 and BTD2 inject electrons into n-type SnO 2 |TiO 2 electrodes and holes into p-type NiO semiconductors from their respective excited states at high efficiencies >60%. Photocurrent densities of chromophore–catalyst assemblies obtained using linear sweep voltammetry (LSV) show that BTD2-sensitized photoanodes generate significantly more photocurrent than BTD1-sensitized electrodes; however, both exhibit similar performances at the photocathode. Photoelectrocatyltic measurements demonstrate that both BTD1 and BTD2 performed similarly, generating Faradaic efficiencies of 39 and 38% at the anode or 61 and 79% at the cathode. Transient absorption measurements suggest that the differences between the LSV and photoelectrocatalytic measurements result from the differences in quantum yields of the photogenerated redox equivalents, which is also a reflection of the varying metal oxide surface conformation. Our findings suggest that BTD2 should be investigated further in photocathodic studies since it has the structural advantage of being incorporated into diverse types of chromophore–catalyst assemblies.

Chromophores↗
DOE OSTI2020

H 2 Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst

Molecular platforms capable of both photochemical H 2 evolution and electrochemical catalyst regeneration hold promise for the efficient generation of solar fuels. Systems studied to date require a significant electrochemical overpotential (η), so no photon energy is being chemically stored. A detailed study of the parameters that govern the electrochemical overpotential of molecular photoelectrocatalysts is presented. By understanding how tuning the solvent, catalyst structure, and acid pK a affect the kinetics and thermodynamics of the potential- and light-driven H 2 evolution reaction, conditions were identified wherein the solvento complex [Cp*Ir(bpy)(NCCH 3 )][PF 6 ] 2 (Cp* is pentamethylcyclopentadienyl; bpy is 2,2'-bipyridine) mediates photoelectrocatalytic H 2 formation at an electrochemical “underpotential” (η < 0) driven by visible light. Under 460 nm illumination and in the presence of the weak organic acid H-PhTMG + (2-phenyl-1,1,3,3- tetramethylguanidinium), the catalyst facilitates H 2 evolution with Faradaic efficiencies up to 90% and overpotentials as low as –90 mV. As a result, mechanistic studies enabled the construction of graphical “maps” that can guide the choice of catalyst and conditions to minimize overpotential, applicable to both photoelectrocatalysts and dark electrocatalysts.

14 SOLAR ENERGY↗
DOE OSTI2020

Excited Carrier Dynamics in a Dye-Sensitized Niobate Nanosheet Photocatalyst for Visible-Light Hydrogen Evolution

Dye-sensitized photocatalysts that consist of a light-absorbing dye and a wide gap oxide semiconductor have been studied extensively as components of solar energy conversion systems. Although surface modification by a metal and/or metal oxide has a significant impact on the photocatalytic efficiency, the mechanism by which these modifications increase the activity has not been fully understood. Here a dye-sensitized H 2 evolution system was constructed by using Pt-intercalated HCa 2 Nb 3 O 10 nanosheets, Ru(II) complex photosensitizers ([Ru(4,4’-(CH 3 ) 2- bpy) 2 (4,4’-(PO 3 H 2 ) 2- bpy)] 2+ and [Ru(4,4’-(CH 3 ) 2- bpy) 2 (4,4’-(CH 2 PO 3 H 2 ) 2- bpy)] 2+ , abbreviated as RuP 2+ and RuCP 2+ ; bpy = 2,2'-bipyridine), and amorphous Al 2 O 3 as building blocks. In the presence of iodide as the electron donor, the H 2 evolution rate from Pt/HCa 2 Nb 3 O 10 nanosheets sensitized by RuP 2+ was increased by modification of the nanosheets with Al 2 O 3 . On the other hand, Al 2 O 3 had a negative impact on the H 2 evolution rate when RuCP 2+ was employed. These hybrid materials were studied by transient diffuse reflectance spectroscopy and steady-state emission spectroscopy. A detailed analysis of the transient absorption profiles of the adsorbed Ru(II) complexes revealed that there are at least three states of the complexes on the nanosheet surface. The transient bleaching of the ground state absorbance had different lifetime components ranging from a few μs to several hundred μs, which mainly reflect back electron transfer rates from HCa 2 Nb 3 O 10 to the oxidized Ru(II) complexes. Here, the Al 2 O 3 modifier could inhibit not only the back electron transfer events but also electron injection from the excited state photosensitizer. Interestingly, the negative effect of Al 2 O 3 on the electron injection rate was negligible in the case of RuP 2+ , which also had a higher H 2 evolution rate. This work highlights that suppressing fast back electron transfer from Pt/HCa 2 Nb 3 O 10 to the oxidized Ru(II) complex, which occurs on a time scale of a few μs, and maximizing the electron injection efficiency are both necessary for improving dye-sensitized H 2 evolution.

14 SOLAR ENERGY↗
DOE OSTI2024

Electronic and Electrochemical Control of Isostructural Ruthenium Hydricities and the Implications for Catalytic Overpotentials

Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (Δ$G$ H- ° ) and $pK$ a of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru(4,4'-R 2 -bpy) 2 (CO)H] + (R = CF 3 , Cl, H, CH 3 , and CH 3 O; bpy = 2,2'-bipyridine) (denoted as (R)Ru-H+). A substantial 22 kcal mol -1 hydricity range is available across five complexes in three stable oxidation states: (R)Ru-H + , (R)Ru-H 0 , and (R)Ru-H - . Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H 2 . Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H 2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. In conclusion, the former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Reconceptualizing the Ir III Role in Metallaphotoredox Catalysis: From Strong Photooxidant to Potent Energy Donor

Dual Ir III /L n Ni II metallaphotoredox catalyzed C(sp 3 )–C(sp 2 ) cross-coupling reactions are widely assumed to proceed by photoinduced single electron transfer steps due to the highly oxidizing Ir III * excited state (Ir III = [Ir(dF(CF 3 )ppy) 2 (dtbbpy)] + [PF 6 ] – ; dF(CF 3 )ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; L n = dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). Using time-resolved absorption and emission spectroscopy, we reveal that energy transfer between Ir III * and various LnNi II precatalysts and intermediates with k q ≥ 10 8 M –1 s –1 also drives catalysis. Specifically, the excited states of L n Ni II dihalide precatalysts/organometallic intermediates accessible by energy transfer appear to drive bond homolysis, halogen radical elimination, and reductive elimination reactions that facilitate formation of cross-coupled products. Energy transfer dynamics consequently circumvent the need for photoinduced electron transfer, thereby extending substrate scopes to coupling partners that cannot be oxidized by Ir III *. Within a cross-electrophile coupling model reaction between 4-bromobenzotrifluoride and bromocyclohexane, energy transfer activates the L n Ni II precatalyst at early reaction times before nucleophilic reductants are present. In the absence of Ir III , direct excitation of L n Ni II (Br) 2 also activates the precatalyst to form a L n Ni II (Br)(Aryl) intermediate. To compare energy transfer and electron transfer kinetics, we determined rate constants for reductive quenching by Br – (k SET = 4.1 × 10 8 M –1 s –1 ) and for the subsequent electron transfer from reduced Ir III•– to L n Ni II (Br) 2 (k SET = 4.1 × 10 7 M –1 s –1 ) using Stern-Volmer analysis and pulse radiolysis, respectively. Energy transfer rate constants are competitive with the electron transfer rate constants and energy transfer is a parallel pathway within metallaphotoredox catalysis. Exploiting the energy transfer mechanism, we demonstrate highly selective cross-electrophile coupling between 4-chlorobenzotrifluoride and bromocyclohexane to form exclusively cross-coupled product. Here, with alkyl-trifluoroborate nucleophiles that do not reductively quench IrIII* emission, transmetalation with L n Ni II (Br/Cl)(Aryl) followed by energy transfer also drives excited state reductive elimination to form C(sp 3 )–C(sp 2 ) cross-coupled product. Similarly, energy transfer rather than Ni II oxidation drives C(sp 2 )–OR reductive elimination, despite the strongly oxidizing ability of Ir III *. In total, these reactions demonstrate energy transfer processes from Ir III * to L n Ni II in metallaphotoredox catalysis that can unlock alternative reactive pathways.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Regeneration of Benzimidazole-Based Organohydrides Mediated by Ru Catalysts

Benzimidazole-based organohydrides (BIHs) are versatile hydride, electron, and proton donors in a variety of artificial photosynthetic systems for the generation of solar fuels. Currently, BIHs are often used in stoichiometric rather than catalytic processes. The catalytic regeneration of BIHs from their oxidized form (BI + ) is an urgent necessity in order to allow the development of recyclable systems and devices, but viable examples are scarce and require large overpotentials. Here, in this study, we report the electrocatalytic regeneration of a series of BIHs with varying hydricities, promoted by a ruthenium half-sandwich complex, [(HMB)Ru(bpy)(H)]+ (HMB = η 6 -C 6 Me 6 , bpy = 2,2′-bipyridine), in the presence of tributylammonium (HNBu 3 + ) as a proton source. This metal hydride was generated in situ from the 2e - reduction/protonation of the corresponding solvento-complex. In comparison to the direct cathodic reduction of BI + in the presence of HNBu 3 + , the use of this ruthenium catalyst allowed efficient regeneration of BIHs at less negative potentials, with Faradaic efficiencies up to 80% and significantly higher conversion yields due to the circumvention of the dimerization pathway and the stepwise reduction and protonation reactions. Finally, the thermodynamic challenges involved in the regeneration of very hydridic BIHs mediated by Ru hydride catalysts were examined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Catalytic Hydrogenation of a Ruthenium Carbonyl to Formyl Enabled by Metal–Ligand Cooperation

Metal formyl complexes are critical intermediates in the reduction of CO to valuable products such as methanol and higher alcohols/hydrocarbons, yet examples of formyl generation via the catalytic hydrogenation of transition metal carbonyl complexes under mild conditions are lacking. The catalytic hydrogenation of a ruthenium carbonyl complex with H 2 to produce a formyl complex is reported here. Two classes of hydrogenation catalysts were compared: bis(diphosphine)-ligated complexes that proceed via termolecular H 2 splitting with an external base and pincer-ligated complexes that proceed via an H 2 splitting mechanism involving metal–ligand cooperativity. The hydride transfer and H 2 splitting steps were evaluated for both classes of catalysts, revealing advantages for catalysts that utilize metal–ligand cooperativity and elucidating conditions to promote formyl generation. Only the pincer-ligated Ir and Ru complexes capable of reacting via pathways involving metal–ligand cooperativity were suitable for catalysis. Using 1–10 mol % of the catalysts (PNP)Ir(H) 2 and (HPNP)Ru(H) 2 (CO) (PNP = ( i Pr 2 PC 2 H 4 ) 2 N – ), which use metal–ligand cooperation to activate H 2 , up to 10 turnovers or up to 71% yield were achieved for the conversion of [Ru(bpy) 2 (CO) 2 ] 2+ (bpy = 2,2′-bipyridine) to the formyl complex [Ru(bpy) 2 (CO)(CHO)] + . The Lewis acid B(C 6 F 5 ) 3 was required as an additive to achieve high yields of the formyl complex using (HPNP)Ru(H) 2 (CO) as a catalyst. In conclusion, the catalytic route avoids the use of expensive stoichiometric reagents, such as borohydride, instead generating metal formyls that are key intermediates in CO reduction schemes with H 2 gas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Real-Time Observation of the Diffusion Mechanism Progression from Liquid to Solid State of Transition Metal Complexes

Copper complexes have recently shown remarkable performance upon conversion from liquid-based to solid-state hole transport materials (HTMs) in mesoscopic solar cells; however, the diffusion mechanism is not clear. Here, we apply an in situ solidification analysis of the charge diffusion and find that the dominant mechanism of [Cu(dmbpy) 2 ] 2+/+ (dmbpy = 6,6'-dimethyl-2,2'-bipyridine) changes from ionic to electronic diffusion. Through use of the modified Dahms–Luff equation, a fast self-exchange rate constant of hole-hopping in the HTM of 8.3 × 10 8 (±5 × 10 7 ) M –1 s –1 is calculated, which indicates a small reorganization energy of 0.47 eV. These findings present a new methodology to analyze the transport mechanism of solids, reveal the mechanism of charge transport in molecular-based HTMs, and offer insight into ways to control the flow of charge in optoelectronic systems

14 SOLAR ENERGY↗
DOE OSTI2026

Incorporation of Ion Transport Chains into Multivariate MOF for Improved Water Oxidation

The climate crisis demands clean energy technologies to cut CO 2 emissions from fossil fuels. Hydrogen fuel cells and solar-driven CO 2 reduction are promising, but both rely on efficient water oxidation. Polypyridyl ruthenium complexes are active catalysts for water oxidation; however, they exhibit poor stability and recyclability. Our group improved performance by embedding these complexes into metal−organic frameworks (MOFs). As water oxidation is pH-dependent, proton management further enhances reactivity. To address the issue, we introduced proton transfer pathways into the MOF structure. Specifically, we incorporated −SO 3 H groups onto the biphenyl linkers of UiO-67 loaded with [Ru(tpy)(dcbpy)OH 2 ]PF 6 catalyst (where tpy = 2,2′:6′,2″-terpyridine; dcbpy = 5,5-dicarboxy-2,2′- bipyridine). The sulfonated MOF exhibited a 2.5-fold increase in oxygen evolution compared to the nonsulfonated analogue. After 1 h of electrolysis, the sulfonated MOF exhibited a turnover number of 25 for oxygen evolution reaction compared to 10 for the native MOF, demonstrating the benefits of built-in proton management.

Catalysts↗
DOE OSTI2022

Controlled Hysteresis of Conductance in Molecular Tunneling Junctions

The problem this paper addresses is the origin of the hysteretic behavior in two-terminal molecular junctions made from an EGaIn electrode and self-assembled monolayers of alkanethiolates terminated in chelates (transition metal dichlorides complexed with 2,2'-bipyridine; BIPY-MCl 2 ). The hysteresis of conductance displayed by these BIPY-MCl 2 junctions changes in magnitude depending on the identity of the metal ion (M) and the window of the applied voltage across the junction. The hysteretic behavior of conductance in these junctions appears only in an incoherent (Fowler–Nordheim) tunneling regime. When the complexed metal ion is Mn(II), Fe(II), Co(II), or Ni(II), both incoherent tunneling and hysteresis are observed for a voltage range between +1.0 V and –1.0 V. When the metal ion is Cr(II) or Cu(II), however, only resonant (one-step) tunneling is observed, and the junctions exhibit no hysteresis and do not enter the incoherent tunneling regime. Using this correlation, the conductance characteristics of BIPY-MCl 2 junctions can be controlled. Furthermore, this voltage-induced change of conductance demonstrates a simple, fast, and reversible way (i.e., by changing the applied voltage) to modulate conductance in molecular tunneling junctions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Machine-Learning-Assisted Synthesis of Polar Racemates

Racemates have recently received attention as nonlinear optical and piezoelectric materials. We report a machine-learning-assisted composition space approach was applied to synthesize the missing M = Ti, Zr members of the Δ,Λ-[Cu(bpy) 2 (H 2 O)] 2 [MF 6 ] 2·3 H 2 O (M = Ti, Zr, Hf; bpy = 2,2'-bipyridine) family (space group: Pna2 1 ). In each (CuO, MO 2 )/bpy/HF(aq) (M = Ti, Zr, Hf) system, the polar noncentrosymmetric racemate (M-NCS) forms in competition with a centrosymmetric one-dimensional chain compound (M-CS) based on alternating Cu(bpy)(H 2 O) 2 2+ and MF 6 2– basic building units (space groups: Ti-CS (Pnma), Zr-CS (P1̅), Hf-CS (P2/n)). Machine learning models were trained on reaction parameters to gain unbiased insight into the underlying statistical trends in each composition space. A human-interpretable decision tree shows that phase selection is driven primarily by the bpy:CuO molar ratio for reactions containing Zr or Hf, and predicts that formation of the Ti-NCS compound requires that the amount of HF present be decreased to raise the pH, which we verified experimentally. Predictive leave-one-metal-out (LOO) models further confirm that behavior in the Ti system is distinct from that of the Zr and Hf systems. The chemical origin of this distinction was probed via fluorine K-edge X-ray absorption spectroscopy. Pre-edge features in the F1s X-ray absorption spectra reveal the strong ligand-to-metal π bonding between Ti(3d – t 2g ) and F(2p) states that distinguishes the TiF 6 2– anion from the ZrF 6 2– and HfF 6 2– anions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

The sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground doublet states ( 2 S o /D o ) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronylnitroxide, NN) covalently attached to a donor acceptor chromophore via a meta-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4’-di-tert-butyl-2,2’-bipyridine, M = Pd II (1-Pd) or Pt II (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light pro-duces an initial exchange-coupled, 3-spin (bpy •- , CAT +• = semiquinone (SQ) and NN • ), charge-separated doublet 2 S 1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2 T 1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. Furthermore, it is postulated that zero-field splitting induced transitions between the chromophoric 2 T 1 and 4 T 1 states (1-Pd and 1-Pt) and spin-orbit induced transitions between 2 T 1 and NN-based quartet states (1-Pt) ac-count for the differences in polarization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re( R bpy)(CO) 3 H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, t Bu, Me, H, Br, COOMe, CF 3 ) to CO 2 and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re( R bpy)(CO) 3 H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗