Natural Assembly of Electroactive Metallopolymers on the Electrode Surface: Enhanced Electrocatalytic Production of Hydrogen by [2Fe-2S] Metallopolymers in Neutral Water.

A molecular catalyst attached to an electrode surface can offer the advantages of both homogeneous and heterogeneous catalysis. Unfortunately, some molecular catalysts constrained to a surface lose much or all of their solution performance. In contrast, we found that when a small molecule [2Fe-2S] catalyst is incorporated into metallopolymers of the form PDMAEMA-g-[2Fe-2S] (PDMAEMA = poly(2-dimethylamino)ethyl methacrylate) and adsorbed to the surface, the observed rate of hydrogen production increases to kobs > 105 s-1 per active site with lower overpotential, increased lifetime, and tolerance to oxygen. Herein, the electrocatalytic performances of these metallopolymers with different length polymer chains are compared to reveal the factors that lead to this high performance. It was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites, but the experiments show that the rates of catalysis per active site are independent of the polymer size. Molecular dynamics modeling reveals that the high performance is a consequence of adsorption of these metallopolymers on the surface with natural assembly that brings the [2Fe-2S] catalytic sites into close contact with the electrode surface while maintaining exposure of the sites to protons in solution. The assembly is conducive to fast electron transfer, fast proton transfer, and a high rate of catalysis regardless of the polymer size. These results offer a guide to enhancing the performance of other electrocatalysts with incorporation into a polymer that provides an optimal interaction of the catalyst with the electrode and solution.

The full spectrum tuning of fluorescent molecules via a one-pot multicomponent reaction.

Fluorogenic probes for imaging enable visualization and analysis of difficult-to-reach cells and organelles. However, there are limited efficient examples of tuning these fluorescent molecules to higher wavelengths. This is vital since different tissues are sensitive to varying wavelength emissions. To address this need, we report the discovery, tuning, structure-photophysical property relationships (SPPR), and time-dependent DFT (TD-DFT) computations of 400-700+ nm fluorescent pyrido[2',1':2,3]imidazo[4,5-c]isoquinolines and substituted imidazo[1,2-a]pyridin-3-amines. The syntheses involve the trimethylsilylcyanide (TMSCN) modified Groebke-Blackburn-Bienaymé (GBB) multicomponent reaction as well as the TMSCN modified GBB combined with subsequent condensation of an aldehyde, and Aza-Friedel-Crafts-Intramolecular Cyclization-Oxidation all in one pot. The SPPR reveals that electron-withdrawing strength in the para-position of the aminopyridine starting material has direct control over the absorption and fluorescence emission wavelengths of these molecules. The TD-DFT computations show the changes in the natural transition orbitals (NTOs) with differing substitutions to the parent molecule that dictate the observed excitations, emissions, and fluorescence intensities. These findings give insights and directions for tuning the fluorescent properties of these motifs for various uses as probes and imaging agents.

Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes.

Electrocatalytic generation of H2 is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H2 generation by 50 mA/cm2 at current densities over 100 mA/cm2 compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]+) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure-activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

FTIR Spectroelectrochemistry of F4TCNQ Reduction Products and Their Protonated Forms.

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

100th Anniversary of Macromolecular Science Viewpoint: High Refractive Index Polymers from Elemental Sulfur for Infrared Thermal Imaging and Optics.

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

Synthesis of Metallopolymers via Atom Transfer Radical Polymerization from a [2Fe-2S] Metalloinitiator: Molecular Weight Effects on Electrocatalytic Hydrogen Production.

Small molecule biomimetics inspired by the active site of the [FeFe]-hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H2 ) generation. However, most of the active-site mimics based on [2Fe-2S] clusters are not water-soluble which limits the use of these electrocatalysts to organic media. Polymer-supported [2Fe-2S] systems, in particular, single-site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H2 generation in aqueous milieu. [2Fe-2S] complexes functionalized within well-defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site-isolation, and improved chemical stability during catalysis. In this report, the synthesis of a new propanedithiolate (pdt)-[2Fe-2S] complex bearing a single α-bromoester moiety for use in atom transfer radical polymerization (ATRP) is demonstrated as a novel metalloinitiator to prepare water-soluble poly(2-dimethylaminoethyl methacrylate) grafted (PDMAEMA-g-[2Fe-2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (Mn = 5-40 kg mol-1 ) and low dispersity (D, Mw /Mn = 1.09-1.36) are prepared, which allows for the first time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enables macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H2 production.

Infrared Fingerprint Engineering: A Molecular-Design Approach to Long-Wave Infrared Transparency with Polymeric Materials.

Optical technologies in the long-wave infrared (LWIR) spectrum (7-14 µm) offer important advantages for high-resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR-transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so-called IR-fingerprint region. We report on a new molecular-design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co-monomers.

Catalytic Metallopolymers from [2Fe-2S] Clusters: Artificial Metalloenzymes for Hydrogen Production.

Reviewed herein is the development of novel polymer-supported [2Fe-2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small-molecule, [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer-supported [2Fe-2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H2 production.

Vibrational Control of Covalency Effects Related to the Active Sites of Molybdenum Enzymes.

A multitechnique spectroscopic and theoretical study of the Cp2M(benzenedithiolato) (M = Ti, V, Mo; Cp = η5-C5H5) series provides deep insight into dithiolene electronic structure contributions to electron transfer reactivity and reduction potential modulation in pyranopterin molybdenum enzymes. This work explains the magnitude of the dithiolene folding distortion and the concomitant changes in metal-ligand covalency that are sensitive to electronic structure changes as a function of d-electron occupancy in the redox orbital. It is shown that the large fold angle differences correlate with covalency, and the fold angle distortion is due to a pseudo-Jahn-Teller (PJT) effect. The PJT effect in these and related transition metal dithiolene systems arises from the small energy differences between metal and sulfur valence molecular orbitals, which uniquely poise these systems for dramatic geometric and electronic structure changes as the oxidation state changes. Herein, we have used a combination of resonance Raman, magnetic circular dichroism, electron paramagnetic resonance, and UV photoelectron spectroscopies to explore the electronic states involved in the vibronic coupling mechanism. Comparison between the UV photoelectron spectroscopy (UPS) of the d2 M = Mo complex and the resonance Raman spectra of the d1 M = V complex reveals the power of this combined spectroscopic approach. Here, we observe that the UPS spectrum of Cp2Mo(bdt) contains an intriguing vibronic progession that is dominated by a "missing-mode" that is composed of PJT-active distortions. We discuss the relationship of the PJT distortions to facile electron transfer in molybdenum enzymes.

[FeFe]-Hydrogenase Mimetic Metallopolymers with Enhanced Catalytic Activity for Hydrogen Production in Water.

Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. By using atom-transfer radical polymerization (ATRP), a new class of [FeFe]-metallopolymers with precise molar mass, defined composition, and low polydispersity, has been prepared. The synthetic methodology introduced here allows facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long-term chemical and aerobic stability. Water soluble functional metallopolymers facilitate electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates an order of magnitude faster than hydrogenases (2.5×105  s-1 ), and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis, with turnover numbers on the order of 40 000 under both anaerobic and aerobic conditions.

Macromolecular Engineering of the Outer Coordination Sphere of [2Fe-2S] Metallopolymers to Enhance Catalytic Activity for H2 Production.

Small-molecule catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes have long struggled to achieve fast rates of hydrogen evolution, long-term stability, water solubility, and oxygen compatibility. We profoundly improved on these deficiencies by grafting polymers from a metalloinitiator containing a [2Fe-2S] moiety to form water-soluble poly(2-dimethylamino)ethyl methacrylate metallopolymers (PDMAEMA-g-[2Fe-2S]) using atom transfer radical polymerization (ATRP). This study illustrates the critical role of the polymer composition in enhancing hydrogen evolution and aerobic stability by comparing the catalytic activity of PDMAEMA-g-[2Fe-2S] with a nonionic water-soluble metallopolymer based on poly(oligo(ethylene glycol) methacrylate) prepared via ATRP (POEGMA-g-[2Fe-2S]) with the same [2Fe-2S] metalloinitiator. Additionally, the tunability of catalyst activity is demonstrated by the synthesis of metallocopolymers incorporating the 2-(dimethylamino)ethyl methacrylate (DMAEMA) and oligo(ethylene glycol) methacrylate (OEGMA) monomers. Electrochemical investigations into these metallo(co)polymers show that PDMAEMA-g-[2Fe-2S] retains complete aerobic stability with catalytic current densities in excess of 20 mA·cm-2, while POEGMA-g-[2Fe-2S] fails to reach 1 mA·cm-2 current density even with the application of high overpotentials (η > 0.8 V) and loses all activity in the presence of oxygen. Random copolymers of the two monomers polymerized with the same [2Fe-2S] initiator showed intermediate activity in terms of current density, overpotential, and aerobic stability.

[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar µ-(SCH2 )2 ER2 Linkers (E=Ge and Sn).

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2 (CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C-Ge and C-Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal-metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2 S2 (CO)6 cores bridged by CH2 SnR2 CH2 , R=Me, Ph, linkers were also obtained and characterized.

Intramolecular electron transfer in bipyridinium disulfides.

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

Solubilizing the most easily ionized molecules and generating powerful reducing agents.

Two very soluble compounds having W2(bicyclic guanidinate)4 paddlewheel structures show record low ionization energies (onsets at 3.4 to 3.5 eV) and very negative oxidation potentials in THF (-1.84 to -1.90 V vs Ag/AgCl). DFT computations show the correlation from the gas-phase ionization energies to the solution redox potentials and chemical behavior. These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful stoichiometric reducing agents in nonpolar, nonprotonated solvents.

The electronic states of pyridine-N-oxide studied by VUV photoabsorption and ab initio configuration interaction computations.

The first vacuum-ultraviolet absorption spectrum of pyridine-N-oxide has been obtained, and has led to the identification of nearly 30 Rydberg states. These states were identified by use of the vibrational envelope ("footprint") of the UV-photoelectron spectrum, and are based on the first to the third ionization energies (IE). The adiabatic IE order, central to the Rydberg state symmetry identification, is confirmed by multi-configuration SCF calculations as: 1(2)B1 < 1(2)B2 < 1(2)A2 < 2(2)B1. Several excited valence state equilibrium structures were determined by multi-configuration SCF and coupled cluster procedures. Multi-reference multi-root CI was used to calculate both Rydberg and valence state vertical excitation energies and oscillator strengths, which were correlated with the experimental measurements.

Substituent effects on the electronic characteristics of pentacene derivatives for organic electronic devices: dioxolane-substituted pentacene derivatives with triisopropylsilylethynyl functional groups.

The intramolecular electronic structures and intermolecular electronic interactions of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (TP-5 pentacene), and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5 pentacene) have been investigated by the combination of gas-phase and solid-phase photoelectron spectroscopy measurements. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. The measurements show that the energies of electron transfer such as the reorganization energies, ionization energies, charge-injection barriers, polarization energies, and HOMO-LUMO energy gaps are strongly dependent on the particular functionalization of the pentacene core. The ionization energy trends as a function of the substitution observed for molecules in the gas phase are not reproduced in measurements of the molecules in the condensed phase due to polarization effects in the solid. The electronic behavior of these materials is impacted less by the direct substituent electronic effects on the individual molecules than by the indirect consequences of substituent effects on the intermolecular interactions. The ionization energies as a function of film thickness give information on the relative electrical conductivity of the films, and all three molecules show different material behavior. The stronger intermolecular interactions in TP-5 pentacene films lead to better charge transfer properties versus those in TIPS pentacene films, and EtTP-5 pentacene films have very weak intermolecular interactions and the poorest charge transfer properties of these molecules.

The electronic states of 1,2,4-triazoles: a study of 1H- and 1-methyl-1,2,4-triazole by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy and a comparison with ab initio configuration interaction computations.

The first vacuum ultraviolet absorption spectrum of a 1,2,4-triazole has been obtained and analyzed in detail, with assistance from both an enhanced UV photoelectron spectroscopic study and ab initio multi-reference multi-root configuration interaction procedures. For both 1H- and 1-methyl-1,2,4-triazoles, the first ionization energy bands show complex vibrational structure on the low-energy edges of otherwise unstructured bands. Detailed analysis of these bands confirms the presence of three ionized states. The 6-7 eV VUV spectral region shows an unusual absorption plateau, which is interpreted in terms of the near degeneracy of the first two ionization energies, leading to a pseudo Jahn-Teller effect. The "fingerprint" of the ionization spectrum yields band origins for several Rydberg states. The configuration interaction study shows that although the equilibrium structure for the first cation is effectively planar, the second cation shows significant twisting of the ring system. Some calculated singlet electronic states also show skeletal twisting in which the ring C-H is substantially out of plane.

Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts.

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp(2)M(bdt) (Cp = η(5)-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the 'leveling' of oxidation state energies in metal-dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology.

Assessment of the electronic structure of 2,2'-pyridylpyrrolides as ligands.

The ligand class 2,2'-pyridylpyrrolide is surveyed, both for its structural features and its electronic structure, when attached to monovalent K, Cu, Ag, Au, and Rh. The influence of pyrrolide ring substituents is studied, as well as the question of push/pull interaction between the pyridyl and pyrrolide halves. The π donor ability of the pyrrolide is found to be less than that of an analogous phenyl. However, in contrast to the phenyl analog, the HOMO is pyrrolide π in character for pyridylpyrrolide complexes of copper and rhodium, while it is conventionally metal localized for planar, d(8) rhodium pyridylphenyl. Monovalent three-coordinate copper complexes show great deviations from Y-shaped toward T-shaped structures, including cases where the pyridyl ligand bonds only weakly.

The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods.

The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series.