Addressing the challenge of carbon-free energy.

This century will witness a major transformation in how energy is acquired, stored, and utilized globally. The impetus for this change comes from the deep impacts that both developed and developing societies have had on our planet's environment during the past century, and the projections going forward of what will happen if we do not act transformatively within the next 2 decades. This paper describes the basis for a meeting held in October 2018 on the need for decarbonization in our energy landscape, and specifically the status and challenges of the science that provides the foundation for such technology. Within the realm of decarbonization in energy generation lies the science of solar energy conversion using new or improved photovoltaic materials and artificial photosynthesis for water splitting and other energy-storing reactions. The intimately related issue of renewable energy storage is being addressed with new strategies, materials, and approaches under current investigation and development. The need to improve the interactions between scientists working on these connected but separately considered challenges and on the transition of scientific achievement to practical application was also addressed, with specific efforts enumerated.

Molecular Nickel Thiolate Complexes for Electrochemical Reduction of CO2 to C1-3 Hydrocarbons.

We report the unprecedented electrocatalytic activity of a series of molecular nickel thiolate complexes (1-5) in reducing CO2 to C1-3 hydrocarbons on carbon paper in pH-neutral aqueous solutions. Ni(mpo)2 (3, mpo=2-mercaptopyridyl-N-oxide), Ni(pyS)3 - (4, pyS=2-mercaptopyridine), and Ni(mp)2 - (5, mp=2-mercaptophenolate) were found to generate C3 products from CO2 for the first time in molecular complex. Compound 5 exhibits Faradaic efficiencies (FEs) of 10.6 %, 7.2 %, 8.2 % for C1 , C2 , C3 hydrocarbons respectively at -1.0 V versus the reversible hydrogen electrode. Addition of CO to the system significantly promotes the FEC1-C3 to 41.1 %, suggesting that a key Ni-CO intermediate is associated with catalysis. A variety of spectroscopies have been performed to show that the structures of nickel complexes remain intact during CO2 reduction.

Semiconductor quantum dot-sensitized rainbow photocathode for effective photoelectrochemical hydrogen generation.

The present study reports the fabrication of CdSe quantum dot (QD)-sensitized photocathodes on NiO-coated indium tin oxide (ITO) electrodes and their H2-generating ability upon light irradiation. A well-established spin-coating method was used to deposit CdSe QD stock solution onto the surface of NiO/ITO electrodes, thereby leading to the construction of various CdSe QD-sensitized photocathodes. The present report includes the construction of rainbow photocathodes by spin-coating different-sized QDs in a sequentially layered manner, thereby creating an energetically favorable gradient for charge separation. The resulting rainbow photocathodes with forward energetic gradient for charge separation and subsequent electron transfer to a solution-based hydrogen-evolving catalyst (HEC) exhibit good light-harvesting ability and enhanced photoresponses compared with the reverse rainbow photocathodes under white LED light illumination. Under minimally optimized conditions, a photocurrent density of as high as 115 µA⋅cm-2 and a Faradaic efficiency of 99.5% are achieved, which is among the most effective QD-based photocathode water-splitting systems.

Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen.

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN2S2) charge transfer complex were synthesized and used as photosensitizers for the generation of H2 from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as Pt-RDM1, Pt-RDM2, and Pt-RDM3. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π-π* absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN2S2 complexes. The shift of the rhodamine π-π* absorption maxima in going from Pt-RDM1 to Pt-RDM3 correlates well with the HOMO-LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H2 generation when attached to platinized TiO2 nanoparticles (Pt-TiO2). After 40 h of irradiation, systems containing Pt-RDM1, Pt-RDM2, and Pt-RDM3 exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine 1π-π* state to the singlet charge transfer (1CT) state of the PtN2S2 chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine 1π-π* state to a charge separated (CS) RDM(0•)-Pt(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN2S2 dyads for H2 generation correlate well with the relative energies of the CS state and the PtN2S23CT state used for H2 production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H2 from aqueous protons.

Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption.

The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)2 (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)2 intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)2 intermediate and Ni(mpo)2 (mpo = 2-mercaptopyridyl- N-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)2 intermediate and Ni(mpo)2 catalyst, a critical reaction intermediate in H2 generation.

Discovery of the magnetic behavior of hemoglobin: A beginning of bioinorganic chemistry.

Two articles published by Pauling and Coryell in PNAS nearly 80 years ago described in detail the magnetic properties of oxy- and deoxyhemoglobin, as well as those of closely related compounds containing hemes. Their measurements revealed a large difference in magnetism between oxygenated and deoxygenated forms of the protein and, along with consideration of the observed diamagnetism of the carbonmonoxy derivative, led to an electronic structural formulation of oxyhemoglobin. The key role of hemoglobin as the main oxygen carrier in mammalian blood had been established earlier, and its allosteric behavior had been described in the 1920s. The Pauling-Coryell articles on hemoglobin represent truly seminal contributions to the field of bioinorganic chemistry because they are the first to make connections between active site electronic structure and the function of a metalloprotein.

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes.

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine dithiolate (PtN2S2) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN2S2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN2S2 moieties), indicating little electronic coupling between them. Connection to TiO2 nanoparticles is carried out by sonication leading to in situ attachment to TiO2 without prior hydrolysis of the ester linking groups to acids. For H2 generation studies, the TiO2 particles are platinized (Pt-TiO2) so that the light absorber (the dyad), the electron conduit (TiO2), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H2O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO2 via a phosphonate or carboxylate attachment shows excellent light-driven H2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼ 40,000 turnover numbers of H2 over 12 d (with respect to dye).

Catalytic Light-Driven Generation of Hydrogen from Water by Iron Dithiolene Complexes.

The development of active, robust systems for light-driven hydrogen production from aqueous protons based on catalysts and light absorbers composed solely of earth abundant elements remains a challenge in the development of an artificial photosynthetic system for water splitting. Herein, we report the synthesis and characterization of four closely related Fe bis(benzenedithiolate) complexes that exhibit catalytic activity for hydrogen evolution when employed in systems with water-soluble CdSe QDs as photosensitizer and ascorbic acid as a sacrificial electron source under visible light irradiation (520 nm). The complex with the most electron-donating dithiolene ligand exhibits the highest activity, the overall order of activity correlating with the reduction potential of the formally Fe(III) dimeric dianions. Detailed studies of the effect of different capping agents and the extent of surface coverage of these capping agents on the CdSe QD surfaces reveal that they affect system activity and provide insight into the continued development of such systems containing QD light absorbers and molecular catalysts for H2 formation.

A comparative study of the photophysics of phenyl, thienyl, and chalcogen substituted rhodamine dyes.

Although rhodamine dyes have been extensively studied for a variety of applications, many details of their photophysics are not yet fully understood, including the possible presence of a charge separated electronic state lying near the optically active excited singlet state and the role of twisting substituent groups in excited-state quenching. To address this, a large library of rhodamine dyes was studied in which the chalcogen is varied from O, to S and Se and the aryl group is either absent (in the pyronin series) or is a phenyl or thienyl substituent. Through an analysis of steady-state absorption spectroscopy, electrochemistry, X-ray crystallography, and quantum mechanical calculations, we show that the lowest unoccupied molecular orbital (LUMO) energy decreases in the O → S → Se series and when a phenyl or thienyl substituent is added. The reduction of the LUMO energy is larger for thienyl species in which the aromatic group has increased torsional flexibility. Excited state lifetimes and fluorescence quantum yields of these dyes in a high and low polarity solvent reveal dramatically different photophysics between chromophores with phenyl and thienyl substituents, due to a combination of torsional and inductive effects. In the pyronin and phenyl-substituted species, non-radiative decay can occur through an amine-to-xanthylium core charge separated state that is stabilized in a highly polar environment. In the thienyl derivatives, a lower energy excited state, which we term S'1, is accessed from S1via rotation of the aryl group and the excited state population rapidly equilibrates between S1 and S'1 in 6-30 ps. Preliminary photochemical hydrogen production data display the potential application of the thienyl derivatives for conversion of solar energy.

Chromophoric Dyads for the Light-Driven Generation of Hydrogen: Investigation of Factors in the Design of Multicomponent Photosensitizers for Proton Reduction.

Two new dyads have been synthesized and studied as photosensitizers for the light-driven generation of H2 from aqueous protons. One of the dyads, Dy-1, consists of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore, denoted as PtN2S2. The two components are connected through an amide linkage on the bdt side of the PtN2S2 complex. The second dyad, Dy-2, contains a diketopyrrolopyrrole dye that is linked directly to the acetylide ligands of a Pt diimine bis(arylacetylide) CT chromophore. The two dyads, as well as the Pt diimine bis(arylacetylide) CT chromophore, were attached to platinized TiO2 via phosphonate groups on the diimine through sonication of the corresponding esters, and each system was examined for photosensitizer effectiveness in photochemical generation of H2 from aqueous protons and electrons supplied by ascorbic acid. Of the three photosensitizers, Dy-1 is the most active under 530 nm radiation with an initial turnover frequency of 260 h(-1) and a total of 6770 turnovers over 60 h of irradiation. When a "white" LED light source is used, samples with Dy-2 and the Pt diimine bis(arylacetylide) chromophore, while not as effective as Dy-1, perform relatively better. A key conclusion is that the presence of a strongly absorbing organic dye increases dyad photosensitizer effectiveness only if the energy of the CT excited state lies below that of the organic dye's lowest excited state; if not, the organic dye does not improve the effectiveness of the CT chromophore for promoting electron transfer and the light-driven generation of H2. The nature of the spacer between the organic dye and the charge transfer chromophore also plays a role in the effectiveness of using dyads to improve light-driven energy-storing reactions.

Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange.

Unique tripodal S-donor capping agents with an attached carboxylate are found to bind tightly to the surface of CdSe nanocrystals (NCs), making the latter water soluble. Unlike that in similarly solubilized CdSe NCs with one-sulfur or two-sulfur capping agents, dissociation from the NC surface is greatly reduced. The impact of this behavior is seen in the photochemical generation of H2 in which the CdSe NCs function as the light absorber with metal complexes in aqueous solution as the H2-forming catalyst and ascorbic acid as the electron donor source. This precious-metal-free system for H2 generation from water using [Co(bdt)2](-) (bdt, benzene-1,2-dithiolate) as the catalyst exhibits excellent activity with a quantum yield for H2 formation of 24% at 520 nm light and durability with >300,000 turnovers relative to catalyst in 60 h.

Fuel from water: the photochemical generation of hydrogen from water.

Hydrogen has been labeled the fuel of the future since it contains no carbon, has the highest specific enthalpy of combustion of any chemical fuel, yields only water upon complete oxidation, and is not limited by Carnot considerations in the amount of work obtained when used in a fuel cell. To be used on the scale needed for sustainable growth on a global scale, hydrogen must be produced by the light-driven splitting of water into its elements, as opposed to reforming of methane, as is currently done. The photochemical generation of H2, which is the reductive side of the water splitting reaction, is the focus of this Account, particularly with regard to work done in the senior author's laboratory over the last 5 years. Despite seminal work done more than 30 years ago and the extensive research conducted since then on all aspects of the process, no viable system has been developed for the efficient and robust photogeneration of H2 from water using only earth abundant elements. For the photogeneration of H2 from water, a system must contain a light absorber, a catalyst, and a source of electrons. In this Account, the discovery and study of new Co and Ni catalysts are described that suggest H2 forms via a heterocoupling mechanism from a metal-hydride and a ligand-bound proton. Several complexes with redox active dithiolene ligands are newly recognized to be effective in promoting the reaction. A major new development in the work described is the use of water-soluble CdSe quantum dots (QDs) as light absorbers for H2 generation in water. Both activity and robustness of the most successful systems are impressive with turnover numbers (TONs) approaching 10(6), activity maintained over 15 days, and a quantum yield for H2 of 36% with 520 nm light. The water solubilizing capping agent for the first system examined was dihydrolipoic acid (DHLA) anion, and the catalyst was determined to be a DHLA complex of Ni(II) formed in situ. Dissociation of DHLA from the QD surface proved problematic in assessing other catalysts and stimulated the synthesis of tridentate trithiolate (S3) capping agents that are inert to dissociation. In this way, CdSe QD's having these S3 capping agents were used in systems for the photogeneration of H2 that allowed meaningful comparison of the relative activity of different catalysts for the light-driven production of H2 from water. This new chemistry also points the way to the development of new photocathodes based on S3-capped QDs for removal of the chemical sacrificial electron donor and its replacement electrochemically in photoelectrosynthetic cells.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions.

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H(2)). Harnessing solar energy to generate H(2) from H(+) is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 11 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to CoL(2)(2-). Subsequent reduction and reaction with H(+) lead to H(2) formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

Cobalt complexes as artificial hydrogenases for the reductive side of water splitting.

The generation of H2 from protons and electrons by complexes of cobalt has an extensive history. During the past decade, interest in this subject has increased as a result of developments in hydrogen generation that are driven electrochemically or photochemically. This article reviews the subject of hydrogen generation using Co complexes as catalysts and discusses the mechanistic implications of the systems studied for making H2. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

From seconds to femtoseconds: solar hydrogen production and transient absorption of chalcogenorhodamine dyes.

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO2 catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore's triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.

Light-driven hydrogen production from aqueous protons using molybdenum catalysts.

Homogeneous light-driven systems employing molecular molybdenum catalysts for hydrogen production are described. The specific Mo complexes studied are six-coordinate bis(benzenedithiolate) derivatives having two additional isocyanide or phosphine ligands to complete the coordination sphere. Each of the complexes possesses a trigonal prismatic coordination geometry. The complexes were investigated as proton reduction catalysts in the presence of [Ru(bpy)3](2+), ascorbic acid, and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis occurs between the Mo(IV)/Mo(III) and Mo(III)/Mo(II) redox couples, around 1.0 V vs SCE. Mechanistic studies by (1)H NMR spectroscopy show that upon two-electron reduction the Mo(CNR)2(bdt)2 complex dissociates the isocyanide ligands, followed by addition of acid to result in the formation of molecular hydrogen and the Mo(bdt)2 complex.

Reversible mechanochromic luminescence at room temperature in cationic platinum(II) terpyridyl complexes.

Reversible mechanochromic luminescence in cationic platinum(II) terpyridyl complexes is described. The complexes [Pt(Nttpy)Cl]X2 (Nttpy = 4'-(p-nicotinamide-N-methylphenyl)-2,2':6',2″-terpyridine, X = PF6 (1), SbF6 (2), Cl (3), ClO4 (4), OTf (5), BF4 (6)) exhibit different colors under ambient light in the solid state, going from red to orange to yellow. All of these complexes are brightly luminescent at both room temperature and 77 K. Upon gentle grinding, the yellow complexes (4-6) turn orange and exhibit bright red luminescence. The red luminescence can be changed back to yellow by the addition of a few drops of acetonitrile to the sample. Crystallographic studies of the yellow and red forms of complex 5 suggest that the mechanochromic response is likely the result of a change in intermolecular Pt···Pt distances upon grinding.

Deactivating unproductive pathways in multichromophoric sensitizers.

The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt(bpy)(bdt) (bpy = 2,2'-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed-metal-ligand-to-ligand' charge transfer ((3)MMLL'CT) state to the Bodipy (3)ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the (3)MMLL'CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the (3)MMLL'CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the (3)MMLL'CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy (3)ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the (3)MMLL'CT state and inducing relaxation to the (3)ππ*.

Nickel pyridinethiolate complexes as catalysts for the light-driven production of hydrogen from aqueous solutions in noble-metal-free systems.

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.