Spectroscopic elucidation of energy transfer in hybrid inorganic-biological organisms for solar-to-chemical production.

The rise of inorganic-biological hybrid organisms for solar-to-chemical production has spurred mechanistic investigations into the dynamics of the biotic-abiotic interface to drive the development of next-generation systems. The model system, Moorella thermoacetica-cadmium sulfide (CdS), combines an inorganic semiconductor nanoparticle light harvester with an acetogenic bacterium to drive the photosynthetic reduction of CO2 to acetic acid with high efficiency. In this work, we report insights into this unique electrotrophic behavior and propose a charge-transfer mechanism from CdS to M. thermoacetica Transient absorption (TA) spectroscopy revealed that photoexcited electron transfer rates increase with increasing hydrogenase (H2ase) enzyme activity. On the same time scale as the TA spectroscopy, time-resolved infrared (TRIR) spectroscopy showed spectral changes in the 1,700-1,900-cm-1 spectral region. The quantum efficiency of this system for photosynthetic acetic acid generation also increased with increasing H2ase activity and shorter carrier lifetimes when averaged over the first 24 h of photosynthesis. However, within the initial 3 h of photosynthesis, the rate followed an opposite trend: The bacteria with the lowest H2ase activity photosynthesized acetic acid the fastest. These results suggest a two-pathway mechanism: a high quantum efficiency charge-transfer pathway to H2ase generating H2 as a molecular intermediate that dominates at long time scales (24 h), and a direct energy-transducing enzymatic pathway responsible for acetic acid production at short time scales (3 h). This work represents a promising platform to utilize conventional spectroscopic methodology to extract insights from more complex biotic-abiotic hybrid systems.

Ultrafast infrared studies of the role of spin states in organometallic reaction dynamics.

The importance of spin state changes in organometallic reactions is a topic of significant interest, as an increasing number of reaction mechanisms involving changes of spin state are consistently being uncovered. The potential influence of spin state changes on reaction rates can be difficult to predict, and thus this class of reactions remains among the least well understood in organometallic chemistry. Ultrafast time-resolved infrared (TRIR) spectroscopy provides a powerful tool for probing the dynamics of spin state changes in organometallic catalysis, as such processes often occur on the picosecond to nanosecond time scale and can readily be monitored in the infrared via the absorptions of carbonyl reporter ligands. In this Account, we summarize recent work from our group directed toward identifying trends in reactivity that can be used to offer predictive insight into the dynamics of coordinatively unsaturated organometallic reaction intermediates. In general, coordinatively unsaturated 16-electron (16e) singlets are able to coordinate to solvent molecules as token ligands to partially stabilize the coordinatively unsaturated metal center, whereas 16e triplets and 17-electron (17e) doublets are not, allowing them to diffuse more rapidly through solution than their singlet counterparts. Triplet complexes typically (but not always) undergo spin crossover prior to solvent coordination, whereas 17e doublets do not coordinate solvent molecules as token ligands and cannot relax to a lower spin state to do so. 16e triplets are typically able to undergo facile spin crossover to yield a 16e singlet where an associative, exothermic reaction pathway exists. The combination of facile spin crossover with faster diffusion through solution for triplets can actually lead to faster catalytic reactivity than for singlets, despite the forbidden nature of these reactions. We summarize studies on odd-electron complexes in which 17e doublets were found to display varying behavior with regard to their tendency to react with 2-electron donor ligands to form 19-electron (19e) adducts. The ability of 19e adducts to serve as reducing agents in disproportionation reactions depends on whether the excess electron density localized at the metal center or at a ligand site. The reactivity of both 16e and 17e complexes toward a widely used organic nitroxyl radical (TEMPO) are reviewed, and both classes of complexes generally react similarly via an associative mechanism with a low barrier to these reactions. We also describe recent work targeted at unraveling the photoisomerization mechanism of a thermal-solar energy storage complex in which spin state changes were found to play a crucial role. Although a key triplet intermediate was found to be required for this photoisomerization mechanism to proceed, the details of why this triplet is formed in some complexes (those based on ruthenium) and not others (those based on iron, molybdenum, or tungsten) remains uncertain, and further exploration in this area may lead to a better understanding of the factors that influence intramolecular and excited state spin state changes.

Exploring the potential of fulvalene dimetals as platforms for molecular solar thermal energy storage: computations, syntheses, structures, kinetics, and catalysis.

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu2 in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔHstorage . Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1',3,3'-tetra-tert-butyl (4), 1,2,2',3'-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and ditungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3 -SiO2 as a good candidate, although catalyst decomposition remains a challenge.

Vibrational cooling dynamics of a [FeFe]-hydrogenase mimic probed by time-resolved infrared spectroscopy.

Picosecond time-resolved infrared spectroscopy (TRIR) was performed for the first time on a dithiolate bridged binuclear iron(I) hexacarbonyl complex ([Fe2(µ-bdt)(CO)6], bdt = benzene-1,2-dithiolate) which is a structural mimic of the active site of the [FeFe]-hydrogenase enzyme. As these model active sites are increasingly being studied for their potential in photocatalytic systems for hydrogen production, understanding their excited and ground state dynamics is critical. In n-heptane, absorption of 400 nm light causes carbonyl loss with low quantum yield (<10%), while the majority (ca. 90%) of the parent complex is regenerated with biexponential kinetics (τ1 = 21 ps and τ2 = 134 ps). In order to understand the mechanism of picosecond bleach recovery, a series of UV-pump TRIR experiments were performed in different solvents. The long time decay (τ2) of the transient spectra is seen to change substantially as a function of solvent, from 95 ps in THF to 262 ps in CCl4. Broadband IR-pump TRIR experiments were performed for comparison. The measured vibrational lifetimes (T1(avg)) of the carbonyl stretches were found to be in excellent correspondence to the observed τ2 decays in the UV-pump experiments, signifying that vibrationally excited carbonyl stretches are responsible for the observed longtime decays. The fast spectral evolution (τ1) was determined to be due to vibrational cooling of low frequency modes anharmonically coupled to the carbonyl stretches that were excited after electronic internal conversion. The results show that cooling of both low and high frequency vibrational modes on the electronic ground state give rise to the observed picosecond TRIR transient spectra of this compound, without the need to invoke electronically excited states.

Reactivity of TEMPO toward 16- and 17-electron organometallic reaction intermediates: a time-resolved IR study.

The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical (TEMPO) has been employed for an extensive range of chemical applications, ranging from organometallic catalysis to serving as a structural probe in biological systems. As a ligand in an organometallic complex, TEMPO can exhibit several distinct coordination modes. Here we use ultrafast time-resolved infrared spectroscopy to study the reactivity of TEMPO toward coordinatively unsaturated 16- and 17-electron organometallic reaction intermediates. TEMPO coordinates to the metal centers of the 16-electron species CpCo(CO) and Fe(CO)4, and to the 17-electron species CpFe(CO)2 and Mn(CO)5, via an associative mechanism with concomitant oxidation of the metal center. In these adducts, TEMPO thus behaves as an anionic ligand, characterized by a pyramidal geometry about the nitrogen center. Density functional theory calculations are used to facilitate interpretation of the spectra and to further explore the structures of the TEMPO adducts. To our knowledge, this study represents the first direct characterization of the mechanism of the reaction of TEMPO with coordinatively unsaturated organometallic complexes, providing valuable insight into its reactions with commonly encountered reaction intermediates. The similar reactivity of TEMPO toward each of the species studied suggests that these results can be considered representative of TEMPO's reactivity toward all low-valent transition metal complexes.

Femtosecond electron solvation at the ionic liquid/metal electrode interface.

Electron solvation is examined at the interface of a room temperature ionic liquid (RTIL) and an Ag(111) electrode. Femtosecond two-photon photoemission spectroscopy is used to inject an electron into an ultrathin film of RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr](+)[NTf2](-)). While much of current literature highlights slower nanosecond solvation mechanisms in bulk ionic liquids, we observe only a femtosecond response, supporting morphology dependent and interface specific electron solvation mechanisms. The injected excess electron is found to reside in an electron affinity level residing near the metal surface. Population of this state decays back to the metal with a time constant of 400 ± 150 fs. Electron solvation is measured as a dynamic decrease in the energy with a time constant of 350 ± 150 fs. We observe two distinct temperature regimes, with a critical temperature near 250 K. The low temperature regime is characterized by a higher work function of 4.41 eV, while the high temperature regime is characterized by a lower work function of 4.19 eV. The total reorganizational energy of solvation changes above and below the critical temperature. In the high temperature regime, the electron affinity level solvates by 540 meV at 350 K, and below the critical temperature, solvation decreases to 200 meV at 130 K. This study will provide valuable insight to interface specific solvation of room temperature ionic liquids.

Switching from Ru to Fe: picosecond IR spectroscopic investigation of the potential of the (fulvalene)tetracarbonyldiiron frame for molecular solar-thermal storage.

A foray into the exploration of Fe analogues of the (fulvalene)tetracarbonyldiruthenium [FvRu2(CO)4] solar-thermal storage system 1⇆2 is described. It was facilitated by the development of a convenient synthetic access to the parent [FvFe2(CO)4] 3a and the more soluble di(tert-butyl)fulvalene derivatives 3c and d. Laboratory time scale irradiations (>400 nm) fail to induce photoisomerization, an observation that is explained by the results of time-resolved IR experiments. They show that photoexcitation generates only the short-lived singlet syn biradical of 3 (and a small amount of decarbonylation product), in the absence of the corresponding triplet species required for the occurrence of rearrangement to 4.

Direct observation of a bent carbonyl ligand in a 19-electron transition metal complex.

The photochemistry of [CpRu(CO)2]2 in P(OMe)3/CH2Cl2 solution has been studied using picosecond time-resolved infrared (TRIR) spectroscopy. Photolysis at 400 nm leads to the formation of 17-electron CpRu(CO)2(•) radicals, which react on the picosecond time scale to form 19-electron CpRu(CO)2P(OMe)3(•) adducts. The TRIR spectra of this adduct display an unusually low CO stretching frequency for the antisymmetric CO stretching mode, suggesting that one carbonyl ligand adopts a bent configuration to avoid a 19-electron count at the metal center. This spectral assignment is supported by analogous experiments on [CpFe(CO)2]2 in the same solvent, combined with DFT studies on the structures of the 19-electron adducts. The DFT results predict a bent CO ligand in CpRu(CO)2P(OMe)3(•), whereas approximately linear Fe-C-O bond angles are predicted for CpFe(CO)2P(OMe)3(•). The observation of a bent CO ligand in the 19-electron ruthenium adduct is a surprising result, and it provides new insight into the solution-phase behavior of 19-electron complexes. TRIR spectra were also collected for [CpRu(CO)2]2 in neat CH2Cl2, and it is interesting to note that no singly bridged [CpRu(CO)]2(µ-CO) photoproduct was observed to form following 400- or 267-nm excitation, despite previous observations of this species on longer time scales.

Insights into the photochemical disproportionation of transition metal dimers on the picosecond time scale.

The reactivity of five transition metal dimers toward photochemical, in-solvent-cage disproportionation has been investigated using picosecond time-resolved infrared spectroscopy. Previous ultrafast studies on [CpW(CO)3]2 established the role of an in-cage disproportionation mechanism involving electron transfer between 17- and 19-electron radicals prior to diffusion out of the solvent cage. New results from time-resolved infrared studies reveal that the identity of the transition metal complex dictates whether the in-cage disproportionation mechanism can take place, as well as the more fundamental issue of whether 19-electron intermediates are able to form on the picosecond time scale. Significantly, the in-cage disproportionation mechanism observed previously for the tungsten dimer does not characterize the reactivity of four out of the five transition metal dimers in this study. The differences in the ability to form 19-electron intermediates are interpreted either in terms of differences in the 17/19-electron equilibrium or of differences in an energetic barrier to associative coordination of a Lewis base, whereas the case for the in-cage vs diffusive disproportionation mechanisms depends on whether the 19-electron reducing agent is genuinely characterized by 19-electron configuration at the metal center or if it is better described as an 18 + δ complex. These results help to better understand the factors that dictate mechanisms of radical disproportionation and carry implications for radical chain mechanisms.

Ultrafast observation of a solvent dependent spin state equilibrium in CpCo(CO).

We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct.

X-ray transient absorption and picosecond IR spectroscopy of fulvalene(tetracarbonyl)diruthenium on photoexcitation.

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

The origin of charge localization observed in organic photovoltaic materials.

Two of the primary hurdles facing organic electronics and photovoltaics are their low charge mobility and the inability to disentangle morphological and molecular effects on charge transport. Specific chemical groups such as alkyl side chains are often added to enable spin-casting and to improve overall power efficiency and morphologies, but their exact influence on mobility is poorly understood. Here, we use two-photon photoemission spectroscopy to study the charge transport properties of two organic semiconductors, one with and one without alkyl substituents (sexithiophene and dihexyl-sexithiophene). We show that the hydrocarbon side chains are responsible for charge localization within 230 fs. This implies that other chemical groups should be used instead of alkyl ligands to achieve the highest performance in organic photovoltaics and electronics.

Time-resolved IR studies on the mechanism for the functionalization of primary C-H bonds by photoactivated Cp*W(CO)3(Bpin).

Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the well-defined systems that reacts under photochemical conditions using both density functional theory calculations and pico- through microsecond time-resolved IR spectroscopy. UV irradiation of Cp*W(CO)(3)(Bpin) (Cp* = C(5)(CH(3))(5); pin = 1,2-O(2)C(2)-(CH(3))(4)) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-Cp*W(CO)(2)(Bpin)(pentane), undergoes C-H bond activation by a sigma-bond metathesis mechanism--in 16 micros, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a sigma-borane complex, Cp*W(CO)(2)(H-Bpin)(C(5)H(11)). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C(5)H(11) ligand. In this configuration, the B-H sigma-bond is broken in favor of a B-C sigma-bond, forming Cp*W(CO)(2)(H)(C(5)H(11)-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form Cp*W(CO)(2)(H) in approximately 170 micros. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex.

Direct observation of photoinduced bent nitrosyl excited-state complexes.

Ground-state structures with side-on nitrosyl (eta (2)-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO) 3(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and eta (2)-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO) 3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.

Ligand rearrangement reactions of Cr(CO)6 in alcohol solutions: experiment and theory.

The ligand rearrangement reaction of Cr(CO)6 is studied in a series of alcohol solutions using ultrafast infrared spectroscopy and Brownian dynamics simulations. Excitation with 266 nm light gives Cr(CO)5 which is quickly solvated by a ligand from the bath. In alcohol solutions, solvation by an alkyl or hydroxyl site can occur; all alkyl bound complexes eventually rearrange to hydroxyl bound complexes. This rearrangement has been described using both an intermolecular (stochastic) and intramolecular (chainwalk) mechanism. Experiments alone do not allow for characterization of the mechanism, and therefore, theoretical calculations were carried out for the first time by modeling the ligand rearrangement as a diffusive walk along a potential defined by the different interaction possibilities. Experiments and simulations were carried out for Cr(CO)6 in 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 2-pentanol, 2-methylbutanol, and 3-methylbutanol. The trends in the theoretical and experimental rearrangement times are similar for all simulations carried out indicating that the two mechanisms have very similar ensemble behavior when bath effects are taken into account. The nature of the mechanism responsible for motion along the alcohol chain is not of primary importance in isolating the kinetic behavior because of the highly diffusive nature of the reaction. Future experimental and theoretical work will be directed at identifying a definitive assignment of the reaction mechanism.

Determination of band curvatures by angle-resolved two-photon photoemission in thin films of C60 on Ag(111).

The thickness-dependent interfacial band structure was determined for thin films of C(60) on Ag(111) by angle-resolved two-photon photoemission spectroscopy. Dispersions of molecular-orbital derived bands (HOMO, LUMO+1, and LUMO+2) were acquired, and limits were placed on their possible effective masses. A group theoretic approach is also incorporated to further understand the properties of these states. The HOMO, LUMO+1, and LUMO+2 bands possess (best-fit) effective masses of -7 m(e), -7 m(e), and -12 m(e), respectively. These values are consistent with theoretical calculations, averaged over the closely spaced subbands for each state, and provide practical limits on the effective fundamental charge-transport properties of C(60) films.

Study of Heat Transfer Dynamics from Gold Nanorods to the Environment via Time-Resolved Infrared Spectroscopy.

Studying the local solvent surrounding nanoparticles is important to understanding the energy exchange dynamics between the particles and their environment, and there is a need for spectroscopic methods that can dynamically probe the solvent region that is in nearby contact with the nanoparticles. In this work, we demonstrate the use of time-resolved infrared spectroscopy to track changes in a vibrational mode of local water on the time scale of hundreds of picoseconds, revealing the dynamics of heat transfer from gold nanorods to the local water environment. We applied this probe to a prototypical plasmonic photothermal system consisting of organic CTAB bilayer capped gold nanorods, as well as gold nanorods coated with varying thicknesses of inorganic mesoporous-silica. The heat transfer time constant of CTAB capped gold nanorods is about 350 ps and becomes faster with higher laser excitation power, eventually generating bubbles due to superheating in the local solvent. Silica coating of the nanorods slows down the heat transfer and suppresses the formation of superheated bubbles.

Ultrafast electron dynamics at metal interfaces: intraband relaxation of image state electrons as friction.

Two-photon photoemission of image potential states above monolayers of p-xylene/Ag(111) shows that electrons with different momenta have very different rise and decay rates as a function of parallel momentum. The dynamics are due to energy and momentum loss (intraband relaxation), which we model as a stochastic process isomorphic to the overdamped motion of a harmonic oscillator. The method extracts a friction coefficient from the data which can be explained by electron-electron scattering in a formalism based on the Lindhard dielectric function. One-electron excitations (interband transistions) dominate the dissipation mechanism, with a smaller contribution from collective electronic excitations (plasmons).

Electron Dynamics and IR Peak Coalescence in Bridged Mixed Valence Dimers Studied by Ultrafast 2D-IR Spectroscopy.

Dynamic IR peak coalescence and simulations based on the optical Bloch equations have been used previously to predict the rates of intramolecular electron transfer in a group of bridged mixed valence dimers of the type [Ru3(O)(OAc)6(CO)L]-BL-[Ru3(O) (OAc)6(CO)L]. However, limitations of the Bloch equations for the analysis of dynamical coalescence in vibrational spectra have been described. We have used ultrafast 2D-IR spectroscopy to investigate the vibrational dynamics of the CO spectator ligands of several dimers in the group. These experiments reveal that no electron site exchange occurs on the time scale required to explain the observed peak coalescence. The high variability in FTIR peak shapes for these mixed valence systems is suggested to be the result of fluctuations in the charge distributions at each metal cluster within a single-well potential energy surface, rather than the previous model of two-site exchange.

Metal/Phthalocyanine Hybrid Interface States on Ag(111).

A phthalocyanine/Ag(111) interface state is observed for the first time using time- and angle-resolved two-photon photoemission. For monolayer films of metal-free (H2Pc) and iron phthalocyanine (FePc) on Ag(111), the state exists 0.23 ± 0.03 and 0.31 ± 0.03 eV above the Fermi level, respectively. Angle-resolved spectra show the state to be highly dispersive with an effective mass of 0.50 ± 0.15 me for H2Pc and 0.67 ± 0.14 me for FePc. Density functional theory calculations on the H2Pc/Ag(111) surface allow us to characterize this state as being a hybrid state resulting from the interaction between the unoccupied molecular states of the phthalocyanine ligand and the Shockley surface state present on the bare Ag(111) surface. This work, when taken together with the extensive literature on the 3,4,9,10-perylene tetracarboxylic dianhydride/Ag interface state, provides compelling evidence that the hybridization of metal surface states with molecular electronic states is a general phenomenon.