Solvent exchange in preformed photocatalyst-donor precursor complexes determines efficiency

Laura M. Kiefer, Kevin J. Kubarych Chem. Sci. (2018) 9, 1527-1533

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In homogeneous photocatalytic reduction of CO2, it is widely assumed that the primary electron transfer from the sacrificial donor to the catalyst is diffusion controlled, thus little attention has been paid to optimizing this step. We present spectroscopic evidence that the precursor complex is preformed, driven by preferential solvation, and two-dimensional infrared spectroscopy reveals triethanolamine (donor)/tetrahydrofuran (solvent) exchange in the photocatalyst's solvation shell, reaching greatest magnitude at the known optimal concentration (∼20% v/v TEOA in THF) for catalytically reducing CO2 to CO. Transient infrared absorption shows the appearance of the singly reduced catalyst on an ultrafast (<70 data-preserve-html-node="true" ps) time scale, consistent with non-diffusion controlled electron transfer within the preformed precursor complex. Identification of preferential catalyst–cosolvent interactions suggests a revised paradigm for the primary electron transfer, while illuminating the pivotal importance of solvent exchange in determining the overall efficiency of the photocycle.

An “Iceberg” Coating Preserves Bulk Hydration Dynamics in Aqueous PEG Solutions

Kimberly R. Daley, Kevin J. Kubarych J. Phys. Chem. B (2017) 121, 10574-10582

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Ultrafast picosecond time scale two-dimensional infrared (2D-IR) spectroscopy of a new water-soluble transition metal complex acting as a vibrational probe shows that over a range of concentration and poly(ethylene glycol) (PEG) molecular mass (2000, 8000, and 20000 Da) the time scale of the sensed hydration dynamics differs negligibly from bulk water (D2O). PEG is well-known to establish a highly stable hydration shell because the spacing between adjacent ethereal oxygens nearly matches water’s hydrogen-bonding network. Although these first-shell water molecules are likely significantly retarded, they present an interface to subsequent hydration shells and thus diminish the largely entropic perturbation to water’s orientational dynamics. In addition to the longer PEGs, a series of concentration-dependent 2D-IR measurements using aqueous PEG-400 show a pronounced hydration slowdown in the vicinity of the critical overlap concentration (c*). Comparison between these dynamical results and previously reported steady-state infrared spectroscopy of aqueous PEG-1000 solutions reveals a strikingly identical dependence on number of water molecules per ethylene oxide monomer, scaled according to the critical overlap concentration.

Interfacial Hydration Dynamics in Cationic Micelles Using 2D-IR and NMR

Ved Prakash Roy, Kevin J. Kubarych J. Phys. Chem. B (2017) 121, 9621-9630

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Using the thiocyanate anion as a vibrational probe chromophore in conjunction with infrared and NMR spectroscopy, we find that SCN– strongly associates with the cationic head group of dodecyltrimethylammonium bromide (DTAB) micelles, both in normal-phase and reverse micelles. In competition with chloride and iodide ions, we find no evidence for displacement of thiocyanate, in accord with the chaotropicity of the Hofmeister ordering, while lending support to a direct interaction picture of its origin. Ultrafast 2D-IR spectroscopy of the SCN– probe in a range of DTAB micelle sizes (w0 = 4 to w0 = 12) shows little if any size dependence on the time scale for spectral diffusion, which is found to be ∼3.5 times slower than in bulk water (both D2O and H2O). Normal-phase micelles studied with 2D-IR exhibit essentially the same spectral dynamics as do reverse micelles, indicating a lack of sensitivity to interfacial curvature. Combined with 1H NMR chemical shift perturbations, we conclude that the SCN– ions tightly associate with the head groups and are partially buried. The 3–4-fold slowdown in spectral diffusion is consistent with the excluded volume model for interfacial perturbation to hydrogen bond reorientation dynamics. On the basis of these observations and comparisons to previous studies of zwitterionic interfaces probed with phosphate transitions, we conclude that the SCN– spectral dynamics in both reverse- and normal-phase micelles is largely dominated by hydration contributions, and offers a promising probe of interfacial hydration at cationic interfaces. Addition of competitive anions alters neither the IR spectra nor the ultrafast dynamics, indicating that SCN– is robustly associated with the head groups.

Oxidation-State-Dependent Vibrational Dynamics Probed with 2D-IR

Peter A. Eckert, Kevin J. Kubarych J. Phys. Chem. A (2017) 121, 2896-2902

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In an effort to examine the role of electronic structure and oxidation states in potentially modifying intramolecular vibrational dynamics and intermolecular solvation, we have used 2D-IR to study two distinct oxidation states of an organometallic complex. The complex, [1,1’-bis(diphenylphosphino)ferrocene]tetracarbonyl chromium (DPPFCr), consists of a catalytic diphenylphosphino ferrocene redox-active component as well as a Cr that can be switched from a Cr(0) to a Cr(I) oxidation state using a chemical oxidant in dichloromethane (DCM) solution. The DPPFCr(1) radical cation is sufficiently stable to investigate with 2D-IR spectroscopy, which provides dynamical information such as vibrational relaxation, intramolecular vibrational redistribution, as well as solvation dynamics manifested as spectral diffusion. Our measurements show that the primarily intramolecular dynamical processes—vibrational relaxation and redistribution—differ significantly between the two oxidation states, with faster relaxation in the oxidized DPPFCr(I) radical cation. The primarily intermolecular spectral diffusion dynamics, however, exhibit no oxidation state dependence. We speculate that the low nucleophilicity (i.e. donicity) of the DCM solvent, which is chosen to facilitate the chemical oxidation, masks any potential changes in solvation dynamics accompanying the substantial decrease in the 2.5 Debye molecular dipole moment of DPPFCr(I) relative to DPPFCr(0) (7.5 Debye).

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12

Nicholas A. Miller, Aniruddha Deb, Roberto Alonso-Mori, Brady D. Garabato, James M. Glownia∥, Laura M. Kiefer, Jake Koralek∥, Marcin Sikorski∥, Kenneth G. Spears, Theodore E. Wiley, Diling Zhu∥, Pawel M. Kozlowski, Kevin J. Kubarych, James E. Penner-Hahn, and Roseanne J. Sension, J. Am. Chem. Soc. (2017) 139, 1894-1899

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Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV–visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co–CN bond and Co–NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Dynamic Flexibility of Hydrogenase Active Site Models Studied with 2D-IR Spectroscopy

Peter A. Eckert, Kevin J. Kubarych J. Phys. Chem. A (2017) 121, 608-615

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Hydrogenase enzymes enable organisms to use H2 as an energy source, having evolved extremely efficient biological catalysts for the reversible oxidation of molecular hydrogen. Small-molecule mimics of these enzymes provide both simplified models of the catalysis reactions and potential artificial catalysts that might be used to facilitate a hydrogen economy. We have studied two diiron hydrogenase mimics, μ-pdt-[Fe(CO)3]2 and μ-edt-[Fe(CO)3]2 (pdt = propanedithiolate, edt = ethanedithiolate), in a series of alkane solvents and have observed significant ultrafast spectral dynamics using two-dimensional infrared (2D-IR) spectroscopy. Since solvent fluctuations in nonpolar alkanes do not lead to substantial electrostatic modulations in a solute’s vibrational mode frequencies, we attribute the spectral diffusion dynamics to intramolecular flexibility. The intramolecular origin is supported by the absence of any measurable solvent viscosity dependence, indicating that the frequency fluctuations are not coupled to the solvent motional dynamics. Quantum chemical calculations reveal a pronounced coupling between the low-frequency torsional rotation of the carbonyl ligands and the terminal CO stretching vibrations. The flexibility of the CO ligands has been proposed to play a central role in the catalytic reaction mechanism, and our results highlight that the CO ligands are highly flexible on a picosecond time scale.

NOESY-Like 2D-IR Spectroscopy Reveals Non-Gaussian Dynamics

Laura M. Kiefer, Kevin J. Kubarych J. Phys. Chem. Lett. (2016) 7, 3819-3824

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We have identified an unexpected signature of non-Gaussian dynamics in a conventional 2D IR measurement on a system with rapid intermolecular vibrational energy transfer. In a ternary mixture of the CO2 reduction photocatalyst, ReCl(bpy)(CO)3, NaSCN, and THF solvent, preferential association between the metal carbonyl catalyst and the NaSCN ion pairs facilitates intermolecular energy transfer on a few picoseconds time scale. Monitoring the cross peak between the highest frequency metal carbonyl band and the CN bands of NaSCN contact ion pairs, we find a striking time evolution of the cross-peak position on the detection axis. This frequency shift, which is due to spectral diffusion following intermolecular energy transfer, occurs with a time scale that is distinct from either the donor or acceptor spectral diffusion measured simultaneously. We argue that the energy transfer, a second-order Förster process, effectively increases the dimensionality of the 2D-IR spectroscopy and thus enables sensitivity to non-Gaussian dynamics.

Histidine Orientation Modulates the Structure and Dynamics of a de Novo Metalloenzyme Active Site

Matthew R. Ross, Aaron M. White, Fangting Yu, John T. King, Vincent L. Pecoraro, Kevin J. Kubarych, J. Am. Chem. Soc. 137 (2015) 10164-10176

The ultrafast dynamics of a de novo metalloenzyme active site is monitored using two-dimensional infrared spectroscopy. The homotrimer of parallel, coiled coil α-helices contains a His3-Cu(I) metal site where CO is bound and serves as a vibrational probe of the hydrophobic interior of the self-assembled complex. The ultrafast spectral dynamics of Cu-CO reveals unprecedented ultrafast (2 ps) nonequilibrium structural rearrangements launched by vibrational excitation of CO. This initial rapid phase is followed by much slower ∼40 ps vibrational relaxation typical of metal-CO vibrations in natural proteins. To identify the hidden coupled coordinate, small molecule analogues and the full peptide were studied by QM and QM/MM calculations, respectively. The calculations show that variation of the histidines’ dihedral angles in coordinating Cu controls the coupling between the CO stretch and the Cu–C–O bending coordinates. Analysis of different optimized structures with significantly different electrostatic field magnitudes at the CO ligand site indicates that the origin of the stretch–bend coupling is not directly due to through-space electrostatics. Instead, the large, ∼3.6 D dipole moments of the histidine side chains effectively transduce the electrostatic environment to the local metal coordination orientation. The sensitivity of the first coordination sphere to the protein electrostatics and its role in altering the potential energy surface of the bound ligands suggests that long-range electrostatics can be leveraged to fine-tune function through enzyme design.

Ultrafast 2D-IR and Simulation Investigations of Preferential Solvation and Cosolvent Exchange Dynamics

Josef A. Dunbar, Evan J. Arthur, Aaron M. White, Kevin J. Kubarych, J. Phys. Chem. B 119 (2015) 6271-6279.

Using a derivative of the vitamin biotin labeled with a transition-metal carbonyl vibrational probe in a series of aqueous N,N-dimethylformamide (DMF) solutions, we observe a striking slowdown in spectral diffusion dynamics with decreased DMF concentration. Equilibrium solvation dynamics, measured with the rapidly acquired spectral diffusion (RASD) technique, a variant of heterodyne-detected photon–echo peak shift experiments, range from 1 ps in neat DMF to ∼3 ps in 0.07 mole fraction DMF/water solution. Molecular dynamics simulations of the biotin–metal carbonyl solute in explicit aqueous DMF solutions show marked preferential solvation by DMF, which becomes more pronounced at lower DMF concentrations. The simulations and the experimental data are consistent with an interpretation where the slowdown in spectral diffusion is due to solvent exchange involving distinct cosolvent species. A simple two-component model reproduces the observed spectral dynamics as well as the DMF concentration dependence, enabling the extraction of the solvent exchange time scale of 8 ps. This time scale corresponds to the diffusive motion of a few Å, consistent with a solvent-exchange mechanism. Unlike most previous studies of solvation dynamics in binary mixtures of polar solvents, our work highlights the ability of vibrational probes to sense solvent exchange as a new, slow component in the spectral diffusion dynamics.

Dynamics of Rhenium Photocatalysts Revealed through Ultrafast Multidimensional Spectroscopy

Laura M. Kiefer, John T. King, Kevin J. Kubarych, Acc. Chem. Res. 48 (2015) 1123-1130

Rhenium catalysts have shown promise to promote carbon neutrality by reducing a prominent greenhouse gas, CO2, to CO and other starting materials. Much research has focused on identifying intermediates in the photocatalysis mechanism as well as time scales of relevant ultrafast processes. Recent studies have implemented multidimensional spectroscopies to characterize the catalyst’s ultrafast dynamics as it undergoes the many steps of its photocycle.

Two-dimensional infrared (2D-IR) spectroscopy is a powerful method to obtain molecular structure information while extracting time scales of dynamical processes with ultrafast resolution. Many observables result from 2D-IR experiments including vibrational lifetimes, intramolecular redistribution time scales, and, unique to 2D-IR, spectral diffusion, which is highly sensitive to solute–solvent interactions and motional dynamics.

Spectral diffusion, a measure of how long a vibrational mode takes to sample its frequency space due to multiple solvent configurations, has various contributing factors. Properties of the solvent, the solute’s structural flexibility, and electronic properties, as well as interactions between the solvent and solute, complicate identifying the origin of the spectral diffusion. With carefully chosen experiments, however, the source of the spectral diffusion can be unveiled.

Within the context of a considerable body of previous work, here we discuss the spectral diffusion of several rhenium catalysts at multiple stages in the catalysis. These studies were performed in multiple polar liquids to aid in discovering the contributions of the solvent. We also performed electronic ground state 2D-IR and electronic excited state transient-2D-IR experiments to observe how spectral diffusion changes upon electronic excitation. Our results indicate that with the original Lehn catalyst in THF, relative to the ground state, the spectral diffusion slows by a factor of 3 in the equilibrated triplet metal-to-ligand charge transfer state. We attribute this slowdown to a decrease in dielectric friction as well as an increase in molecular flexibility. It is possible to partially simulate the charge transfer by altering the electron density moderately by adding electron donating or withdrawing substituents symmetrically to the bipyridine ligand. We find that unlike the significant electronic structure change induced by MLCT, such small substituent effects do not influence the spectral diffusion. A solvent study in THF, DMSO, and CH3CN found there to be an explicit solvent dependence that we can correlate to the solvent donicity, which is a measure of its nucleophilicity. Future studies focused on the solvent effects on spectral diffusion in the crucial photoinitiated state can illuminate the role the solvent plays in the catalysis.

Biomolecular hydration dynamics probed with 2D-IR spectroscopy: From dilute solution to a macromolecular crowd

John T. King, Evan J. Arthur, Derek G. Osborne, Charles L. Brooks III, Kevin J. Kubarych, Chinese Chemical Letters 26 (2015) 435-438

Although it is well known that water is essential for biological function, it has been a challenge to determine how water behaves near biomacromolecular interfaces, and what role water plays in influencing the dynamics of the biochemical machinery. By adopting a vibrational labeling strategy coupled with ultrafast two-dimensional infrared (2D-IR) spectroscopy, it has recently become possible to study hydration dynamics, site specifically at the surface of proteins and model membranes. We review our recent progress in measuring hydration dynamics in contexts ranging from small-molecule solutes to biomacromolecules in dilute, viscous, and crowded environments.

Solvent-Dependent Dynamics of a Series of Rhenium Photo-Activated Catalysts Measured with Ultrafast 2D-IR

Laura M. Kiefer, Kevin J. Kubarych J. Phys. Chem. A 119 (2015) 959-965

The spectral dynamics of a series of rhenium photocatalysts, fac-Re(4,4′-R2-bpy)(CO)3Cl, where R = H, methyl, t-butyl, and carboxylic acid, as well as Re(1,10-phenanthroline)(CO)3Cl were observed in multiple aprotic solvents using two-dimensional infrared spectroscopy (2DIR). The carbonyl vibrational stretching frequencies showed slight variations due to the electron-donating or -withdrawing nature of the substituents on the bipyridine. The different substituents had minimal to no influence on the spectral diffusion time scales of the compounds within a particular solvent, but among the three different solvents investigated (DMSO, THF, and CH3CN), we find the spectral diffusion times to correlate with the solvent’s donor number (DN). Because the donicity is a measure the Lewis basicity of the solvent, these findings may help establish a more complete dynamical picture of the photocatalysis, where the first chemical step following optical excitation is electron transfer from a sacrificial donor to the rhenium complex.

Monitoring equilibrium reaction dynamics of a nearly barrierless molecular rotor using ultrafast vibrational echoes

Ian A. Nilsen, Derek G. Osborne, Aaron M. White, Jessica M. Anna, Kevin J. Kubarych J. Chem. Phys. 141 (2014) 134313

Using rapidly acquired spectral diffusion, a recently developed variation of heterodyne detected infrared photon echo spectroscopy, we observe ∼3 ps solvent independent spectral diffusion of benzene chromium tricarbonyl (C6H6Cr(CO)3, BCT) in a series of nonpolar linear alkane solvents. The spectral dynamics is attributed to low-barrier internal torsional motion. This tripod complex has two stable minima corresponding to staggered and eclipsed conformations, which differ in energy by roughly half of kBT. The solvent independence is due to the relative size of the rotor compared with the solvent molecules, which create a solvent cage in which torsional motion occurs largely free from solvent damping. Since the one-dimensional transition state is computed to be only 0.03 kBT above the higher energy eclipsed conformation, this model system offers an unusual, nearly barrierless reaction, which nevertheless is characterized by torsional coordinate dependent vibrational frequencies. Hence, by studying the spectral diffusion of the tripod carbonyls, it is possible to gain insight into the fundamental dynamics of internal rotational motion, and we find some evidence for the importance of non-diffusive ballistic motion even in the room-temperature liquid environment. Using several different approaches to describe equilibrium kinetics, as well as the influence of reactive dynamics on spectroscopic observables, we provide evidence that the low-barrier torsional motion of BCT provides an excellent test case for detailed studies of the links between chemical exchange and linear and non-linear vibrational spectroscopy.

Note: the first author, Ian Nilsen, carried out this research while he was an undergraduate student majoring in chemistry and in chemical engineering. This is quite an achievement!

Equilibrium Excited State Dynamics of a Photo-Activated Catalyst Measured with Ultrafast Transient 2DIR

Laura M. Kiefer, John T. King, Kevin J. Kubarych J. Phys. Chem. A 118 (2014) 9853-9860

A detailed understanding of photocatalyzed reaction dynamics requires a sensitive means of investigating the transient catalytically active species. Ideally, the method should be able to compare the electronically excited photocatalyst directly to the ground state species. We use equilibrium and transient two-dimensional infrared (2DIR and t-2DIR) spectroscopy to study the ground and excited state spectral dynamics of [Re(CO)3(bpy)Cl] in tetrahydrofuran (THF). We leverage the long-lived triplet excited state of the molecule to re-establish an equilibrated state relative to intersystem crossing dynamics and external solvent fluctuations, allowing access to the dynamics experienced by the excited state photocatalyst. The decay of frequency correlations within the excited triplet state species differs significantly from the ground state (slower by a factor of 3), indicating that the electronic excitation and subsequent metal-to-ligand charge transfer and associated structural changes are sufficient to perturb the spectral dynamics as sensed by the carbonyl ligands. In addition, we observe a 2-fold slowdown in ground state spectral dynamics around the in-phase symmetric vibrational mode compared to the two lower frequency, out-of-phase symmetric and asymmetric modes. Following electronic absorption and metal-to-ligand charge transfer the symmetry of the vibrational modes are disrupted, and all vibrational modes experience inhomogeneous broadening and spectral diffusion. The qualitative change in broadening mechanisms arises from the charge redistribution, indicating that direct comparisons of vibrational spectral dynamics on different electronic states—reported here for the first time—can be highly sensitive indicators of changes in electronic structure and in the concomitant solvation dynamics that underlie the microscopic details of charge transfer reactions.