We present the femtosecond time- and angle-resolved photoemission spectroscopy of a liquid beam of aqueous solution. A liquid laminar flow 25 micron in diameter is introduced into a high-vacuum photoelectron spectrometer and interrogated using the pump-probe method. The photoelectron kinetic energy distribution is measured by rotating the probe laser polarization with respecto to an electron detection axis with a small detecion solid angle. The observed time- and angle-resolved photoelectron kinetic energies exhibit electron binding energies and characters of transient electronic states of solutes near the liquid surface. The method provides novel information for understanding solution chemistry.

We have employed 2D electronic photon echo spectroscopy to study intramolecular charge-transfer dynamics in eosin B. After preparation of the first excited singlet state (S₁) with 40-fs excitation pulses at 520 nm, the nitro group (-NO₂) in eosin B undergoes excited state torsional motion towards a twisted intramolecular charge transfer (TICT) state. As the viscosity of the surrounding solvent increases, the charge-transfer rate decreases because the twisting of the -NO₂ group is hindered. These conclusions are supported by the time evolution of the 2D spectrum, which provides a direct measure of the the ground-to-excited-state energy gap time-correlation function, M(t). In comparison to the inertial and diffusive solvation time scales exhibited by eosin Y, which lacks the nitro group, the M(t) function for eosin B exhibits under the same conditions an additional component on the 150-fs timescale that arises from quenching of the S₁ state by crossing to the TICT state. These results indicate that 2D electronic spectroscopy can be used as a sensitive probe of the rate of charge transfer in a molecular system and of the coupling to the motions of the surrounding solvent. (Supported by grant DE-SC0010847 from the Department of Energy, Office of Basic Energy Sciences, Photosynthetic Systems program.)

The infrared spectra of aqueous solutions of NaOH and other strong bases exhibit a broad continuum absorption for frequencies between 800-3500 cm-1, which is attributed to the strong interactions of the hydroxide ion with its solvating water molecules. To provide molecular insight into the origin of the broad continuum absorption feature, we have performed ultrafast pump-probe and 2DIR experiments on aqueous NaOH by exciting the O—H stretch vibrations and probing the response from 1350-3800 cm-1 using a newly developed sub-70 fs broadband mid-infrared source. These experiments, in conjunction with harmonic vibrational analysis of OH–(H2O)n clusters, reveal that O—H stretch vibrations of aqueous hydroxides arise from coupled vibrations of multiple water molecules solvating the ion. We classify the vibrations of the hydroxide complex by symmetry defined by the relative phase of vibrations of the O—H bonds hydrogen bonded to the ion. Although spectral broadening does not allow us to distinguish 3- and 4-coordinate ion complexes, we find a resolvable splitting between asymmetric and symmetric stretch vibrations, and assign the 2850 cm-1 peak infrared spectra of aqueous hydroxides to asymmetric stretch vibrations.

The technique of two-dimensional infrared (2DIR) spectroscopy is used to expose the chemical dynamics of various concentrations of polymers and their monomers in heterogeneous mixtures. An environmentally relevant heterogeneous mixture, which inspires this study, is hydraulic fracturing liquid (HFL). Hydraulic fracking is a technique used to extract natural gas from shale deposits. HFL consists of mostly water, proppant (sand), an emulsifier (guar), and other chemicals specific to the drilling site. Utilizing a metal carbonyl as a probe, we observe the spectral dynamics of the polymer, guar, and its monomer, mannose, and compare the results to see how hydration dynamics change with varying concentration. Another polymer, Ficoll, and its monomer, sucrose, are also compared to see how polymer size affects hydration dynamics. The two results are as follows: (1) Guar experiences collective hydration at high concentrations, where as mannose experiences independent hydration; (2) no collective hydration is observed for Ficoll in the same concentration range as guar, possibly due to polymer shape and size. HFL experiences extremely high pressure during natural gas removal, so future studies will focus on how increased pressure affects the hydration dynamics of polymers and monomers.

Silica sol-gel syntheses are among the most widely used and studied synthetic methods in the materials field. However there lacks a fundamental understanding of how these materials gel and what is happening dynamically inside of the pores of these materials. In this study, we adapt a typical silica sol gel synthesis so as to introduce an intrinsic probe (a SiH stretch) throughout the material. Using these probes and two-dimensional infrared spectroscopy (2DIR) we have monitored the molecular motions (solution dynamics) of the solution as it approaches gel formation. After gel formation we can then study the solution dynamics inside the pores of these materials.

Figure Single molecule experiments based on Förster resonance energy transfer (FRET) are now capable of detecting energy funneling in branched molecules. Here we present the synthesis, as well as the optical characterization of a dendron coupled to two donor dyes (Cy3) and one acceptor dye (Cy5). Characterization of the dendron by ensemble absorption and emission spectroscopy shows that the molecule is capable of light harvesting; yielding a FRET signal from the acceptor that is greater than expected for a single donor. Additionally, we investigated an energy transfer cascade upon UV excitation of the conjugated backbone, resulting in several competing energy transfer pathways with the same total energy transfer as direct FRET. The first pathway is FRET from the backbone to Cy3 and resulting FRET to Cy5, with the competing pathway that allows direct energy transfer to Cy5 from the backbone via superexchange. Structural simulations in solution, as well as direct imaging by scanning tunneling microscopy show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for imaging single molecule studies of light harvesting.

Figure The realization of common materials transformations in nanocrystalline systems is fostering the development of novel nanostructures and allowing a deep look into the atomistic mechanisms involved. Galvanic corrosion is one such transformation. We studied galvanic replacement within individual metal nanoparticles by using plasmonic spectroscopy. This proved to be a powerful approach to studying materials transformations in the absence of ensemble averaging. Individual nanoscale units act as domains that can be interrogated optically in isolation, whereas the averaging of all such domains provides a bulk reaction trajectory. Single-nanoparticle reaction trajectories showed that a Ag nanoparticle exposed to Au³⁺ makes an abrupt transition into a nanocage structure. The transition is limited by a critical structural event, which we identified by electron microscopy to comprise the formation of a nanosized void, similar to the pitting process commonly observed in the corrosion of metals. Trajectories also revealed a surprisingly strong nonlinearity of the reaction kinetics, which we explain by a model involving the critical coalescence of vacancies into a growing void. The critical void size for galvanic exchange to spontaneously proceed was found to be 20 atomic vacancies. In the future we hope to extend this approach to examine a wide variety of materials transformations and chemical reactions.

Re(bpy)(CO)₃Cl is a well studied CO₂ reduction catalyst, known for its ability as both a photosensitizer and a catalyst with a high quantum yield and product selectivity. The catalysis reaction is initiated by a 400 nm excitation, followed by an intersystem crossing (ISC) and re-equilibration in the lowest triplet state. We utilize the quasi-equilibrium nature of this long-lived triplet metal-to-ligand charge-transfer (³MLCT) state to completely characterize the solvent dynamics using the technique of transient two-dimensional infrared (t-2DIR) spectroscopy to extract observables such as the frequency-frequency correlation function (FFCF), an equilibrium function. The electronic ground state solvent dynamics are characterized using equilibrium two-dimensional infrared spectroscopy (2D IR). Our technique allows us to independently observe the solvent dynamics of different electronic states and compare them. In this study, three carbonyl stretching modes were utilized to probe both the intramolecular and solvent environments in each electronic state. In the electronic ground state, the totally symmetric mode exhibits pure homogeneous broadening and a lack of spectral dynamics, while the two other modes have similar FFCF decay times of  ∼  1.5 ps. In the ³MLCT, however, all three modes experience similar spectral dynamics and have a FFCF decay time of  ∼  4.5 ps, three times slower than in the electronic ground state. Our technique allows us to directly observe the differences in spectral dynamics of the ground and excited electronic states and allows us to attribute the differences to specific origins such as solvent-solute coupling and molecular flexibility.

Femtosecond transient-grating spectroscopy with optical heterodyne detection was employed to observe separately the time evolution of the absorption and dispersion components of the third-order nonlinear optical signal following resonant excitation of the S₂ (¹B_u⁺) states of β-carotene in benzonitrile and peridinin in methanol using 40-fs pulses centered at 520 nm. The absorption and dispersion components exhibit distinctively different time profiles owing to the population of intermediate states. An initial intermediate state is populated on an ultrashort ( < 30 fs) time scale in both carotenoids. Owing to the fast red-shifting of the stimulated emission part of the S₂-state transient grating signal, we suggest that the intermediate state arises from vibrational displacements on the S₂-state potential surface that eventually yield twisted or bent conformations. Motions of the molecule of this type in the S₂-state would contribute to a mixing of the diabatic S₂ and S₁ electronic states and would promote the formation of intramolecular charge-transfer character. Both of these effects would enhance the efficiency of energy transfer from the S₁ state to the (B)Chl Q_y state in photosynthetic light-harvesting proteins. The time-resolved transient-grating spectra obtained for peridinin in the peridinin–chlorophyll a protein from Amphidinium carterae suggest a more rapid formation of the intermediate than for peridinin in methanol. This finding suggests that the conformation of the peridinin chromophore is controlled in the binding site to optimize the formation of the twisted intermediate upon excitation of the S₂ state. (Supported by grant DE-SC0010847 from the Department of Energy, Office of Basic Energy Sciences, Photosynthetic Systems program.)

Substitution of oxygen by a sulfur atom in the natural DNA and RNA bases gives rise to a family of derivatives commonly known as the thiobases. Upon excitation with UV radiation, the natural bases are able to quickly and efficiently dissipate the imparted energy as heat to their surroundings. Thiobases, on the other hand, relax into a long-lived triplet excited state in quantum yields that approach unity. This finding has both fundamental and biological relevance because the triplet state plays a foremost role in the photochemistry of the thiobases, this is especially important in the current medicinal applications of thiobase derivatives. Using femtosecond transient absorption spectroscopy, we are able uncover the ultrafast dynamics leading to the population of this reactive triplet state. In particular, I will present our results on how the site of sulfur substitution and the degree of substitution impact these dynamics and I will compare these experimental results to some recent computational work. Pinning down the excited-state dynamics of the thiobases is important to furthering the understanding of dynamics in natural DNA/RNA bases, as well as to the discovery of thiobase derivatives with desirable therapeutic properties. The authors acknowledge the CAREER program of the National Science Foundation (Grant No. CHE-1255084) for financial support.

Conformational heterogeneity and dynamics impact protein function, but their investigation is limited by the availability of methods for characterizing rapidly fluctuating protein states with both high spatial and temporal resolution. Multidimensional infrared spectroscopy is emerging as a powerful technique that directly probes the structural dynamics on fast timescales. To overcome the spectral congestion inherent to protein spectra that restricts application of infrared spectroscopy to protein systems, we incorporate into proteins vibrational probes with spectrally isolated frequencies and local-mode character that make possible rigorous analysis of protein environments and dynamics with infrared spectroscopy. In particular, both heme-bound carbon monoxide and nitriles selectively incorporated as unnatural amino acids are used as probes of cytochrome P450. 2D IR spectroscopy is used to investigate the impact of protein dynamics and conformational heterogeneity on the selectivity of its catalytic activity. Comparative studies of mutants of cytochrome P450 are used to unravel the contribution of specific residues to the protein's dynamics.

Second harmonic generation (SHG) microscopy was integrated with synchrotron X-ray diffraction (XRD) to test the Ostwald Rule of Stages, in which is hypothesized that crystals dynamically transition through metastable polymorphs before settling on the most thermodynamically favored form. The presence or absence of metastable forms has historically been challenging to probe due to the stochastic randomness of crystal nucleation coupled with the relatively short time-frame over which the metastable forms may survive. In this work, inkjet printing of a racemic amino acid solutions results in rapid solvent evaporation, placing crystallization under kinetic rather than thermodynamic control. SHG microscopy is used to rapidly and selectively identify the positions of metastable crystal forms. Coupling this measurement with synchrotron XRD allows diffraction analysis to be performed on individual inkjet printed dots of only a few pg of total material, prepared from single  1 pL droplets. In studies of amino acids, we have shown that the homochiral crystals emerge when printed, while those same solutions exclusively generate the racemic co-crystals upon slow solvent evaporation.

Intracellular nicotinamide adenine dinucleotide (NADH) is a key cofactor in energy metabolism pathways and a myriad of oxidation-reduction reactions in living cells. The crowded milieu of these cells with organelles and macromolecules influences many biological processes such as biomolecular diffusion, protein-protein and protein-substrate interactions, and protein folding. In this contribution, I will highlight our recent findings on the role of macromolecular crowding on biochemical reaction between NADH and selected dehydrogenases in both living cells and in controlled macromolecules-rich environment. In addition, multiscale diffusion (rotational and translational) of a small fluorophore will be used to understand the role of non-specific binding, heterogeneity in microenvironmental viscosity in crowded solutions. Our experimental approach is a combination of fluorescence lifetime imaging microscopy, time-resolved anisotropy and fluorescence correlation spectroscopy. The broader impacts of these results will be discussed within the context of energy metabolism and biophysics in the crowded milieu of living cells.