Gregory M. Greetham

Ultra: A Unique Instrument for Time-Resolved Spectroscopy

We report the development of a high-sensitivity time-resolved infrared and Raman spectrometer with exceptional experimental flexibility based on a 10-kHz synchronized dual-arm femtosecond and picosecond laser system. Ultrafast high-average-power titanium sapphire lasers and optical parametric amplifiers provide wavelength tuning from the ultraviolet (UV) to the mid-infrared region. Customized silicon, indium gallium arsenide, and mercury cadmium telluride linear array detectors are provided to monitor the probe laser intensity in the UV to mid-infrared wavelength range capable of measuring changes in sample absorbance of ΔOD ∼ 10−5 in 1 second. The system performance is demonstrated for the time-resolved infrared, two-dimensional (2D) infrared, and femtosecond stimulated Raman spectroscopy techniques with organometallic intermediates, organic excited states, and the dynamics of the tertiary structure of DNA.

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Toward control of electron transfer in donor-acceptor molecules by bond-specific infrared excitation

Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation. Vibrational excitation can modulate electron transfer probabilities in real time. Big impact from a well-placed shake Since the advent of ultrashort laser pulses, chemists have sought to steer reaction trajectories in real time by setting particular molecular vibrations in motion. Using this approach, Delor et al. have demonstrated a markedly clear-cut influence on electron transfer probabilities along the axis of a platinum complex. The complex comprised donor and acceptor fragments—which respectively give and take electrons upon ultraviolet excitation—bridged together by triply bonded carbon chains linked to the metal center. By selectively stimulating the carbon triple-bond stretch vibration with an infrared pulse, the authors could induce substantial changes in the observed electron transfer pathways between the fragments. Science, this issue p. 1492

Vibrationally quantum-state-specific reaction dynamics of H atom abstraction by CN radical in solution

Molecular vibrations in a solution-phase reaction are detected at a level of detail rivaling that of gas-phase studies. Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.

Comparing molecular photofragmentation dynamics in the gas and liquid phases

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.

On the mechanism of vibrational control of light-induced charge transfer in donor–bridge–acceptor assemblies

Nuclear-electronic (vibronic) coupling is increasingly recognized as a mechanism of major importance in controlling the light-induced function of molecular systems. It was recently shown that infrared light excitation of intramolecular vibrations can radically change the efficiency of electron transfer, a fundamental chemical process. We now extend and generalize the understanding of this phenomenon by probing and perturbing vibronic coupling in several molecules in solution. In the experiments an ultrafast electronic-vibrational pulse sequence is applied to a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, for which infrared excitation of selected bridge vibrations during ultraviolet-initiated charge separation alters the yields of light-induced product states. The experiments, augmented by quantum chemical calculations, reveal a complex combination of vibronic mechanisms responsible for the observed changes in electron transfer rates and pathways. The study raises new fundamental questions about the function of vibrational processes immediately following charge transfer photoexcitation, and highlights the molecular features necessary for external vibronic control of excited-state processes.

Photoexcitation of the blue light using FAD photoreceptor AppA results in ultrafast changes to the protein matrix.

Photoexcitation of the flavin chromophore in the BLUF photosensor AppA results in a conformational change that leads to photosensor activation. This conformational change is mediated by a hydrogen-bonding network that surrounds the flavin, and photoexcitation is known to result in changes in the network that include a strengthening of hydrogen bonding to the flavin C4═O carbonyl group. Q63 is a key residue in the hydrogen-bonding network, and replacement of this residue with a glutamate results in a photoinactive mutant. While the ultrafast time-resolved infrared (TRIR) spectrum of Q63E AppA(BLUF) is characterized by flavin carbonyl modes at 1680 and 1650 cm(-1), which are similar in frequency to the analogous modes from the light activated state of the wild-type protein, a band is also observed in the TRIR spectrum at 1724 cm(-1) that is unambiguously assigned to the Q63E carboxylic acid based on U-(13)C labeling of the protein. Light absorption instantaneously (<100 fs) bleaches the 1724 cm(-1) band leading to a transient absorption at 1707 cm(-1). Because Q63E is not part of the isoalloxazine electronic transition, the shift in frequency must arise from a sub picosecond perturbation to the flavin binding pocket. The light-induced change in the frequency of the Q63E side chain is assigned to an increase in hydrogen-bond strength of 3 kcal mol(-1) caused by electronic reorganization of the isoalloxazine ring in the excited state, providing direct evidence that the protein matrix of AppA responds instantaneously to changes in the electronic structure of the chromophore and supporting a model for photoactivation of the wild-type protein that involves initial tautomerization of the Q63 side chain.

Determination of the photolysis products of [FeFe]hydrogenase enzyme model systems using ultrafast multidimensional infrared spectroscopy.

Ultrafast transient 2D-IR (T-2D-IR) spectroscopy has been used to study the photolysis products of the [FeFe]hydrogenase enzyme model compound (μ-propanedithiolate)Fe(2)(CO)(6) in heptane solution following irradiation at ultraviolet wavelengths. Observation of coupling patterns between the vibrational modes of the photoproduct species formed alongside examination of the appearance time scales of these signals has uniquely enabled assignment of the photoproduct spectrum to a single pentacarbonyl species. Comparison of the vibrational relaxation rate of the photoproduct with that of the parent is consistent with the formation of a solvent adduct at the vacant coordination site, while anisotropy data in conjunction with density functional theory simulations indicates substitution in an axial rather than equatorial position. No firm evidence of additional short-lived intermediates is seen, indicating that the subsequent chemistry of these species is likely to be strongly defined by the nature of the first solvation shell.

Time-resolved multiple probe spectroscopy

Time-resolved multiple probe spectroscopy combines optical, electronic, and data acquisition capabilities to enable measurement of picosecond to millisecond time-resolved spectra within a single experiment, using a single activation pulse. This technology enables a wide range of dynamic processes to be studied on a single laser and sample system. The technique includes a 1 kHz pump, 10 kHz probe flash photolysis-like mode of acquisition (pump-probe-probe-probe, etc.), increasing the amount of information from each experiment. We demonstrate the capability of the instrument by measuring the photolysis of tungsten hexacarbonyl (W(CO)(6)) monitored by IR absorption spectroscopy, following picosecond vibrational cooling of product formation through to slower bimolecular diffusion reactions on the microsecond time scale.

Vibrational relaxation and microsolvation of DF after F-atom reactions in polar solvents

Deuterium fluoride gets born shivering Modern spectroscopic techniques can analyze collisions between gas phase molecules in exquisite detail, highlighting exactly which vibrations and rotations come into play. However, much chemistry of interest takes place in solution, where its harder to tease out what happens. Dunning et al. applied infrared spectroscopy to study solution-phase formation of deuterium fluoride (DF) from F atoms, a longstanding test bed of gas phase dynamics. The DF product vibrated for a surprisingly long time before dissipating its energy to the surrounding solvent molecules. Science, this issue p. 530 Infrared spectroscopy reveals the dynamics of vibrational energy flow from product to solvent in a bimolecular reaction. Solvent-solute interactions influence the mechanisms of chemical reactions in solution, but the response of the solvent is often slower than the reactive event. Here, we report that exothermic reactions of fluorine (F) atoms in d3-acetonitrile and d2-dichloromethane involve efficient energy flow to vibrational motion of the deuterium fluoride (DF) product that competes with dissipation of the energy to the solvent bath, despite strong solvent coupling. Transient infrared absorption spectroscopy and molecular dynamics simulations show that after DF forms its first hydrogen bond on a subpicosecond time scale, DF vibrational relaxation and further solvent restructuring occur over more than 10 picoseconds. Characteristic dynamics of gas-phase F-atom reactions with hydrogen-containing molecules persist in polar organic solvents, and the spectral evolution of the DF products serves as a probe of solvent reorganization induced by a chemical reaction.