Henrik T. Lemke

Tracking excited-state charge and spin dynamics in iron coordination complexes

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers

Adv. Mater. 2010, 22, 2371–2375 2010 WILEY-VCH Verlag G Field-effect transistors (FETs) based on conjugated polymers and small molecules have been of extensive fundamental and practical interest for more than two decades. In terms of fundamental charge transport properties organic semiconductors have been recently shown to be intrinsically ambipolar, i.e., able to accumulate and transport both holes and electrons within the same material under suitable biasing conditions and device configurations. The discovery of the intrinsic ambipolar charge transport properties in common semiconducting polymers was made possible by the understanding of the crucial role played by electronegative trapping groups in the dielectric, such as hydroxyl groups on the surface of a SiO2 gate dielectric. [10] Ambipolar charge transport is not only of fundamental, but also of practical interest as it enables the realization of novel device architectures such as complementary-like voltage inverters with a single organic semiconductor as well as ambipolar light-emitting field-effect transistors (LFETs). Here we report the general observation of ambipolar charge transport characteristics in a series of regioregular polyselenophene-based polymers. Compared to the well-studied polythiophenes, which appear among the most promising solution processable organic semiconductors, polyselenophenes were recently developed as analogue systems providing several advantages over their predecessors. The highest occupied molecular orbital (HOMO) of polythiophenes has little contribution from the sulfur heteroatom, whereas the lowest unoccupied molecular orbital (LUMO) has significant electron density on the heteroatom. Polyselenophenes were initially developed as promising alternatives to polythiophenes for solar cell applications, mainly because of their reduced optical band gaps and their enhanced photostability due to the lower lying LUMO. For FET applications we expect the hole transport to be similar to that of polythiophenes, while the lower lying LUMO in polyselenophenes should result in improved electron transport due to enhanced electron injection from metal electrodes and lower susceptibility of electrons to trap states and oxidation. The regioregular polyselenophenes investigated in this work were: (1) poly(3,300-di-n-alkylterselenophene) (PSSS) of three different alkyl side-chains, namely PSSS-C10, PSSS-C8, and PSSS-C6; and (2) poly(3-octyl)selenophene (P3OS) (Fig. 1). We employed identical top-gate, bottom contact (TGBC) configurations with gold source-drain electrodes for all polymers. For ambipolar FETs the TGBC device configuration offers several advantages over a bottom-gate/bottom-contact (BGBC) configuration: (i) the freedom to select different gate dielectrics to minimize irreversible charge trapping at the semiconductordielectric interface and to act as encapsulation for the FET channel, and (ii) a lower contact resistance due to reduction of current-crowding effects. PSSS is the selenium analogue of the previously reported poly(3,300-dialkylterthiophene) (PTT) with a ‘‘spaced-out’’ distribution of the alkyl side-chains along the polymer backbone. 28] PTT was reported to readily self-assemble into a threedimensional lamellar p-stacking arrangement with an ‘‘edge-on’’

Direct observation of ultrafast collective motions in CO myoglobin upon ligand dissociation

Observing ultrafast myoglobin dynamics The oxygen-storage protein myoglobin was the first to have its three-dimensional structure determined and remains a workhorse for understanding how protein structure relates to function. Barends et al. used x-ray free-electron lasers with femtosecond short pulses to directly observe motions that occur within half a picosecond of CO dissociation (see the Perspective by Neutze). Combining the experiments with simulations shows that ultrafast motions of the heme couple to subpicosecond protein motions, which in turn couple to large-scale motions. Science, this issue p. 445, see also p. 381 Time-resolved crystallography at an x-ray laser reveals ultrafast structural changes in myoglobin upon ligand dissociation. [Also see Perspective by Neutze] The hemoprotein myoglobin is a model system for the study of protein dynamics. We used time-resolved serial femtosecond crystallography at an x-ray free-electron laser to resolve the ultrafast structural changes in the carbonmonoxy myoglobin complex upon photolysis of the Fe-CO bond. Structural changes appear throughout the protein within 500 femtoseconds, with the C, F, and H helices moving away from the heme cofactor and the E and A helices moving toward it. These collective movements are predicted by hybrid quantum mechanics/molecular mechanics simulations. Together with the observed oscillations of residues contacting the heme, our calculations support the prediction that an immediate collective response of the protein occurs upon ligand dissociation, as a result of heme vibrational modes coupling to global modes of the protein.

Ultrafast three-dimensional imaging of lattice dynamics in individual gold nanocrystals.

Distorted Nanoparticle Nanoparticles have found many applications in modern technology; however, the full characterization of individual particles is challenging. One of the most interesting mechanical properties is the particles response to lattice distortion. This property has been probed for ensembles of nanoparticles, but the required averaging may distort the results. Clark et al. (p. 56, published online 23 May; see the Perspective by Hartland and Lo) were able to image the generation and subsequent evolution of coherent acoustic phonons from an individual perturbed gold nanocrystal on the picosecond time scale. An x-ray free-electron laser is used to probe the elastic modes of a gold nanocrystal. [Also see Perspective by Hartland and Lo] Key insights into the behavior of materials can be gained by observing their structure as they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a “pump-probe” experiment with the ensuing transients being probed stroboscopically with femtosecond pulses of visible light, x-rays, or electrons. Here we report three-dimensional imaging of the generation and subsequent evolution of coherent acoustic phonons on the picosecond time scale within a single gold nanocrystal by means of an x-ray free-electron laser, providing insights into the physics of this phenomenon. Our results allow comparison and confirmation of predictive models based on continuum elasticity theory and molecular dynamics simulations.

Architecture of the synaptotagmin-SNARE machinery for neuronal exocytosis.

Synaptotagmin-1 and neuronal SNARE proteins have central roles in evoked synchronous neurotransmitter release; however, it is unknown how they cooperate to trigger synaptic vesicle fusion. Here we report atomic-resolution crystal structures of Ca2+- and Mg2+-bound complexes between synaptotagmin-1 and the neuronal SNARE complex, one of which was determined with diffraction data from an X-ray free-electron laser, leading to an atomic-resolution structure with accurate rotamer assignments for many side chains. The structures reveal several interfaces, including a large, specific, Ca2+-independent and conserved interface. Tests of this interface by mutagenesis suggest that it is essential for Ca2+-triggered neurotransmitter release in mouse hippocampal neuronal synapses and for Ca2+-triggered vesicle fusion in a reconstituted system. We propose that this interface forms before Ca2+ triggering, moves en bloc as Ca2+ influx promotes the interactions between synaptotagmin-1 and the plasma membrane, and consequently remodels the membrane to promote fusion, possibly in conjunction with other interfaces.

X-ray and optical wave mixing

Light–matter interactions are ubiquitous, and underpin a wide range of basic research fields and applied technologies. Although optical interactions have been intensively studied, their microscopic details are often poorly understood and have so far not been directly measurable. X-ray and optical wave mixing was proposed nearly half a century ago as an atomic-scale probe of optical interactions but has not yet been observed owing to a lack of sufficiently intense X-ray sources. Here we use an X-ray laser to demonstrate X-ray and optical sum-frequency generation. The underlying nonlinearity is a reciprocal-space probe of the optically induced charges and associated microscopic fields that arise in an illuminated material. To within the experimental errors, the measured efficiency is consistent with first-principles calculations of microscopic optical polarization in diamond. The ability to probe optical interactions on the atomic scale offers new opportunities in both basic and applied areas of science.

Guest–host interactions investigated by time-resolved X-ray spectroscopies and scattering at MHz rates: Solvation dynamics and photoinduced spin transition in aqueous Fe(bipy)3 2+

We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.

Spectral encoding of x-ray/optical relative delay.

We present a new technique for measuring the relative delay between a soft x-ray FEL pulse and an optical laser that indicates a sub 25 fs RMS measurement error. An ultra-short x-ray pulse photo-ionizes a semiconductor (Si(3)N(4)) membrane and changes the optical transmission. An optical continuum pulse with a temporally chirped bandwidth spanning 630 nm-710 nm interacts with the membrane such that the timing of the x-ray pulse can be determined from the onset of the spectral modulation of the transmitted optical pulse. This experiment demonstrates a nearly in situ single-shot measurement of the x-ray pulse arrival time relative to the ultra-short optical pulse.

Goniometer-based femtosecond crystallography with X-ray free electron lasers

Significance The extremely short and bright X-ray pulses produced by X-ray free-electron lasers unlock new opportunities in crystallography-based structural biology research. Efficient methods to deliver crystalline material are necessary due to damage or destruction of the crystal by the X-ray pulse. Crystals for the first experiments were 5 µm or smaller in size, delivered by a liquid injector. We describe a highly automated goniometer-based approach, compatible with crystals of larger and varied sizes, and accessible at cryogenic or ambient temperatures. These methods, coupled with improvements in data-processing algorithms, have resulted in high-resolution structures, unadulterated by the effects of radiation exposure, from only 100 to 1,000 diffraction images. The emerging method of femtosecond crystallography (FX) may extend the diffraction resolution accessible from small radiation-sensitive crystals and provides a means to determine catalytically accurate structures of acutely radiation-sensitive metalloenzymes. Automated goniometer-based instrumentation developed for use at the Linac Coherent Light Source enabled efficient and flexible FX experiments to be performed on a variety of sample types. In the case of rod-shaped Cpl hydrogenase crystals, only five crystals and about 30 min of beam time were used to obtain the 125 still diffraction patterns used to produce a 1.6-Å resolution electron density map. For smaller crystals, high-density grids were used to increase sample throughput; 930 myoglobin crystals mounted at random orientation inside 32 grids were exposed, demonstrating the utility of this approach. Screening results from cryocooled crystals of β2-adrenoreceptor and an RNA polymerase II complex indicate the potential to extend the diffraction resolution obtainable from very radiation-sensitive samples beyond that possible with undulator-based synchrotron sources.

Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser.

Light absorption can trigger biologically relevant protein conformational changes. The light-induced structural rearrangement at the level of a photoexcited chromophore is known to occur in the femtosecond timescale and is expected to propagate through the protein as a quake-like intramolecular motion. Here we report direct experimental evidence of such ‘proteinquake’ observed in myoglobin through femtosecond X-ray solution scattering measurements performed at the Linac Coherent Light Source X-ray free-electron laser. An ultrafast increase of myoglobin radius of gyration occurs within 1 picosecond and is followed by a delayed protein expansion. As the system approaches equilibrium it undergoes damped oscillations with a ~3.6-picosecond time period. Our results unambiguously show how initially localized chemical changes can propagate at the level of the global protein conformation in the picosecond timescale.