On the assessment of time-resolved diffraction results

Agreement factors based on the ratio of intensities R collected in dynamic structure pump–probe crystallography experiments are shown to be analogous to the factors widely used in standard crystallographic refinements. Fourier photodifference maps allow the visualization of the externally induced structural changes in the crystal, but also can be used during refinement to monitor its progress. Photodeformation maps are modified to separate the photo-induced structural change from the effect of the temperature increase on laser exposure.

Analysis of multicrystal pump-probe data sets. I. Expressions for the RATIO model

The application of the RATIO method in processing/analysis prior to structure refinement requires an appropriate ratio model for modeling the light response. Such a model is discussed, taking into account both geometric and thermal light-induced changes.

Fleeting Excited States by Synchrotron and In-House Pump-Probe Diffraction

While the geometry of the initial state of chemical reactions is now being studied by electron diffraction methods [1] with further efforts underway at XFEL's, the properties of longer lived molecular excited states remain of crucial importance for the design of photoluminescent devices [2] and as precursors in photochemical reactions and electron transfer into semiconductor nanoparticles. Triplet lifetimes in crystals are temperature dependent, but typical in the ns-ms range. If high luminescence is of importance, the lower part of this range is preferable, for other applications longer lifetimes may be desirable. In the former case time-resolved (TR) synchrotron experimentation, preferable with Laue techniques, is the method of choice, whereas for lifetimes above about 10 µs monochromatic in-house TR methods are increasingly feasible due to the continuing increase in the brightness of commercial X-ray sources and in the efficiency of detectors. The in-house method implemented in our laboratory uses a fast mechanical shutter synchronized with up to 100 kHz high repetition-rate lasers, corresponding to a pulse separation of 10 µs, as discussed in detail in a presentation by Kaminski. A number of synchrotron [3] and in-house experiments on mono- bi- and tetra-nuclear organometallic complexes have been completed and are compared. New methods for inter-sample scaling of multi-sample data sets and for visualization of the geometrical changes increase the sensitivity of the TR methods and allow examination of geometry changes on excitation at conversion percentages of only a few %. Further details will be presented in separate presentations. Research funded by the National Science Foundation (CHE1213223). BioCARS Sector 14 at APS is supported by NIH (RR007707). The Advanced Photon Source is funded by the Office of Basic Energy Sciences, U.S. Department of Energy, (W-31-109-ENG-38).

On the assessment of time-resolved diffraction results

Dynamic-structure crystallography, and in particular photocrystallography, is a field of great interest because of its numerous applications. The field covers the studies of reactions in crystals including non-reversible reactions, phase transitions and the structure change on excitation leading to short-lifetime species. Data collected during dynamic-structure crystallography experiments require appropriate tools to perform data analysis. The relative scaling of different sets of data recurs in time-resolved pump-probe data collection in which crystals often disintegrate before a full set of data can be collected. We discuss relative scaling and its use with the Ratio method [1], in which ratios of observed intensities collected with and without light-exposure are used as observables. The scaling method has been developed specifically for pump-probe Laue data, but is not limited to polychromatic data sets. The visualization of the externally induced structure changes in the crystal environment is of importance to check the presence of a structural response (see Fig. a) and hence to define a starting point for the structure refinement. For this purpose, ratio-based Fourier difference maps have been defined. After the structure refinement using the software LASER [2], the quality of the light-exposed structure model can be checked in direct space by visualizing the ratio-based Fourier residual maps, and, in a complementary manner, in reciprocal space by performing statistical analysis on the least-squares (LS) residual vector components with respect to the corresponding H-vectors (see Fig. b). The latter allows detection of any outliers or unexpected tendencies such as dependence of the residual vector components on the resolution or the angular direction in the reciprocal space. Application to the complex [Cu(I)-(1,10-phenanthroline-N,N')bis(triphenylphosphine)][BF4] will be presented [3]. Research supported by the National Science Foundation (CHE1213223).

Direct observation of the excited state structure of a Ag(I)-Cu(I) complex

Heterodentate coordination complexes have been extensively studied because of their rich electronic and luminescent properties, which are of importance in the design of molecular devices. The short metal-metal contacts found in such complexes determine the nature of the lowest lying emissive states, and must be explored in order to understand their physical properties. Recent advances in time-resolved (TR) synchrotron techniques supported by specific data collection strategies and data processing procedures [1] allow for elucidation of molecular excited state geometries in the solid state. The approach has been so far successfully applied to several high-quality Laue-data sets collected at the 14-ID BioCars beamline at the Advanced Photon Source.[2] In this contribution we present synchrotron TR experiment results obtained for a new solvent-free crystal form of a model complex containing Ag(I) and Cu(I) (Ag2Cu2L4, L = 2-diphenylphosphino-3-methylindole).[3] This system exhibits red solid-state luminescence with a lifetime of about 1 µs. This is one of the shortest-lived excited states we have studied so far with the Laue technique. The relatively short lifetime goes along with significant structural changes observed upon irradiation, such as, the Ag...Ag distance shortening of about 0.2 Å in the excited state. The results clearly show strengthening of the Ag...Ag interactions suggesting a bond formation upon excitation. The photocrystallographic findings are supported by spectroscopic measurements and quantum computations. The results confirm the triplet nature of the emissive state originating mainly from a ligand-to-metal charge transfer. Research funded by the NSF (CHE1213223). BioCARS Sector 14 is supported by NIH, National Center for Research Resources (RR007707). APS is funded by the U.S. DOE, Office of Basic Energy Sciences (W-31-109-ENG-38). KNJ is supported by the Polish Ministry of Science and Higher Education through the "Mobility Plus" program.

First steps towards time-resolved 'in-house' X-ray diffraction experiments

High-intensity X-ray sources, such as synchrotrons or X-ray free electron lasers, providing up to 100 ps time-resolution allow for studying very short-lived excited electronic states in molecular crystals. Some recent examples constitute investigations of Rh...Rh bond shortening,[1] or metal-to-ligand charge transfer processes in CuI complexes.[2] Nevertheless, in cases in which the lifetime of excited state species exceeds 10 μs it is now possible, due to the dramatic increase in the brightness of X-ray sources and the sensitivity of detectors, to use laboratory equipment to explore structural changes upon excitation. Consequently, in this contribution we present detailed technical description of the 'in-house' X-ray diffraction setup allowing for the laser-pump X-ray-probe experiments within the time-resolution at the order of 10 μs or larger. The experimental setup consists of a modified Bruker Mo-rotating-anode diffractometer, coupled with the high-frequency Nd:YAG laser (λ = 355 nm). The required synchronization of the laser pulses and the X-ray beam is realized via the optical chopper mounted across the beam-path. Chopper and laser capabilities enable high-repetition-rate experiments reaching up to 100 kHz. In addition, the laser shutter is being directly controlled though the original diffractometer software, allowing for collection of the data in a similar manner as done at the synchrotron (alternating light-ON & light-OFF frames). The laser beam itself is split into two allowing for improved uniform light delivery onto the crystal specimen. The designed setup was tested on the chosen set of crystals exhibiting rather long-lived excited state, such as, the Cu2Br2L2 (L = C5H4N-NMe2) complex, for which the determined lifetime is about 100 μs at 90 K. The results shall be presented. Research is funded by the National Science Foundation (CHE1213223). KNJ is supported by the Polish Ministry of Science and Higher Education through the "Mobility Plus" program.

Selective time-dependent changes of Cu(DPPE)(DMP)·PF₆ on photoexcitation

Thanks to their potential applications as light-emitting devices, chemical sensors and dye-sensitized solar cells, heteroleptic copper (I) complexes have been extensively studied. Cu(DPPE)(DMP)·PF₆ (dppe= 1,2-bis(diphenylphosphino)ethane; dmp = 2,9-dimethyl-1,10-phenanthroline) crystallizes in the monoclinic system, P2₁/c, with two independent molecules in the asymmetric unit. Previous studies on this system [1,2] show strong temperature-dependent emission. The complex was studied at 90K under 355nm laser excitation. At this temperature, the luminescence decay for Cu(DPPE)(DMP)·PF₆ is biexponential with lifetimes of ~3μs and ~28μs. Two time-resolved X-ray diffraction techniques were applied for studies: (1) a Laue technique at BioCARS ID-14 beamline at the Advanced Photon Source, and (2) monochromatic diffraction at a newly constructed in-house pump-probe monochromatic facility at the University at Buffalo. Structural changes determined with the two methods are in qualitative agreement; discrepancies in position of the Cu and P atoms were observed. The molecular distortions were smaller than those determined at 16K in the earlier synchrotron study by Vorontsov et al. [2]. Photodeformation maps (see Figure below), in which the increase in temperature on photoexcitation has been eliminated, clearly illustrate the photoinduced atomic shifts for both data sets. Results will be compared with those obtained for other studied heteroleptic copper (I) complexes, for instance Cu[(1,10-phenanthroline-N,N′) bis(triphenylphosphine)]·BF₄ [3]. The in-house pump-probe facility is discussed by Radoslaw Kaminski at this meeting. Research funded by the National Science Foundation (CHE1213223). BioCARS Sector 14 at APS is supported by NIH (RR007707). The Advanced Photon Source is funded by the Office of Basic Energy Sciences, U.S. Department of Energy, (W-31-109-ENG-38). KNJ is supported by the Polish Ministry of Science and Higher Education through the "Mobility Plus" program.