Time-resolved Crystallographic Developments Utilising Detector Gating Techniques

The X-ray scattering process occurs on the time scale of about 10-18 seconds; the complete data collection is in the order of hours at synchrotron sources and consequently gives a time-averaged structure of the crystalline material. Previously on beamline I19 at Diamond Light Source we have used a method which involves mechanically chopping the X-ray beam to produce a pulsed source. The pulsed X-ray beam can then be used to probe the crystal a short period after the sample has been photo-activated by a laser beam. This method can be repeated changing the period between the laser (pump) and X-ray pulse (probe) until the entire time series is obtained. Beamline I19 in collaboration with the Dynamic Structural Sciences Consortium at the Research Complex at Harwell have designed a novel strategy to collect an entire time-series (zero to 100 ms) in one data collection utilising the fast image collection time of the Pilatus detector. The 300K Pilatus detector has a readout out time of 2.7 ms and can be gated down to 200 ns. This means that we can use this gating (instead of the mechanical chopper) to obtain single crystal time-resolved structures. This technique shortens the data collection time and as the entire series is obtained from one crystal during the same data collection, this reduces decay and scaling issues.

Single crystal synchrotron X-ray diffraction for imaging hydrogen bond evolution

Hydrogen bonding is a valuable intermolecular interaction in "engineering" solid-state materials. This is because of the directionality and relative strength (1) of these bonds. Hydrogen bonds enable charge and energy transfer, via H-bond evolution, in a range of biological and chemical systems (2). Recent work has demonstrated that single crystal X-ray diffraction can be used to image the evolution of hydrogen bonds, including variable temperature proton migration and proton disorder processes. In particular, in a recent study of the temperature dependent proton disorder in hydrogen bonded 3,5-dinitrobenzoic acid (3,5-DNBA) dimers, the proton disorder deduced from data collected on an X-ray laboratory source is in agreement with that found from neutron data (3). This work focuses on variable temperature single crystal synchrotron X-ray diffraction, for the imaging of evolving hydrogen bonds. The development of appropriate methodology is important here, particularly as previous studies have involved laboratory X-ray sources only. Results will be presented from variable temperature data collections on I19, at the Diamond Light Source, and on beamline 11.3.1, at the Advanced Light Source (ALS), on systems such as 3,5-DNBA and co-crystals of benzimidazole, both exhibiting proton disorder across hydrogen bonding interactions. Synchrotron X-ray diffraction measurements have also been used to follow the change in the position of a proton within an intramolecular [N-H···N]+ hydrogen bond across a range of proton-sponge molecular complexes. Importantly, it has been possible to visualise the evolving hydrogen atom position in Fourier difference electron density maps generated from the synchrotron data. In particular, for the 35-DNBA study, the clearest picture of the evolving hydrogen atom position is observed in those generated from data collected at the ALS; even clearer than that observed in X-ray laboratory and neutron measurements on the same system.

Photocrystallographic and kinetic studies of metastable linkage isomers

The ability of a molecular system to reversibly convert between two distinct states on photoactivation is desirable for many real-world applications, including photo-switchable device media. When induced in the solid-state, the process can be studied by crystallography and specifically photocrystallographic methods have contributed greatly to this diverse research area. The process of linkage isomerism has proved popular for photocrystallographic study. While the atomic rearrangements involved in the process are large enough to be determined from diffraction data, the changes are also moderate enough that crystal integrity is often maintained. Examples of species studied by these methods include nitrite, sulfur dioxide and nitrosyl coordination complexes [1]. Our research has focused on metal–nitrite complexes displaying nitro–nitrito isomerism and has aimed toward the rational design of new systems to undergo maximum conversion. Recently we have studied a selection of nickel–amine systems, including [Ni(Et4dien)(η2-O,ON)(η1-NO2)]. This complex shows diverse linkage isomeric behaviour on thermal and photochemical treatment [2], with 100% nitro–nitrito conversion achieved on excitation at λ = 500nm. The thermal and photochemical processes have been studied in detail [3] and the results of temperature studies, steady-state and pseudo-steady-state photocrystallographic experiments and solid-state kinetic studies are presented. The findings are discussed in relation to the key information they provide on the steric and kinetic factors that may influence the isomerisation process in the single-crystal.

Towards Controlling the Valance Tautomer Interconversion Character by Solvation

Valence tautomers are bi-stable functional molecular materials in which it is observed charge transference between redox active ligands and a metallic center followed by the change of spin of the metal. Valence tautomerism (VT) interconversion is entropically driven and induced by external stimuli such as irradiation by light and soft X-rays and/or changes in temperature and pressure. VT interconversion is also associated with remarkable variations in optical and magnetic properties and and can be modulated with slight chemical changes [1]. Typical examples of valence tautomers are coordination compounds of Co and o-dioxolenes ligands [2]. Crystals of the [Co(diox)(4-X-py)2], where diox = 3,5-di-t-butylcatecholate/3,5-di-t-butylsemiquinonate, X=CN/NO2, py= pyridine, were initially studied with respect to low spin (LS) to high spin (HS) thermo and photoinduced VT interconversion. It was reported that [Co(diox)(4-CN-py)2] crystals become HS-Co3+ at temperatures below 110 K with cooperative VT interconversion whereas [Co(diox)(4-NO2-py)2] crystals present non cooperative VT interconversion [3]. Toluene and benzene solvates of [Co(diox)(4-NO2-py)2 and [Co(diox)(4-CN-py)2 have been prepared to investigate further the solvation effects on the VT interconversion properties. Analysis of the single crystal X-ray diffraction data obtained during cooling and heating at temperatures ranging from 293 K to 90 K indicated that [Co(diox)(4-CN-py)2] toluene and benzene as well as [Co(diox)(4-NO2-py)2].toluene solvate crystals undergo into non cooperative VT interconversion. The [Co(diox)(4-NO2-py)2].benzene crystal show a highly cooperative VT interconversion with a pronounced hysteresis. Intermolecular interactions between inlayer [Co(diox)(4-X-py)2] molecules are responsible for the VT interconversion in all compounds, however the VT interconversion cooperativity seems to be related with the strength of the Car-H...Odiox interactions and with the correspondent Co-Co separation. Thus solvation plays a key role in the definition of the VT interconversion nature. Acknowledgments: FAPEMIG, CNPq and CAPES grant 10030-12-3.