Nanosecond time-resolved crystallography of photo-induced species: case study and instrument development for high-resolution excited-state single-crystal structure determination

Abstract

This work describes one of the first stages in the development of time-resolved photo-induced small-molecule single-crystal diffraction, whereby transient electron density perturbations, with lifetimes down to the nanosecond level, can be resolved at the atomic level. Knowledge of such ephemeral electronic effects is likely to yield key information regarding the origins of certain important physical properties, e.g. luminescent and non-linear optical effects, since it will allow the dynamics of electron density to be identified and quantified, and it is this that underpins such phenomena in a given molecule. The experimental methodology employs phase-locking pump–probe techniques such that the inherent time-structure of a synchrotron X-ray beam (nanoseconds) is harnessed and time-gated in-phase with a femtosecond laser. The resultant beams, made coincident on the crystal in a periodic manner, and a diffraction pattern are recorded as a function of the Bragg angle, θ. Such technology is based upon the pioneering work carried out in sub-nanosecond time-resolved crystallography of macromolecular biological moieties (non-atomic resolution) at the ESRF, although one crucial difference here is the use of monochromatic irradiation and oscillatory motion rather than Laue ‘snapshot’ methodology, so that atomic resolution is possible. The experimental details of a case study conducted on ID9 at the ESRF, France, are described, whereby the feasibility of the excited-state structure determination of a luminescent rhenium carbene complex, [HNCH₂CH₂NHCRe(2,2′-bipyridine)(CO)₃]Br, is realised. Key experimental parameters that are required for the success of such an experiment are discussed in the light of this study, together with other feasibility work conducted at the SRS, UK, and in the laboratory. Plans, designs and tests for the implementation of this technique in the UK, first at the SRS, and then at DIAMOND, the forthcoming UK synchrotron, are described, in particular with reference to the world-leading potential that DIAMOND could lend toward the development of this technique.