Single-crystal X-ray diffraction studies of photo-induced molecular species.
This tutorial review gathers together the recent developments in single-crystal X-ray diffraction that are starting to enable one to quantify directly the nature of light-induced electronic perturbations in chemical structures. Such structural information is key to understanding many photo-activated chemical processes and physical properties, and a description of the scientific impetus behind this incipient area of structural science, from academic and industrial perspectives, is given. Photoisomerism processes, solid-state photochemical reactions and spin-cross-over magnetic transitions, that have long-lived or irreversible light-induced states, are best understood by unravelling their three-dimensional structures measured in situ in their photo-converted state. A review of steady-state laser-induced single-crystal X-ray diffraction studies conducted, to date, and the experimental methodologies used in order to realise such structures, is presented. The structural characterisation of more transient photo-induced species (down to picosecond lifetimes) is paramount to a better understanding of the materials that undergo high-speed electronic switching, which make operative much of the electronics and optics industry, since there exists an inherent relationship between the excited-state structure and the physical properties exhibited. Prime examples include excited-state structures of molecular conductors and luminescent materials with potential applications as molecular wires, light-emitting diodes, non-linear optics, triboluminescence and electroluminescence. Previously, only indirect and qualitative interpretations of the nature of these excited-states could be formulated via spectroscopic techniques, but the developments in ms-ps time-resolved laser pump, X-ray probe single-crystal diffraction techniques, described herein, are overcoming this barrier, affording results that are entirely quantitative via a three-dimensional structural representation. In this regard, a review of structures of transient species studied to date is presented along with a discussion of the key experimental parameters that are required for a successful experiment, in terms of the X-ray, laser and sample characteristics. The importance of auxiliary spectroscopic work and complementary theoretical calculations is also briefly discussed. The paper concludes with a future outlook on new possible X-ray sources that will facilitate such work and extend it to structural studies on even more ephemeral species in the future.
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK. jmc61@cam.ac.uk
Jacqueline M
JM
Cole