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.