X-ray diffraction of photolytically induced molecular species in single crystals

Abstract

We review developments in X-ray diffraction of single crystals that begin to enable one to quantify directly the nature of electronic perturbations induced by light in chemical structures. Such structural information is key to understanding many chemical processes and physical properties activated with light, and the scientific impetus behind this incipient area of structural science is described from academic and industrial perspectives. Photoisomerisation, photochemical reactions in the solid state and spin-crossing magnetic transitions that have enduring or irreversible states induced with light are best understood by unravelling their three-dimensional structure measured in situ in their states converted by light. Investigations conducted with single-crystal X-ray diffraction of structures in a laser-induced steady state and the experimental methods used to realise such structures are reviewed. The structural characterisation of transient photo-induced species (down to picosecond lifetime) is paramount to improve understanding of materials that undergo rapid electronic switching, which make operative much of the electronic and optical industry, as there exists an inherent relationship between the structure of the excited state and the physical properties exhibited. Prime instances include structures of molecular conductors and luminescent materials in their excited states with prospective applications as molecular wires, light-emitting diodes, non-linear optical components, triboluminescent and electroluminescent devices. Only indirect and qualitative interpretations of the nature of these excited states were formerly formulated with spectrometric 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 quantitative via a three-dimensional structural representation. Structures of transient species are reviewed and the key experimental parameters that are required for a successful experiment, in terms of characteristics of the X-rays, laser and sample are discussed. The importance of auxiliary spectroscopic experiments is also described. A future outlook on possible X-ray sources to facilitate such work and to extend it to structural studies on even more ephemeral species concludes this review. © 2008 Springer.