Chemical X-ray photodiffraction: Principles, examples, and perspectives

X-ray photodiffraction (in the chemical literature also referred to as photocrystallography), which is based on the combination of X-ray diffraction methods with samples excited by UV or visible light to solve fundamental photochemical or photophysical issues, has developed in the last couple of decades into a very promising technique for direct observation of photoinduced chemical species in the solid state. The capability of providing direct information on very small perturbations in atomic positions and thus on the minute changes in molecular geometry during (or as a consequence of) photoexcitation appears to be the most important asset of this emerging analytical technique. When combined with other physicochemical methods, X-ray photodiffraction can be a powerful tool for analysis of steady-state photoinduced structures as well as slow or very fast time-dependent phenomena. Despite being a very useful approach, however, due to a number of practical requirements that it places with regard to the system to be studied, at the present stage of developments the technique is not widely and indiscriminately applicable to any photoinduced process. In some particular chemical systems the inherent pitfalls could be practically overcome by practical or theoretical means. In this short chapter, the basic principles of X-ray photodiffraction are briefly summarized, and the prospects of its application to "physical" and "chemical" problems is illustrated with selected examples from recent literature. Some possible future developments and alternative approaches with this and related methods are also presented.