Photocrystallography

This review describes the development and application of a new crystallographic technique that is starting to enable the three‐dimensional structural determination of molecules in their photo‐activated states. So called `photocrystallography' has wide applicability, particularly in the currently exciting area of photonics, and a discussion of this applied potential is put into context in this article. Studies are classified into four groups: photo‐structural changes that are (i) irreversible; (ii) long‐lived but reversible under certain conditions; (iii) transient with photo‐active lifetimes of the order of microseconds; (iv) very short lived, existing at the nanosecond or even picosecond level. As photo‐structural changes relative to the `ground state' can be subtle, this article necessarily concentrates on small‐molecule single‐crystal X‐ray diffraction given that high atomic resolution is possible. That said, where it is pertinent, references are also made to related major advances in photo‐induced macromolecular crystallography. The review concludes with an outlook on this new research area, including the future possibility of studying even more ephemeral, femtosecond‐lived, photo‐active species.