Scientific News

Ultrafast laser switching in a photochromic film visualised by time-resolved X-ray diffraction

February 2010 - A (2+2) photo-cycloaddition reaction in an organic crystal film was triggered by a fast laser pulse and studied with X-ray diffraction. This reaction and its thermally-activated reverse reaction were followed on the picosecond timescale, and intricate details of the reaction kinetics were revealed.

Photoinduced processes in organic crystals and their potential for applications in a variety of photosensitive devices has made them the focus of much research not only in optical data storage but also in template-controlled synthesis in which one of the reaction steps is photo-induced. Solid-state photochemical reactions are highly dependent on the geometry of the reacting compounds and their products. Much experimental work has been undertaken to understand the thermodynamical properties of solids undergoing solid-state reactions [4], the influence of the spatial arrangements of the molecules within the lattice, optimisation of reaction conditions (i.e., type of solvent for dispersal of crystals), and temperature dependence. However, very little is known about the kinetics of such reactions. Time-resolved crystallography permits the structural changes in such solid state reactions to be followed as a function of time, revealing the reaction mechanism and its dependency on external parameters. Photoinduced solid-state reactions are either reversible or irreversible, and they may involve homogeneous single-crystal to single-crystal dimerisation or heterogeneous dimerisation, where the long-range order is destroyed.

More information