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The ultrafast structural dynamics of the [2+2] photocycloaddition of alpha-styrylpyrylium trifluoromethanesulfonate (TFMS) has been studied in great detail. During the photoreaction, optical and infrared spectroscopy confirms that crystals of alpha-styrylpyrylium change color. Since the reaction is reversible, it has been suggested to be used as an organic holographic storage device. The present photocrystallographic studies (with high spatial resolution) allow for an electron density analysis of the overall reaction kinetics, revealing the mechanism of bond-breaking and bond-formation. It could furthermore be proved how the reaction is influenced by the rearrangement of the surrounding moieties. Picosecond time-resolved X-ray diffraction studies allow for the monitoring the photoreaction in crystalline thin films under experimental conditions where the transformation times are greatly enhanced. These investigations are discussed in the context of the photocrystallographic results. It has been found that alpha-styrylpyrylium TFMS undergoes an ultrafast photoreaction to the dimer product state and back-reaction to the monomer reactant state which is temperature driven. The present experiments indicate that TFMS reacts on time scales which are the fundamental limiting ones of two-quantum systems and therefore has the potential to be used as an ultrafast organic molecular switcher.

Time-Resolved X-ray Diffraction of the Photochromic α-Styrylpyrylium Trifluoromethanesulfonate Crystal Films Reveals Ultrafast Structural Switching