Ring-Closing Reaction in Diarylethene Captured by Femtosecond Electron Crystallography

Abstract

The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials.

Femtosecond single-electron diffraction

“…We attribute this to some additional noise of our single-electron detection scheme 44 . Nevertheless, for many of the spots, the signal-to-noise ratio achieved here would allow seeing changes of a few percent in Bragg spot intensities, comparable to conventional multi-electron approaches 6,10,64 . There is no significant difference in noise between the one-electron and the ten-electron measurement.…”

“…In practice, however, the repetition rate is limited by the thermal load imposed by the excitation process 3 to a few hundreds of kHz. We note that single-electron diffraction at 1–10 kHz used in the majority of UED experiments 6,41,42 would require net exposure times on the order of weeks.…”

Optical fiber-based photocathode

“…Ultrafast electron diffraction (UED) is a highly successful technique for structural dynamics investigation of phase transitions, electron-phonon coupling, and chemical reactions. [1][2][3][4][5][6][7][8][9] Metal photocathodes are currently the most commonly employed electron sources due to their robustness, ultrafast temporal response, and compatibility with high electric fields. They provide sub-picosecond electron pulses with coherence lengths up to a few nanometers.…”

Ultrabright femtosecond electron sources: perspectives and challenges towards the study of structural dynamics in labile systems

“…UED has been applied to a variety of crystalline samples and provided detailed information about photoinduced structural processes. However, its application to labile organic materials has only been recently realized 3,4 . Source brightness has been one of the most challenging tasks to extend UED to effectively monitor molecular motion in labile organic systems.…”