Ultrafast diffraction of transient molecular structures in radiationless transitions

Radiationless transitions in molecules are ubiquitous in photophysical, chemical, and biological systems. Because such transitions are usually ultrafast in nature, the determination of transient structures is extremely difficult, particularly for complex molecules with many degrees of freedom. Here, we report the direct determination of the molecular structure during the ultrafast, nonradiative transition in aromatic pyridine, excited above the so-called channel three threshold, which marks the onset of an anomalous decay. The approach we invoked is ultrafast electron diffraction (UED), developed in this laboratory to image transient molecular structures in real time. The diffraction results reveal the breakage of "old bonds" near 1.4 and 2.4 Angstrom and the formation of "new bonds" near 1.3 and 1.5 Angstrom and at distances greater than similar to3.5 Angstrom, thus permitting the dominant intermediate species to be identified. Unexpectedly, a ring-opened diradical structure resulting from C-N bond scission was observed and was found to form with a rate of (17 ps)(-1). This hitherto unknown intermediate must be considered as a new channel for energy dissipation in pyridine and possibly analogous molecules. These results represent a clear demonstration of the potential of UED for studying structural dynamics of radiationless transitions in complex molecular systems.