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 …