Small molecule structures and their applications rely
on good
knowledge of their atomic arrangements. However, the crystal structures
of these compounds and materials, which are often composed of fine
crystalline domains, cannot be determined with single-crystal X-ray
diffraction. Three-dimensional electron diffraction (3D ED) is already
becoming a reliable method for the structure analysis of submicrometer-sized
organic materials. The reduction of electron beam damage is essential
for successful structure determination and often prevents the analysis
of organic materials at room temperature, not to mention high temperature
studies. In this work, we apply advanced 3D ED methods at different
temperatures enabling the accurate structure determination of two
phases of Pigment Orange 34 (C34H28N8O2Cl2), a biphenyl pyrazolone pigment that
has been industrially produced for more than 80 years and used for
plastics application. The crystal structure of the high-temperature
phase, which can be formed during plastic coloration, was determined
at 220 °C. For the first time, we were able to observe a reversible
phase transition in an industrial organic pigment in the solid state,
even with atomic resolution, despite crystallites being submicrometer
in size. By localizing hydrogen atoms, we were even able to detect
the tautomeric state of the molecules at different temperatures. This
demonstrates that precise, fast, and low-dose 3D ED measurements enable
high-temperature studies the door for general in situ studies of nanocrystalline materials at the atomic level.