Single-Crystal-to-Single-Crystal Radical-Ion Chain Reactions under the Electron Microscope: A Chemical Design to Repurpose Electron Beam Damage

Crystals designed to undergo electron beam-induced single-crystal-to-single-crystal
(SC–SC) reactions were used for structural elucidation by microelectron
diffraction (3D ED) and the results compared with those from crystals
where the same reaction was expected to disrupt the crystal lattice.
We confirmed that microcrystals of rctt-tetrakis(4-pyridyl)cyclobutane
with 2,4-dichlororesorcinol (bpeD·dcr) or 2,4,6-trichlorophenol
(bpeD·tcp) formed by supramolecular templating underwent
smooth SC–SC cyclobutane splitting reactions to form trans-4,4′-bipyridylethene (bpe) in
their corresponding crystalline complexes bpeD·dcr and bpeD·tcp. We also showed that the reactions
are initiated by sporadic electron beam ionization events that are
propagated by subsequent electron transfer steps. Electron beam damage
was greatly mitigated, and the crystal structures could be solved
in various steps of the reaction and through completion. By contrast,
nontemplated crystals of rctt-tetrakis(4-pyridyl)cyclobutane
complexed with iso-coumarin carboxylic acid (bpeD·icCA) and crystals of pure bpe underwent
sluggish, non-SC–SC reactions, and displayed diffraction patterns
that decayed more rapidly. These results suggest that structural damage
caused by ionization-induced chemical changes may play a significant
role in crystal decay for samples exposed to the high-energy beam
of an electron microscope.