Controlling Chemical Reactivity Under the Electron Microscope: Single‐Crystal‐to‐Single‐Crystal Splitting of Thymine Dimers Initiated by Electron Beam Ionization and Amplified by Electron Transfer‐Mediated Chain Reactions

ABSTRACT Three crystalline thymine dimers (TDs) obtained by solid‐state topochemical photodimerization were selected to explore radical ion‐initiated solid‐to‐solid splitting reactions when exposed to the high‐energy beam of an electron microscope. While samples of cis ‐ syn ‐thymine dimer ( c,s ‐TD) were available from a previous study, samples of cis ‐ syn ‐N,N‐dimethyl‐thymine dimer ( c , s ‐TD‐Me) and cis ‐ anti ‐N,N‐dimethyl‐thymine dimer ( c , a ‐TD‐Me) were obtained by photoreaction of crystalline N,N‐dimethyl‐thymine (T‐Me). The unexpected formation of c , a ‐TD‐Me was rationalized on a topochemical basis based on a high‐resolution x‐ray structure of T‐Me refined with DFT‐calculated non‐spherical scattering factors. When exposed to the electron beam (30–300 kV), recrystallized dimers underwent the expected splitting reaction back to T and T‐Me as the only products. Electron beam‐induced reactions studied as a function of electron fluence, beam energy, and temperature confirmed an ionization‐induced chain reaction propagated by a single electron transfer (SET) process with a chain length up to 80 000 product molecules per electron. Microcrystal electron diffraction (3D ED) analysis of the photodimers allowed crystal structure determination with approximately 30% of the monomers formed in situ. A long‐lived transient formed in single crystals of c , s ‐TD‐Me was assigned by EPR as the TD‐Me •+ , in support of the suggested reaction mechanism.