Controlling Chemical Reactivity Under the Electron Microscope: Single-Crystal-to-Single-Crystal Splitting of Thymine...

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.