Crystallization-Induced Dimerization and Solution-Phase Bond Dissociation of Stable Dibenzoolympicenyl Radicals

Crystallization of organic materials can lead to different assembly structure with different reactivity, but this phenomenon is rarely observed for delocalized hydrocarbon radicals. This report introduces a crystallization-induced radical-radical coupling reaction, which employs a series of stable nonplanar organic π-radicals as reactants. Six stable radical congeners are synthesized, resulting in radical-radical coupling at the allenyl radical site during crystallization to produce close-shell dimers. This coupling reaction is absent in the solution phase, which highlights the importance of preorganization in the lattice. Remarkably, the attempts of cocrystallization of different congeners yielded homocoupling products instead of cross-coupling products. In specific cases, two distinct polymorphs are observed and their reactivity is different according to the distance of the reaction sites. Theoretical calculations indicate that the transition from a metastable preorganized monomer to a dimer is barrierless and spontaneous. The dimer could regenerate free radicals by heating or photoirradiation in the solution phase. This discovery may lead to controllable molecular switches.