Light-Mediated C−C σ-Bond Driven Crystallization of a Phenalenyl Radical Dimer

Polymorphismthe phenomenon that a given compound forms more than one crystalline
arrangement of the molecules in the solid state plays a crucial role in understanding organic conductors,
superconductors, and magnets. We have found that solutions of a new phenalenyl radical can give rise to
two (nonpolymorphic) crystalline forms depending on whether the crystallization is allowed to proceed in
the presence or absence of light. In both cases the crystals take the form of black shining blades and are
indistinguishable by optical microscopy. We have fully characterized these crystalline forms, and we show
that they differ by the presence or absence of a C−C σ-bond between the unpaired electrons of the parent
radical. These molecular forms crystallize from the same solvent to give rise to a σ-dimerized insulator and
a monomeric radical semiconductor as dictated by the presence or absence of light.