Rich Polymorphism of an Organic Electron Donor–Acceptor Compound

Organic compounds that possess more than four polymorphs are rare. Herein, we report that the compound, PTZ-hexyl-NMI, with a phenothiazine (PTZ) electron donor and a 1,8-naphthalimide (NMI) electron acceptor bonded to opposite ends (α, ω-) of a flexible hexyl chain via the respective heterocyclic N atoms crystallizes from organic solvents in seven different polymorphs depending on solvent, concentration, crystallization temperature, and cooling rates of the solutions. Static low temperature crystallizations (P21/c) form, whereas higher temperature crystallizations give an orange orthorhombic form (Pbca). The Pbca form undergoes an irreversible phase transition at 260 K to the monoclinic (P21/c) form that is accompanied by an anti-/gauche-conformational change of hexyl atoms, thus providing a new example of conformational polymorphism. Dynamic cooling of ethanol solutions produces other crystal forms, although only a yellow orthorhombic (P212121) form could be reproducibly prepared in bulk. The different solid forms of PTZ-hexyl-NMI vary in color along the spectrum from yellow to orange, including instances of yellow/orange dichroism. The orange and dichroic forms exhibit close donor–acceptor aromatic stacking interactions, whereas the yellow forms are more loosely packed. In addition to the conformational flexibility of the hexyl chain, the polymorphic behavior arises due to a variety of comparable-strength noncovalent interactions. Namely, comparable CH···O weak hydrogen bonding interactions that possess either PTZ, naphthalimide, or hexyl hydrogen donors give rise to different intermolecular (polymeric) connectivities. Similarly, different types (donor–donor, acceptor–acceptor, and donor–acceptor) and geometries of pi-stacking interactions (exacerbated by the butterfly-shaped phenothiazine donor that differentiates the top from bottom sides of the molecule) contribute to the polymorphism of the compound.