Fast and Slow Motions Drive Elusive Polymorphic Transitions in a Naphthalene-Diimide Crystal

Thermally induced solid-state polymorphic transitions in an n-hexyl-functionalized naphthalene-diimide organic semiconductor (NDI-C6) are investigated using atomistic molecular dynamics (MD) simulations. NDI-C6 exhibits two stable polymorphs: the α phase at room temperature and the γ phase at higher temperatures. The MD simulations reproduce the structural stability of both phases and provide a refined description of their unit-cell parameters and side-chain conformations, which have been only partially resolved experimentally. The α-to-γ transition proceeds through a sequential, nonconcerted two-step mechanism. An initial fast (subnanosecond) process involves the collective rearrangement of the NDI cores, followed by a slow process (several ns) characterized by side-chain extension. These results provide an atomistic picture of polymorphism in NDI-C6 and establish a robust computational protocol for investigating labile phase transitions in the broader class of NDI-based n-type soft organic semiconductors.