Martensitic-like transition between liquid crystalline and crystalline phases of a prototypical discotic organic semiconductor

Transitions between crystalline phases occur either through the nucleation and growth mechanism, a process that is slow and destructive, or through the diffusion-less and order preserving Martensitic route. We demonstrate here that for canonical discotic organic semiconductor HAT6, the transition between the liquid crystalline columnar hexagonal phase (Col H ) and the crystalline solid can occur through a mechanism that exhibits the hallmarks of Martensitic transformations: orientational correlations between parent and daughter phases, reversibility, and ultrafast kinetics. To access Martensitic-like solidification, the Col H phase of HAT6 is biaxially aligned in microchannels and crystallization is induced on deep supercooling or fast cooling rates. Selection of a Martensitic-like transformation requires cooling rates fast enough to avoid an equilibrium phase transition but slow enough to avoid vitrification. The transition mechanism is studied using a combination of polarized optical microscopy and X-ray scattering. At the largest accessible supercooling, the Col H → Crystal phase transition occurs at speeds of ~100 µm/s, which is seven orders of magnitude greater than the theoretical prediction for growth from isotropic melts. Our work suggests that Martensitic-like transformations can occur between liquid crystals and crystals and are therefore more general than previously believed. Further, we demonstrate that Martensitic-like transformations of anchored liquid crystals can be used to grow biaxially aligned crystals of organic molecules over arbitrarily long distances. As macroscopic lattice alignment is desirable for devices like transistors and as several high-performance molecular semiconductors exhibit a Col H phase, our results hold general significance for organic electronics.