The Transient Chiral Phase of 1,1′-Binaphthyl

1,1′-Binaphthyl is an axially chiral molecule that exists as two stable atropisomers (R and S), which can interconvert through rotation of the naphthyl units about the biaryl axis across a potential energy barrier. Different crystal structures form upon cooling from the molten state on a solid surface. At low cooling rates (2 °C min–1), the well-known racemic phase forms, whereas at high cooling rates greater than 10 °C min–1, an unknown chiral phase appears. This new phase is also formed in thin films prepared by solution processing. Infrared thermography during melt crystallization, combined with the appearance of this new phase through thin film deposition, highlights the importance of kinetic effects due to reduced induction time for crystallization. Detailed studies by differential scanning calorimetry and temperature-dependent in situ X-ray powder diffraction reveal that the new phase is transient at room temperature, since a spontaneous transition to the other well-known stable chiral phase occurs. The new chiral phase crystallizes in the same space group P41212 as the stable chiral phase, with slightly enlarged unit cell volume but with a different molecular conformation. The phase transition is based on continuous nucleation and an Avrami exponent of n = 2, indicating a one-dimensional crystal growth mechanism. This work demonstrates how crystallization kinetics influence the crystallization of conglomerates from a racemic melt or solution via a transient polymorphic state.