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.