A thermo-responsive structural switch and colossal anisotropic thermal expansion in a chiral organic solid.

Trianglimine, a chiral triangular-shaped hexaimine, exists in at least three apohost polymorphic forms. Form I had been previously obtained by crystallisation from a mixture of dichloromethane and acetonitrile and we have now crystallised Form II from acetone. Both forms possess similar packing arrangements, but Form II undergoes a reversible phase transition to Form III, as well as colossal anisotropic positive thermal expansion. Form I does not exhibit any remarkable thermal properties.