The role of nucleation site in determining the morphology of the product crystal in a structural phase transition

The Diels-Alder reaction between 11-cyano-1,6-methano[10]annulene and 4-methyl-1,2,4-triazoline-3,5-dione yields the exo and endo isomers of the adduct 2,3,5,5a,9a,10-hexahydro-2-methyl-1,3-dioxo-5,10-etheno-5a,9a-methano-1H-[1,2,4]triazolo[1,2-b]phthalazine-14-carbonitrile (CMAT). CMAT(II) (the crystal of the exo isomer) transforms to CMAT(I) (the crystal of the endo isomer) during a thermally induced phase transition in the solid state. Two modes of transition were observed in crystals of CMAT(II) taken from different batches: a single-crystal-to-single-crystal transition and a single-crystal-to-polycrystal transition. The identification of the factor that determines the mode of transition was based on a structural comparison at room temperature and at circa 380 K, on thermal-motion analyses at both temperatures and on an examination of the defect content using X-ray topography. No substantial differences, concerning these aspects, were detected between crystals from the two batches and the various modes of transition were attributed to different sites of nucleation. A single nucleus at the edge of the crystal plates results in a single-crystal product, while nucleation at holes on the plate surface leads to a polycrystalline material.