Abstract
The phase transitions and polymorphism of three 4-aminopyridine-based indolocarbazole analogues are analyzed with respect to symmetry relationships and twinning. Seven polymorphs were structurally characterized using single-crystal diffraction. 5NICz (the indolo[3,2,1-jk]carbazole derivative with the C atom in the 5-position replaced by N) crystallizes as a P21/a high-temperature (270 K) polymorph and as a Pca21 low-temperature (150 K) polymorph. Even though their space-group symmetry is not related by a group-subgroup relationship, the local symmetries of both belong to the same order-disorder (OD) groupoid family. Both are polytypes of a maximum degree of order and are twinned by point operations of the other polytype. 2NICz (C atom in the 2-position replaced by N) likewise crystallizes in a high-temperature (Pcca, 280 K) polymorph and a low-temperature (P21/c, 150 K) polymorph. Here, the space-group symmetries are related by a group-subgroup relationship. The low-temperature phase is twinned by the point operations lost on cooling. The crystal structure of bulk 2,5NICz (N-substitution at the 2- and 5-positions) was unrelated to 2NICz and 5NICz and no phase transition was observed. Isolated single crystals of a different polymorph of 2,5NICz, isotypic with 2NICz, were isolated. However, the analogous phase transition in this case takes place at distinctly higher temperatures (> 300 K).
Highlights
Symmetry relationships are crucial in understanding and describing phase transitions (Muller, 2013)
Seven polymorphs were structurally characterized using singlecrystal diffraction. 5NICz crystallizes as a P21/a high-temperature (270 K) polymorph and as a Pca21 low-temperature (150 K) polymorph. Even though their space-group symmetry is not related by a group–subgroup relationship, the local symmetries of both belong to the same order–disorder (OD) groupoid family
We present the structural phase transitions of three 4-aminopyridine derivatives of indolo[3,2,1-jk]carbazole (ICz), whereby C atoms para to the N atom of ICz are replaced by an N atom
Summary
Symmetry relationships are crucial in understanding and describing phase transitions (Muller, 2013). Numerous incommensurate phases feature a lock-in phase transition to a periodic (and higher-symmetry) LT structure on cooling (Cummins, 1990). In such a case, both phases are derived from a higher-symmetry prototype structure, which may exist at high temperatures or may be purely hypothetical. The bulk 2,5NICz-1 crystallizes in a structure unrelated to 5NICz and 2NICz. We could not find any evidence of a phase transition in the solid state for this polymorph.
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