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T. Kader, B. Stöger, J. Fröhlich:
"The phase transitions of 4-aminopyridine-based indolocarbazoles: twinning, local- and pseudo-symmetry";
Acta Crystallographica Section B, 75 (2019), 97 - 106.

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).

phase transition; polymorphism; polytypism; twinning; order-disorder (OD) theory.

http://dx.doi.org/10.1107/S2052520618017341