Thermal Single Crystal to Single Crystal Transformation among Crystal Polymorphs in 2‑Dimethylamino-5,7-bis(trifluoromethyl)-1,8-naphthyridine and in a 1‑Quinoline Analogue

2-Dimethylamino-5,7-bis­(trifluoromethyl)-1,8-naphthyridine 1 was prepared as a new solid-state fluorophore. Recrystallization
of 1 from CH2Cl2/n-hexane afforded three crystal polymorphs, G, BM, and BO, with the space group (crystal class) C2/c (monoclinic), P21/c (monoclinic), and Pbca (orthorhombic), respectively, at 23 °C. DSC curves for G showed one endothermic peak at 110 °C in the high temperature
region and two pairs of endo- and exothermic peaks at ca. 18 and 7
°C in the low temperature region, which were assigned as crystal
phase transitions. In the high temperature region, a reversible transformation
from G to B (a mixture of BM and BO) by heating at 110 °C and from B to G by grinding was observed with alteration of the
emitted color. In the low temperature region, X-ray crystallography
suggested that G transformed to G2 (10 °C)
and G4 (−50 °C) with a subtle alteration
of molecular arrangements through thermal single crystal to single
crystal interconversion. Eventually, 1 provided five
crystal polymorphs, G, G2, G4, BM, and BO, containing eight crystallographically
independent molecules. In contrast, the 1-quinoline analogue, 2, provided two crystal polymorphs, 2α (23
°C) and 2β (−123 °C).