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

Abstract

2-Dimethylamino-5,7-bis­(trifluoromethyl)-1,8-naphthyridine <b>1</b> was prepared as a new solid-state fluorophore. Recrystallization of <b>1</b> from CH₂Cl₂/<i>n</i>-hexane afforded three crystal polymorphs, <b>G</b>, <b>BM</b>, and <b>BO</b>, with the space group (crystal class) <i>C</i>2/<i>c</i> (monoclinic), <i>P</i>2₁/<i>c</i> (monoclinic), and <i>Pbca</i> (orthorhombic), respectively, at 23 °C. DSC curves for <b>G</b> 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 <b>G</b> to <b>B</b> (a mixture of <b>BM</b> and <b>BO</b>) by heating at 110 °C and from <b>B</b> to <b>G</b> by grinding was observed with alteration of the emitted color. In the low temperature region, X-ray crystallography suggested that <b>G</b> transformed to <b>G2</b> (10 °C) and <b>G4</b> (−50 °C) with a subtle alteration of molecular arrangements through thermal single crystal to single crystal interconversion. Eventually, <b>1</b> provided five crystal polymorphs, <b>G</b>, <b>G2</b>, <b>G4</b>, <b>BM</b>, and <b>BO</b>, containing eight crystallographically independent molecules. In contrast, the 1-quinoline analogue, <b>2</b>, provided two crystal polymorphs, <b>2α</b> (23 °C) and <b>2β</b> (−123 °C)