Crystal Structures, Thermal Properties, and Emission Behaviors of N,N‑R-Phenyl-7-amino-2,4-trifluoromethylquinoline Derivatives: Supercooled Liquid-to-Crystal Transformation Induced by Mechanical Stimuli

N,N-R-Phenyl-7-amino-2,4-trifluoromethylquinoline derivatives (R = Me (1), Et (2), isopropyl (3), and Ph (4)) were prepared as a new type of fluorophore responsive to external stimuli. 1, 2, 3, and 4 were obtained as single crystals including three crystal polymorphs (, , and ) of 1 and two ( and ) of 2. In 4, a phase transition from 4173 and 490 between 173 and 90 K was observed. The solid-state emission showed a red shift by 30–58 nm compared with the emission in n-hexane, and their emission properties depended on the molecular arrangements. The modes of molecular arrangements for , , and were a slipped parallel (SP), head-to-tail γ-type herringbone (HT-γ-HB), and head-to-head γ-type herringbone (HH-γ-HB); those for and were HT-γ-HB and head-to-tail dimer (HT-dimer), and that for 3 was head-to-tail columnar (HTC). 4173 and 490 were similar HT-γ-HB. The crystal-to-crystal transformations from to and from to were observed by heating and grinding the crystal, respectively, with emittance changes. After melting, on cooling, all crystals formed supercooled liquid (SCL) and then glass states. In the SCL state, molecules were amorphous and were quickly crystallized by a mechanical stimulus such as scratching. By taking advantage of the difference of emitting intensity between the SCL and the crystal states for 1, “writing” and “erasing” of a letter with scratching and heating, respectively, were demonstrated.