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 (1, 1β, and 1γ) of 1 and two (2 and 2β) of 2. In 4, a phase transition from 4₁₇₃ and 4 ₉₀ 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 1, 1β, and 1γ 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 2 and 2β 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 1γ to 1β and from 2β to 2 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.