Crystal structures and emitting properties of trifluoromethylaminoquinoline derivatives: Thermal single-crystal-to-single-crystal transformation of polymorphic crystals that emit different colors

2,4-Trifluoromethylquinoline (TFMAQ) derivatives that have amine (1), methylamine (2), phenylamine (3), and dimethylamine (4) substituents at the 7-position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorphs of 2 (crystal GB and YG) and 3 (crystal B and G) were obtained and characterized by X-ray crystallography. In solution, TFMAQ derivatives emitted relatively strong fluorescence (λ^f _max=418-469 nm and φ_f(s)=0.23-0.60) depending on the solvent polarity. From Lippert-Mataga plots, δμ values in the range of 7.8-14 D were obtained. In the crystalline state, TFMAQ derivatives emitted at longer wavelengths (λ^f_max=464-530 nm) with lower intensity (φ_f(c)=0.01-0.28) than those in n-hexane solution. The polymorphous crystals of 2 and 3 emitted different colors: 2, λ^f_max=470 and 530 nm with φ_f(c)=0.04 and approximately 0.01 for crystal GB and YG, respectively; and 3, λ ^f_max=464 and 506 nm with φ_f(c)=0.28 and approximately 0.28 for crystal B and G, respectively. In both crystal polymorphs of 2 and 3, crystals GB and G showed emission color changes by heating/melting/cooling cycles that were representative. By following the color changes in heating at the temperature below the melting point with X-ray diffraction measurements and X-ray crystallography, the single-crystal-to- single-crystal transformations from crystal GB to YG for 2 and from crystal B to G for 3 were revealed.