Crystalline transformations of dinaphthyridinylamine derivatives with alteration of solid-state emission in response to external stimuli

Dinaphthyridinylamine derivatives, (1,8Nap)₂NR (R = H (1), Me (2) and Ph (3)), were prepared as fluorophores in response to external stimuli. From crystallization, single crystals (1) containing water molecules as crystal solvents and crystal polymorphs (3-α and 3-β) containing MeOH molecules were obtained. In the crystal packing of 1, a γ-herringbone (γ-HB) fashion with π-π interaction was observed. In contrast, the packing of 3-α was in a HB fashion with intermolecular CH-N interaction, while the packing of 3-β was in a slipped columnar fashion with π-π interaction between the naphthyridine planes. The observed difference of crystal packings influenced the solid-state emission spectra. Crystals 1, 2 and 3 showed piezochromic behaviors, in which by thoroughly grinding the crystals, the solid-state emission blue color (λfmax = ca. 450 nm) turned into yellowish green (λfmax = ca. 510 nm) for the amorphous solid. In addition, crystals 3-α and 3-β showed various phase transitions including single-crystal-to-single-crystal (SC-to-SC) transformation in response to external stimuli. Crystal 3-β desorbed the MeOH molecules under ambient atmosphere to provide crystal 3-β′ (λfmax = 449 nm), while the resulting crystal 3-β′ returned to crystal 3-β upon exposure to MeOH vapor, indicating vapochromic behavior. Furthermore, crystal 3-β′ was heated at ca. 160 °C to afford crystal 3-α (SC-to-SC transformation). The gummy oil in the resulting supercooled liquid (SCL) state after melting the crystals was scratched by using a fine wire at and above 80 °C (>T_g, glass temperature) to form crystal 3-α with blue emission (SCL-to-SC transformation).