Crystal polymorphs of 1,8-naphthyridine derivative, being anti and syn conformers, show a reversible transformation from anti to syn by heating and from syn to anti by grinding with the alteration of emittance intensity, and notably, thermal transformation from anti to syn conformer took place in single-crystal-to-single-crystal (SC-to-SC) form, which was confirmed by a single crystal X-ray crystallography.

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 (1α, 1β, and 1γ) of 1 and two (2α and 2β) of 2. In 4, a phase transition from 4 173 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 1α, 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 2α and 2β were HT-γ-HB and head-to-tail dimer (HT-dimer), and that for 3 was head-to-tail columnar (HTC). 4 173 and 4 90 were similar HT-γ-HB. The crystal-to-crystal transformations from 1γ to 1β and from 2β to 2α 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.

2-Dimethylamino-5,7-bis(trifluoromethyl)-1,8-naphthyridine 1 was prepared as a new solid-state fluorophore. Recrystallization of 1 from CH 2 Cl 2 /n-hexane afforded three crystal polymorphs, G, BM, and BO, with the space group (crystal class) C2/c (monoclinic), P2 1 /c (monoclinic), and Pbca (orthorhombic), respectively, at 23 °C. DSC curves for G 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 G to B (a mixture of BM and BO) by heating at 110 °C and from B to G by grinding was observed with alteration of the emitted color. In the low temperature region, X-ray crystallography suggested that G transformed to G2 (10 °C) and G4 (−50 °C) with a subtle alteration of molecular arrangements through thermal single crystal to single crystal interconversion. Eventually, 1 provided five crystal polymorphs, G, G2, G4, BM, and BO, containing eight crystallographically independent molecules. In contrast, the 1-quinoline analogue, 2, provided two crystal polymorphs, 2α (23 °C) and 2β (−123 °C). ■ EXPERIMENTAL SECTIONGeneral Methods. Infrared spectra were recorded on a JASCO 420 FT-IR spectrometer. 1 H NMR spectra were measured on a JEOL

Bis[2,4-di(trifluoromethyl)quinoline-7-yl]amine (1), bis[2,4-di(trifluoromethyl)quinoline-7-yl]methylamine (2), bis[2,4-di(trifluoromethyl)quinoline-7-yl]phenylamine derivatives, Q2 NPhX; X=NO2 (3 a), I (3 b), H (3 c), OMe (3 d), and NH2 (3 e), tris[2,4-di(trifluoromethyl)quinoline-7-yl]amine (4), and bis[2,4-di(pentafluoroethyl)quinoline-7-yl]-4-nitrophenylamine (5) were prepared as functional fluorophores. On irradiating the solution samples, 1 showed no noticeable alteration, whereas 2, 3 a-d, and 4 showed emission color changes from yellowish green to blue, indicating that a photoreaction took place. Analyses of the photoproduct based on absorption and emission spectra, (1) H NMR spectra, and X-ray crystallography indicated that photocyclization reactions occurred regioselectively and quantitatively to form bent-bent dipyridocarbazoles. In 3 a-d, the reaction rates depended on the solvent polarity and the substituent on the benzene ring. The photoreactions were accelerated with decreasing solvent polarity and with increasing electron-withdrawing character of the substituents. The photocyclization of triquinolineamine 4 was faster than that of 3 a in all solvents. The results of semiempirical quantum-chemical PM6 calculations suggested that the observed regioselective photocyclization could be explained by stabilization of the excited triplet transition state for the bent-bent form because of the molecular geometry with the CH-NQ hydrogen bonds. The solution of 5 in MeOH displayed photoreaction-induced self-assembly behavior to form twisted tape-like fibers of width 200 nm, as determined by TEM imaging.

The dynamics of photoexcited carriers in standard TiO 2 photocatalytic powders has been studied by investigating the time decay and temperature dependence of the photoluminescence. A biexponential decay curve in a time-resolved signal suggests that there are different formation processes of the self-trapped exciton states. The peak position of the luminescence spectrum shifted to a higher energy at room temperature. On the basis of these experimental results, we propose a relaxation model of photoexcited carriers.