Structure determinations of three fluorescent organic pigments by powder diffraction and micro-crystal structure analysis

Bisazomethine compounds with 2-hydroxy-naphthalene moieties belong to the few organic pigments which show fluorescence in the solid state. The condensation products of hydrazine with 4-formyl-3-hydroxy-2-naphthoic acid-4'-methoxyanilide (C38H30N4O6), as well as the corresponding 4'-chloro and 2',5-dichloro derivatives (C36H24O2N4O4 and C36H22Cl4N4O4) are almost insoluble in all common solvents, even at temperatures of 200 degrees C. The 4'-methoxy derivative exists in at least two polymorphs. Small crystals of the alpha-phase could be obtained from N-methylpyrrolidone at 150 degrees C. A crystal with a size of just 1 x 2 x 10 mu m(3) was sufficient to determine the crystal structure by micro-focus synchrotron-X-ray diffraction. The phenyl groups are disordered. The crystal structure of the beta-phase of the 4'-methoxy derivative, as well as the structures of the two other derivatives were determined from X-ray powder diffraction data. The crystal structures were solved by real-space methods using the program DASH, the Rietveld refinement was done with the program TOPAS. The 2',5'-dichloro derivative crystallises in P (1) over bar, Z = 1, all other phases in P2(1)/c with Z = 2. All compounds form intramolecular hydrogen bonds, which explain the solid state fluorescence.