Two polymorphs of a diketopyrrolopyrrole dye, one yellow and the other orange, were obtained.

The bisazomethine dyes derived from diaminomaleonitrile and aminobenzaldehydes, 2,3-bis[(E)-4-(dialkylamino)benzylideneamino]fumaronitrile derivatives, are known to have potential as dyes for forming J-aggregate vapour-deposited films, even though appropriate conditions for preparation of their pure J-aggregate films are unknown. In this study, an attempt was made to investigate the effects of alkoxy substitution on the crystal structures of eight bisazomethine dyes; our final goal was to obtain pure J-aggregates of bisazomethine dyes in a crystalline state. The focus of this study is on the relationship between changes in the molecular structure and the crystal structure, characterized by a two-dimensional molecular stacking layer and its alignment. We found that the interlayer distance between adjacent two-dimensional stacking layers can be controlled by alkoxy substitution, when the substituent chain length is less than C10, without any significant structural changes in the stacking layer. Our observation was confirmed by lattice energy calculations from an energetic perspective. Scheme 1 Chemical structures of dyes 1-8.

The terminal dimethyl and diethyl substituents of the bisazomethine derivatives were found to affect the formation of J-aggregate-like 2-D molecular arrangements.

Crystal structures of three bisazomethine dyes with dipropyl, dibutyl, and dihexyl substituents on their terminal amino groups are reported. To systematically interpret the effects of the terminal dialkyl substituents on the low-dimensional arrangements of the π–π stacked molecules, the structural features of the molecular geometries and the low-dimensional arrangements were compared with those in the reported crystal structure of two bisazomethine dyes, i.e. with terminal dimethylamino and diethylamino groups. Lattice energy calculations were also carried out to interpret the substitution effects from an energetic perspective. In the crystal structures of all five dyes, one-dimensional arrangements of the π–π stacked molecules were found. The slip angles between the π–π stacked molecules constituting the characteristic one-dimensional arrangements of the five bisazomethine dyes were distributed in the range of 24.66(4)–79.34(7)°. The lengths of the alkyl chains and projections of the terminal dialkyl substituents from the molecular planes in the five bisazomethine dyes were found to play significant roles in determining the slip angles between the π–π stacked molecules and the distances between the molecules aligned along the long molecular axes.