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Diaminodicyanopyrazine chromophoric system was developed in 1990’s [1]. The derivatives with bulky substituents on the amino groups were found to exhibit a series of polymorphs with different colours [2]. On the other hand, two polymorphs with different fluorescent properties were obtained in the dye having methyl substituents on the amino groups 1, which is shown in the inset of Figure 1. These two polymorphs are coloured red, but their hues are different, one is dark (1D) and the other is bright (1B). DSC measurement indicated that 1B is the stable form and the irreversible phase transition from 1D to 1B undergoes. The solid state fluorescence spectra of 1D and 1B are shown in Figure 1. These spectra were determined by a conventional fluorophotometer using an accessory for powder samples. The emission maximum of 1B appears around 637 nm, while that of 1D shifts to the longer wavelength region with a considerable decrease in the intensity. In order to explain this spectral difference, X-ray structure analysis was performed. Although there was the difference in the planarity of the chromophoric system, molecular orbital calculations showed that there is no remarkable difference in their electronic states corresponding to those in the visible region. The difference was also recognized in their molecular arrangement. In 1D, molecules adopt a herringbone structure in which the molecules are stacked along the b-axis with an interplanar distance of 3.35Å. In 1B, molecules are also stacked to form a molecular column. However the large interplanar distance of about 7.08Å was observed between the stacking molecules, because molecules in the neighbouring columns are slightly inserted between the stacking molecular pair. An interplanar distance and a degree of overlap between stacking chromophoric systems are known to be correlated with fluorescent properties in the solid state [3]. The present difference in fluorescent properties between two polymorphs can be thus attributed to the different molecular arrangement in crystals.

Two polymorphs with different fluorescent properties of 2,5-bis(dimethylamino)-3,6-dicyanopyrazine dye