No abstract

Diketopyrrolopyrroles are typical hydrogen-bonded pigments of red color on the basis of the skeleton of 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP). Our previous investigation revealed that the 1:1 mixed crystal (MX-DPP) composed of DPP and its tert-butylphenyl derivative (BTB-DPP) gives practically the same electronic spectra as well as X-ray diffraction diagrams as those of the hybrid compound (MTB-DPP) of DPP and BTB-DPP. To elucidate the above coincidence, electronic structure has been studied in the present investigation in evaporated films and single crystals of MX-DPP and MTB-DPP with major focus on the crystal structure and intermolecular interactions. According to the structure analysis, the space groups of both crystals are the same (P1 h) and the crystallographic parameters are also quite similar. In both crystals, the same molecular environment (in other words, "local similarity") prevails throughout the crystal as characterized by two pairs of "tert-butyl group and phenyl ring". These facts lead us to conclude that the local similarity is the determinant for the coincidence of the crystallographic and electronic structures.

Source of materialDiketo-diphenylpyrrolopyrrole (DPP) and diketo-bis(tert-butylphenyl)pyrrolo pyrrole (BTB-DPP) were obtained from Ciba Specialty Chemicals. The 1:1 mixed crystal (MX-DPP) was prepared by deprotonation of DPP and BTB-DPP with sodium hydroxide, followed by protonation with hydrochloric acid [1]. MX-DPP was then purified by sublimation at about 610 K, using two-zone furnace [2]. Organic pigments are quite insoluble in organic solvents. Thus, after a long optimization of the growth parameters, it is possible to grow single crystals from the vapor phase in a closed system. Red platelet crystals of the title compound were obtained after 48 hours. Experimental detailsThe present crystal is extremely small and the crystallinity is rather poor. Therefore, only isotropic refinement has been carried out on MX-DPP. Nevertheless, the bond parameters seem to be quite reasonable. Refinement was done using 769 reflections with I obs > 1 s(I obs ). However, the threshold expression of I obs > 2 s(Iobs) was used for calculating Rgt(F) (409 reflections). No additional peaks were found on difference Fourier maps. DiscussionThe 1:1 mixed crystal (MX-DPP) contains two kinds of diketopyrrolopyrroles known as industrially important red pigments: diketo-diphenylpyrrolopyrrole (DPP) [3,4] and diketobis(tert-butylphenyl)pyrrolopyrrole (BTB-DPP) [5]. We have recently found that MX-DPP composed of DPP and BTB-DPP gives practically the same electronic spectra as well as the X-ray diffraction diagrams as those of the hybrid compound of DPP and BTB-DPP; diketo-mono(tert-butylphenyl) pyrrolopyrrole (MTB-DPP) [1,6]. The present coincidence of these physical properties is of great scientific interest and also of industrial interest, because the color specified by expensive MTB-DPP (due to the synthesis of asymmetrical molecules) can be achieved by inexpensive DPP and BTB-DPP (due to the synthesis of symmetrical molecules). Previously, we have reported the crystal structure of MTB-DPP [7]. This shows that there is a NH···O hydrogen bond network and the molecules are arranged in a fashion "bricks in a brick wall". In addition, the t-butyl group of MTB-DPP faces the phenyl ring of the neighboring one. The structure of the present MX-DPP is found to be isomorphous with that of MTB-DPP. The two component molecules are shown in top figure. Each molecule has a symmetry of C i. The two phenyl rings on each side of the heterocyclic ring system are twisted symmetrically in the same direction: 13°in DPP and 11°i n BTB-DPP with respect to the heterocyclic system. The both molecules are stacked alternately in a fashion "bricks in a brick wall" just as found in MTB-DPP [7]. On each molecular plane of DPP and BTB-DPP, there are chains of intermolecular hydrogen bonds between the NH group of one molecule and the O atom of the neighboring one. This forms a two-dimensional hydrogen Z. Kristallogr. NCS 218 (2003)