Source of materialThe title compound was synthesized by the method previously described [1], The sample was purified by sublimation at about 610 K, using a two-zone furnace [2]. However, the single crystals were quite small and their cry stallinity was poor as reflected in the rather high R values. The single crystals were grown from the vapor phase in a closed system. After 48 hours, a number of red platelet crystals were obtained. DiscussionDiketopyrrolopyrrole (abbreviated to DPP) pigments are industrially important red pigments based on the novel diketopyrrolopyrrole chromophore [3]. DPPs are also used as colorants for imaging areas as well as red color filters for LCD applications. Up to now, five DPP-derivatives are on the market and most of their structures have already been analyzed [3][4][5][6][7]. The title compound (MTB-DPP) is an asymmetrical molecule characterized by an extremely large bathochromic shift on going from solution to the solid state as compared with that of other DPP analogues. As shown in the figure, the molecule is characterized by the C1 symmetry. The two phenyl rings on each side of the heterocyclic ring system are twisted asymmetrically in opposite directions ("propeller type"): about 11.1 ° at the C10-C13 bond and about 14.2° at the C4-C7 bond as in the case of /m-butylphenyl derivative (TB-DPP) [6] and biphenyl derivative (BP-DPP) [8], On the other hand, the twisting of the phenyl rings takes place in the same direction in other DPPs (C, symmetry) [4,5,7]. In addition, the heterocyclic ring system is not entirely planar, but is folded in the middle with a dihedral angle of 177.7° as in TB-DPP and BP-DPP, while it is completely planar in other DPPs. Due to this deformed structure, a slight dipole moment of about 0.16 D appears perpendicular to the molecular plane. The molecules are stacked in a "bricks in a brick wall" fashion as in BP-DPP. On the (a,c) molecular plane, 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 bond network as found in all DPP pigments [4][5][6][7][8] and causes the very good thermal and solvent resistance of the hydrogen bonded DPP pigments.

droxide, 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 7 0 bs > 1 o(Iobs)· However, the threshold expression of 7 0 bs > 2 a(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 dikctobis(ferf-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(rerf-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-0 hydrogen bond network and the molecules are arranged in a fashion "bricks in a brick wall". In addition, the r-butyl group of MTB-DPP faces the phenyl ring of the neighboring one.

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