Stability and structural aspects of diketopyrrolopyrrole pigment and its N-alkyl derivatives

An unsubstituted sample, three symmetrically N-substituted samples (methyl, butyl and heptyl) and two asymmetrically N-substituted samples (butyl and heptyl) of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione (DPP) were investigated using thermogravimetry and differential scanning calorimetry to reveal the influence on physical-chemical properties of different alkyl chains and symmetry of N-substitution. Stability tests revealed that in all cases the substitution brought significant destabilization of the structure in comparison with the unsubstituted DPP molecule. It was demonstrated that the length of the substituting alkyl chain is a crucial factor in the stability of N-alkyl derivatives; the shorter the alkyl chain was, the less stable was the derivative. Further, the symmetrical derivates were less stable than the asymmetrical ones. Unlike the unsubstituted DPP molecule, all the derivatives showed remarkable sensitivity to different cooling regimes which lead to the revealing of monotropical polymorphism in the symmetrical butyl and heptyl derivatives crystalline structure. (C) 2010 Elsevier Ltd. All rights reserved.