Stability and physical structure tests of piperidyl and morpholinyl derivatives of diphenyl-diketo-pyrrolopyrroles (DPP)

Crystalline structure, thermo-oxidative and thermal stability of symmetrical and asymmetrical piperidyl and morpholinyl derivatives of both N-substituted and non-N-substituted butyl diphenyl-diketo-pyrrolopyrrole (DPP) pigments were studied using differential scanning calorimetry (DSC) and thermogravimetry (TG). Except for the asymmetrical morpholine DPP derivative, all the samples showed melting peaks which were relatively close to their degradation temperatures (from 260 to 430 °C). Using DSC, monotropic polymorphism was revealed in the symmetrical piperidyl-N-butyl-derivative which confirmed earlier observation about tendency of symmetrical N-alkyl DPP derivates to form several crystalline structures. TG carried out under nitrogen atmosphere served for distinguishing of evaporation/sublimation and degradation temperatures. Temperatures of evaporation/sublimation were typically 10–30 °C lower in comparison with temperatures of thermal degradation. The highest thermal (450 °C) and thermo-oxidative stability (around 360 °C) showed the DPP derivatives containing morpholine moieties with no alkyl substitution on NH-group of DPP core. The presence of the latter was found to be the most destabilizing factor. Piperidyl group showed more stabilizing effect due to its polar character and its influence on π–π intermolecular interactions of neighbouring phenyl groups. The highest stabilizing effect of morpholine moiety on DPP structure was explained based on the presence of polar oxygen atom in that group. The preparations of 3,6-di-(4-morpholinophenyl)-2,5-dihydro-pyrrolo3,4-c]pyrrole-1,4-dione and 3-(phenyl)-6-(4-morpholinophenyl)-2,5-dihydropyrrolo3,4-c]pyrrole-1,4-dione are reported.