Structure and Raman spectra of pyridyl substituted diketo-pyrrolo-pyrrole isomers and polymorphs

A complete series of symmetrical and unsymmetrical isomeric pairs of 4- and 2-pyridyl substituted diketo-pyrrolo-pyrroles was synthesized. Both polymorphs of 3,6-bis-(4′-pyridyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione were prepared. Asymmetrical 3-phenyl-6-pyridine-2′-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione was synthesized for the first time and X-ray structure of its monocrystal was determined. Density functional theory calculations of the ground state geometry were compared with experimental X-ray diffraction data. Theoretical Raman spectra enabled to assign the main peaks of the experimental ones for all four pyridyl DPP derivatives. Crucial spectral features, which reflect the molecular and crystal (H-bond invoked) asymmetry are CO (pyrrolinone) stretching, NH (pyrrolinone) bending and trigonal (hetero)aryl bending. Sublimation temperatures of 2-pyridyl derivatives are significantly lower than for 4-pyridyl derivatives, in which stronger in-plane NH(pyrrolinone)CO(pyrrolinone) intermolecular H-bonding dominates.