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In situ cryo-crystallization was used to obtain single crystals of the low-melting tetrafluoropyrimidine and trifluorotriazine in order to explore the molecular aggregation of nitrogen-containing aromatic compounds with weak directing substituents in the solid state. A comparative crystallographic study of the series hexafluorobenzene, pentafluoropyridine, tetrafluoropyrimidine and trifluorotriazine was performed to clarify an influence of a successive substitution of C-F groups by a further nitrogen atom on the crystal architecture. Due to the absence of the hydrogen atoms different supramolecular motifs based on short F···F contacts were observed in all crystal structures. [ABSTRACT FROM AUTHOR]