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A red zwitterionic co-crystal of acetaminophen and 2,4-pyridinedicarboxylic acid.

We report on a co-crystal of acetaminophen (APAP) and 2,4-pyridinedicarboxylic acid (PDA). The co-crystal was discovered by screening using the solution-mediated phase transformation (SMPT) technique. Despite the bulk solids of each component being white in color, the new co-crystal phase exhibited a red color. The new phase was analyzed using single-crystal X-ray diffraction and identified as (APAP).(PDA).(1). Structural analysis revealed PDA to exist in a hitherto unreported zwitterionic form in the co-crystal. A structural analysis of pure PDA revealed the presence of the zwitterion form in (PDA).(H(2)O) (2), as well. The components of 1 self-assemble as a three-dimensional (3D) hydrogen-bonded network with a pronounced 2D structure. The origin of the red color was investigated using density functional theory calculations, which demonstrate a decreasing pi-pi(*) separation involving the components of the solid.