Diversity through Similarity: A World of Polymorphs, Solid Solutions, and Cocrystals in a Vial of 4,4′-Diazopyridine

4,4'-Diazopyridine (AP) is largely known for its photophysical properties, and it is extensively studied in solution as molecular switch. By contrast, AP is not similarly explored in the solid state. We here discuss the fine crystallographic interpretation of the electron density maps combined with occupancy refinement of solid solutions, cocrystals and solid solution of cocrystals of AP and some halogenated derivatives, obtained as a result of recrystallization experiments. AP halogenated derivatives are contained as impurities in the purchased starting material and present a remarkable propensity to give cocrystals or solid solutions along with AP. A new AP polymorph (AP2) has also been identified, and phase evolution as a function of temperature has been investigated by differential scanning calorimetry and variable temperature X-ray powder diffraction.