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1-Acetyl-3-phenyl-5-(1-pyrenyl)-pyrazoline(APPP) was synthesizedand formed four types of different crystals under different crystallizationconditions: guest-free crystal (I, α polymorph),APPP·chloroform solvate (II), APPP·phenol cocrystal(III), and APPP·acetic acid solvate (IV). Twisted structures and different stacking modes of APPP moleculeswere found in the four crystals. Single crystal X-ray analyses revealedpyrene fluorophores adopt an edge-to-face π-stacked arrangementin crystal I, mononer arrangement in crystals II and III, and face-to-face slipped π-stacked arrangementin crystal IV. Another phase (β polymorph) wasdiscovered during DSC experiments of crystals IIand IVand obtained by desolvation of crystal II.However, solvent-mediated phase transitions revealed β polymorphis a metastable phase at room temperature, and no single crystal couldbe isolated. The optical–physical properties of these solidswere investigated. Crystal Ishows a broad emission band(λmax= 419 nm) with a red shift of about 40 nm relativeto its flurescence in solvents, in which the vibrational featuresare less clear. The spectra of crystals IIand IIIand β polymorph are similar to each other, exhibitingthe shortest λem(λmax= 399–400nm), and the fluorescence spectra with vibrational features are closeto pyrene fluorescence in solvents. In contrast, crystal IVshows structureless and broad emission spectrum with the longestλem(λmax= 452 nm) among thesecrystals. The difference in their optical–physical propertiesis closely related to the different arrangements of pyrene fluorophores.The monomer arrangements of pyrene fluorophores may be responsiblefor the shortest λem, higher emission quantum yields,and longer lifetimes. For a given organic luminescent material, thestrategy based on the solvates and cocrystals can not only tune theoptical–physical properties but also be helpful to find suitablepolymorphic phases. [ABSTRACT FROM AUTHOR]