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Regulation of Arrangements of Pyrene Fluorophores via Solvates and Cocrystals for Fluorescence Modulation

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journal contribution
posted on 18.02.2016 by Qi Feng, Mingliang Wang, Baoli Dong, Jing He, Chunxiang Xu
1-Acetyl-3-phenyl-5-(1-pyrenyl)-pyrazoline (APPP) was synthesized and formed four types of different crystals under different crystallization conditions: 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 molecules were found in the four crystals. Single crystal X-ray analyses revealed pyrene fluorophores adopt an edge-to-face π-stacked arrangement in crystal I, mononer arrangement in crystals II and III, and face-to-face slipped π-stacked arrangement in crystal IV. Another phase (β polymorph) was discovered during DSC experiments of crystals II and IV and obtained by desolvation of crystal II. However, solvent-mediated phase transitions revealed β polymorph is a metastable phase at room temperature, and no single crystal could be isolated. The optical–physical properties of these solids were investigated. Crystal I shows a broad emission band (λmax = 419 nm) with a red shift of about 40 nm relative to its flurescence in solvents, in which the vibrational features are less clear. The spectra of crystals II and III and β polymorph are similar to each other, exhibiting the shortest λemmax = 399–400 nm), and the fluorescence spectra with vibrational features are close to pyrene fluorescence in solvents. In contrast, crystal IV shows structureless and broad emission spectrum with the longest λemmax = 452 nm) among these crystals. The difference in their optical–physical properties is closely related to the different arrangements of pyrene fluorophores. The monomer arrangements of pyrene fluorophores may be responsible for the shortest λem, higher emission quantum yields, and longer lifetimes. For a given organic luminescent material, the strategy based on the solvates and cocrystals can not only tune the optical–physical properties but also be helpful to find suitable polymorphic phases.