Regulation of Arrangements of Pyrene Fluorophores
via Solvates and Cocrystals for Fluorescence Modulation
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Abstract
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 (<b>I</b>, α polymorph),
APPP·chloroform solvate (<b>II</b>), APPP·phenol cocrystal
(<b>III</b>), and APPP·acetic acid solvate (<b>IV</b>). 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 <b>I</b>, mononer arrangement in crystals <b>II </b>and <b>III</b>, and face-to-face slipped π-stacked arrangement
in crystal <b>IV</b>. Another phase (β polymorph) was
discovered during DSC experiments of crystals <b>II</b> and <b>IV</b> and obtained by desolvation of crystal <b>II</b>.
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 <b>I</b> shows a broad emission band
(λ<sub>max</sub> = 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 <b>II</b> and <b>III</b> and β polymorph are similar to each other, exhibiting
the shortest λ<sub>em</sub> (λ<sub>max</sub> = 399–400
nm), and the fluorescence spectra with vibrational features are close
to pyrene fluorescence in solvents. In contrast, crystal <b>IV</b> shows structureless and broad emission spectrum with the longest
λ<sub>em</sub> (λ<sub>max</sub> = 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 λ<sub>em</sub>, 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