Tuning solid-state fluorescence of pyrene derivatives via a cocrystal strategy

1-Acetyl-3-(4-methoxyphenyl)-5-(1-pyrenyl)-pyrazoline (AMPP) was synthesized and crystallized to yield three types of crystals with different host-guest structures: AMPP crystal (I), AMPP-phenol (II) and AMPP- 2-naphthol) cocrystals (III). Twisted pi-conjugated structures and different stacking modes of AMPP molecules were found in the three crystals. In crystal I, pyrene fluorophores adopt a face-to-face pi-stacked arrangement, while pyrene fluorophores in crystals II and III adopt the monomer arrangements because AMPPs were widely separated by means of entrapment of guest molecules in the two lattices. Investigation of the optical-properties of the three crystals reveals that they are closely related to the arrangements of the pyrene fluorophores. Face-to-face pi-pi stacking of the pyrene fluorophores in crystal I cause a broad band emission with a red shift of 40-50 nm relative to pyrene fluorescence in solution. However, the emission spectrum of crystal II is similar to that of crystal III; both of them exhibit two sharp bands with a small red shift of 20 nm relative to pyrene fluorescence in solution, which are induced by the monomer arrangements of pyrene fluorophores and overlaps of pi-orbitals between benzene and pyrene fluorophores. These results demonstrate that using a cocrystal strategy to regulate packing modes of pyrene fluorophores is an effective way for exploring highly blue-emissive pyrene derivatives.