Tuning Solid-State Fluorescence of a Twisted π-Conjugated Molecule by Regulating the Arrangement of Anthracene Fluorophores

1-(5-(Anthracen-9-yl)-3-(4-methoxyphenyl)-4,5-dihydropyrazol-1-yl)ethanone was synthesized and crystallized to provide three types of crystals with different fluorescent colors and host-guest structures. Crystal structure analysis reveals that this compound possesses twisted pi-conjugated structure and different degrees of distortion depending on guest molecules in the three crystal structures. The anthracene fluorophore stacking modes are regulated from monomer arrangement to face-to-face pi-stacked arrangement by means of the entrapment of organic acid molecules in the lattice. The vibrational spectroscopy, thermal behaviors, diffuse reflectance absorption spectroscopy, solid-state fluorescence properties, and fluorescence quantum yields and lifetimes of the three types of crystals were investigated. Such properties are closely related to the fluorophore stacking modes and intermolecular electronic interactions in crystals. The pi-stacked geometries of anthracene fluorophores are responsible for the red-shifted emissions and longer fluorescence lifetimes. It indicates that the optical properties of organic materials could be modulated by entrapping different guest molecules in lattice.