Structural Insight of Anthracene Orientation by Halogen Substitution: Impact on Solid-State Fluorescence and Stimuli-Induced Fluorescence Switching

The orientation of an anthracene π-conjugated aromatic group and excimer formation significantly influenced the fluorescence efficiency and functions of the resulting smart fluorescent materials. Herein, we have synthesized an alpha-cyanostyrene-integrated anthracene (1) fluorophore and investigated a halogen (F, C, and Br) effect on anthracene arrangement and solid-state fluorescence. Naphthalene-type excimer formation was observed in para-F and para-Cl, whereas unsubstituted 1 and meta-F and meta-Cl produced anthracene-type excimer. meta-F showed strong π···π interaction and higher overlapping of anthracene units, but meta-Cl exhibited alternative π···π and C–H···π interactions and produced isolated discrete dimers. However, Br isomers showed good separation of anthracene units without any significant overlapping. The strong π···π stacking arrangements of anthracene in meta-F resulted in complete quenching of fluorescence, whereas good separation of anthracene in Br isomers led to enhanced solid-state fluorescence (ΦF = 61.9%). Contrary to a traditional heavy-atom quenching effect, bromo-derivatives showed high fluorescence efficiency because of their structural arrangement. The columnar stacking arrangements of anthracene units caused off–on fluorescence switching upon crushing/heating or solvent exposure. Powder X-ray diffraction studies indicated the reversible transformation between crystalline and amorphous/partial amorphous phases upon crushing and heating/solvent exposure, which caused reversible fluorescence switching. Overall, the present studies provided structural insight for controlling anthracene orientation via halogen substitution for developing efficient solid-state smart fluorescent materials.