Pyridine nitrogen position controlled molecular packing and stimuli-responsive solid-state fluorescence switching: supramolecular complexation facilitated turn-on fluorescence

Abstract

The fluorophore structure and supramolecular interactions play important roles in the molecular conformation and packing in the solid state that strongly influence the solid-state fluorescence properties. Herein, we report the synthesis of pyridylacetonitrile integrated anthracene fluorophores ((Z)-3-(anthracen-9-yl)-2-(pyridin-2-yl)acrylonitrile (2-APy), (Z)-3-(anthracen-9-yl)-2-(pyridin-3-yl)acrylonitrile (3-APy) and (Z)-3-(anthracen-9-yl)-2-(pyridin-4-yl)acrylonitrile (4-APy)) and pyridine nitrogen position controlled stimuli-responsive fluorescence switching. 3-APy exhibited strongly enhanced solid state fluorescence (Φ_F = 45.83%) compared to 2-APy (Φ_F = 1.68%) and 4-APy (Φ_F = 6.05%). Moreover, 3-APy displayed reversible pressure/heating induced off–on fluorescence switching whereas the quenched fluorescence of 2-APy and 4-APy by crushing did not recover after heating. Solid state structural analysis revealed a twisted molecular conformation between anthracene and pyridylacetonitrile in all three molecules. However, the pyridine nitrogen position significantly influenced the extent of twisting and molecular packing. The highly twisted molecular conformation and intermolecular interactions facilitated good separation of anthracene molecules without π–π stacking in the crystal lattice of 3-APy. In contrast, 2-APy showed a face-to-face arrangement of pyridine rings whereas anthracene and pyridine rings were overlapped in the 4-APy crystal lattice contributing to weak fluorescence. Powder X-ray diffraction (PXRD) of 3-APy showed reversible transformation between crystalline and amorphous phases upon crushing and heating, causing reversible off–on fluorescence switching. The strong solid-state fluorescence and pyridine unit of 3-APy were used to demonstrate halochromic reversible fluorescence switching. Further the weak fluorescence of 2-APy and 4-APy was strongly enhanced via supramolecular co-crystal formation with p-toluenesulfonic acid (PTSA) which hindered aromatic overlapping and close packing in the crystal lattice. Overall, the present work studied the impact of subtle positional change of nitrogen on the molecular aggregation and solid-state fluorescence properties.