Solid-state red fluorescence of intramolecularly ring-fused donor-π-acceptor-type fluorinated diphenylacetylenes achieved by enhancing intramolecular charge transfer properties

Developing efficient red fluorescent materials is crucial in various fields, such as medicine, chemical biology, and organic electronics. Molecular design strategies involving the extension of pi-conjugation lower the solid-state fluorescence efficiency because of the energy gap law and stacking. Precisely controlling the intramolecular charge transfer properties can be a practical method for inducing fluorescence redshift. This study examines the design and synthesis of ring-fused donor-pi-acceptor-type fluorinated diphenylacetylenes-with donor units suppressing intramolecular rotation-and their photophysical properties. Target molecules with a ring-fused aromatic-amine donor unit emit strong yellow fluorescence in dilute solutions. Changing the donor unit to a ring-fused aliphatic amine causes the fluorescence wavelength to undergo a redshift, significantly reducing the fluorescence efficiency. In the solid state, fluorinated diphenylacetylene, possessing a julolidine unit, emitted 643 nm-long red fluorescence wavelengths with high fluorescence efficiency. On mechanical grinding, this fluorescence wavelength undergoes a slight blueshift. However, the red fluorescence is maintained, and the fluorescence quantum yield is 0.20, which is promising for facilitating future research on solid-state red-fluorescent materials.