α-Cyanostilbene-modified 1,8-naphthalimide dye with long wavelength emission: synthesis, characterization, self-absorption + ESIPT, and controllable and reversible solid fluorescence

The development of 1,8-naphthalimides (NPIs) with tunable and controllable solid fluorescence has received unprecedented attention in many fields. Herein, we report the design, synthesis, and optical behaviors of a novel α-cyanostilbene-modified 1,8-naphthalimide dye constructed from the Schiff base condensation reaction between the salicylaldehyde-analogue α-cyanostilbene luminogen and the NPI derivative obtained through the selective imidization of 1,8-naphthalic anhydride with 3-aminobenzylamine. Its well-defined structure was confirmed by FT-IR, HR-ESI-MS, and ¹H/¹³C NMR spectroscopies. In the solution state, the steady-state absorption and emission results revealed that the self-absorption between the NPI core and α-cyanostilbene group occurred when the NPI core was excited (under the excitation at 340 nm), accompanied by the ESIPT process of the α-cyanostilbene unit because the α-cyanostilbene unit was excited after self-absorption. Although it did not exhibit particularly remarkable aggregation-induced emission (AIE) features in THF/water solvent mixtures as the water content was increased, it showed controllable and reversible solid fluorescence. In particular, its crystallization in EtOH solvent at room temperature afforded an orange solid in natural light, emitting bright red fluorescence under UV_365 nm illumination. However, crystallization in an EtOH/H₂O mixture (1 : 1, v/v) at room temperature afforded a yellow solid in natural light, emitting dull red fluorescence under UV_365 nm illumination. Switching of the emission colors between bright red and dull red was highly reversible by crystallizing it in EtOH and EtOH/H₂O (1 : 1, v/v), respectively. Furthermore, the solid obtained by crystallization in EtOH displayed reversible acid/base stimuli-responsive fluorescence. This work provides a strategy for constructing controllable and reversible NPI-based fluorescent materials.