In the quest to engineer multifunctional fluorescent materials, 1,8-naphthalimide (NPI) and salicylaldehyde Schiff base molecules were considered as the promising fluorophores because of their fascinating vistas. Herein, from 1,8-naphthalic anhydride precursor, a novel salicylaldehyde Schiff-base-functionalized 1,8-naphthalimide (NPI) luminophore, where NPI core and salicylaldehyde Schiff base were judiciously integrated through a flexible methylene spacer, was elaborately designed and successfully synthesized in a high yield of 90 %. The structure-property relationship was meticulously lucubrated by comparing the photophysical properties of it and its analogues. Mainly, it enjoyed excited-state intramolecular proton transfer (ESIPT) process upon photoexcitation in solution state. Interestingly, it showed the vividly fluorochromic behaviours at the unimolecular level in the aggregation/solid/film states. Specifically, it exhibited multiple self-assembly stages in the proportionally mixed solutions of THF/H2O with different water fractions (fw), accompanying fluorescence transitions from aggregation-induced emission (AIE)+ESIPT-based emission to J-type aggregation-induced emission to H-type aggregation caused quenching. Moreover, its film fabricated by casting EtOH solution (10- 5 M) onto the glass slide displayed thickness-dependent fluorescence colour changes and reversible acid/base stimuli-responsive fluorescence. Furthermore, its solid obtained by crystallization in the EtOH/water (1/1, v/v) mixtures (approximately 10- 2 M) exhibited the reversible mechanochromic luminescence. This work not only realized efficient and controllable aggregation/solid/film fluorescence from a salicylaldehyde Schiff-base-functionalized 1,8-naphthalimide luminophore but also provided a strategy for constructing NPI-based smart fluorescent materials.
