<p>A functional fluorescent molecule, 4-bromo-1,8-naphthalimide derivative (BN1B) was designed and synthesized and used as a fluorescent probe for detecting Cr(VI) in aqueous media. The fluorescence intensity of BN1B decreased with increasing Cr(VI) concentration, and the detection limits were 3.64 × 10⁻⁶ mol·L^⁻1 for CrO₄^2⁻ and 2.98 × 10⁻⁶ mol·L^⁻1 for Cr₂O₇^2⁻. Moreover, a distinct fluorescence color change was observed for BN1B solutions containing CrO₄^2⁻ or Cr₂O₇^2⁻ under 356&#xa0;nm UV irradiation, producing pale yellow and orange yellow emissions, respectively. The probe features simple preparation, low cost, fast response, and high sensitivity. Exploiting the favorable optical properties of BN1B, a supramolecular assembly, BN1B@Q[8], was constructed with cucurbit[8]uril (Q[8]). In this assembly, the benzene rings and part of the naphthalene moiety of BN1B are encapsulated by Q[8], forming a 2:1 host-guest inclusion complex. Furthermore, a film was prepared by incorporating BN1B@Q[8] into a polyvinyl alcohol (PVA) matrix at a mass ratio of 1:50. The film effectively isolated the assembly from atmospheric oxygen and moisture, suppressed non-radiative transitions, and induced long-lived room-temperature phosphorescence with a lifetime τ = 55.19&#xa0;ms. The use of this phosphorescent film in anti-counterfeiting highlights its potential for expanding applications of luminescent materials in encryption and related fields.</p> Graphical abstract <p></p>