A High-Toughness NDI-Based Fluorescent Polyurea Based on Gradient Energy Dissipation

1,5-Naphthalene diisocyanate (NDI) has garnered considerable attention in the design of functional materials due to its excellent photophysical properties and corrosion resistance. However, materials synthesized from NDI typically exhibit poor toughness, limiting their broader applications. To overcome these limitations, this study synthesized a polyurea material that combines both high toughness and fluorescence properties by constructing the polymer backbone through the rapid reaction of NDI with amines, followed by covalent cross-linking between hard domains. The material demonstrates a mechanical strength of 24.8 MPa and an ultrahigh toughness of 505.25 MJ/m3. Its exceptional toughness stems from a gradient energy dissipation mechanism, which is governed by the dissociation of strong and weak hydrogen bonds (with bond dissociation energies of 76.64 kJ/mol and 64.89 kJ/mol, respectively) and the decomposition of concentrated hard domains. Additionally, the naphthalene rings in NDI possess a planar rigid conjugated structure, and through covalent cross-linking among hard domains, the nonradiative transition is restricted, and the radiative transition is augmented, thereby enabling the material to exhibit intense and stable blue fluorescence under 365 nm UV lamps. The material can still fluoresce in the presence of a strong acid and a strong base, where the strong base enhances the fluorescence intensity, while the strong acid has the opposite effect. By modifying the form of the urea group surrounding the naphthalene ring, the acid–base effect influences the distribution of the electron cloud of the naphthalene ring and subsequently alters the fluorescence intensity. This study offers a novel concept for preparing NDI-based polyurea materials with high toughness and stable fluorescence properties.