Long-wavelength dual-state luminescent naphthalimide derivatives for bio-imaging

Developing fluorescent materials with long-wavelength emission, a large Stokes shift, dual-state emission, and long emissive lifetime is highly challenging. Herein, we combined the donor-pi-acceptor (D-pi-A) structure, alkyl chain, and J-aggregation to obtain four-in-one long-wavelength emission dyes. By introducing a triphenylamine or N,N-dimethylaminomine moiety into naphthalimide to construct large conjugate D-pi-A structures, long-wavelength luminescence with a large Stokes shift can be obtained in the solution state due to the large conjugate structure and the intramolecular charge transfer effect. The distorted donor structure and long alkyl chains inhibit the face-to-face stacking of molecules to achieve long-wavelength fluorescence in the powder state. Moreover, the naphthalimide group with a planar rigid structure can specifically drive molecules to form J-aggregates in the powder state, prolonging the excited state lifetimes for a longer fluorescence lifetime. The longest emission wavelength was up to 831 nm with the largest Stokes shift of 383 nm in the solution of compound 2. However, the longest fluorescence lifetime of 7.28 ns can be observed in the powder state of compound 1. Hence, these naphthalimide derivatives can effectively be applied to viscosity-responsive fluorescence lifetime imaging. This work provides a new strategy for developing multi-state and long-lifetime emissive long-wavelength luminescent molecules. A series of naphthalimide derivatives with long-wavelength emission, large Stokes shift, dual-state emission, and long emissive lifetime were obtained by constructing D-pi-A structures applying in viscosity-responsive fluorescence lifetime imaging.