The synthesis and photophysics of two new aminopropenyl naphthalene diimide (SANDI) dyes are reported. A general and convenient method for the synthesis of the precursor mono-, di-, and tetrabrominated 1,4,5,8-naphthalene tetracarboxylic dianhydrides is described. The two core-substituted SANDIs exhibit many of the photophysical properties required for fluorescence labeling applications including high photostability and high fluorescence quantum yields (>0.5) in the visible region of the spectrum. The emission wavelength is sensitive to the number of substituents on the NDI core, and the fluorescence decay times are in the range of approximately 8-12 ns for both compounds in the solvents investigated. Preliminary fluorescence emission data from single molecules of the compounds embedded in poly(methyl methacrylate) films are also reported and show that single molecules have very low yields of photobleaching, particularly the di-substituted system. Furthermore, only a small proportion (<10 %) of the single molecules studied display fluorescence intermittencies or "blinks" in their photon trajectory. The compounds appear to be excellent candidates for applications at the single molecule level, for example, as FRET labels.