Convenient construction of fluorescent markers for lipid droplets with 1,8-naphthalimide unit

1,8-Naphthalimide is widely applied in the construction of probes because it is easy to be modified. However, 1,8-naphthalimide has been reported rarely in the application of organelle labelling especially lipid droplets (LDs). Herein, three types of amine compounds, aliphatic amine (a, b), aromatic amine (c, d) and heterocyclic amine (e, f), were chosen to construct fluorescent dyes (1a-1f) with 4-bromo-N-butylnaphthalimide (1) by Ullmann reaction. The maximum emission wavelength ranged from 490 nm to 550 nm. What's more, 1a-1e showed high fluorescence quantum yields (0.54-0.93) in chloroform and 1e-1f had excellent photostability (>92%). In addition, the cell imaging experiment showed that they could label lipid droplets for HeLa cancer cells and L929 normal cells with extremely low toxicity. This work realizes the classical fluorophore 1,8-naphthalimide to synthesize a series of LDs markers without complex synthesis steps.