A novel fluorescent bis-lactam scaffold presenting high photostability and brightness

As the use and importance of fluorescent materials in biomedical fields increase, the demand for stable and bright fluorescent materials in biological environment is increasing. Herein, a new fluorescent scaffold that satisfies the demands named 1,5-naphthyridine-2,6-dione (NTD) was developed and used for staining cells. Thienyl-flanked NTD (NTDT) exhibited photoluminescence quantum yield of 0.98 and brightness of 4.01 × 10⁴, which was 1.2–1.6 times larger than those of diketopyrrolopyrrole-based model compound (DPPT). K_r and k_nr values of NTDT were twice larger and one order of magnitude smaller than those of DPPT, respectively. Theoretical calculation supported the better photophysical properties via lower reorganization energy and 1.9 times of oscillator strength of NTDT than those of DPPT, originated from rigid and planar structure of NTDT. Especially, NTDT showed no degradation and 62% maintenance of fluorescence in organic solvent and HeLa cells, respectively, while DPPT showed below 50% and 35% maintenances, respectively at the same light irradiation condition. Based on the synthetic flexibility of NTD core, colors are tuned from blue to red and solubility is adjusted for aqueous environment. As a result, various fluorophores emitting blue to red are developed and red, green, and blue staining is successfully performed in the HeLa cells.