Fluorescent probes based 1,8-naphthalimide-nitrogen heterocyclic for monitoring the fluctuation of mitochondrial viscosity

Abnormal mitochondrial viscosity could lead to diabetes and neurodegenerative diseases, therefore three probes 1a-c for monitoring the fluctuation of viscosity were designed and synthesized in this article. Probes 1a-c were composed of 1,8-naphthalimide derivatives and benzoxazole, benzothiophene or oxazole [4,5-b] pyridine structure through carbon-carbon double bond connection, and their emission wavelengths increased sequentially (1a: 580 nm; 1b: 614 nm; 1c: 630 nm). The single bond between the nitrogen-containing heterocycle and the carbon-carbon double bond served as the viscosity response site. When the viscosity increased from 0.95 cP to 945 cP, the fluorescence intensity of probes 1a-c increased 18.67-fold, 20.34-fold and 18.37-fold, respectively. In 99% glycerol, the fluorescence quantum yields of probes 1a-c were 85.4%, 80.8% and 34.3% respectively. In addition, the excited state lifetime of probe 1c in 80% glycerol (725 ps) was higher than that in 40% glycerol (203 ps). More importantly, the influence of solvent polarity, pH and some potential interfering substances on probes 1a-c were negligible. And cell experiments showed that probes 1a-c could be used for monitoring the fluctuation of the mitochondrial viscosity in HeLa cells.