Effect of substituents on Stokes shift of BODIPY and its application in designing bioimaging probes

Abstract BODIPY-based probes have excellent fluorescence properties. However, small Stokes shifts approximately 5–15 nm greatly affect their detection sensitivity. In this study, we compared the Stokes shifts of reported BODIPY-based probes with various of substituents, and found that the phenyl groups on the specific position of BODIPY core could expand the Stokes shift of BODIPY-based probes, and methoxy groups on these phenyl substituents could enhance such effects. Then, by quantum chemical calculations, we found that the number of methoxy groups might also have obvious effect on the Stokes shift of BODIPY. Taking nitric oxide (NO) as analyte, 4,4-difluoro-8-(3,4-diaminophenyl)-3,5-bis(2,4-dimethoxyphenyl)-4-bora-3a,4a-diaza-s-indancene (DMOPB) with diaminophenyl substituents has been designed and synthesized. Compared with monomethoxy-phenyl substituted BODIPY-based probes (MOPBs) in our previous work, Stokes shift of DMOPB was expanded by 10 nm when using dimethoxyphenyl instead of monomethoxyphenyl, which is basically consistent with the quantum chemistry calculation of 11 nm. DMOPB can react with NO in only 2 min to form the triazole DMOPB-T with a fluorescence quantum yield of 0.32. An excellent linear relationship was observed in the range of NO concentration from 0.5 μM to 4 μM and the detection limit was 1 nM. The experimental results indicate that DMOPB with high sensitivity, excellent selectivity, low toxicity and dark background can be a great candidate for imaging NO in cells and tissues. Considering the lack of practical way to increase Stokes shift of small-molecule fluorescent probes based on specific fluorophore, the proposed strategy has great potential for the designing of probes with large Stokes shift.