Engineering the BASHY Dye Platform toward Architectures with Responsive Fluorescence

A set of nine boronic-acid-derived salicylidenehydrazone (BASHY) complexes has been synthesized in good to very good chemical yields in a versatile three-component reaction. In an extension to previous reports on this dye platform, the focus was put on the electronic modification of the vertical positions of the salicylidenehydrazone backbone. This enabled the observation of fluorescence quenching by photoinduced electron transfer (PeT), which can be reverted by the addition of acid in organic solvent (OFF-ON fluorescence switching). The resulting emission is observed in the green-to-orange spectral region (maxima at 520-590 nm). In contrast, under physiological pH conditions in water, the PeT process is inherently decativated, thereby enabling the observation of fluorescence in the red-to-NIR region (maxima at 650-680 nm) with appreciable quantum yields and lifetimes. The latter characteristic supported the application of the dyes in fluorescence lifetime imaging (FLIM) of live A549 cells.

Automated summary

A series of boronic-acid-derived salicylidenehydrazone (BASHY) complexes were synthesized in good yields through a versatile three-component reaction. These complexes exhibit fluorescence switching behavior by photoinduced electron transfer, which can be controlled by the addition of acid. In organic solvent, the fluorescence is observed in the green-to-orange region, while in water under physiological pH conditions, the fluorescence is observed in the red-to-NIR region with appreciable quantum yields and lifetimes. These characteristics make these dyes suitable for fluorescence lifetime imaging of live cells.