Readily Accessible and Brightly Fluorogenic BODIPY/NBD-Tetrazines via SAr Reactions.

We describe SAr reactions of some commercial amino-tetrazines and halo-dyes, which give efficiently quenched BODIPY/NBD-tetrazines ( < 0.01) in high yields and, importantly, with high purities affordable via simple silica gel chromatography only. The dyes exhibit large Stokes shifts, moderate environmental sensitivity, and emission enhancements (up to 193-fold) upon Tz ligation with BCN─a strained dienophile. They successfully serve as labels for HSA protein premodified with BCN, resulting in bright blue-green emission upon ligation.