Thiofluor 623 (Compound 3) is a fluorescent turn-on probe that can be used for the selective sensing and bioimaging of thiols. Thiofluor 623 displays excellent immunity to interference from nitrogen and oxygen nucleophiles. Thiofluor 623 is essentially nonfluorescent in the absence of thiols, which cleave the probe and release the red-emissive donor-acceptor fluorophore (Ex=563 nm, Em=623 nm)^[1].