4,4′-Diarylsulfanyl-2,2′,5,5′-tetraoxybiaryl Derivatives as a Water-Soluble Fluorescent Dye

4,4′-Disulfanyl-2,2′,5,5′-tetrahydrobiaryl
(5,5′-disulfanyl hydroquinone dimer) derivatives were readily
synthesized from benzoquinone and thiols via an oxidative coupling
reaction. The hydroquinone dimers showed strong fluorescence upon
excitation at 330 nm, and it was observed that the presence of the
sulfanyl groups at the C4 and C4′ positions is important for
achieving strong photoluminescence. The tetrapotassium salts of the
hydroquinone dimers also showed good water solubility, but the fluorescence
disappeared rapidly on dissolution in water. 2,2′- and 5,5′-protected
biaryls were synthesized. The dipotassium salt of the 5,5′-dimethoxy-2,2′-dihydroxy
derivative was observed to show good and stable fluorescence in water,
while the dipotassium salt of the 2,2′-dimethoxy-5,5′-dihydroxy
derivative showed less water solubility. Introduction of propargyl
groups was demonstrated to provide a convenient method for installing
amino acids derivatives. This derivatization afforded potentially
useful compounds for attaching the biologically active fragment to
the fluorescent unit.