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.