Arsenical Fluorescent Agents and Assays

A substituted, optionally hydro-, optionally hetero-, monoarsenical anthracene compound comprising a 4′ rotation blocking group and a 5′ arsenic, and that exhibits a detectable increase or shift in fluorescence when the arsenic reacts with two thiols of a rotation-blocking binding target molecule, the compound having the structure I:

• wherein:
• R1 is O, S, NRα, or CRα2;
• R2 is C or N;
• R3 is H, optionally substituted-, optionally hetero-alkyl, optionally substituted-, optionally hetero-alkenyl, optionally substituted-, optionally hetero-alkynyl, optionally substituted-, optionally hetero-aryl, or optionally substituted-, optionally hetero-alkoxy;
• R4 and R5 are independently carbonyl, carbonothioyl, NRα2, ORα, or SRα;
• R6 and R7 are Rα, halide, ORα, NRα2, ORα, SRα, or nitro (—NO2);
• R8 and R9 are Rα, halide, ORα, NRα2, ORα, SRα, or nitro (—NO2);
• R10 is a rotational blocking group;
• R11 is an arsenic protecting group displaced by reaction of the arsenic with the thiols of the target molecule;
• Rα groups are independently H, optionally substituted-, optionally hetero-alkyl, optionally substituted-, optionally hetero-alkenyl, optionally substituted-, optionally hetero-alkynyl, optionally substituted-, optionally hetero-aryl, or optionally substituted-, optionally hetero-alkoxy;
• wherein one or more of the pairs R4-R6, R6-R8, R5-R7, and R7-R9 may be covalently joined in one or more rings;
• and tautomers, anhydrides and salts of the compound.