NOVELTY - A water-soluble rylene diimide dye is new.    USE - The new water-soluble rylene diimide dye is useful as fluorescent label for biomolecules, preferably as site-specific fluorescent label for proteins (claimed). The rylene diimide dye is used for specific N-terminal labeling of proteins or specific labeling of polyhistidin tagged proteins (claimed). The rylene diimide dye may be used in dye lasers, fluorescence solar collectors, liquid crystals, cold light sources, material testing, photographic processes, and illumination and display elements; and as labeling groups in analytical processes, tracer in scintillation counters, photoconductors, and as semiconductors.    ADVANTAGE - The water-soluble rylene diimide dye has improved properties regarding reactivity with biomolecules such as higher reactivity and/or site-specifity, coupling efficiency and labeling yield of biomolecules to be labeled; and allows tracking of single interfacial enzymes (such as phospholipases) with their complex catalytic pathways and modes of operation on their native substrate.    DETAILED DESCRIPTION - A water-soluble rylene diimide dye of formula (I) is new.    R1=organic residue which is attached to N atom via C atom;    n=1-6;    R2=-(CH2)m-A-(CH2)o-E;    m, o=0-20;    A=linking group;    E=coupling group including isothiocyanate, activated ester, carbonyl azide, sulfonyl chloride, aldehyde, alkyl halide, haloacetamide, maleimide, nitrilotriacetic acid, disulfide, thiosulfate, thiol, or thioester; and    R4-R7=H or organic residue which carries negatively or positively charged group(s) in neutral medium or hydrophilic uncharged group(s); and at least two of R4-R7 being organic residue which carries negatively or positively charged group(s) in neutral medium or hydrophilic uncharged group(s).    INDEPENDENT CLAIMS are included for:    (1) preparation of rylene diimide dye of formula (I);    (2) a conjugate comprising compound (I) and binding partner, preferably biomolecule; and    (3) purification of bioconjugate containing biomolecule and rylene diimide dye (I), by providing labeling solution comprising bioconjugate and non-reacted dye; adding solid support to the labeling solution, in which the non-reacted dye covalently couples to the solid support; and isolating the bioconjugate by removing the solid support.