CoraFluor 1, an amine-reactive terbium-based compound, serves as a potent donor in time-resolved Förster resonance energy transfer (TR-FRET) assays, including applications in target engagement and ternary complex assessments. Its formulation, featuring a PFP ester, allows efficient bonding with primary amines, thus facilitating the labeling of various biomolecules like antibodies, nanobodies, proteins, and peptides. Notably, CoraFluor 1 surpasses other TR-FRET donors in brightness, assay sensitivity, and stability within biological environments. Additionally, it is fully compatible with prevalent acceptor dyes, including FAM, TMR, and Cyanine 5, SE, making it an ideal choice for high-throughput screening endeavors.