BDP 581/591 NHS ester, a borondipyrromethene dye (Ex=585 nm, Em=594 nm), exhibits a relatively long fluorescence lifetime and significant two-photon excitation cross-section. This compound is applicable in fluorescence polarization analysis, interacts with reactive oxygen species (ROS) to modify fluorescence, and, as an NHS ester derivative, binds to the primary and secondary amine groups of proteins, peptides, and various molecules. Storage instructions highlight the importance of protecting it from light.