The invention relates to fluorescent dyes that are substituted or unsubstituted derivatives of 1-(isoindolyl)methylene-isoindole that are bound through both isoindole nitrogens to a boron difluoride moiety, forming a fluorescent dibenzopyrrometheneboron difluoride compound ##STR1## that is further substituted by bathochromic substituents that are aryl or heteroaryl moieties further substituted by an additional aryl or heteroaryl, that is itself optionally further substituted by an additional aryl or heteroaryl. These aryl and heteroaryl groups are separated by a covalent bond, or by an ethenyl, butadienyl or hexatrienyl linkage. The dyes of the invention are particularly useful as labels for carriers, particularly polymeric microparticles. The resulting microparticles have a long-wavelength fluorescence emission, and possess utility for tracing flow in biological systems, particularly in tracing blood flow.