We have presented synthesis, optical properties and biomedical applications of a series of polyester-tethered fluorophores of pyrrolopyrrole cyanine (PPcy) derivatives and their colloidal nanoparticles dispersed in water. We found that both the chemical composition and chain length of polyesters affect the intermolecular aggregation among PPcy fluorophores and the optical properties of the nanoparticles. Relatively short chains of polycaprolactone or polydecalactone tethered from PPcy promote the formation of Jaggregates when co-precipitating in water in the presence of amphiphilic block copolymers (e.g., PCL37-b-POEGMA60) as the stabilizer. Such J-aggregation could be precisely tuned by variation of the polymer chain lengths. In contrast, non-fluorescent Haggregation was observed in poly(lactide)-tethered PPcy while the other conditions were kept similar to those PPcy tethered by polycaprolactone. Both J-and H-aggregates of PPcy confined in the colloidal nanoparticles thermoresponsive and reversible aggregate-to-monomer transitions. The enhanced fluorescence intensity of such J-aggregating nanoparticles vs. the monomers under the same concentration resembles nature's strategy of circumventing concentration quenching in photosynthetic lightharvesting systems. Moreover, these polyester-tethered PPcy nanoparticles are broadly useful in biomedical applications, such as NIR fluorescence or photoacoustic imaging and phototherapy of tumor models in animals.