Herein, we report a series of polyester-tethered molecular chromophores by controllable polymerization from the two ends of a pyrrolopyrrole cyanine (PPcy) derivative as the initiator (Scheme 1, S1a). The chemical structures and chain-lengths of the polyesters play a critical role in fine-tuning the excitonic coupling among PPcy chromophores within colloidal nanoparticles in water. Moreover, we present a new type of thermo-switchable near-infrared (NIR) emission caused by the reversible transformation between monomeric (M) and J-aggregate states of the fluorophores. We further demonstrate the potential of these thermosensitive materials for applications such as tumor imaging and the high efficiency of phototherapy.