Improved Fluorescence and Brightness of Near-Infrared and Shortwave Infrared Emitting Polymer Dots for Bioimaging Applications

Development of near-infrared (NIR) and shortwave infrared (SWIR) emitting fluorophores is central to the fluorescence-based bioimaging. While conjugated polymer nanoparticles (polymer dots) are one of the promising fluorophores for this application, obtaining polymer dots that show bright fluorescence, especially in the SWIR wavelength region, has been challenging. Here, we report a generalized approach to obtain bright polymer dots through a systematic characterization of photophysical properties of NIR and SWIR emitting polymer dots. Detailed photophysical characterization of a series of polymer dots fabricated using polycarbazole- (PCz-) based conjugated polymers that adopt bent and twisted conformation reveals that the fluorescence brightness of the PCz-based polymer dots is determined by subtle balance between fluorescence quenching due to polymer chain interaction inside the particles and the twisting between the donor and acceptor moieties of the conjugated polymers inside the particles. Our results provide important insight into the rational design of highly fluorescing SWIR-emitting polymer dots.