Multivariable Design of a Minimalist Conjugated Polymer Nanoparticle with Bright NIR-II Emission

Near-infrared (NIR) emitting materials underpin emerging medical diagnostics and therapeutic bionanotechnologies. In particular, the NIR-II window (1000 – 1700 nm) allows for enhanced penetration depth and image clarity in vivo by minimizing light-tissue interactions. Organic materials are advantageous due to their synthetic tunability and potential for greater biocompatibility, however the energy gap law poses fundamental limitations on accessible quantum yields (Φ) due to the exponential increase in nonradiative decay rates as the bandgap narrows. Conjugated polymer nanoparticles (CPNs) in particular offer biocompatibility, photostability, and remarkable absorption cross-sections (ε), the latter of which can offset the reduced Φ of the organic material to afford brilliant emission (ε × Φ). Here we investigate a structurally simple, scalable furan-flanked diketopyrrolopyrrole material (PDFT) with peak emission at 950 nm, and explore the role of backbone architecture, molecular weight, and CPN particle size on NIR-II emission brightness. PDFT CPNs featured extraordinary brightness and photostability in solution and in vivo, outperforming indocyanine green (ICG) and many contemporary NIR-II emitters. Through rational structural modifications across multiple length scales, the NIR-II brightness was enhanced by a factor of ~3.2. This performance, combined with synthetic accessibility and scalability positions PDFT as a promising material class for next-generation NIR-II bionanotechnologies.