Singlet Fission in Core-Shell Micelles of End-Functionalized Polymers

Singlet fission is the process in aggregates of molecular semiconductors where the initial product of light absorption (a singlet exciton) is converted into two correlated spin-triplet excitons. While most studies of singlet fission are conducted on assemblies of small molecule singlet fission chromophores, polymer self-assembly has yet to be explored as a means of creating nanostructures conducive for singlet fission. In this work, we use solution self-assembly of mono- and difunctionalized polymers to create core-shell micelles that display efficient singlet fission. The polymers are synthesized by copper(I)-catalyzed "click" chemistry between a 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) alkyne precursor and the corresponding azide-terminated poly(ethylene glycol) (PEG) polymer. Spontaneous solution self-assembly creates starlike and flowerlike core-shell micelles that are characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) experiments. Ultrafast transient absorption spectroscopy and time-resolved fluorescence experiments evidence nearly equivalent singlet fission dynamics in starlike and flowerlike micelles. Studies on mixed micelles of the Pn-functionalized polymer with a C₁₆-PEG surfactant reveal how triplet pair formation and decay rates vary with micelle composition. The core-shell micelles developed herein demonstrate the potential of polymer self-assembly for creating functional singlet fission nanostructures and provide insight into how secondary components and solubilizing blocks influence singlet fission dynamics and triplet pair losses in self-assembled systems.