Nanoparticle–Size–Dependent Singlet Fission and Exciton Dynamics in Amorphous TIPS-Pentacene

Aqueous nanoparticle (NP) dispersions are commonly used as model systems for spectro- scopic study of singlet exciton fission (SF) in acenes such as 6,13-(triisopropylsilylethynyl) pentacene (TIPS-Pn). However, the potential for particle size effects to complicate interpretation of results in such model systems is generally ignored. In this work, we study amorphous TIPS-Pn NP dispersions prepared by the re-precipitation method over a range of particle sizes. Time-resolved fluorescence and femtosecond transient absorption spectroscopy show that exciton dynamics in these systems depend significantly upon particle size. Kinetic analysis reveals that SF becomes slower at smaller NP sizes, while triplet exciton decay (through both correlated triplet pair relaxation and geminate triplet-triplet annihilation) accelerates. These significant size-dependent effects are ascribed to increased morphological disorder within smaller NPs, weakening the intermolecular couplings which control SF and triplet migration. A non-radiative singlet quenching channel separate from SF is also identified, which has not been previously reported for NPs of SF-capable chromophores. This non-radiative singlet decay becomes a significant relaxation pathway at small particle sizes, substantially reducing SF yields. Interestingly, exciton kinetics in 81-nm NPs approach those of bulk amorphous TIPS-Pn, suggesting that NPs of this size or larger are likely good models for bulk TIPS-Pn. This work demonstrates that particle-size effects are significant for small NPs of SF chromophores, and must be accounted for in order to accurately model bulk materials with such NP dispersions.