Morphological Evolution and Singlet Fission in Aqueous Suspensions of TIPS-Pentacene Nanoparticles

We report the observation of singlet fission in aqueous suspensions of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) nanoparticles (NPs) synthesized using the reprecipitation method. By altering the synthesis conditions we are able to fabricate NPs which evolve from a system of poorly coupled to highly coupled chromophores. This morphological evolution can also be suppressed for a period of several months. Absorption spectra confirm that the particles evolve over time, displaying increased intermolecular interaction, if the initial reaction conditions seeded a polycrystalline sample. We correlate these differences in morphologies to different rates of singlet state decay, where higher intermolecular interaction drives a more rapid rate of decay. Ultrafast time-resolved photoluminescence spectroscopy confirms a short first excited singlet state lifetime (<2 ps), and transient absorption spectroscopy is used to probe the generation of triplets. We find that NPs with greater interchromophore coupling are less efficient at singlet fission. This is surprising and contrasts with previous reports of fission in TIPS-Pn. It is suggested that the slow morphological evolution used to generate highly coupled chromophores also introduces singlet exciton traps. We observe a persistent singlet signal in transient absorption measurements and a long-lived fluorescence anisotropy component, supporting this hypothesis. As such, it is clear that both long-range and short-range order play significant roles in the efficacy of singlet fission. A rapid initial fluorescence polarization dephasing is also observed (<1 ps), suggesting that excitons rapidly migrate over crystalline grain boundaries or within amorphous regions.