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.

Photochemical upconversion in a quasi-solid gel proceeds with an identical efficiency to an otherwise identical liquid composition.

Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigated in this work through the design and synthesis of four new 9,10-diphenylanthracene (DPA) derivatives by employing platinum octaethylporphyrin as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core exhibit high fluorescence quantum yields. Interestingly, large differences in the triplet fusion upconversion performance were observed between the derivatives with the meta-substituted dimer showing the closest performance to the DPA reference. The differences are discussed in terms of the statistical probability for obtaining a high-energy singlet excited state from triplet fusion, f, for both inter- and intramolecular processes and the effect of magnetic field on the upconversion efficiency. These results demonstrate the challenges to be overcome in improving triplet fusion upconversion efficiency based on multichromophoric emitter systems.

The excited-state dynamics of 6,13-bis­(triisopropylsilylethynyl)­pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M–1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.