TIPS anthracene a singlet fission or triplet fusion material?

Triisopropylsilylethynyl-substituted acenes (TIPS-acenes) have received prominent attention in the field of singlet fission, the pentacene derivative being an exothermic singlet fission material, and the tetracene being a prototypical endothermic material. Little attention has been given to TIPS-anthracene, which is expected to exhibit exothermic triplet-triplet annihilation, despite literature reports to the contrary. We show that there is some evidence for singlet fission in TIPS-anthracene solutions, and that it does exhibit triplet-triplet annihilation. We apply anti-Stokes action spectroscopy to determine the upconversion efficiency of a composition of TIPS-anthracene and platinum octaethylporphyrin. At a bias equivalent to 0.86 suns, the composition exhibited an annihilation efficiency of 3.2%, which may be compared to diphenylanthracene, which yielded 9.2% under the same conditions. We attribute the low efficiency to a combination of a shorter triplet lifetime and low lying T2 and T3 states. The results are supported by ab initio quantum chemical calculations

Power Dependence of the Magnetic Field Effect on Triplet Fusion: A Quantitative Model

Two strategies for improving solar energy efficiencies, triplet fusion and singlet fission, rely on the details of triplet−triplet interactions. In triplet fusion, there are several steps, each of which is a possible loss mechanism. In solution, the parameters describing triplet fusion collisions are difficult to inspect. Here we show that these parameters can be determined by examining the magnetic field dependence of triplet fusion upconversion. We show that there is a reduction of the magnetic field effect for perylene triplet fusion as the system moves from the quadratic to linear annihilation regimes with an increase in laser power. Our data are modeled with a small set of parameters that characterize the triplet fusion dynamics. These parameters are cross-validated with molecular dynamics simulations. This approach can be applied to both solution and solid state materials, providing a tool for screening potential annihilators for photon upconversion.Roslyn Forecast, Elham M. Gholizadeh, Shyamal K. K. Prasad, Simon Blacket, Patrick C. Tapping, Dane R. McCamey, Murad J. Y. Tayebjee, David M. Huang, Jared H. Cole, and Timothy W. Schmid

Singlet Fission in Concentrated TIPS Pentacene Solutions The Role of Excimers and Aggregates

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 encounter