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.