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Endothermic singlet fission is hindered by excimer formation

Abstract

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet–triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

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Figure 1: Schematic Marcus–Hush–Morse free-energy surface of the SF–TTA process.
Figure 2: Emission spectra of concentrated TIPS-Tc solutions.
Figure 3: TRPL of concentrated TIPS-Tc in toluene. The emission spectrum reverts to being dominated by the S1 state after excimer decay.
Figure 4: Extracted spectra and their time-dependent weightings.
Figure 5: Experimental and simulated TA heat maps with extracted spectra and time dependences.

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Acknowledgements

T.W.S. acknowledges the Australian Research Council for a Future Fellowship (FT130100177). This work was supported by the Australian Research Council (Centre of Excellence in Exciton Science CE170100026, DP160103797, LE0989747).

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Contributions

C.B.D. and L.F. performed the measurements. C.B.D., J.K.G. and L.F. analysed the data. A.J.P. synthesized the TIPS-Tc material. J.E.A. designed and provided the TIPS-Tc material. M.J.C. designed and provided the PdPQ4 material. T.W.S., J.K.G. and L.F. wrote the manuscript. J.K.G. and C.B.D. designed the figures with input from T.W.S. C.B.D. and P.C.T. performed the TA experiments. T.W.K. provided access to and advice on the TA experiments, and critically read the manuscript. T.W.S. conceived the experiments and performed the modelling.

Corresponding author

Correspondence to Timothy W. Schmidt.

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The authors declare no competing financial interests.

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Dover, C., Gallaher, J., Frazer, L. et al. Endothermic singlet fission is hindered by excimer formation. Nature Chem 10, 305–310 (2018). https://doi.org/10.1038/nchem.2926

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