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Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters

Abstract

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet–triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy–time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

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Fig. 1: Chemical structures and change in dipole moment upon excitation for the investigated TADF emitters.
Fig. 2: Ultrafast TA and PL measurements of TXO-TPA and 4CzIPN in toluene solutions.
Fig. 3: IVS of TXO-TPA and 4CzIPN in toluene solutions.
Fig. 4: Quantum-chemical calculations on TXO-TPA in a toluene solvent.
Fig. 5: The calculated vibrational mode evolution of TXO-TPA in a toluene solvent.
Fig. 6: The impact of the toluene solvent dynamics on the rISC process of TXO-TPA.

Data availability

The data that support the plots within this paper are available at the University of Cambridge Repository: https://doi.org/10.17863/CAM.85068.

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Acknowledgements

A.J.G. and R.H.F. acknowledge support from the Simons Foundation (grant no. 601946) and the Engineering and Physical Sciences Research Council (EPSRC) (EP/M01083X/1 and EP/M005143/1). This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (R.H.F., grant agreement no. 670405; A.R., grant agreement no. 758826). A.R. thanks the Winton Programme for the Physics of Sustainability for funding. A.P., Y.O. and D.B. were supported by the European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska Curie Grant agreement 748042 (MILORD project). Computational resources in Mons were provided by the Consortium des Équipements de Calcul Intensif, funded by the Fonds de la Recherche Scientifiques de Belgique (FNRS) under grant no. 2.5020.11, as well as by the Tier-1 supercomputer of the Fedération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under grant agreement no. 1117545. D.B. is a FNRS Research Director. R.P. acknowledges financial support from an EPSRC Doctoral Prize Fellowship. A.J.S. acknowledges the Royal Society Te Apārangi and the Cambridge Commonwealth European and International Trust for their financial support. Y.O. acknowledges funding by the FNRS under grant no. F.4534.21 (MIS-IMAGINE). L.-S.C. acknowledges funding from the University of Science and Technology of China (USTC) Research Funds of the Double First-Class Initiative and the National Natural Science Foundation of China (grant no. 52103242), and this work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. S.F. is grateful for support from an EPSRC Doctoral Prize Fellowship and the Winton Programme for the for the Physics of Sustainability. We thank C. Schnedermann for useful discussions.

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A.J.G, R.H.F. and D.B. conceived the work. A.J.G. performed the TA measurements. A.J.G., R.P. and A.J.S. carried out the IVS measurements. S.F. performed the transient grating PL measurements. E.W.E., T.H.T. and B.H.D. carried out the time-resolved PL measurements. E.W.E. measured the PL quantum efficiency. T.H.T. performed the Raman spectroscopy. A.J.G., E.W.E and L.-S.C. fabricated the samples used in the work. A.P. carried out the quantum-chemical calculations. A.M.A., G.D.S, Y.O. and D.B. discussed the calculation results. A.R., R.H.F. and D.B. supervised their group members involved in the project. A.J.G., A.P., G.D.S., R.H.F. and D.B. wrote the manuscript with input from all authors.

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Correspondence to Alexander J. Gillett or David Beljonne.

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Gillett, A.J., Pershin, A., Pandya, R. et al. Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters. Nat. Mater. 21, 1150–1157 (2022). https://doi.org/10.1038/s41563-022-01321-2

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