(Invited) Kinetic Prediction of Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

Abstract

Reverse intersystem crossing (RISC), the endothermic spin-flip process of producing an emissive singlet exciton from a non-emissive triplet exciton, is of increasing interest for applications in organic electroluminescent materials, photocatalysts, and biomedical probes. Theoretical prediction of the rate constant of RISC (k RISC) may enable an efficient priori screening of new materials in chemical spaces; yet, the understanding of its kinetics in vastly different structures is challenging. Here, we demonstrate a theoretical expression that reproduces experimental k RISC ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid.1 The present model also leads to a newly designed TADF molecule with high k RISC of 108 s–1.2 1. N. Aizawa, Y. Harabuchi, S. Maeda, Y.-J. Pu, Nat. Commun. 11, 3909 (2020).2. N. Aizawa, A. Matsumoto, T. Yasuda, Sci. Adv. 7, 5769 (2021). Figure 1