Abstract
Factors influencing the rate of reverse intersystem crossing (k rISC) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third‐generation heavy‐metal‐free organic light‐emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states and consequently research has focused almost exclusively on the energy gap between these two states. Herein, we use a model spin‐vibronic Hamiltonian to reveal the crucial role of non‐Born‐Oppenheimer effects in determining k rISC. We demonstrate that vibronic (nonadiabatic) coupling between the lowest local excitation triplet (3LE) and lowest charge transfer triplet (3CT) opens the possibility for significant second‐order coupling effects and increases k rISC by about four orders of magnitude. Crucially, these simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the Born‐Oppenheimer approximation for the future development of high‐performing systems.
Highlights
Factors influencing the rate of reverse intersystem crossing in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third-generation heavy-metal-free organic light-emitting diodes (OLEDs)
We demonstrate that vibronic coupling between the lowest local excitation triplet (3LE) and lowest charge transfer triplet (3CT) opens the possibility for significant second-order coupling effects and increases krISC by about four orders of magnitude
These simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the BornOppenheimer approximation for the future development of high-performing systems
Summary
Adachi and co-workers[7,8] recently demonstrated the effectiveness of the TADF approach for third-generation heavymetal-free OLEDs. This harvests the triplet excited states by exploiting thermal energy to drive population transfer from the triplet to the singlet states so that they can emit as singlet states via delayed fluorescence. This approach, adopted by Adachi and co-workers, defines the TADF equilibrium as depending exclusively on the energy gap between the singlet and triplet states and crucially, as shown in Equation (1), independent of the coupling between
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