Thermally activated delayed fluorescence (TADF) is a cornerstone of third generation OLED design, enabling up to 100% internal quantum efficiency through reverse intersystem crossing (RISC).[1] However, in flexible donor-acceptor systems, RISC rates are sensitive to geometry-dependent fluctuations in energy gaps and spin-orbit couplings (SOCs).[2] Traditional static models relying on single optimized geometries often fail to capture the experimental reality of these flexible molecules, as they not only ignore the distribution of accessible excited-state conformations but also RISC pathways governed by higher-order spin-vibronic coupling via intermediate higher triplet states[3]. To address this, we present an automated multi-scale quantum-chemical workflow that moves beyond static geometry approximations and bridges the gap between microscopic structural dynamics and macroscopic kinetic rates. The pipeline integrates large-scale metadynamics sampling[4] of S1 and T1 states as well as S1–T2 crossings[5] with structural clustering to identify key representative macrostates. We then employ efficient electronic structure theory[6] to refine geometries, energies, and SOC matrix elements across the resulting conformational space. Integrating these local contributions, our approach calculates the global RISC rate as a Boltzmann-weighted sum of all local direct and T2-mediated pathways. This framework accurately captures molecular flexibility and provides a physically rigorous and scalable tool for the reliable prediction of TADF RISC rates across a diverse chemical space.
Jonas Weiser