Towards Efficient Modeling of Non-Radiative Decay in Extended INVEST: Overcoming Computational Challenges in Quantum Dynamics Simulations

In the last years, an increasing number of fully organic molecules capable of thermally activated delayed fluorescence (TADF) have been reported, often with very small or even inverted singlet-triplet (INVEST) energy gaps.1 These molecules typically exhibit complex photophysics, due to the close energy levels of multiple singlet and triplet states, which create various transition pathways towards emission.2 A predictive model for the rates of these transitions is thus essential for assessing the suitability of new materials for light-emitting devices. Quantum Dynamics (QD) calculations are ideal for this purpose, as they include quantum effects, without the limitations of firstorder perturbative approaches, also allowing taking into account more than two electronic states at once. However, the huge computational demands of QD methodologies, especially for large molecules, currently limit their use as a standard tool, necessitating the development of novel methodologies.3 To address this problem, we here employ a strategy that allows including almost the whole set of the vibrational coordinates by selecting the key elements of the Hilbert space that significantly impact dynamics, thereby hugely reducing the computational burden.4.5 Application of this protocol to two relatively large INVEST molecules reveals that internal conversion in these systems is very fast, making indirect emissive pathways a possible channel for the population of the S1 state.5 More importantly, this study demonstrates that the dynamics can be accurately described even with a significantly reduced vibrational space, thus allowing to perform quantum dynamics calculations that yield accurate transition rates in a few minutes of computational time.5