Benchmarking DFT Functionals for Excited-State Calculations of Donor-Acceptor TADF Emitters: Insights on the Key Parameters Determining Reverse Inter-System Crossing

David Hall; Juan Carlos Sancho-García; Anton Pershin; David Beljonne; Eli Zysman-Colman; Yoann Olivier

doi:10.1021/acs.jpca.2c08201

Benchmarking DFT Functionals for Excited-State Calculations of Donor-Acceptor TADF Emitters: Insights on the Key Parameters Determining Reverse Inter-System Crossing

J Phys Chem A. 2023 Jun 1;127(21):4743-4757. doi: 10.1021/acs.jpca.2c08201. Epub 2023 May 17.

Authors

David Hall^{1

2}, Juan Carlos Sancho-García³, Anton Pershin⁴, David Beljonne², Eli Zysman-Colman¹, Yoann Olivier⁵

Affiliations

¹ Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, KY16 9ST St Andrews, U.K.
² Laboratory for Chemistry of Novel Materials, University of Mons, 7000 Mons, Belgium.
³ Department of Physical Chemistry, University of Alicante, E-03080 Alicante, Spain.
⁴ Wigner Research Centre for Physics, P.O. Box 49, 1121 Budapest, Hungary.
⁵ Laboratory for Computational Modeling of Functional Materials, Namur Institute of Structured Matter, University of Namur, Rue de Bruxelles, 61, 5000 Namur, Belgium.

PMID: 37196185
DOI: 10.1021/acs.jpca.2c08201

Abstract

The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse inter-system crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here, we compare the results of widely used density functional theory (DFT) functionals, CAM-B3LYP, LC-ωPBE, LC-ω*PBE, LC-ω*HPBE, B3LYP, PBE0, and M06-2X, against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled Cluster (SCS-CC2), in 14 known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm-Dancoff Approximation (TDA) together with CAM-B3LYP, M06-2X, and the two ω-tuned range-separated functionals LC-ω*PBE and LC-ω*HPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S₁, and triplet T₁ and T₂ excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T₁ and T₂ is not as accurately captured as compared to S₁. We also investigated the impact of the optimization of S₁ and T₁ excited states on ΔE_ST and the nature of these states for three different functionals (PBE0, CAM-B3LYP, and M06-2X). We observed large changes in ΔE_ST using CAM-B3LYP and PBE0 functionals associated with a large stabilization of T₁ with CAM-B3LYP and a large stabilization of S₁ with PBE0, while ΔE_ST is much less affected considering the M06-2X functional. The nature of the S₁ state barely evolves after geometry optimization essentially because this state is CT by nature for the three functionals tested. However, the prediction of the T₁ nature is more problematic since these functionals for some compounds interpret the nature of T₁ very differently. SCS-CC2 calculations on top of the TDA-DFT optimized geometries lead to a large variation in terms of ΔE_ST and the excited-state nature depending on the chosen functionals, further stressing the large dependence of the excited-state features on the excited-state geometries. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution.