Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters─Properly Accounting for Electron Correlation Is Key!

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

doi:10.1021/acs.jctc.2c00141

Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters─Properly Accounting for Electron Correlation Is Key!

J Chem Theory Comput. 2022 Aug 9;18(8):4903-4918. doi: 10.1021/acs.jctc.2c00141. Epub 2022 Jul 5.

Authors

David Hall^{1

2}, Juan Carlos Sancho-García³, Anton Pershin⁴, Gaetano Ricci⁵, 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,Budapest 1121, Hungary.
⁵ Laboratory for Computational Modeling of Functional Materials, Namur Institute of Structured Matter, Université de Namur, Rue de Bruxelles, 61, 5000 Namur, Belgium.

PMID: 35786892
DOI: 10.1021/acs.jctc.2c00141

Abstract

With the surge of interest in multiresonant thermally activated delayed fluorescent (MR-TADF) materials, it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔE_ST (i.e., the energy difference between the lowest singlet S₁ and triplet T₁ excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-dependent density functional theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔE_ST as a result of a poorer account of Coulomb correlation as compared to SCS-CC2. Very interestingly, the use of a metric to assess the importance of higher order excitations in the SCS-CC2 wavefunctions shows that Coulomb correlation effects are substantially larger in the lowest singlet compared to the corresponding triplet and need to be accounted for a balanced description of the relevant electronic excited states. This is further highlighted with coupled cluster singles-only calculations, which predict very different S₁ energies as compared to SCS-CC2 while T₁ energies remain similar, leading to very large ΔE_ST, in complete disagreement with the experiments. We compared our SCS-CC2/cc-pVDZ with other wavefunction approaches, namely, CC2/cc-pVDZ and SOS-CC2/cc-pVDZ leading to similar performances. Using SCS-CC2, we investigate the excited-state properties of MR-TADF emitters showcasing large ΔE_T2T1 for the majority of emitters, while π-electron extension emerges as the best strategy to minimize ΔE_ST. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge-transfer singlet state (S₂). This work highlights the importance of using wavefunction methods in relation to MR-TADF emitter design and associated photophysics.