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

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.

Highly Efficient Green and Red Narrowband Emissive Organic Light‐Emitting Diodes Employing Multi‐Resonant Thermally Activated Delayed Fluorescence Emitters**

“…According to our previous study, the SRCT should dominate the S 1 emission process under a low polarity environment. [8,16] The predicted S 1 /T 1 energies for 3TPA-DiKTa and 3DPA-DiKTa are 3.16/2.93 and 2.72/2.46 eV, respectively, with corresponding ΔE ST values of 0.23 eV and 0.26 eV, which is similar to DiKTa (0.27 eV).…”

Multiple‐Resonance Extension and Spin‐Vibronic‐Coupling‐Based Narrowband Blue Organic Fluorescence Emitters with Over 30% Quantum Efficiency

“…Note that that the inclusion of higher‐order electron correlation is especially important in correctly capturing the electronic structure of MR‐TADF molecules. [ 30 ] We also note that only atomic sites with a gross charge greater than 0.01 confirmed within the atomic volume are shown herein (Figures S1,S2, Supporting Information). By summing the positive and negative atomic charges, ≈30% of the charges shifted atomically upon excitation, regardless of the π‐system extension.…”

Contributing to Biochemistry and Optoelectronics: Pyrrolo[1′,2′:2,3]imidazo[1,5-a]indoles and Cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles via [3+2] Annulation of Acylethynylcycloalka[b]pyrroles with Δ1-Pyrrolines

“…On the other hand, it is known that in solid media organic molecules with a small singlet–triplet (S-T) gap and a donor–acceptor (D-A) character can exhibit thermally activated delayed fluorescence (TADF) [ 29 , 30 ]. The mechanism is based on the thermal upconversion of the triplet excitons into singlets via the reverse intersystem crossing.…”