Accurate Prediction of Vertical Emission from Excited-State Tuning of Range-Separated Density Functional Theory

Abstract

We report for the first time an excited-state (EX) tuning approach to accurately predict vertical emissions (VEs) based on the study of 36 medium and large organic molecules (∼66 heavy atoms) used mostly in thermally activated delayed fluorescence applications. Compared to the conventional ground-state (GS) tuning of the rangeseparation parameter (ω) in long-range-corrected functionals, which by far provides accurate optical properties (compared to B3LYP, M062X, CAM-B3LYP, ωB97XD, etc.), the proposed EX tuning approach [LC-ωPBE/6-31G(d)] predicts more accurate VEs in the density functional theory (DFT) framework. The mean absolute errors (MAEs) of VEs reduced by 20%, from 0.25 eV with GS tuning to 0.20 eV with EX tuning, against the experimental benchmarks. We find that further improvement in accuracy is possible if accurate EX geometries are employed in the EX tuning approach. For nine smaller molecules (for which higher level optimized EX geometries are available at CCSDR(3) and CC3), MAEs of VEs reduced from 0.32 to 0.10 eV, a nearly 70% improvement (against theoretical benchmarks) employing higher level EX geometries compared to DFT EX geometries. Tuning the range-separation parameter based on the optimized EX geometry enables us to capture the delocalization patterns of EX accurately, which results in accurate prediction of VEs. These results prove that the EX-tuning approach in combination with accurate EX geometries provides the most accurate VEs comparable to CC3 benchmarks.

Evaluation of using the time-dependent density functional theory in studying the fluorescence properties of coumarin derivatives

“…30 The order of accurately predicting VAEs and VEEs of 36 organic compounds decreases in the order of M06-2X, B3LYP, oB97X-D, and LC-oPBE. 29 The results presented in Fig. 2 show that all surveyed DFT functionals exhibited a moderately linear correlation between It should be noted that this trend is consistent with a previous publication in which reference data were obtained from the CC2.…”

“…MAEs for VEEs were 0.52, 0.43, 0.60, 0.96, 0.25, and 0.20 eV for B3LYP, M06-2X, oB97X-D, LC-oPBE, GS-tuned, and ES-tuned LC-oPBE, respectively. 29 While the tuning approach offers better accuracy than default DFT functionals, it is complex and time-consuming because of the need to fine-tune the HF exchange fraction by adjusting the range separation parameter (o). 29 These studies are primarily useful for enhancing default DFT functionals in calculation software.…”

“…29 While the tuning approach offers better accuracy than default DFT functionals, it is complex and time-consuming because of the need to fine-tune the HF exchange fraction by adjusting the range separation parameter (o). 29 These studies are primarily useful for enhancing default DFT functionals in calculation software. As a result, further investigation is necessary to assess the accuracy of existing default DFT functionals in predicting fluorescence spectra.…”

“…This approach, which includes both ground state (GS) and excited state (ES) tuning, determines the optimal HF exchange fraction by adjusting the range separation parameter ( ω ) in LC-hybrids. 29 In predicting the vertical absorption of 17 organic molecules, Bredas et al found that tuned LC-hybrids outperformed other DFT functionals, with MAEs of 0.15 eV for tuned LC-hybrids, around 0.40 eV for global hybrids (B3LYP, M06-2X), and approximately 1.00 eV for PBE, M06HF, and LC-ωPBE. 30 Similarly, when predicting the vertical absorption energy (VAEs) and VEEs of 36 organic compounds, Talapunur Vikramaditya et al reported that the ES and GS tuning approaches outperformed other DFT functionals.…”

“…MAEs for VEEs were 0.52, 0.43, 0.60, 0.96, 0.25, and 0.20 eV for B3LYP, M06-2X, ωB97X-D, LC-ωPBE, GS-tuned, and ES-tuned LC-ωPBE, respectively. 29…”