Abstract
Luminescent organic semiconducting doublet-spin radicals are unique and emergent optical materials because their fluorescent quantum yields (Φfl) are not compromised by spin-flipping intersystem crossing (ISC) into any dark high-spin states. The multiconfiguration nature of these radicals challenges their electronic structure calculations in the framework of single-reference density functional theory (DFT) and introduces room for method improvement. In the present study, we extend our earlier development of ML-ωPBE, a range-separated hybrid (RSH) exchange−correlation (XC) functional constructed using the stacked ensemble machine learning (SEML) algorithm, from closed-shell organic semiconducting molecules to doublet-spin organic semiconducting radicals. We assess its performance for a new test set of 64 radicals from five categories based on the original training set of 3,926 molecules. Interestingly, ML-ωPBE agrees with the first-principles OT-ωPBE functional regarding the molecule-dependent range-separation parameter (ω), with a small mean absolute error (MAE) of 0.0197 a0−1 but saves the computational cost by 2.46 orders of magnitude. This result demonstrates outstanding domain adaptation capacity of ML-ωPBE among various organic semiconducting species. To further assess the predictive power of ML-ωPBE, we also compare its performance on absorption and fluorescence energies (Eabs and Efl) evaluated using time-dependent DFT (TDDFT) with nine conventional functionals. For most radicals, ML-ωPBE reproduces experimental measurements of Eabs and Efl with small MAEs of 0.222 and 0.121 eV, only marginally different from OT-ωPBE. Our work illustrates a successful extension of the SEML framework from closed-shell molecules to open-shell radicals and will open the venue for calculating optical properties using single-reference TDDFT.
Supplementary materials
Title
Supporting Information: Accurate Electronic and Optical Properties of Organic Doublet Radicals Using Machine Learned Range-Separated Functionals
Description
Brief revisit of the SEML model; error statistics of ML-ωPBE and other XC functionals in optical properties; and configurations of frontier MOs and NTOs (PDF).
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Title
Molecular Geometries in XYZ Coordinates
Description
Optimized D0 geometries for 48 radicals in the test subset of Eabs; and optimized D1geometries for 16 radicals in the test subset of Efl (ZIP).
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Title
SMILES Strings and Range Separation Parameters
Description
SMILES strings, experimental measurements of Eabs and Efl, values of ωOT and ωML for all 64 radicals in the external test set (XLSX).
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Supplementary weblinks
Title
Stacked Ensemble Machine Learning for Range-Separation Parameters
Description
Source codes and original dataset for the stacked ensemble machine learning (SEML) model in the construction of ML-wPBE functional developed in the Lin group.
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