Accurate Photophysics of Organic Radicals From Machine Learned Range-Separated Functionals

Abstract

Luminescent doublet-spin organic semiconducting radicals are emergent and unique candidates for organic light-emitting diodes because their internal quantum efficiency is not limited by intersystem crossing into any non-emissive high-spin state. The multi-configurational nature of their electronic structures challenges the usage of single-reference density functional theory (DFT), but the problem can be mitigated by designing more powerful exchange-correlation (XC) functionals. In an earlier study, we developed a molecule-dependent range-separated functional, referred to as ML-ωPBE, using a stacked ensemble machine learning framework. a In the present study, we assessed the performance ML-ωPBE for 64 organic semiconducting radicals from four categories, when similar radicals are absent from the training set. Compared to the firstprinciples OT-ωPBE functional, ML-ωPBE reproduced the molecule-dependent range-separation parameter, ω, with a small mean absolute error (MAE) of 0.0214 a −1 0 . Using single-reference time-dependent DFT (TDDFT), ML-ωPBE exhibited outstanding behaviors in absorption and fluorescence energies for most radicals in question, with small MAEs of 0.22 and 0.12 eV compared to experimental sources, and approached the accuracy of OT-ωPBE (0.22 and 0.11 eV). Our results demonstrated excellent generalizability and transferability of our ML-ωPBE functional from closed-shell organic semiconducting molecules to open-shell doublet-spin organic semiconducting radicals.

Turning on Organic Radical Emitters

“…ground-state DFT and excited-state time-dependent DFT (TDDFT) methods remain attractive for studying organic radicals, despite the previously mentioned theoretical difficulties. [25][26][27][28][29][30][31]33,[53][54][55][56][57][58][59] Due to the possible spin-contamination issues in the unrestricted version of TDDFT for open-shell molecules, explicitly spin-adapted TDDFT (X-TDDFT) is typically employed. Additionally, spin-polarized DFT calculations incorporating many-body perturbative corrections at the G 0 W 0 level have also been utilized.…”