State-Specific Density Functional Theory for Computational Screening of Organic Optoelectronic Materials

Abstract

In recent years, organic light-emitting diodes (OLED) have rapidly grown in relevance due to their high color purity, mechanical flexibility, and energy efficiency, making them attractive options for applications in the display industry. For the design process of OLEDs, the computational modeling of the electronically excited-state properties of emitters devices has become an crucial part of the research and development pipeline. This thesis addresses the limitations and challenges associated with emitter modeling via common computational methods, i.e., time-dependent density functional theory (TD-DFT), by presenting alternative state-specific excited-state ΔSCF approaches within the framework of density functional theory (DFT). Specifically, I investigate the unrestricted open-shell and two-determinant restricted open-shell Kohn-Sham methods, herein called ΔUKS and ΔROKS respectively, for their accuracy in calculating singlet-triplet (ST) energy gaps and fluorescence energies in three different classes of emitters, namely donor-acceptor thermally activated delayed fluorescence (DA-TADF), multiresonance TADF (MR-TADF), and inverted singlet-triplet gap (INVEST) systems.<br /> Starting with DA-TADF emitters, I compiled a new and diverse benchmark set of 27 TADF emitters, for which very accurate experimental ST gap references were available. Despite the fact that the charge-transfer (CT) states in DA-TADF emitters are notoriously difficult to model, due to the difficulties of capturing orbital relaxation and solvation effects, both ΔUKS and ΔROKS combined with a standard polarizable continuum model (PCM) achieved sub-chemical accuracy in the reproduction of experimental ST gaps. I additionally tested both ΔDFT approaches for reproducing experimental fluorescence energies in the STGABS27 benchmark set. Both ΔUKS and ΔROKS in combination with a nonequilibrium PCM for modeling solvation effects produce fluorescence energies close to experiment, better than TD-DFT.<br /> The robustness of ΔDFT approaches for challenging systems encouraged me to examine the special class of INVEST emitters which feature an inversion of the singlet-triplet energy gap. To assess the capabilities of ΔDFT methods, I compiled two partially new benchmark sets: the smaller INVEST15 benchmark for testing different ΔDFT methods and establishing an accurate reference method, and the larger NAH159 benchmark, for validating the best-performing ΔDFT on a more diverse set. On both benchmark sets ΔUKS established itself as the superior ΔDFT approach, being able to not only qualitatively but also quantitatively reproduce high-level WFT reference results.<br /> Finally, I investigate the performance of ΔDFT approaches for ST gaps and fluorescence energies of MR-TADF emitters. On a literature benchmark of 35 MR-TADF emitters with experimental references, the accuracy of computed ST gaps was in strong favor for ΔUKS compared to ΔROKS yet again, cementing ΔUKS as the more reliable and robust state-specific excited-state method. ΔUKS-calculated ST gaps reached chemical accuracy against experimental data, whereas ΔROKS failed to accurately reproduce trends in the benchmark set. For fluorescence energies of MR-TADF emitters, both ΔUKS and ΔROKS performed well, with a slight edge in accuracy towards ΔUKS. An particularly important development from this work is the density functional, FX175-ωPBE, which in combination with ΔUKS displays excellent accuracy for ST gaps and fluorescence energies across all tested benchmark set. Overall, the presented work establishes ΔDFT methods, foremost ΔUKS, as a valuable computational tool for the calculation and modeling of photophysical properties, enabling efficient and accurate screening of optoelectronic OLED materials.