TDDFT versus GW/BSE Methods for Prediction of Light Absorption and Emission in a TADF Emitter

Abstract

Design concepts for organic light emitting diode (OLED) emitters, which exhibit thermally activated delayed fluorescence (TADF) and thereby achieve quantum yields exceeding 25%, depend on singlet−triplet splitting energies of order kT to allow reverse intersystem crossing at ambient temperatures. Simulation methods for these systems must be able to treat relatively large organic molecules, as well as predict their excited state energies, transition energies, singlet−triplet splittings, and absorption and emission cross sections with reasonable accuracy, in order to prove useful in the design process.Here we compare predictions of TDDFT with M06-2X and ωB97X-D exchange-correlation functionals and a G o W o @HF/BSE method for these quantities in the well-studied DPTZ-DBTO2 TADF emitter molecule. Geometry optimization is performed for ground state (GS) and lowest donor−acceptor charge transfer (CT) state for each functional. Optical absorption and emission cross sections and energies are calculated at these geometries. Relaxation energies are on the order of 0.5 eV, and the importance of obtaining excited state equilibrium geometries in predicting delayed fluorescence is demonstrated. There are clear trends in predictions of G o W o @HF/BSE, and TDDFT/ωB97X-D and M06-2X methods in which the former method favors local exciton (LE) states while the latter favors DA CT states and ωB97X-D makes intermediate predictions. G o W o @HF/BSE suffers from triplet instability for LE states but not CT states relevant for TADF. Shifts in HOMO and LUMO levels on adding a conductor-like polarizable continuum model dielectric background are used to estimate changes in excitation energies on going from the gas phase to a solvated molecule.

Spectroscopic Identification of the Charge Transfer State in Thiophene/Fullerene Heterojunctions: Electroabsorption Spectroscopy from GW/BSE Calculations

“…The Exciton code and methods used for GW/BSE calculations in this work have been described elsewhere. , Calculations on ammonia-borane systems were performed using 6-311G basis sets for B, H and N and the CIS method in Exciton. Molecular geometry optimizations for thiophene and C 60 systems were performed with the B3LYP functional and def2-TZVP basis sets using the Gaussian16 code .…”

Intramolecular Charge Transfer and Stimuli-Responsive Emission in Cholesterol-Appended Phenothiazine–Cyanostyryl-Based Donor–Acceptor Systems

“…These observations are consistent with the literature and experimental findings. Therefore, while the TD-DFT results may not be precise, they provide a qualitative understanding, enabling the derivation of meaningful conclusions. , …”