Getting the Right Twist: Influence of Donor-Acceptor Dihedral Angle on Exciton Kinetics and Singlet-Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

TL;DR

This study investigates how the donor-acceptor dihedral angle influences exciton kinetics and singlet-triplet gap in a deep blue TADF emitter, combining experimental and computational methods to optimize OLED performance.

Contribution

It provides new insights into the relationship between molecular geometry and TADF properties, highlighting the importance of torsion angles for efficient OLED emitters.

Findings

Dihedral angle significantly affects singlet-triplet gap and oscillator strength.

Experimental energy gap aligns with torsion angles of 70-75 degrees.

High-temperature evaporation leads to a distribution of torsion angles in films.

Abstract

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86{\deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{\deg}-75{\deg}. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.