Are Heptazine-Based Organic Light-Emitting Diode (OLED) Chromophores Thermally Activated Delayed Fluorescence (TADF) or Inverted Singlet-Triplet (IST) Systems?

TL;DR

This study investigates the electronic properties of heptazine-derived OLED chromophores HAP-3MF and HAP-3TPA, revealing HAP-3MF as the first inverted singlet-triplet (IST) emitter used in OLEDs and analyzing their excited-state mechanisms.

Contribution

It provides the first computational evidence of HAP-3MF as an IST system and explores the excited-state topography of both chromophores using ab initio methods.

Findings

HAP-3MF is an inverted singlet-triplet (IST) system.

HAP-3MF is the first IST emitter implemented in an OLED.

HAP-3TPA exhibits near-degenerate S1 and T1 states, bordering TADF and IST systems.

Abstract

Two chromophores derived from heptazine, HAP-3MF and HAP-3TPA, were synthesized and tested as emitters in light-emitting diodes (OLEDs) by Adachi and coworkers. Both emitters were shown to exhibit quantum efficiencies which exceed the theoretical maximum of conventional fluorescent OLEDs. The enhanced emission efficiency was explained by the mechanism of thermally activated delayed fluorescence (TADF). In the present work, the electronic excitation energies and essential features of the topography of the excited-state potential-energy surfaces of HAP-3MF and HAP-3TPA have been investigated with a wave-function based ab initio method (ADC(2)). It is found that HAP-3MF is an inverted singlet-triplet (IST) system, that is, the energies of the S1and T1states are robustly inverted in violation of Hund's multiplicity rule. Notably, HAP-3MF presumably is the first IST emitter which was implemented in an OLED device. In HAP-3TPA, on the other hand, the vertical excitation energies of the S1 and T1 states are essentially degenerate. The excited states exhibit vibrational stabilization energies of similar magnitude along different relaxation coordinates, resulting in adiabatic excitation energies which also are nearly degenerate. HAP-3TPA is found to be a chromophore at the borderline of TADF and IST systems. The spectroscopic data reported by Adachi and coworkers forHAP-3MF and HAP-3TPA are analysed in the light of these computational results.