posted on 2021-07-19, 19:13authored byAndrzej
L. Sobolewski, Wolfgang Domcke
Two chromophores derived from heptazine,
HAP-3MF and HAP-3TPA,
were synthesized and tested as emitters in light-emitting diodes (OLEDs)
by Adachi and co-workers. 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 S1 and T1 states 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 co-workers for HAP-3MF
and HAP-3TPA are analyzed in light of these computational results.