Inverting the singlet-triplet energy gap in triangle-shaped molecules: a matter of symmetry and correlation

G. Ricci1, J.-C. Sancho-García2, Y. Olivier1

1 Laboratory for Computational Modeling of Functional Materials, Namur Institute of Structured Matter, Université de Namur
2 Department of Physical Chemistry, University of Alicante

The discovery of triangle-shaped organic compounds exhibiting an inversion of the energy gap between the lowest singlet (S1) and triplet (T1) excited states (INVEST) can certainly represent a new way to foster the performances of light-emitting devices, going beyond the common TADF strategy. In our works, we carried out a computational investigation on a set of triangle-shaped compounds showing how the conventional Time-Dependent Density Functional Theory (TD-DFT) is unable to tackle this inversion and employing correlated wavefunction methods1, 2 or double-hybrid functionals3 is key to properly describe the electron correlation effects driving the S1-T1 inversion. However, the most of the INVEST compounds suffer from a vanishing oscillator strength, making them unsuitable for light-emitting devices applications, and the few synthesized ones are built relying on the sole heptazine core.

In light of this, we made a step forward by establishing design rules that aimed at guiding the identification of molecular emitters with a negative ΔEST and non-zero oscillator strength. In our work4, we systematically assessed the effect of different point groups (D3h, C2v, C3v and C3h) to show how the peculiar orbital localization that minimizes the exchange interaction, and thus favors a negative ΔEST, is intrinsically dictated by the symmetry. We showed how relying on the C2v symmetry is the only way to conciliate a negative ΔEST and a non-zero oscillator strength.

  1. Sanz-Rodrigo J., Ricci G., Olivier Y. and Sancho-García J.C., J. Phys. Chem. A, 2021, 125, 513-522.
  2. Ricci G., San-Fabian E., Olivier Y. and Sancho-García J.C., ChemPhysChem, 2021, 22, 553-560.
  3. Sancho-García J.C, Brémond E., Ricci G., Pérez-Jiménez A. J., Olivier Y. and Adamo C., J. Chem. Phys., 2022, 156, 034105
  4. Ricci G., Sancho-García J.C and Olivier Y., J. Mater. Chem. C, 2022, 10, 12680-12698.