The role of spin polarization and dynamic correlation in singlet-triplet gap inversion of heptazine derivatives

TL;DR

This paper explores how spin polarization and dynamic correlation influence the unusual negative singlet-triplet gap in heptazine derivatives, proposing a simple predictive model and validating it with advanced quantum chemistry methods.

Contribution

It introduces a perturbation theory-based descriptor for predicting negative ST gaps and demonstrates its effectiveness for heptazine derivatives.

Findings

Spin polarization favors singlet states over triplet states.

The proposed descriptor effectively prescreens candidate molecules.

Multireference methods accurately predict ST gaps when combined with dynamic correlation.

Abstract

The new generation of proposed light-emitting molecules for OLEDs has raised a considerable research interest due to its exceptional feature-a negative singlet-triplet (ST) gap violating the Hund's multiplicity rule in the excited S1 and T1 states. We investigate the role of spin polarization in the mechanism of ST gap inversion. Spin polarization is associated with doubly excited determinants of certain types, whose presence in the wavefunction expansion favors the energy of the singlet state more than that of the triplet. Using a perturbation theory-based model for spin polarization, we propose a simple descriptor for prescreening of candidate molecules with negative ST gaps and prove its usefulness for heptazine-type molecules. Numerical results show that the quantitative effect of spin polarization is approximately inverse-proportional to the HOMO-LUMO exchange integral. Comparison of single- and multireference coupled- cluster predictions of ST gaps shows that the former methods provide good accuracy by correctly balancing the effects of doubly excited determinants and dynamic correlation. We also show that accurate ST gaps may be obtained using a complete active space model supplemented with dynamic correlation from multireference adiabatic connection theory.