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The new generation of proposed light-emitting molecules for OLEDs has raiseda considerable research interest due to its exceptional feature-a negativesinglet-triplet (ST) gap violating the Hund's multiplicity rule in the excitedS1 and T1 states. We investigate the role of spin polarization in the mechanismof ST gap inversion. Spin polarization is associated with doubly exciteddeterminants of certain types, whose presence in the wavefunction expansionfavors the energy of the singlet state more than that of the triplet. Using aperturbation theory-based model for spin polarization, we propose a simpledescriptor for prescreening of candidate molecules with negative ST gaps andprove its usefulness for heptazine-type molecules. Numerical results show thatthe quantitative effect of spin polarization is approximatelyinverse-proportional to the HOMO-LUMO exchange integral. Comparison of single-and multireference coupled- cluster predictions of ST gaps shows that theformer methods provide good accuracy by correctly balancing the effects ofdoubly excited determinants and dynamic correlation. We also show that accurateST gaps may be obtained using a complete active space model supplemented withdynamic correlation from multireference adiabatic connection theory.

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