Role of Spin Polarization and Dynamic Correlation in Singlet–Triplet Gap Inversion of Heptazine Derivatives

The new generation of proposed light-emitting molecules
for organic
light-emitting diodes (OLEDs) has raised considerable research interest
due to its exceptional featurea negative singlet–triplet
(ST) gap violating 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 wave function 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 decreases linearly with the increasing
highest occupied molecular orbital–lowest unoccupied molecular
orbital (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.