Insights Into Symmetry and Substitution Patterns Governing Singlet-Triplet Energy Gap in the Chemical Space of...

Molecules that violate Hund's rule by exhibiting an inverted singlet-triplet gap (STG), where the first excited singlet ( $S_1$ ) lies below the triplet ( $T_1$ ), are rare but hold great promise as efficient fifth-generation light emitters. Azaphenalenes (APs) represent one of the few known molecular classes capable of such inversion of the $S_1$ / $T_1$ energy ordering, yet a systematic exploration of all unique APs is lacking. Here, we investigate 104 distinct APs and classify them based on their adherence to or deviation from Hund's rule using $S_1$ - $T_1$ gaps computed with the second-order coupled-cluster method employing the Laplace transform (L-CC2). To capture substitution-dependent pseudo-Jahn-Teller distortions that are inadequately described by MP2 and DFT methods, we employ a focal-point extrapolation scheme to obtain near-CCSD(T)/cc-pVTZ-quality geometries. We find 3 APs to undergo $D_{3h}\to C_{3h}$ and 10 to show $C_{2v}\to C_s$ symmetry lowering, leading to a total of 117 configurations of 104 unique APs. Our study identifies top candidates with inverted STGs, revealing how substitution and symmetry-lowering modulate these gaps to uncover new stable AP cores that provide promising targets for designing molecular light-emitters.