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

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 focal-point extrapolation scheme to obtain near-CCSD(T)/cc-pVTZ-quality geometries. We find three APs to undergo $D{\rm 3h} \rightarrow C{\rm 3h}$ and ten to show $C{\rm 2v}\rightarrow C{\rm s}$ 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.

💡 Research Summary

This paper presents a comprehensive computational investigation of azaphenalenes (APs), a class of heterocyclic molecules capable of violating Hund’s rule by exhibiting an inverted singlet‑triplet gap (STG), where the first excited singlet (S₁) lies energetically below the first triplet (T₁). Such “INVEST” molecules are of great interest for fifth‑generation organic light‑emitting diodes (OLEDs) that rely on delayed fluorescence from inverted singlet‑triplet states (DFIST).

The authors first enumerate the entire chemical space of APs by considering all possible nitrogen (N) substitutions on the nine peripheral positions of the cycl