Unlocking Inverted Singlet-Triplet Gap in Alternant Hydrocarbons with Heteroatoms

TL;DR

This work tackles the challenge of inverted singlet-triplet gaps ($STG$) by conducting a large-scale, multi-level high-throughput virtual screening of 30,797 BN-substituted PAHs to identify DFIST candidates. The four-level workflow (level-1 LR-TD-DFT, level-2 L-CC2, level-3 geometry refinement, level-4 final excited-state calculations) yields 72 dynamically stable BNPAHs with $STG<0$, including non-planar BN-helicenes whose negative gaps arise from through-space CT. Among these, 46 molecules exhibit robust inverted gaps with $STG< -0.015$ eV and nonzero oscillator strengths, signaling potential MR-DFIST emitters and expanding design motifs beyond the previously known triangular cores. The findings demonstrate rich structure-property relationships, categorize candidates into four classes based on topology and B/N motif, and provide concrete targets for synthesis and experimental validation, advancing DFIST design in heteroatom-embedded aromatic systems.

Abstract

Fifth-generation organic light-emitting diodes exhibit delayed fluorescence even at low temperatures, enabled by exothermic reverse intersystem crossing from a negative singlet-triplet gap (STG), where the first excited singlet lies anomalously below the triplet. This phenomenon -- termed delayed fluorescence from inverted singlet and triplet states (DFIST) -- has been experimentally confirmed only in two triangular molecules with a 12-annulene periphery and a central nitrogen atom. Here, we report a high-throughput virtual screening of 30,797 BN-substituted polycyclic aromatic hydrocarbons derived from 77 parent scaffolds (2--6 rings). Using a multi-level workflow combining structural stability criteria with accurate L-CC2 excited-state calculations, we identify 72 heteroaromatic candidates with STGs$<0$. Notably, this includes BN-helicenes, where inversion arises from through-space charge-transfer states. Several systems exhibit non-zero oscillator strengths, supporting their potential as fluorescent emitters. Our findings reveal new design motifs for DFIST beyond known frameworks, expanding the chemical space for next-generation emitters based on heteroatom-embedded aromatic systems.

Figures (7)

• Figure 1: Set of 77 smallest polycyclic aromatic hydrocarbons (PAHs) comprising up to six benzene rings, adapted from Ref. chakraborty2019chemical. These serve as parent scaffolds for the BNPAH chemical space explored in this work. Molecules include linear, angular, and helical topologies. Names and SMILES are collected in Table S1 of the Supplementary Information (SI).
• Figure 2: Data distribution and high-throughput virtual screening workflow: (A) Distribution of 30,797 BNPAH molecules categorized by PAH size in terms of the number of rings. (B) Correlation between STGs of 644 BNPAH molecules predicted by TDA/SCS-PBE-QIDH and L-CC2 methods. Out of the 2,032 molecules with negative STGs at the DFT level, 644 molecules with S$_1$ energies $>1$ eV at the L-CC2 level are retained for further analysis. TP (true positives) and FP (false positives) indicate the classification accuracy of TDA/SCS-PBE-QIDH in predicting negative STGs, with L-CC2 serving as the reference. (C) High-throughput workflow for identifying DFIST candidates with reliable minimum-energy geometries and accurate excited-state characteristics.
• Figure 3: Dependence of L-CC2 STGs on the distance between B and N atoms in 644 BNPAH molecules. Typical distances for 2-bond separations (1,3) are shaded in magenta, while 3-bond separations are distinguished as 1,4-cis and 1,4-trans (analogous to cis/trans butadiene) and shaded in orange and blue, respectively. Distances for beyond 3-bond separations are shaded in red.
• Figure 4: B, N-substituted pyrenes with azaphenalene and boraphenalene cores: Variation of frontier MOs shown along with the L-CC2/cc-pVDZ transition energies of S$_1$ and T$_1$ states along with STG (in eV). Also given are the $\Lambda$-indices for S$_1$ and T$_1$ transitions and the corresponding frontier MOs calculated with TDA-SCS-PBE-QIDH/cc-pVDZ.
• Figure 5: Structures and L-CC2/cc-pVDZ-level excited-state properties of 10 BN-benzo[a]pyrene molecules derived from PAH #21 (see Figure \ref{['fig:PAH']}). White$|$green$|$black$|$blue atoms denote H$|$B$|$C$|$N.