Heptazine, Cyclazine, and Related Compounds: Chemically-Accurate Estimates of the Inverted Singlet-Triplet Gap

Abstract

Molecules that violate Hund's rule and exhibit an inverted gap between the lowest singlet $S_1$ and triplet $T_1$ excited states have attracted considerable attention due to their potential applications in optoelectronics. Amongst these molecules, the triangular-shaped heptazine, and its derivatives, have been in the limelight. However, conflicting reports have arisen regarding the relative energies of $S_1$ and $T_1$. Here, we employ highly accurate levels of theory, such as CC3, to not only resolve the debate concerning the sign but also quantify the magnitude of the $S_1$-$T_1$ gap. We also determine the 0-0 energies to evaluate the significance of the vertical approximation. In addition, we compute reference $S_1$-$T_1$ gaps for a series of 10 related molecules. This enables us to benchmark lower-order methods for future applications on larger systems within the same family of compounds. This contribution can serve as a reliable foundation for the design of triangular-shaped molecules with enhanced photophysical properties.

8 pages, 3 figures, supporting information available

References in corpus (4)

• QUESTDB: a database of highly-accurate excitation energies for the electronic structure community
• Are Heptazine-Based Organic Light-Emitting Diode (OLED) Chromophores Thermally Activated Delayed Fluorescence (TADF) or Inverted Singlet-Triplet (IST) Systems?
• Connections and performances of Green's function methods for charged and neutral excitations
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• Noncollinear Spin-Flip TDDFT for Potential Energy Surface Crossings: Conical Intersections and Spin Crossings
• Unlocking Inverted Singlet-Triplet Gap in Alternant Hydrocarbons with Heteroatoms
• Rank-reduced equation-of-motion coupled cluster triples: an accurate and affordable way of calculating electronic excitation energies
• Leveraging the Bias-Variance Tradeoff in Quantum Chemistry for Accurate Negative Singlet-Triplet Gap Predictions: A Case for Double-Hybrid DFT
• A Mountaineering Strategy to Excited States: Accurate Vertical Transition Energies and Benchmarks for Substituted Benzenes
• Rank-reduced equation-of-motion coupled cluster formalism with full inclusion of triple excitations