Symmetry-induced singlet-triplet inversions beyond azaphenalenes: new molecular emitters from known chemistry J. Terence Blaskovits , Marc H. Garner, Clémence Corminboeuf Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering; École polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland Molecules with inversion of the lowest singlet (S 1 ) and triplet (T 1 ) excited-state energies are highly promising for the development of active materials for organic light-emitting diodes (OLEDs), as fluorescence from the S 1 state is no longer impeded by population transfer to the thermodynamically favored T 1 state. [1] And yet, azaphenalenes are the only class of molecules where these inversions have been identified to date. We have recently introduced a curated database tailored to materials design applications, consisting of over 117,000 experimentally-reported organic crystal structures and their associated computed optical properties. [2] Following a pre-screening of this database for molecules with very small (though not inverted) S 1 -T 1 gaps based on time- dependent density functional theory, second-order approximate coupled-cluster and equations-of-motion coupled- cluster methods are employed to refine the excited state energies. We recover compounds with known inversions in the azaphenalene class, confirming the validity of our screening methodology, and identify two further classes with this behavior. [3] The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet- triplet inversions occur in the third excited singlet (S 3 ) state. The second class consists of two D 2h -symmetry non-alternant hydrocarbons, a fused azulene dimer and a bicalicene, whose lowest excited singlet states violate Hund’s rule. Due to the connectivity of the polycyclic structure, they achieve this symmetry through aromatic stabilization, while retaining the inverted character of the normally unstable antiaromatic D 2h -pentalene core. [3] These hydrocarbons show promise as the next generation of building blocks for OLED emitters. In this talk, we will present the virtual screening protocol and discuss the origin of singlet-triplet inversions in the identified molecules. References 1. N. Aizawa, et al.; Delayed fluorescence from inverted singlet and triplet excited states. Nature, 2022, 609, 502–506. 2. J. T. Blaskovits, S. Vela, R. Laplaza, C. Corminboeuf; Data-driven discovery of organic electronic materials enabled by hybrid top-down/bottom-up design. 2022, Under review; ChemRxiv preprint: 10.26434/chemrxiv–2022–88t3 3. J. T. Blaskovits, M. H. Garner, C. Corminboeuf; Symmetry-Induced Singlet-Triplet Inversions in Non-Alternant Hydrocarbons. 2022, Under review; ChemRxiv preprint: https://doi.org/10.26434/chemrxiv-2022-ghmvg
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