Symmetry‐Induced Singlet‐Triplet Inversions in Non‐Alternant Hydrocarbons**

Abstract

Molecules with inversion of the singlet and triplet excited‐state energies are highly promising for the development of organic light‐emitting diodes (OLEDs). To date, azaphenalenes are the only class of molecules where these inversions have been identified. Here, we screen a curated database of organic crystal structures to identify existing compounds for violations of Hund's rule in the lowest excited states. We identify two further classes with this behavior. The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet‐triplet inversions occur in the third excited singlet state. The second class consists of two D2h‐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. These hydrocarbons show promise as the next generation of building blocks for OLED emitters.

Identification of Unknown Inverted Singlet–Triplet Cores by High-Throughput Virtual Screening

“…To provide several comparisons with known molecules optimized at the same level of theory, an aromatic molecule such as naphthalene shows BLA ≈0.05 Å, slightly lower than that of molecule g, while pentalene has BLA ≈0.15 Å. Molecule e , which led to the formulation of the symmetry-driven IST mechanism, shows alternating bonds of approximately the same length, with BLA ≈0.016 Å, in agreement with experiments and calculations highlighting its aromatic features . We, thus, conclude that fusion with other cores is not the only mechanism that can drive the conversion of an unstable antiaromatic core to a stable aromatic one, showing that, in case of molecule g , this conversion occurs by substitution of a carbon atom with a heteroatom.…”

“…Pentalene is known to give rise to singlet–triplet inversion in its planar D 2h geometry, which is, however, antiaromatic, and thus unstable. Recently, Blaskovits identified molecule e (CSD identifier: COLDEM) through a screening procedure of a CSD-based data set, arguing that the planarity of the pentalene core was driven by fusion with a 7-membered ring, giving rise to an azulene dimer where the stabilization of the pentalene core is symmetry-driven and leads to singlet–triplet inversion, taking origin from the pentalene frontier orbitals . The same idea proposed by Blaskovits applies to molecule f , where the pentalene core is fused to a 7-membered ring.…”

“…Nevertheless, to identify further design rules to achieve IST, one needs to discover other cores displaying such feature, something that cannot be achieved if the chemical space where the search is performed is defined by the known examples. New cores with unusual properties can be identified via virtual screening of datasets of organic compounds not originally designed for a particular function. , This “repurposing” approach was adopted by us to identify several families of new lead compounds to be used as singlet fission materials, TADF emitters, non-fullerene acceptors, and charge and exciton transport materials. , An analogous step in the direction of IST candidates was recently taken by Blaskovits et al; screening a set of known organic molecules, they identified two new classes of IST molecules, one of which seems particularly promising for applications in OLEDs, and where the singlet–triplet inversion mechanism is driven by high symmetry and stabilization through aromaticity.…”

Double-bond delocalization in non-alternant hydrocarbons induces inverted singlet–triplet gaps

“…Pentalene is an archetypical antiaromatic molecule and the ground-state structure distorts into a C 2h symmetry geometry with significant bond-length alternation and positive E (S 1 –T 1 ). 20,21,61 As shown in Fig. 3 (left), the large positive gap correlates with the overlapping HOMO and LUMO.…”

“…1b) has an inverted gap of E(S 1 -T 1 ) = −14 meV at the EOM-CCSD/ aug-cc-pVDZ level. 20 We attributed the inverted gap to its D 2hsymmetry pentalene core, which ensures non-overlapping HOMO and LUMO. [20][21][22][23] Unlike pentalene, which distorts into C 2h -symmetry to alleviate antiaromaticity, the highly symmetric structure of isopyrene is aromatically stabilized by the pattern of 5-and 7-membered rings.…”