Violation of Hund's multiplicity rule in the lowest excited singlet–triplet pairs of cyclic bicalicene and its higher homologues

Shining Light on Inverted Singlet–Triplet Emitters

“…Indeed, in these systems the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) marginally overlap, and the lowest energy transition has a dominant multiresonant charge-transfer (MR-CT) character, with the electronic charge being transferred from the HOMO to the LUMO. In these conditions, the exchange integral, which is responsible for the ST splitting, is tiny, and the spin polarization correction, which is typically negligible, may be large enough to invert the ST gap. − …”

Correlation vs. exchange competition drives the singlet–triplet excited-state inversion in non-alternant hydrocarbons

“…The molecules dicyclohepta[cd,gh]pentalene (1) and dicyclopenta-[ef,ki]heptalene (2) are non-benzenoid non-alternant hydrocarbons, see Fig. 1, historically predicted as candidates violating Hund's rule in their lowest singlet (S 1 ) and triplet (T 1 ) excited states.…”

Computational Design of Multiple Resonance-Type BN Molecules for Inverted Singlet and Triplet Excited States

“…Later on, researchers at RIKEN demonstrated experimentally that HzTFEX 2 (Figure S1), a heptazine derivative, exhibits a negative Δ E ST and, consequently, an rISC rate faster than the ISC rate as estimated by temperature-dependent transient photoluminescence spectroscopy . As the INVEST has been mostly driven by the computational chemistry community, there have been many reports on the theoretical and computational design of molecules showing a negative Δ E ST , but the molecular structures have been largely limited to the boron- and/or nitrogen-doped phenalene-type structures such as cyclazine and heptazine derivatives, − except for a few reports of polyaromatic hydrocarbon core (PAH) compounds such as pentalene, COLDEM azulene-type alternant hydrocarbon molecule (compound 3 in Figure S1) is predicted to show a negative Δ E ST . − …”