Rational Design of Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet

One implication of Hund’s first rule for molecules is that the first excited triplet is for the large majority of molecules more stable than the first excited singlet. Only a handful of structural motifs are known to violate it. Recently, the potential of such molecules for emissive materials has been appreciated as their inverted singlet-triplet gaps enable favorable triplet exciton harvesting and low triplet exciton population to be utilized in both efficient and long-lived organic light-emitting diodes. Herein, we report bottom-up design of small organic molecules with inverted singlet-triplet gaps, i.e., INVEST molecules, via the Ring-Bonds-Substitutions (RiBS) ruleset which we introduce in this work. It relies on a few simple rules based on orbital interactions in alternant hydrocarbons to both minimize the exchange integral between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and maximize dynamic spin polarization. Using RiBS, we find many new molecules violating Hund’s first rule in their first excited states thereby greatly expanding the chemical space of potential inverted singlet-triplet gap (INVEST) emitters.