AUTHOR=Li Jie , Li Zhi , Liu Hui , Gong Heqi , Zhang Jincheng , Yao Yali , Guo Qiang 

TITLE=Organic molecules with inverted singlet-triplet gaps

JOURNAL=Frontiers in Chemistry

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2022.999856

DOI=10.3389/fchem.2022.999856

ISSN=2296-2646

ABSTRACT=<p>According to Hund’s multiplicity rule, the energy of the lowest excited triplet state (T<sub>1</sub>) is always lower than that of the lowest excited singlet state (S<sub>1</sub>) in organic molecules, resulting in a positive singlet-triplet energy gap (Δ<italic>E</italic><sub>ST</sub>). Therefore, the up-converted reverse intersystem crossing (RISC) from T<sub>1</sub> to S<sub>1</sub> is an endothermic process, which may lead to the quenching of long-lived triplet excitons in electroluminescence, and subsequently the reduction of device efficiency. Interestingly, organic molecules with inverted singlet-triplet (INVEST) gaps in violation of Hund’s multiplicity rule have recently come into the limelight. The unique feature has attracted extensive attention in the fields of organic optoelectronics and photocatalysis over the past few years. For an INVEST molecule possessing a higher T<sub>1</sub> with respect to S<sub>1</sub>, namely a negative Δ<italic>E</italic><sub>ST</sub>, the down-converted RISC from T<sub>1</sub> to S<sub>1</sub> does not require thermal activation, which is possibly conducive to solving the problems of fast efficiency roll-off and short lifetime of organic light-emitting devices. By virtue of this property, INVEST molecules are recently regarded as a new generation of organic light-emitting materials. In this review, we briefly summarized the significant progress of INVEST molecules in both theoretical calculations and experimental studies, and put forward suggestions and expectations for future research.</p>