Intramolecular singlet fission and triplet exciton harvesting in tetracene oligomers for solar energy conversion

Abstract

Singlet fission (SF), which can convert one singlet exciton into two triplet excitons, has the potential to enhance the solar energy conversion efficiency in devices such as photovoltaic cells and processes such as photocatalysis. Recently, intramolecular SF (iSF), which is observed in covalently linked molecular oligomers, has attracted significant research interest due to its unique advantages in revealing the SF mechanism and practical applications. In the case of exothermal chromophores (their singlet state energy (E_S1) is more than twice their triplet state energy (E_T1), such as pentacene), efficient iSF is easily achieved. However, efficient iSF is limited in endothermic/isothermal chromophores (E_S1 ≤ 2E_T1, such as tetracene). Compared to exothermal systems, the energy loss in the SF of endothermic/isothermal systems is much lower, which is more favorable for enhancing the solar energy conversion efficiency. Therefore, achieving efficient iSF in endothermic/isothermal systems is necessary. Unfortunately, the design strategy for efficient iSF based on endothermic/isothermal chromophores is not clear. Herein, we summarize the SF studies on covalently linked tetracene oligomers and analyze the key influencing factors on iSF in these systems. Additionally, we review the harvesting of the two formed triplets from the iSF of tetracene oligomers. It is hoped that the results of this review will provide some new insights into the design of highly efficient iSF materials based on endothermic/isothermic chromophores for practical applications.