Multi-exciton generation of organic chromophores, i.e., singlet fission (SF), is highly promising for photoenergy conversion such as photocatalysis and photovoltaics. Among the reported high-yield SF systems, many systems possess only short-lived triplet excited states derived from correlated triplet pair intermediates, but not individual doubled triplet excitons. However, high-yield and long-lived individual triplet exciton generation is favorable for achieving efficient photoenergy conversion. In this Focus Review, we discuss the molecular design strategy for high-yield and long-lived individual triplet exciton generation through intramolecular SF utilizing a series of acene derivatives such as hexacene, pentacene, and tetracene. In addition to clarifying the structural and vibronic characters required for high-yield individual triplet exciton generation, specific examples of the photoenergy conversion far exceeding 100% quantum yields of subsequent electron-transfer products and singlet oxygen generation are presented.
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