The few known reports and the likely prospects of finding new efficient routes to exciton fission from higher excited singlet states, S ( > 1), are reviewed. Aggregates of molecules that have large S-S electronic energy spacings and/or emit measurable "contra-Kasha" emission may offer further opportunities. Among these, electronically excited molecular systems that exhibit known efficient (T + T) triplet-triplet annihilation producing S could exhibit efficient singlet fission in aggregates when appropriately substituted to meet the necessary energy requirements. The potential problem of loss of triplet excitons 2T → T + S triplet-triplet annihilation following (S + S) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.
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