Prospects for useful fission from singlet states higher than S 1 in aggregated organic chromophores.

The few known reports and the likely prospects of finding new efficient routes to exciton fission from higher excited singlet states, S n ( n > 1), are reviewed. Aggregates of molecules that have large S 2 -S 1 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 1 + T 1 ) triplet-triplet annihilation producing S 2 could exhibit efficient singlet fission in aggregates when appropriately substituted to meet the necessary energy requirements. The potential problem of loss of triplet excitons via 2T 1 → T n >1 + S 0 triplet-triplet annihilation following (S 2 + S 0 ) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.