Prospects for useful fission from singlet states higher than S1 in aggregated organic chromophores

Abstract

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 S₂–S₁ electronic energy spacings and/or emit measurable “contra-Kasha” emission may offer further opportunities. Among these, electronically excited molecular systems that exhibit known efficient (T₁ + T₁) triplet–triplet annihilation producing S₂ 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 2T₁ → T_n>1 + S₀ triplet–triplet annihilation following (S₂ + S₀) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.