Quintet Formation, Exchange Fluctuations, and the Role of Stochastic Resonance in Singlet Fission

Singlet fission describes the spin-conserving production of two triplet excitons from one singlet exciton. The existence of a spin-2 (quintet) triplet-pair state as a product of singlet fission is well established in the literature, and control of quintet formation is an important step towards applying singlet fission in photovoltaics and quantum information. However, a definitive mechanism for quintet formation is yet to be established, which makes it difficult to design materials for optimal quintet formation. Here we outline a mechanism in which inter-triplet exchange-coupling fluctuations drive fast and efficient quintet formation. We show that quintet formation is possible even in the strong-exchange regime, in accordance with recent experimental prediction. We evaluate the performance of this quintet formation mechanism in two regimes of conformational freedom, and relate quintet dynamics to material properties of singlet fission molecules. Quintet formation is a part of a photochemical energy conversion process, which could be exploited to help achieve higher efficiency values for photovoltaics. Here, the authors propose a stochastic and coherent resonance mechanism for formation of the quintet spin state in singlet fission materials, that is still viable in the strong-exchange regime.