Mechanism of the singlet fission: probing the intermediates and the dissociation toward independent triplets – SingletFission

Singlet fission (SF) is a spin-allowed process involving at least two organic chromophores in which a photo-generated singlet exciton (S1S0) can spontaneously down-convert to give two lower-energy triplet excitons (T1+T1) with a theoretical quantum yield of triplet formation up to 200%. Hence, it offers the possibility to surpass the efficiency limit of single-junction solar cells beyond the Shockley–Queisser limit of 33% to nearly 45%. SF has thus recently received a strongly growing interest for solar energy conversion. But despite numerous efforts on SF research, the number of chromophores able to generate SF is still limited and actual pathways followed by the system to reach individual triplets remain yet unclear.
In the simplest model, the singlet excited state evolves towards the formation of an intermediate formally referred to as triplet pair (T1T1). The latter pair decouples, losing spin coherence and forming two individual triplet states. Currently, there are two accepted mechanisms leading to SF, but their exact nature and the evolution of the intermediates are a matter of intense debate: (i) formation of the triplet pair (T1T1) from the singlet exciton (S1S0) mediated by charge-transfer states or (ii) direct formation of (T1T1) from (S1S0). Despite the acceptance of this simplest model, much controversy has recently appeared with the participation of different intermediates as quintet state, excimer, which can all be interpreted as limiting cases of a complex mixing of states. A full mechanistic description with a detailed interpretation of all intermediate processes leading to independent triplets is still far from achieved. In this context, the goal of our project is to characterize the superimposed multiple states in the singlet fission process, to develop a comprehensive model of the dissociation, and to propose the optimal conditions to achieve independent triplets.
Two families of model systems (acene and rylene) which are well known to exhibit singlet fission will be studied. We will investigate the nature of the intermediate states and the dissociation depending on several physico-chemical factors: endothermic/exothermic processes, strong/weak excitonic coupling, as well as the role of vibronic/vibrational couplings and of the disorder. The multidisciplinary SingletFission project will enable us to gain more insight on the fundamental mechanistic events governing singlet fission process.
We will use a combination of advanced time-resolved spectroscopies from gas phase to bulk and will complement the experimental results with extensive theoretical modeling. Singlet fission research in the gas phase is particularly novel and will provide the intrinsic relaxation dynamics of isolated systems. Such studies are therefore an important and complementary link between theoretical calculations and condensed phase experiments to uncover the processes at play in SF. The combination of Femtosecond Stimulated Raman Scattering and transient absorption spectroscopies will be essential to describe precisely the dynamics of the system in condensed phase, to understand the involvement of intermediate species, and to disentangle the keys to shift the energy towards the desired pathway, in our case, maximizing long-living triplet formation. A continuous feedback loop between theoretical and experimental results from gas and condensed phases will enable the understanding of the role of the different physico-chemical parameters, the nature of intermediate states and the mechanism of SF. Understanding such fundamental processes leading to singlet fission stands as the stepping stone toward the development of more efficient photosystems.