Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley-Queisser limit in photovoltaics [1,2]. Recent results have shown that the process is strongly governed by the coupling of electronic and vibrational degrees of freedom [3-5] and strongly coupled triplet pair states [6,7]. This gives rise to a host of interesting phenomena such as multi-molecular conical intersections at which singlets convert to entangled triplet pair states and the formation of exchange-coupled spin-2 quintet states. In this talk we will present recent results from our investigations into the coupled triplet pair in these systems using a combination of ultrafast spectroscopy and high magnetic fields. These results help to elucidate the nature of the wavefunction of the coupled triplet pair state and how it can be engineered via molecular design.
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