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The process of singlet fission (SF) produces two triplet excited states (T 1 + T 1 ) from one singlet excited exciton (S 1 ) and a molecule in its ground state (S 0 ). It, thus, possesses the potential to boost the solar cell efficiency above the thermodynamic Shockley-Queisser limit of 33%. A key intermediate in the SF mechanism is the singlet correlated triplet pair state 1 (T 1 T 1 ). This state is of great relevance, as its formation is spin-allowed and, therefore, very fast and efficient. Three fundamentally different pathways to formation of 1 (T 1 T 1 ) have been documented so far. The factors that influence which mechanism is associated with which chromophore, however, remain largely unknown. In order to harvest both triplet excitons independently, a decorrelation of the correlated triplet pair state to two individual triplets is required. This second step of the SF process implies a change in the total spin quantum number. In the case of a dimer, this is usually only possible if the coupling between the two pentacenes is sufficiently weak. In this study, we present two platinum-bridged pentacene dimers in which the pentacenes are coupled strongly, so that spin-decorrelation yielding (T 1 + T 1 ) was initially expected to be outcompeted by triplet-triplet annihilation (TTA) to the ground state. Both platinum-bridged pentacene dimers undergo quantitative formation of the (T 1 T 1 ) state on a picosecond timescale that is unaffected by the internal heavy-atom effect of the platinum. Instead of TTA of (T 1 T 1 ) to the ground state, the internal heavy-atom effect allows for 1 (T 1 T 1 )- 3 (T 1 T 1 ) and 1 (T 1 T 1 )- 5 (T 1 T 1 ) mixing and, thus, triggers subsequent TTA to the (T 1 S 0 ) state and minor formation of (T 1 + T 1 ). A combination of transient absorption and transient IR spectroscopy is applied to investigate the mechanism of the (T 1 T 1 ) formation in both dimers. Using a combination of experiment and quantum chemical calculations, we are able to observe a transition from the CT-mediated to the direct SF mechanism and identify relevant factors that influence the mechanism that dominates SF in pentacene. Moreover, a combination of time-resolved optical and electron paramagnetic resonance spectroscopic data allows us to develop a kinetic model that describes the effect of enhanced spin-orbit couplings on the correlated triplet pair state.

Influence of the heavy-atom effect on singlet fission: a study of platinum-bridged pentacene dimers