Abstract
The process of singlet fission (SF) produces two triplet excited states (T1 + T1) from one singlet excited exciton (S1) and a molecule in its ground state (S0). 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(T1T1). 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(T1T1) 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 (T1 + T1) 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 (T1T1) state on a picosecond timescale that is unaffected by the internal heavy-atom effect of the platinum. Instead of TTA of (T1T1) to the ground state, the internal heavy-atom effect allows for 1(T1T1)-3(T1T1) and 1(T1T1)-5(T1T1) mixing and, thus, triggers subsequent TTA to the (T1S0) state and minor formation of (T1 + T1). A combination of transient absorption and transient IR spectroscopy is applied to investigate the mechanism of the (T1T1) 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.
Highlights
Singlet ssion (SF) is a fundamental photophysical process that describes the spin conserved reaction of an singlet excited state (S1) with a molecule in its electronic ground state (S0) to provide a singlet spin-correlated triplet pair state 1(T1T1) (eqn (1)).[1]S1 + S0 / (T1T1)[1 ] (1)11130 | Chem
Compounds mono-Pt, trans-Pt, and cis-Pt are stable under ambient laboratory conditions for days, they slowly decompose over several days in solution under exposure to both oxygen and light
The utilization of two platinum-bridged pentacene dimers that show efficient singlet ssion allows us to study the in uence of intersystem crossing (ISC) on the transient states that are involved in singlet fission (SF)
Summary
Singlet ssion (SF) is a fundamental photophysical process that describes the spin conserved reaction of an singlet excited state (S1) with a molecule in its electronic ground state (S0) to provide a singlet spin-correlated triplet pair state 1(T1T1) (eqn (1)).[1]S1 + S0 / (T1T1)[1 ] (1)11130 | Chem. Singlet ssion (SF) is a fundamental photophysical process that describes the spin conserved reaction of an singlet excited state (S1) with a molecule in its electronic ground state (S0) to provide a singlet spin-correlated triplet pair state 1(T1T1) (eqn (1)).[1]. The energetic requirement for SF is that the energy of the singlet states (S1 + S0) or (S1S0) should be close to or larger than the energy of the triplet product 1(T1T1).[2] As the total spin does not change during this rst step of the SF process, it is spinallowed; this is fundamentally different from conventional triplet formation via intersystem crossing (ISC). If the coupling between the triplet states is weak enough, the 1(T1T1) state can subsequently undergo spin evolution mixing with the quintet spin-correlated triplet pair state, 5(T1T1), and eventually decorrelation that results in two individual triplet states (T1 + T1) (eqn (2)).[4,5,6]
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