Experimental Evidence of Direct Exchange Interaction Mediating Intramolecular Singlet Fission in Weakly-Coupled Dimers

The electronic interaction between an optically active singlet state (S_1S_0) and a dark state of singlet multiplicity, known as correlated triplet pair (^1[TT]), plays a crucial role in the effective transformation from S_1S_0 to ^1[TT] during intramolecular singlet fission (iSF). This process is understood through mechanisms such as direct exchange coupling and incoherent processes that involve super-exchange coupling through charge-transfer states. However, most insights into these mechanisms are derived from theoretical studies due to the difficulties in obtaining experimental evidence. In this study, we investigate the excited-state interactions between S_1S_0 and ^1[TT] in spiro-conjugated iSF sensitizers by employing transient two-dimensional electronic spectroscopy. This approach allows us to focus on the early stages of the conversion from S_1S_0 to ^1[TT]. Upon optical excitation, a superposition of S_1S_0 and ^1[TT] is created, which gradually transitions to favor ^1[TT] within the characteristic time frames of iSF. The observed high-order signals indicate circular repopulation dynamic that effectively reinitiates the iSF process from higher energy electronic states. Our findings, supported by semi-quantum-mechanical simulations of the experimental data, suggest the presence of a direct iSF mechanism in the dimers, facilitated by weak non-adiabatic coupling between S_1S_0 and ^1[TT]. This experiment provides new insights into the equilibrium between the two electronic states, a phenomenon previously understood primarily through theoretical models.