2024/03/08 by Oskar Kefer, Kefer, Oskar, Pavel V. Kolesnichenko +9
Physics and Astronomy · Biochemistry, Genetics and Molecular Biology · #Advanced Chemical Physics Studies #Spectroscopy and Quantum Chemical Studies #DNA and Nucleic Acid Chemistry
paper · pdf · doi:10.48550/arxiv.2403.05163
The electronic interaction between an optically active singlet state (S1S0) and a dark state of singlet multiplicity, known as correlated triplet pair (1[TT]), plays a crucial role in the effective transformation from S1S0 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 S1S0 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 S1S0 to 1[TT]. Upon optical excitation, a superposition of S1S0 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 S1S0 and 1[TT]. This experiment provides new insights into the equilibrium between the two electronic states, a phenomenon previously understood primarily through theoretical models.