Spin Mixing in Intramolecular Singlet Fission: A First-Principles-Based Quantum Dynamical Study

We investigate the dynamical interplay between the different triplet-pair spin states that are formed in the intramolecular singlet fission process in a series of pentacene-based dimers covalently bonded to a phenylene linker in ortho, meta, and para positions. Using first-principles calculations and a density matrix quantum dynamical approach we show that the spin dipole–dipole interaction leads to significant population of the quintet spin manifold in these regioisomers when the singlet, triplet and quintet triplet-pair states are quasidegenerate. Furthermore, we also show that the relative arrangement of the pentacene-like moieties has a profound impact on the dynamics of the spin-mixing process, affecting both the relative population of the different spin-states involved in the dynamics and the time scale of the process.