Multifaceted Control of Triplet-Pair Spin Dynamics in Intramolecular Singlet Fission at Room Temperature

The spin evolution dynamics of the correlated triplet pair (TT) decide the fate of the number of triplet (T1) excitons after photoexcitation in singlet fission (SF) materials, yet accurately resolving its relaxation pathways remains elusive. In this work, we systematically elucidate the spin dynamics of intramolecular SF in covalently linked TIPS-pentacene dimers with different inter-triplet exchange coupling (J) values and their dynamic fluctuations by externally perturbing the total spin Hamiltonian via the solventmediated external heavy-atom effect and the magnetic field effect. Based on transient absorption spectroscopy, we quantitatively demonstrate that the responsiveness to both perturbations is strictly governed by the J regime. While certain geometries facilitate or suppress spin mixing across spin manifolds depending on the J values, non-planar conformations eventually open an irreversible 3TT → T1 kinetic sink, channeling the initial population into a single T1 rather than two uncorrelated triplets (T1 + T1). Our findings reveal the mechanistic origins of spin-state dissipation, establishing a molecular framework for precisely steering spin evolution in exciton-multiplying architectures.