Ultrafast Vibronic Dynamics of Singlet Fission: From Molecular Movies to Wavefunction Projection*

The complex dynamics of ultrafast photoinduced reactions such as singlet fission are governed by their evolution along vibronically coupled potential energy surfaces. Here, I will decribe our recent work on both understading this coupling and manipulating it using ultrafast optical spectrscopy. Combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. Our combined experimental and theoretical approach reveals the atomic- scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. In other singlet fission systems, polydiacetylene and carotenoids, we experimentally demonstrate that S1 state (21Ag-) is a superposition state with strong contributions from spin-entangled pairs of triplet excitons (1(TT)). We further show that optical manipulation of the S1 (21Ag-) wavefunction using triplet absorption transitions allows selective projection of the 1(TT) component into a manifold of spatially separated triplet-pairs with lifetimes enhanced by up to one order of magnitude and whose yield is strongly dependent on the level of inter- chromophore coupling.