Abstract
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. It is now often possible to identify such processes, but a detailed depiction of the crucial nuclear degrees of freedom involved typically remains elusive. Here, 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. We assign the tuning and coupling modes, quantifying their relative intensities and contributions, and demonstrate how these modes coherently synchronise to drive the reaction. 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. This will enable mechanistic insight on a detailed structural level, with the ultimate aim to rationally design molecules to maximise the efficiency of photoinduced reactions.
Highlights
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces
To gain structural insight into this vibrationally coherent mechanism, we modelled the full quantum dynamics of fission employing a recently developed Tree Tensor Network state (TTNS) approach[48] which accounts for 252 vibrational modes spanning 110–1680 cm−1 and their respective couplings to 5 excited electronic states
We employed structurally sensitive excitedstate Raman spectroscopy to uncover the transfer of vibrational wavepackets from S1 to 1TT, mandating a vibrationally coherent reaction mechanism despite there being no direct coupling between these states
Summary
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. Despite remarkable progress in the optical manipulation of vibrational and electronic states[3,5] and the identification of vibronically coherent processes[1,8,11,12], the precise molecular mechanisms and associated structural changes remain largely elusive and subject to competing interpretations This uncertainty stems from a disparity between experimental and theoretical methods. There is no clear determination of what motions drive the process, how this coupling occurs, or whether the reported vibrational coherence is important in achieving a high reaction yield or a consequence of the ultrafast nature of the reaction with no functional importance These problems are typical of the more general study of non-Born-Oppenheimer dynamics, and they constitute a key bottleneck in materials understanding and design
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