Beating maps of singlet fission: Simulation of coherent two-dimensional electronic spectroscopy by Davydov ansatz in organic molecules.

ABSTRACT

The coherent two-dimensional (2D) electronic spectra for both the intra- and inter-molecular singlet fission (SF) processes in organic molecules are simulated by the Davydov ansatz combined with the Frenkel-Dirac time-dependent variational algorithm. By virtue of the dynamical approach, we are able to identify the signals of triplet excitation in the excited-state absorption contribution of the 2D spectra. In order to discuss whether a mediative charge-transfer (CT) state is necessary to SF, we increase the CT-state energy and find, in a theoretical manner, that the beating signal related to the triplet is inhibited. The vibronic coherence is then studied in the beating maps for both the ground and excited states. Except for the normal beating modes adhering to the relevant electronic state, we observe signals that are explicitly related to the triplet excitations. The pathways of transition corresponding to these signals are clarified in the respective Feynman diagram, which can help the experimenters determine the physical origin of relevant measurements.