  The coherent two-dimensional (2D) electronic spectra with respect to the
singlet fission (SF) process in organic molecular aggregates are simulated by
the Davydov ansatz combined with the Frenkel-Dirac time-dependent variational
algorithm. By virtue of the full-quantum dynamical approach, we are able to
identify the signals of triplet excitation in the excite-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, 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 state. 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.

Mesoscale and Nanoscale Physics