Exciton-exciton annihilation in a molecular trimer: Wave packet dynamics and 2D spectroscopy

We theoretically study the exciton-exciton annihilation (EEA) in a molecular trimer MMM. The system is treated within a model of electronic states, and the coupling to a bath is incorporated using the quantum jump method. Two situations of initial excitation are compared. In the first one, a two-photon process populates configurations M^*M^*M and MM^*M^* so that two excitons reside on neighboring monomers M. Then, EEA can immediately proceed. In contrast, if the trimer initially is in the local configuration M^*MM^*, exciton diffusion must occur before the annihilation process can take place. For the trimer, this excitonic motion takes place on a very short time scale. In both cases, wave packets are prepared which show a different quantum dynamics where the latter depends on the couplings and decay rates. It is documented how fifth-order coherent two-dimensional spectroscopy can be used to directly map the EEA as a function of time.