Annihilation Dynamics of Molecular Excitons Measured at a Single Perturbative Excitation Energy

Exciton-exciton annihilation (EEA) is a ubiquitous phenomenon, which may limit the efficiency of photovoltaic devices. Conventional methods of determining EEA time scales rely on measuring the intensity dependence of third-order signals. In this work, we directly extract the annihilation rate of molecular excitons in a covalently joined molecular trimer without the need to perform and analyze intensity dependent data by employing fifth-order coherent optical spectroscopy signals emitted into ±2k- 1 2k- 2 + k- 3 phase matching directions. Measured two-dimensional line shapes and their time traces are analyzed in the framework of the many-body version of the Frenkel exciton model, extended to incorporate annihilation dynamics. Combining double-sided Feynman diagrams with explicit simulations of the fifth-order response, we identify a single peak as a direct reporter of EEA. We retrieve an annihilation time of 30 fs for the investigated squaraine trimer.