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 ±2k⃗1 ∓ 2k⃗2 + k⃗3 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.

Department of Chemistry Loughborough University Loughborough LE11 3TU United Kingdom

Faculty of Science and Engineering Zernike Institute for Advanced Materials University of Groningen Nijenborgh 4 9747AG Groningen The Netherlands

Institut für Organische Chemie Center for Nanosystems Chemistry Universität Würzburg Am Hubland 97074 Würzburg Germany

Institute of Physics Faculty of Mathematics and Physics Charles University Ke Karlovu 5 Prague 121 16 Czech Republic

Professur für Dynamische Spektroskopien Fakultät für Chemie Technische Universität München Lichtenbergstrasse 4 D 85748 Garching b München Germany

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