Tracing the steps of photoinduced chemical reactions in organic molecules by coherent two-dimensional electronic spectroscopy using triggered exchange.

We establish coherent triggered-exchange two-dimensional (TE2D) electronic spectroscopy as an expansion of pump-repump-probe transient absorption spectroscopy and uniquely elucidate the role of higher-lying electronic states in ultrafast photochemistry. As an example, this is demonstrated for a molecular switch present in two ring-open conformations. The formation of a new species-the radical cation-is observed and its precursor state is identified via TE2D.