Signatures of Exciton Delocalization and Exciton-Exciton Annihilation in Fluorescence-Detected Two-Dimensional Coherent Spectroscopy.

Incoherently detected coherent multidimensional spectroscopy is rapidly gaining popularity, promising a different application range and sensitivity than its traditional counterpart. While measuring the same response, the two methods are not equivalent. We present calculations of the fluorescence-detected coherent two-dimensional (F-2DES) spectra of a molecular heterodimer. We compare how the F-2DES technique differs from standard coherently detected two-dimensional (2DES) spectroscopy in measuring exciton delocalization. We analyze which processes contribute to cross-peaks in the zero-waiting-time spectra obtained by the two methods. Strictly on the basis of time-dependent perturbation theory, we study how in both methods the varying degree of cancellation between perturbative contributions gives rise to cross-peaks and we identify exciton annihilation and exciton relaxation contributions to the cross-peak in the zero-waiting-time F-2DES. We propose that time-gated fluorescence detection can be used to isolate the annihilation contribution to F-2DES both to retrieve information equivalent to 2DES spectroscopy and to study the annihilation contribution itself.