LAUSR

Summary Two-dimensional electronic spectroscopy (2D ES) probes the energies of chromophores and their coupling, and therefore, the technique is now widely used for studying excitation-energy-transfer mechanisms. The ubiquity and importance of Forster resonance energy transfer (FRET) demand a thorough study of its signatures in 2D ES. Here, using principles from noisy-light spectroscopy to account for the spontaneous transfer event, we describe a model of the signals that arise from FRET in 2D ES. This model yields three theoretical results regarding the energy-transfer peak, which are consistent with our laboratory measurements. A second unanticipated finding is that the solvation processes that give rise to the dynamic Stokes shift of the acceptor signal also induce a relatively slower redshift of the energy-transfer signal, revealing a fundamental difference in solvation relaxation between photoexcited and FRET-excited acceptor molecules. The signatures derived from the model serve as a benchmark for ongoing work on energy-transfer mechanisms. Here, Turner and colleagues develop a noisy-light-inspired model of FRET in 2D ES to account for the vibrations that activate it. Crucially, they reveal several unreported features that distinguish FRET from other signals. These signatures are fundamental and immediately applicable to ongoing work on photosynthetic complexes, solar nanomaterials, and other renewable energy focal points. Future interdisciplinary work ranging from studies of energy transfer in organisms to medically applicable bio-imaging methodologies will take advantage of these spectral and temporal signatures.