Ultrafast Multidimensional Spectroscopy: Principles and Applications to Photosynthetic Systems

We present the utility of 2-D electronic spectroscopy for the investigation of energy transfer dynamics in photosynthetic light-harvesting systems. Elucidating ultrafast energy transfer within photosynthetic systems is difficult due to the large number of molecules and complex environments involved in the process. In many spectroscopic methods, these systems appear as overlapping peaks with broad linewidths, obscuring the details of the dynamics. 2-D spectroscopy is a nonlinear, ultrafast method that yields a correlation map between excitation and emission energies, and can track incoherent and coherent energy transfer processes with femtosecond resolution. A 2-D spectrum can provide important insight into the structure and the mechanisms behind the excited state dynamics. We review the principles behind 2-D spectroscopy and describe the content of a 2-D electronic spectrum. Several recent applications of this technique to the major light-harvesting complex of Photosystem II are presented, including monitoring the time scales of energy transfer processes, investigation of the excited state energies, and determination of the relative orientations of the excited state transition dipole moments.