Quantum Coherence and Energy Landscapes in Photosynthetic Systems Investigated with Two-Dimensional Electronic Spectroscopy

Two-dimensional (2D) electronic spectroscopy has recently emerged as a powerful technique for the study of complex photodynamics in a variety of condensed phase systems. The application of this technique to both photosynthetic pigment-protein complexes and chromophore solutions has provided insight into their intricate excitation energy transfer mechanisms and landscapes. Analysis of beating peak amplitudes in 2D spectra of the Fenna-Matthews-Olson bacteriochlorophyll complex combined with changing lineshapes has revealed signals consistent with excitonic coherence. In addition, the long lifetime of the coherence indicates a reversible, wavelike motion of energy through the complex as opposed to the classical hopping picture. This quantum-mechanical behavior may explain the near unity quantum efficiency of excitation energy transfer observed in networks of photosynthetic complexes. The inclusion of a noncollinear optical parametric amplifier producing broad bandwidth pulses enables the exploration of excitonic coherence in Light Harvesting Complex II, the most abundant antenna complex in higher plants. Long-lived quantum coherence is again observed suggesting this to be a universal phenomenon in natural photosynthetic systems. To expand upon these findings, a coherence power spectrum is produced. This novel technique allows the first direct experimental determination of the excitonic energy levels. In another set of experiments, the building an adaptive, pulse-shaping apparatus allows optimal compression of the broad bandwidth pulses for use in probing the debated electronic structure of β-carotene. Oscillating lineshapes reveal coupling of electronic states to high energy vibrational modes while the short pulses allow the initial dynamics of this system to be studied in unprecedented detail revealing several new features whose origins are still under investigation