Abstract:
Two-dimensional electronic spectroscopy (2DES) resolves energy and charge transfer dynamics with femtosecond temporal resolution along multiple spectral dimensions – excitation, emission and coherence axes. Salient features such as 2D cross-peaks (CPs) between coherently coupled transitions and quantum beat maps can, in principle, reveal complex energy transfer pathways and the vibrational-electronic couplings that may be responsible for driving these processes. However, 2DES methods relying on detection of coherently radiated electric field are largely inapplicable to complex light harvesting systems such as intact photosynthetic cells and nanotubes due to overwhelming scatter. I will describe our recent efforts towards introducing 2DES methods that can circumvent the above challenges and demonstrate their applications on intact photosynthetic cells and light harvesting nanotubes that closely mimic naturally occurring chlorosome nanotubes.
I will introduce a repetition-rate scalable approach1 to conventional 2DES that relies on purely conventional optics and electronics with shot-to-shot detection at any repetition rate, only limited by the camera speed. I will demonstrate how the additional knob of polarization-control2 allows us to decipher the presence of strong intraband couplings within the overlapping vibrational-electronic (vibronic) bands of light harvesting nanotubes. These observations suggest that vibronic couplings may indeed survive at room temperature in large aggregates to drive ultrafast internal conversion.
I will then show3 how incoherent detection of the 2D signal through non-linear fluorescence, relying not on coherently radiated electric field but rather on signal generation through incoherent exciton-exciton annihilation, allows for first such observations of exciton diffusion inside cyanobacterial cells across the intact photosynthetic machinery, thereby bypassing the significant complications that are inevitably introduced upon isolating proteins from the native cell environment. Together these suites of white-light 2DES approaches provide a powerful platform to characterize a variety of complex systems with future avenues for introducing spatial resolution as well.
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