How does electronic spectroscopy measure nanoscale energy transfer and excitation localization?

When the pigment nuclear modes respond to electronic excitation in a pigment-protein complex, two effects are important. First, the energy transfer mechanism is potentially more complex: excitation energy can flow from one pigment to another through vibronic resonances, and energy is only subsequently dissipated to the environment during slow vibrational relaxation. In this mechanism, the actual energy transfer between pigments can be much faster than the observed energy relaxation. Second, optical measurements probe a mixture of nuclear and electronic responses, and some of the observed dynamics result from dominantly vibrational processes confined to single pigments. Long-lived vibrational wavepackets can produce spectral oscillations that obscure the true survival lifetime for delocalized electronic superposition states.

We have found that vibronic resonances, and the delocalization of vibronic states, can be very sensitive to the strong environmental fluctuations typical in protein environments at biologically relevant temperatures. While energy transfer calculations are often performed in one of either the perturbative-environment (Redfield) or perturbative-coupling (Forster) limits, these approximations are invalid to properly treat the dynamics in many photosynthetic systems, where the environmental fluctuations, pigment interactions, and energy gaps are all similar in size. For this reason, we use the power of parallel computing to solve numerically exact equations of motion for the quantum dynamics and spectroscopic response of simple mixed electronic-vibrational systems.

Dimer Relaxation

Helpful Background Reading:

  1. Quantum Coherence in Photosynthesis, Akihito Ishizaki and Graham R. Fleming, ANNUAL REVIEW OF CONDENSED MATTER PHYSICS 3:333-61, 2012.
  2. Unified treatment of quantum coherent and incoherent hopping in electronic energy transfer: Reduced hierarchy equation approach, Akihito Ishizaki and Graham R. Fleming, THE JOURNAL OF CHEMICAL PHYSICS 130, 234111, 2009