Theory of Light-Harvesting and Optical Spectra: Challenges and Recent Developments

Thomas Renger, Julian Adolphs, Manuel Berrer, Alexander Klinger, T.-C. Dinh, Frank Müh
Johannes Kepler University Linz, Austria

The equal magnitude of excitonic and exciton-vibrational coupling provides one challenge for the description of optical spectra and excitation energy transfer in pigment-protein complexes. From a normal mode analysis of the spectral density of the exciton-vibrational coupling [1] it is seen that the modulation of excitonic couplings is one order of magnitude smaller than that of the site energies. This inequality together with static disorder reveals a dominant contribution of the diagonal elements of the exciton-vibrational coupling in the basis of delocalized exciton states that can be exploited in the development of theory [2,3]. The small off-diagonal elements give rise to non-secular effects that are found to redistribute oscillator strength from the strong to the weak exciton transitions [2] and cause lifetime broadening of optical lines. The latter is often taken into account by Redfield or Modified Redfield theory. Both theories assume that nuclear relaxation is fast. We have developed a theory, termed Non-Equilibrium Modified Redfield (NeMoR) theory that takes into account the finite relaxation time of nuclei. Interestingly, at low temperatures the NeMoR results are better approximated by Redfield than by Modified Redfield theory due to a fortuitous error compensation. This error compensation is exploited in our microscopic theory of resonant and non-resonant hole-burning spectroscopy of multi-pigment protein complexes [4], which allows for a quantitative analysis of those experiments over a spectral range that covers all exciton states and not just the lowest.

Finally, I want to comment on another challenge concerning the off-diagonal elements of the exciton-vibrational coupling, namely the dynamic localization of the exciton-vibrational wavefunction. We find evidence for these localization effects in circular dichroism and circular polarized luminescence spectra. These spectra have been used to re-investigate the energy sinks in the CP43 [5] and CP47 subunits of photosystem II.

References

[1] T. Renger, A. Klinger, F. Streinecker, M. Schmidt am Busch, J. Numata, F. Müh, J. Phys. Chem. B 2012, 116, 14565-14580.
[2] T. - C. Dinh, T. Renger, J. Chem. Phys. 2015, 142, 034104.
[3] T. - C. Dinh, T. Renger, J. Chem. Phys. 2016 (under revision).
[4] J. Adolphs, M. Berrer, T. Renger, J. Am. Chem. Soc. 2016, 138, 2993−3001.
[5] J. Hall, T. Renger, R. Picorel, E. Krausz, Biochim. Biophys. Acta 2016, 1857, 115-128.