Coherent control for spectroscopy and manipulation of biological dynamics

Motivated originally by the goal of steering a photoreaction into desired product channels, the concept of coherent control is to adapt the spectral and temporal characteristics of the excitation light to the inherent molecular resonances and dynamics, such that these can be selectively addressed and manipulated. In the last decade, the ultrafast dynamics of many atomic and molecular quantum systems in the gas and condensed phase have been controlled successfully. Motivations in chemistry are now 1) to perform spectroscopy by coherent control, which requires a deeper understanding of control mechanisms, 2) to treat more complex, biological photoreactions, and 3) the pragmatic use of coherent control techniques, for example, for pulse compression or enhanced contrast in multiphoton microscopy. As examples for 1) and 2) we review here the combined effort and interplay of conventional spectroscopy and coherent control experiments, applied to the energy flow in the light-harvesting complex LH₂ from bacterial photosynthesis. Closed-loop control experiments allowed the characteristic coupling frequency of internal conversion in the carotenoid in LH₂ to be extracted. Open-loop three-pulse control experiments, on the other hand, could directly observe an anticipated Raman-excited carotenoid ground state. As a variant of difference spectroscopy, coherent control has thus served to gain complementary spectroscopic knowledge about the energy flow in carotenoids by comparing natural to manipulated dynamics. Finally, we propose future coherent control experiments on the electronic state structure of carotenoids and discuss prospects of coherent control for other biological chromophores.