Revealing molecular mechanisms of photosynthetic light harvesting with ultrafast optical spectroscopy

Photosynthesis, a key process for life on earth, unfolds a complex sequence of light-triggered events, involving interactions between various chromophores within large supramolecular architectures. These photoinduced processes range from the highly efficient directional energy flow that governs the light-harvesting functionality, to the unidirectional charge separation in the reaction center, to the activation of non-radiative energy dissipation channels upon excess illumination. Investigation of the molecular mechanisms and the dynamics of such processes is important both for a fundamental understanding of nature and for the design of biomimetic systems for solar energy harvesting. Ultrafast optical spectroscopy provides a wealth of information on these photophysical processes and has therefore been extensively used in photosynthesis studies. This review serves a dual purpose: to introduce the application to photosynthesis to the experts in ultrafast spectroscopy, and to explain to the experts in photosynthesis the power of ultrafast optical spectroscopy to answer key biochemical questions. The paper begins by outlining the working principles and experimental implementations of three commonly used techniques to study photosynthetic light harvesting: picosecond time-resolved photoluminescence (TRPL), femtosecond transient absorption (TA) spectroscopy and femtosecond two-dimensional electronic spectroscopy (2DES). It then proceeds to present selected examples, taken from our own recent work, which demonstrate the ability of TRPL, TA, 2DES to reveal specific aspects of the molecular mechanisms of photosynthesis. It concludes by discussing perspectives for novel technological developments and the studies that they will enable.