From coherent to vibronic light harvesting in photosynthesis

Photosynthetic organisms are a remarkable example of nanoscale engineering and have mastered the process of solar energy harvesting over billions of years of evolution. Therefore, researchers seek insights from the light collection mechanisms of photosynthetic machinery. The initial energy transfer stage of photosynthesis, which begins with light absorption and leads to charge separation, is remarkably robust in conditions of strong energetic disorder, extreme physiological temperatures, and low light flux - very different from conventional solar conversion materials [1-3]. However, determining the key principles which are responsible for efficient conversion is a challenging task due to the complexity of the photosynthetic systems. The field encountered a fascinating lead in 2007 when oscillatory features were discovered in two-dimensional electronic spectroscopic data - the optical analogue of 2D NMR - and were assigned to quantum coherence between donor and acceptor electronic states [4^••]. In this review, we describe the evolution in our understanding of quantum effects in photosynthetic energy transfer. A vibronic model is described to demonstrate the current opinion on how quantum effects can optimize energy transfer.