Vibrationally-mediated dynamics in beta-carotene probed with broadband 2D electronic spectroscopy

The functions of carotenoids in photosynthetic systems range from photoprotection and light harvesting to structure. Despite the ubiquitious presence of these polyenes in natural systems, and almost four decades of research, the electronic structure of carotenoids is still intensely debated [1 ,2]. The ultrafast internal conversion from the optically bright S2 state to the lower energy optically 'dark' (i.e., the transition is not allowed in one-photon absorption from S0) state, S1, is proposed to proceed via a conical intersection [3) . However, controversy surrounds the existence and role of other dark states lying energetically intermediate to S2 and S1 [2,4]. The unique ability of two-dimensional (2D) spectroscopy to resolve emission as a function of excitation wavelength makes it an ideal tool to study carotenoids such as beta-carotene