Abstract
The non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis. Despite the continuous theoretical and experimental efforts to clarify the ultrafast dynamics of this process, it still represents the object of an intense investigation because the ultrafast timescale and the congestion of the involved states makes its characterization particularly challenging. Here we exploit 2D electronic spectroscopy and recently developed data analysis tools to provide more detailed insights into the mechanism of internal conversion within the Q-bands of chlorophyll a. The measurements confirmed the timescale of the overall internal conversion rate (170 fs) and captured the presence of a previously unidentified ultrafast (40 fs) intermediate step, involving vibronic levels of the lowest excited state.
Highlights
The non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis
The 2D electronic spectroscopy (2DES) characterization of chla discussed in this work gives a new and detailed mechanistic insight on the relaxation and dephasing dynamics of internal conversion process within the Q-bands
The analysis of the dynamics response by means of recently developed methodologies confirmed the timescale of the overall internal conversion rate (170 fs) and captured the presence of an intermediate step involving vibronic states S1,n, in particular a state at 510 cm−1 above the vibrational ground state of electronic state S1
Summary
The non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis. Particular attention has been devoted to the ultrafast relaxation dynamics of chla, in the context of its possible involvement in quantum mechanisms of energy and charge transport in biological complexes[9,10,11,12,13,14,15,16,17,18,19]. In these photosynthetic proteins, the characteristic signatures resulting from the peculiar intra-chromophore vibrational and electronic structure can overlap with the most interesting collective behavior resulting from exciton interaction. The first, proposed in the ‘60s’31, identifies the Qx component with the signal at 17400 cm−1, while the second, from the ‘80s’32, assigns that transition to the feature at 17000 cm−1
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