Assignment Of The Q-Bands Of The Chlorophylls: Coherence Loss Via Q(X) - Q(Y) Mixing

Nature Publishing Group
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Journal Article
Scientific Report, 2013, 3 pp. 1 - 8
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We provide a new and definitive spectral assignment for the absorption, emission, high-resolution fluorescence excitation, linear dichroism, and/or magnetic circular dichroism spectra of 32 chlorophyllides in various environments. This encompases all data used to justify previous assignments and provides a simple interpretation of unexplained complex decoherence phenomena associated with Q(x)-> Q(y) relaxation. Whilst most chlorophylls conform to the Gouterman model and display two independent transitions Q(x) (S-2) and Q(y) (S-1), strong vibronic coupling inseparably mixes these states in chlorophyll-a. This spreads x-polarized absorption intensity over the entire Q-band system to influence all exciton-transport, relaxation and coherence properties of chlorophyll-based photosystems. The fraction of the total absorption intensity attributed to Q(x) ranges between 7% and 33%, depending on chlorophyllide and coordination, and is between 10% and 25% for chlorophyll-a. CAM-B3LYP density-functional-theory calculations of the band origins, relative intensities, vibrational Huang-Rhys factors, and vibronic coupling strengths fully support this new assignment.
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