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Differential infrared spectroscopy allows to select only those chemical vibrations involved in a light induced reaction. The structure and environment of unstable and short lived excited states can be probed by using step scan FTIR time resolved spectroscopy. In this chapter we present an application of time resolved FTIR step scan spectroscopy to the light harvesting complexes involved in the collection of solar energy in photosynthesis. The time resolved data are analysed using a global and target analysis procedure which allows identification of the dynamic and the spectral properties of short lived intermediates such as triplet states. Triplet state of chlorophyll a (Chl a) can react with oxygen and lead to the formation of singlet oxygen. Carotenoids avoid this reaction via triplet excitation energy transfer (TEET) and quench the triplet of Chl a. The peridinin chlorophyll protein (PCP), an algal light harvesting complex, which binds Per and Chl a is a good system to study photoprotection mechanism by infrared spectroscopy. Indeed Per and Chl a have both conjugated carbonyl groups that are efficient probes of the molecular state in the infrared. We first investigated by step scan spectroscopy the TEET reaction of Per and Chl a in solvent to get their respective spectral signature. Such a study leads to the identification of several mechanisms associated with the formation of triplet states in solution. Secondly the triplet formation is observed in two different PCP complexes leading to the unexpected conclusion that the Per triplet state is delocalised over the Chl a. In a third part we reveal that the same process of triplet sharing between Chl and carotenoid is also present in higher plants, in sharp contrast with purple bacteria for which the triplet is fully localised on the carotenoid. Our finding strongly suggests that in higher plants and algae a much stronger interaction between carotenoids and chlorophylls is at the basis of photoprotection, and represents an example of molecular adaptation in oxygenic photosynthesis.

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Time-Resolved FTIR Difference Spectroscopy Reveals the Structure and Dynamics of Carotenoid and Chlorophyll Triplets in Photosynthetic Light-Harvesting Complexes