Abstract
This chapter will review the recent results in the field of carotenoid photophysics and relate excited-state properties of carotenoids to their potential roles in the dissipation of the singlet-excited state of chlorophyll a resulting in non-photochemical quenching (NPQ) of chlorophyll fluorescence. Investigations into the structure and dynamics of excited states of carotenoids have revealed potential mechanisms regarding the involvement of spectroscopically forbidden “dark” excited states in carotenoid relaxation pathways. The generally accepted model for deactivation of excited states of carotenoids following photo-excitation invokes three states, the ground state S0, the first excited state S1, considered “dark” because the S0→S1 transition is forbidden by symmetry, and the second excited state S2, where the S0→S2 transition is strongly allowed. In addition to these states, a number of other excited states have been proposed to be involved in carotenoid de-excitation on the basis of theoretical computations and ultrafast spectroscopy. Yet, the properties of these additional states, and even their very existence, are still being questioned. We will describe the current state of knowledge regarding energetics and dynamics of carotenoid excited states and molecular factors that control these properties, which include the π-electron conjugated chain length, various attached functional groups, configuration and conformation of the molecules, and, perhaps most importantly, carotenoid interaction with the local environment. Many of these factors tune the excited-state spectra and dynamics of carotenoids, and there is accumulating evidence that they are crucial for carotenoid function in photosynthetic systems.
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