A carotenoid can efficiently quench the Qy energy of phthalocyanine molecule. Target analysis provides evidence for the pivotal role of the carotenoid excited state in the quenching by showing that the spectrum of the quenching species resembles the carotenoid S1 spectrum. However, energy transfer involving the carotenoid S1 state alone cannot be solely responsible for the quenching because the process is solvent polarity dependent. This chapter performs a transient absorption measurement on a model carotenoid with 10 double bonds to gain further insights into the process. Solvent polarity-dependent shape changes that cannot be ascribed to the Sl state are detected. Similar changes are reported for several substituted carotenoids and assigned to an intramolecular charge transfer state. Results show that carotenoids can quench tetrapyrrole singlet excited states by means of energy transfer to optically forbidden carotenoid states. Expanding the conjugated system of the carotenoid by one double bond turns the carotenoid from a nonquencher into a strong quencher.
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