| Abstract
| - We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads madeup of a zinc phthalocyanine (Pc) covalently linked to carotenoids with 9, 10, or 11 conjugated carbon−carbon double bonds, referred to as dyads 1, 2, and 3, respectively. We assessed the energy transfer andexcited-state deactivation pathways following excitation of the strongly allowed carotenoid S2 state as a functionof the conjugation length. The S2 state rapidly relaxes to the S* and S1 states. In all systems we detected anew pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediatestate in the S2 → S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type ofcollective carotenoid−Pc electronic state is observed that may correspond to a carotenoid excited state(s)−Pc Q exciplex. The exciplex is only observed upon direct carotenoid excitation and is nonfluorescent. Indyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet−singlet energy transfer to Pc,making the process very efficient (>90%) while for dyads 2 and 3 the S1 energy transfer channel is precludedand only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singletexcited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicatingthat the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.
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