Energy transfer, excited-state deactivation, and exciplex formation in artificial caroteno-phthalocyanine light-harvesting antennas

We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads made up 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 and excited-state deactivation pathways following excitation of the strongly allowed carotenoid S-2 state as a function of the conjugation length. The S-2 state rapidly relaxes to the S* and S-1 states. In all systems we detected a new pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediate state in the S-2 -> S-1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective 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. In dyad 1, two carotenoid singlet excited states, S-2 and S-1, contribute to singlet-singlet energy transfer to Pc, making the process very efficient (> 90%) while for dyads 2 and 3 the S-1 energy transfer channel is precluded and only S-2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.