title: Energy transfer, excited-state deactivation, and exciplex formation in artificial caroteno-phthalocyanine light-harvesting antennas.
abstract: 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 S2 state as a function of the conjugation length. The S2 state rapidly relaxes to the S* and S1 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 S2-->S1 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, 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 precluded and only S2 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.
authors.affiliation: Department of Biophysics, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, Amsterdam 1081 HV, The Netherlands.
authors.forename: Rudi
authors.initials: R
authors.lastname: Berera
authors.forename: Ivo H M
authors.initials: IH
authors.lastname: van Stokkum
authors.forename: Gerdenis
authors.initials: G
authors.lastname: Kodis
authors.forename: Amy E
authors.initials: AE
authors.lastname: Keirstead
authors.forename: Smitha
authors.initials: S
authors.lastname: Pillai
authors.forename: Christian
authors.initials: C
authors.lastname: Herrero
authors.forename: Rodrigo E
authors.initials: RE
authors.lastname: Palacios
authors.forename: Mikas
authors.initials: M
authors.lastname: Vengris
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
authors.forename: Devens
authors.initials: D
authors.lastname: Gust
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Moore
authors.forename: Ana L
authors.initials: AL
authors.lastname: Moore
authors.forename: John T M
authors.initials: JT
authors.lastname: Kennis
mesh_terms: D040761:Biomimetic Materials / Q000138:chemical synthesis / Q000737:chemistry*; D002338:Carotenoids / Q000138:chemical synthesis / Q000737:chemistry*; D004735:Energy Transfer; D007211:Indoles / Q000138:chemical synthesis / Q000737:chemistry*; D054833:Isoindoles; D045342:Light-Harvesting Protein Complexes / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D007211:Indoles; D054833:Isoindoles; D045342:Light-Harvesting Protein Complexes; D002338:Carotenoids; C013647:phthalocyanine