title: Two-photon study on the electronic interactions between the first excited singlet states in carotenoid-tetrapyrrole dyads.
abstract: Electronic interactions between the first excited states (S(1)) of carotenoids (Car) of different conjugation lengths (8-11 double bonds) and phthalocyanines (Pc) in different Car-Pc dyad molecules were investigated by two-photon spectroscopy and compared with Car S(1)-chlorophyll (Chl) interactions in photosynthetic light harvesting complexes (LHCs). The observation of Chl/Pc fluorescence after selective two-photon excitation of the Car S(1) state allowed sensitive monitoring of the flow of energy between Car S(1) and Pc or Chl. It is found that two-photon excitation excites to about 80% to 100% exclusively the carotenoid state Car S(1) and that only a small fraction of direct tetrapyrrole two-photon excitation occurs. Amide-linked Car-Pc dyads in tetrahydrofuran demonstrate a molecular gear shift mechanism in that effective Car S(1) → Pc energy transfer is observed in a dyad with 9 double bonds in the carotenoid, whereas in similar dyads with 11 double bonds in the carotenoid, the Pc fluorescence is strongly quenched by Pc → Car S(1) energy transfer. In phenylamino-linked Car-Pc dyads in toluene extremely large electronic interactions between the Car S(1) state and Pc were observed, particularly in the case of a dyad in which the carotenoid contained 10 double bonds. This observation together with previous findings in the same system provides strong evidence for excitonic Car S(1)-Pc Q(y) interactions. Very similar results were observed with photosynthetic LHC II complexes in the past, supporting an important role of such interactions in photosynthetic down-regulation.
authors.affiliation: Technische Universität Braunschweig, Institute for Physical and Theoretical Chemistry, Department for Biophysical Chemistry, Hans-Sommer-Strasse 10, 38106 Braunschweig, Germany.
authors.forename: Pen-Nan
authors.initials: PN
authors.lastname: Liao
authors.forename: Smitha
authors.initials: S
authors.lastname: Pillai
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: Peter J
authors.initials: PJ
authors.lastname: Walla
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry; D004583:Electrons; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry; D015394:Molecular Structure; D017785:Photons; D011789:Quantum Theory; D013237:Stereoisomerism; D045725:Tetrapyrroles / Q000737:chemistry*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045725:Tetrapyrroles; D002734:Chlorophyll; D002338:Carotenoids