title: Excited-state dynamics of monomeric and aggregated carotenoid 8'-apo-β-carotenal.
abstract: Excited-state properties of monomeric and aggregated carbonyl carotenoid 8'-apo-β-carotenal were studied by means of femtosecond transient absorption spectroscopy. For monomers, the polarity-dependent behavior characteristic of carotenoids with conjugated carbonyl group was observed. In n-hexane the S(1) lifetime is 25 ps, but it is shortened to 8 ps in methanol. This shortening is accompanied by the appearance of new spectral bands in transient absorption spectrum. On the basis of analysis of the transient absorption spectra of monomeric 8'-apo-β-carotenal in n-hexane and methanol, we propose that the polarity-induced spectral bands are due to the S(1)(A(g)(-))-S(3)(A(g)(+)) transition, which is enhanced upon breaking the symmetry of the molecule. This symmetry breaking is caused by the conjugated carbonyl group; it is much stronger in polar solvents where the S(1) state gains significant charge-transfer character. Upon addition of water to methanol solution of 8'-apo-β-carotenal we observed formation of aggregates characterized  by either blue-shifted (H-aggregate) or red-shifted (J-aggregate) absorption spectrum. Both aggregate types exhibit excited-state dynamics significantly different from those of monomeric 8'-apo-β-carotenal. The lifetime of the relaxed S(1) state is 20 and 40 ps for the H- and J-aggregates, respectively. In contrast to monomers, aggregation promotes formation of triplet state, most likely by homofission occurring between tightly packed molecules within the aggregate.
authors.affiliation: Institute of Physics and Biophysics, Faculty of Science, University of South Bohemia, Branišovská 31, České Budějovice, Czech Republic.
authors.forename: Milan
authors.initials: M
authors.lastname: Durchan
authors.forename: Marcel
authors.initials: M
authors.lastname: Fuciman
authors.forename: Václav
authors.initials: V
authors.lastname: Šlouf
authors.forename: Gürkan
authors.initials: G
authors.lastname: Keşan
authors.forename: Tomáš
authors.initials: T
authors.lastname: Polívka
mesh_terms: D000042:Absorption; D002338:Carotenoids / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: C110652:beta-apo-8'-carotenal; D002338:Carotenoids