| 1. |
- Ruseckas, Arvydas, et al.
(author)
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Conformations and Photophysics of a Stilbene Dimer.
- 2003
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In: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 107:40, s. 8029-8034
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Journal article (peer-reviewed)abstract
- Photophysical studies of pseudo-p-distyryl[2.2]paracyclophane, a model molecule of a stilbene dimer arranged in a brick-wall geometry, in tetrahydrofurane solution indicate the existence of at least two conformers in the ground state. The conformer A with the smallest optical gap of 3 eV has a short radiative lifetime of 3.3 ns, while the conformer B with an optical gap about 0.3 eV larger is much more abundant in solution and its radiative lifetime is about 10 times longer. Ab initio calculations show that the ground-state energy has a shallow minimum with respect to the torsional angle between the styryl side group and the paracyclophane core around the zero position, with the lowest energy conformation at a torsional angle of -21. On this basis, A and B are assigned to flat and twisted conformations, respectively. Conformer B shows a partial decay of excited-state absorption and fluorescence with a time constant ranging from 5 to 30 ps, depending on excitation photon energy. We attribute this to isomerization of the stilbene moiety, which initially keeps conjugation in the ground state. The isomerization product is long-lived and tends to accumulate in solution.
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| 2. |
- Ruseckas, Arvydas
(author)
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Dynamics of Excitation Transfer and Charge Separation in Polymeric Semiconductors
- 1999
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Doctoral thesis (other academic/artistic)abstract
- Dynamics of optically excited electronic states have been studied in solid films and solutions of semiconducting polymers and model compounds. Transient absorption, monitored in the spectral range from 0.9 eV to 2.5 eV with a time resolution of about 100 fs, was the main experimental technique. Transient anisotropy, obtained from different polarizations of light, was employed to trace the dipole moment dynamics of the probed optical transitions. Dynamics of electron-hole pairs created by direct photoexcitation to the charge transfer state of electron donor-acceptor complexes was studied in sensitised films of a photoconducting polymer poly(epoxypropyl-carbazole) and in model solutions. Fast hole transfer (within 100 fs) from the parent carbazolyl moiety to the neighbouring carbazolyls in films is concluded from transient anisotropy measurements. This is supported by the charge recombination dependence on excitation photon energy and acceptor concentration. Comparison of charge recombination kinetics with Monte Carlo simulations suggests the separation distance between the thermalized charge pairs to be in the range of 10-20 Å. Correlation is observed between the charge separation distance and the probability of radiative recombination. Emissive intra-chain singlet states are found to be the predominant primary excitations in polythiophene conjugated polymers. A red - shift of emission spectra with time and a gradual decay of the anisotropy indicate excitation transfer to the lower energy sites in the bulk films and in the isolated polymer chains in solution. Very fast excitation transfer between adjacent chains on a time scale of 100 fs is observed in films with dense packing. Short inter-chain distance (about 4 Å) allows for the formation of inter-chain charge pairs, which are generated on a femtosecond time scale with about 20% efficiency. Mixed inter-chain exciton – charge transfer states are proposed to be precursors for charge pairs. Finally, electron transfer from the excited model molecule terthiophene into an electron accepting TiO2-SiO2 hybrid polymer matrix is found to proceed on a time scale of 1 ps. Charge separation is slower than in polymer systems. That result is explained by a fact that the primary excited state is localized on a single molecule, while it is considered delocalized in conjugated polymer systems at early time after excitation.
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| 3. |
- Ruseckas, Arvydas, et al.
(author)
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Luminescence quenching by inter-chain aggregates in substituted polythiophenes
- 2001
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In: Journal of Photochemistry and Photobiology, A: Chemistry. - 1873-2666 .- 1010-6030. ; 144:1, s. 3-12
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Journal article (peer-reviewed)abstract
- Time-resolved photo luminescence spectra measured in solid films of two polythiophene derivatives with different chain packing allow to distinguish emission of intra-chain excitations from the luminescence of inter-chain aggregates. Aggregate luminescence is red shifted by about 0.1 eV relative to intra-chain emission and shows vibronic coupling to the C=C bond stretch with the Huang-Phys factor of S approximate to 1.5, which is twice bigger than that of the intra-chain emission. Combining time resolved luminescence data with femtosecond transient absorption, we show that the dynamic quenching of the luminescence in films with dense chain packing is mainly due to excitation energy transfer to aggregates. The radiative lifetime of the lowest excited state of the aggregate with the optical gap of 1.84-1.9 eV is estimated to be about 20 ns. The aggregate contribution to the total luminescence in the polythiophene films with dense chain packing is about 50% and does not change significantly with temperature. (C) 2001 Elsevier Science B.V. All rights reserved.
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| 4. |
- Voerman, Sofie E., et al.
(author)
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Dominance of photo over chromatic acclimation strategies by habitat-forming mesophotic red algae
- 2023
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In: Proceedings of the Royal Society of London. Biological Sciences. - : Royal Society. - 0962-8452 .- 1471-2954. ; 290:2008
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Journal article (peer-reviewed)abstract
- Red coralline algae are the deepest living macroalgae, capable of creating spatially complex reefs from the intertidal to 100+ m depth with global ecological and biogeochemical significance. How these algae maintain photosynthetic function under increasingly limiting light intensity and spectral availability is key to explaining their large depth distribution. Here, we investigated the photo- and chromatic acclimation and morphological change of free-living red coralline algae towards mesophotic depths in the Fernando do Noronha archipelago, Brazil. From 13 to 86 m depth, thalli tended to become smaller and less complex. We observed a dominance of the photo-acclimatory response, characterized by an increase in photosynthetic efficiency and a decrease in maximum electron transport rate. Chromatic acclimation was generally stable across the euphotic-mesophotic transition with no clear depth trend. Taxonomic comparisons suggest these photosynthetic strategies are conserved to at least the Order level. Light saturation necessitated the use of photoprotection to 65 m depth, while optimal light levels were met at 86 m. Changes to the light environment (e.g. reduced water clarity) due to human activities therefore places these mesophotic algae at risk of light limitation, necessitating the importance of maintaining good water quality for the conservation and protection of mesophotic habitats.
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| 5. |
- Voerman, Sofie E., et al.
(author)
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Red algae acclimate to low light by modifying phycobilisome composition to maintain efficient light harvesting
- 2022
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In: BMC Biology. - : BioMed Central (BMC). - 1741-7007. ; 20:1
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Journal article (peer-reviewed)abstract
- Background: Despite a global prevalence of photosynthetic organisms in the ocean’s mesophotic zone (30–200+ m depth), the mechanisms that enable photosynthesis to proceed in this low light environment are poorly defined. Red coralline algae are the deepest known marine benthic macroalgae — here we investigated the light harvesting mechanism and mesophotic acclimatory response of the red coralline alga Lithothamnion glaciale.Results: Following initial absorption by phycourobilin and phycoerythrobilin in phycoerythrin, energy was transferred from the phycobilisome to photosystems I and II within 120 ps. This enabled delivery of 94% of excitations to reaction centres. Low light intensity, and to a lesser extent a mesophotic spectrum, caused significant acclimatory change in chromophores and biliproteins, including a 10% increase in phycoerythrin light harvesting capacity and a 20% reduction in chlorophyll-a concentration and photon requirements for photosystems I and II. The rate of energy transfer remained consistent across experimental treatments, indicating an acclimatory response that maintains energy transfer.Conclusions: Our results demonstrate that responsive light harvesting by phycobilisomes and photosystem functional acclimation are key to red algal success in the mesophotic zone.
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