Electrical discharges in water and aqueous solutions

Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Elektrické výboje ve vodě a vodných roztocích

Matematicko-fyzikální fakultaFaculty of Mathematics and Physic

Effect of Isomerization on Excited-State Dynamics of Carotenoid Fucoxanthin

Ultrafast transient absorption spectroscopy and single-wavelength anisotropy measurements were used to study the effect of isomerization on the excited-state properties of fucoxanthin in polar and nonpolar solvents. The excitation wavelengths were 477 nm for all-trans-fucoxanthin, and 333 and 477 nm for cis-fucoxanthin. All transient absorption spectra of the fucoxanthin isomers in polar solvents show intramolecular charge transfer (ICT) state features, typical for carbonyl carotenoids. Global analysis of the data requires an additional fitting component, originated from the presence of blue and red forms of fucoxanthin in a polar protic solvent. Here we demonstrate that the ICT state decays faster than the S1 state, due to the significant contribution of the red form to the ICT state dynamics. The isomerization does not affect the S1 lifetime, but induces a larger difference between the S1- and ICT-state lifetimes in cis-fucoxanthin, which is likely caused by alterations of ICT coupling to either the S1 or S0 states; the S∗-state signal is more pronounced for cis-isomers in a nonpolar solvent

Excited state properties of aryl carotenoids

Excited-state properties of aryl carotenoids, important components of light harvesting antennae of green sulfur bacteria, have been studied by femtosecond transient absorption spectroscopy. To explore effects of the conjugated aryl group, we have studied a series of aryl carotenoids with conjugated phi-ring, chlorobactene, β-isorenieratene and isorenieratene, and compared them with their non-aryl counterparts γ-carotene and β-carotene, which contain β-ring. Changing β-ring to phi-ring did not reveal any changes in absorption spectra, indicating negligible effect of the phi-ring on the effective conjugation length. This observation is further supported by the carotenoid S1 lifetimes. In n-hexane, the S1 lifetime of chlorobactene having one phi-ring is 6.7 ps, while the S1 lifetime of the β-ring analog, γ-carotene is 5.4 ps. The same effect is observed for the series β-carotene (two β-rings), β-isorenieratene (one β- and one phi-ring) and isorenieratene (two phi-rings) whose S1 lifetimes in n-hexane are 8.2, 10.3 and 12.7 ps, respectively. The systematically longer lifetimes of aryl carotenoids show that the additional conjugated C=C bonds at the -ring do not contribute to the conjugation length. The S1 lifetimes of aryl carotenoids were slightly shortened in benzene, indicating π–π stacking interaction between the phi-ring and benzene.This research was supported by grants from the Czech Ministry of Education (MSM6007665808, MSM0021620835 and AV0Z50510513), and the Grant Agency of the Czech Academy of Sciences (IAA608170604). JBA thanks the Spanish Ministry of Science and Innovation (Ref. BF2007-68107-C02-02/BMC) and the AVCR-CSIC joint program(Ref. 2008CZ0004) for financial support.Peer reviewe

Spectroscopic properties of the S1 state of linear carotenoids after excess energy excitation

Properties of the S1 state of neurosporene, spheroidene and lycopene were studied after excess energy excitation in the S2 state. Excitation of carotenoids into higher vibronic levels of the S2 state generates excess vibrational energy in the S1 state. The vibrationally hot S1 state relaxes faster when carotenoid is excited into the S2 state with excess energy, but the S1 lifetime remains constant regardless of which vibronic level of the S2 state is excited. The S∗ signal depends on excitation energy only for spheroidene, which is likely due to asymmetry of the molecule, facilitating conformations responsible for the S∗ signal

Role of B800 in carotenoid-bacteriochlorophyll energy and electron transfer in LH2 complexes from the purple bacterium Rhodobacter sphaeroides

The role of the B800 in energy and electron transfer in LH2 complexes has been studied using femtosecond time-resolved transient absorption spectroscopy. The B800 site was perturbed by application of lithium dodecyl sulfate (LDS), and comparison of treated and untreated LH2 complexes from Rhodobacter sphaeroides incorporating carotenoids neurosporene, spheroidene, and spheroidenone was used to explore the role of B800 in carotenoid to bacteriochlorophyll-a (BChla) energy transfer and carotenoid radical formation. Efficiencies of the S-1-mediated energy transfer in the LDS-treated complexes were 86, 61, and 57% in the LH2 complexes containing neurosporene, spheroidene, and spheroidenone, respectively. Analysis of the carotenoid S-1 lifetimes in solution, LDS-treated, and untreated LH2 complexes allowed determination of B800/B850 branching ratio in the S-1-mediated energy transfer. It is shown that B800 is a major acceptor, as approximately 60% of the energy from the carotenoid S-1 state is accepted by B800. This value is nearly independent of conjugation length of the carotenoid. In addition to its role in energy transfer, the B800 BChla is the only electron acceptor in the event of charge separation between carotenoid and BChla in LH2 complexes, which is demonstrated by prevention of carotenoid radical formation in the LDS-treated LH2 complexes. In the untreated complexes containing neurosporene and spheroidene, the carotenoid radical is formed with a time constant of 300-400 fs. Application of different excitation wavelengths and intensity dependence of the carotenoid radical formation showed that the carotenoid radical can be formed only after excitation of the S-2 state of carotenoid, although the S-2 state itself is not a precursor of the charge-separated state. Instead, either a hot S-1 state or a charge-transfer state lying between S-2 and S-1 states of the carotenoid are discussed as potential precursors of the charge-separated state

Ultrafast spectroscopy tracks carotenoid configurations in the orange and red carotenoid proteins from cyanobacteria.

International audienceA quenching mechanism mediated by the orange carotenoid protein (OCP) is one of the ways cyanobacteria protect themselves against photooxidative stress. Here, we present a femtosecond spectroscopic study comparing OCP and RCP (red carotenoid protein) samples binding different carotenoids. We confirmed significant changes in carotenoid configuration upon OCP activation reported by Leverenz et al. (Science 348:1463-1466. doi: 10.1126/science.aaa7234 , 2015) by comparing the transient spectra of OCP and RCP. The most important marker of these changes was the magnitude of the transient signal associated with the carotenoid intramolecular charge-transfer (ICT) state. While OCP with canthaxanthin exhibited a weak ICT signal, it increased significantly for canthaxanthin bound to RCP. On the contrary, a strong ICT signal was recorded in OCP binding echinenone excited at the red edge of the absorption spectrum. Because the carbonyl oxygen responsible for the appearance of the ICT signal is located at the end rings of both carotenoids, the magnitude of the ICT signal can be used to estimate the torsion angles of the end rings. Application of two different excitation wavelengths to study OCP demonstrated that the OCP sample contains two spectroscopically distinct populations, none of which is corresponding to the photoactivated product of OCP

Excited-State Dynamics of Monomeric and Aggregated Carotenoid 8′-Apo-β-carotenal

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 <i>n</i>-hexane the S₁ 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 <i>n</i>-hexane and methanol, we propose that the polarity-induced spectral bands are due to the S₁(A_g^–)–S₃(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 S₁ 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 S₁ 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

Unveiling the role of upper excited electronic states in the photochemistry and laser performance of: anti -B18H22

13 pags., 7 figs., -- This article is part of the themed collection: Journal of Materials Chemistry C HOT PapersIn the search for innovative new light sources, the discovery that solutions of the boron hydride anti-B18H22 generate photostable blue laser emission stands out in its significance as the first laser borane. Surprisingly, though, the laser performance of anti-B18H22 (∼10% efficiency) does not match the expectations based on its exceptional photophysical properties (Φf = 0.97 and high photostability). To understand this contradiction, we herein present an investigation into the upper excited states of the anti-B18H22 photophysical system, which we suggest to be the most relevant factor to its laser performance. The use of computational quantum chemistry, laser and UV-vis spectroscopy, NMR spectroscopy, and mass spectrometry unveil the role of the upper excited states on the laser performance of anti-B18H22, showing that efficient excited state absorption (ESA) leads to the population of these states, and results not only in the loss of laser efficiency, but also in the activation of chemically reactive relaxation pathways and the formation of photochemically produced novel molecular species. The likely composition of these photoproducts, formed upon prolonged high intensity laser irradiation, is inferred from their molecular masses, NMR properties, and calculated natural orbitals. Together, these results are of key importance to the complete understanding of the anti-B18H22 photophysical system and provide valuable information to chemists and laser physicists working to mitigate deficiencies and enhance the performance of the next generation of borane lasers and borane-based photoactive materials.This work has been supported by the Czech Science Foundation (project No. 18-20286S). L. C. acknowledges financial support from the Spanish Ministerio de Economı´a y Competitividad (MINECO) through Grant number MAT2017-83856-C3-1. A. F.-M. is grateful to Generalitat Valenciana and the European Social Fund for a postdoctoral contract (APOSTD/2019/149). D. R.-S. is thankful to the Spanish MINECO/FEDER for financial support through the Ramon y Cajal fellowship (RYC-2015-19234) and the Unit of Excellence Maria de Maeztu (MDM-2015-0538). Support from the project CTQ2017-87054-C2-2-P (Ministerio de Ciencia e Innovacion) is gratefully acknowledged