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Excited State Dynamics of Selected All-trans C-40 Xanthophyll Carotenoids
No full textWe investigated the excited state dynamics of selected all-trans C-40 xanthophyll carotenoids by ultrafast pump-supercontinuum probe (PSCP) spectroscopy in the spectral range 350-770 nm upon photoexcitation by a femtosecond laser (ca. 80 fs pulse length) near 500 nm in acetone. The following carotenoids were selected considering a systematic variation of the number and position of carbonyl (CO) and hydroxyl (OH) functional groups on the beta-ionone rings and the change of the effective conjugation length of the polyene system: beta-cryptoxanthin (1), echinenone (2), 3-hydroxyechinenone (3), 3'-hydroxyechinenone (4), canthaxanthin (5), adonirubin (6) and astaxanthin (7). The carotenoids featuring an increasing conjugation length, for example, (1), (3) and (6), showed a systematically shorter S-1 lifetime of 8.7, 6.2 and 4.7 ps, respectively. Carotenoids with OH groups adjacent to a CO group showed slightly broader steady-state and transient absorption bands, but the influence on the S-1 lifetime was minor compared to the case of CO substitution. Moreover, in all cases we observed clearly visible "S signals", namely a longer-lived characteristic S-0 -> S-2 red-edge absorption and a negative signal in the S-0 -> S-2 bleach region. This spectral signature is assigned to highly vibrationally excited molecules in the ground electronic state S-0 which are generated by internal conversion from S-1. The S-0 species cool with a time constant of 9.2-9.6 Ps in the case of the keto-substituted carotenoids (2)-(7), whereas we obtain a larger value of 13.2 Ps for (1) which does not have a keto group. Our findings are supported by results from a global kinetic analysis procedure
On-line in-situ characterization of CO2 RESS processes for benzoic acid, cholesterol and aspirin
No full textRapid expansions of supercritical solutions (RESS) of benzoic acid, cholesterol and aspirin in supercritical CO2 have been used to investigate the influence of a systematic variation of the pre-expansion temperature and pressure, the distance from the RESS nozzle and the amount of added co-solvent on properties like the average particle diameter D-av and the width of the particle size distribution sigma. The properties of the CO2 expansion have been characterized by a 1-dimensional flow-field model using the Span - Wagner equation of state. Particle detection was performed on-line and in-situ using laser-based three wavelength extinction measurements (3-WEM). For benzoic acid we found a decrease in Dav with increasing pre-expansion pressure, and an increase in Dav with increasing pre-expansion temperature. This is probably due to a lower mass flow rate, which is associated with a lower pre-expansion pressure or higher pre-expansion temperature. This in turn results in a longer residence time in the expansion region and thus a longer particle growth time. Furthermore, a decrease in pre-expansion pressure or an increase in pre-expansion temperature is associated with a decrease in saturation, corresponding to an increase in the critical particle radius and a decrease in the nucleation rate. The size of the benzoic acid particles ranged from about 100 to 500 nm. In addition, we found no obvious correlation between Dav and the distance from the RESS nozzle for benzoic acid and aspirin particles. The particle size was roughly 350 nm and 160 nm for these two solutes, respectively. Obviously, the particle growth processes have already ceased not too far away from the Mach disc. In addition, for cholesterol expansions in CO2 there was no correlation between the amount and type of co-solvent added. Particle sizes of similar to 100 nm were obtained for methanol, ethanol and isopropanol co-solvents. This is most likely due the low solubility of cholesterol in supercritical CO2, compared with molecules such as benzoic acid, which results in a change of D-av which is too small to be detected using 3-WEM
Investigation of the S-1/ICT -> S-0 internal conversion lifetime of 4 '-apo-beta-caroten-4 '-al and 8 '-apo-beta-caroten-8 '-al: Dependence on conjugation length and solvent polarity
No full textThe ultrafast internal conversion (IC) dynamics of aldehyde-substituted apocarotenoids (n'-apo-beta-caroten-n'-als with n = 4, 8 and 12) have been investigated in a systematic variation of conjugation length and solvent polarity using time-resolved femtosecond transient absorption spectroscopy. After excitation to the S-2 state with different excess energies, the subsequent intramolecular dynamics were investigated at several probe wavelengths covering the S-0 -> S-2 and S-1/ICT -> S-n absorption bands. Time constants tau(1) for the internal conversion process S-1/ICT -> S-0 of 4'-apo-beta-caroten-4'-al and 8'-apo-beta-caroten-8'-al have been newly measured. We compared these results with our earlier measurements for 12'-apo-beta-caroten-12'-al (D.A. Wild, K. Winkler, S. Stalke, K. Oum, T. Lenzer Phys. Chem. Chem. Phys. 2006, 8, 2499). In the case of the aldehyde with the longest conjugation (4'-apo-beta-caroten-4'-al), tau(1) is almost independent of solvent polarity (4-5 ps), whereas a significant reduction of tau(1) from 22.7 to 8.6 ps for the shorter 8'-apo-beta-caroten-8'-al and an even more pronounced reduction from 220 to 8.0 ps for 12'-apo-beta-caroten-12'-al were observed when the solvent medium was changed from n-hexane to methanol, respectively. In n-hexane, tau(1) of the apocarotenals is strongly dependent on the conjugation length and this can be well understood in terms of an energy gap law description where the S-1-S-0 energy differences were estimated from their steady-state fluorescence spectra. In highly polar solvents, the IC to S-0 is very fast, irrespective of the conjugation length. This is probably due to the stabilization of an intramolecular charge transfer (ICT) state in 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al. In the case of 4'-apo-beta-caroten-4'-al, such an influence of an ICT state is presumably less important than for the other two apocarotenals
Solvent effects on the S-0(1(1)Ag(-)) -> S-2(1(1)B(u)(+)) transition of beta-carotene, echinenone, canthaxanthin, and astaxanthin in supercritical CO2 and CF3H
No full textSolvent-induced spectral shifts of the four C-40 carotenoids, beta-carotene, echinenone, canthaxantin, and astaxanthin, have been studied in supercritical CO2 and CF3H. In situ absorption spectroscopic analysis was used to determine the maximum peak position of the electronic transitions from the ground state (1(1)A(g)(-)) to the S-2 state (1(1)B(u)(+)) of the carotenoids. The medium polarizability function, R(n) = (n(2)-1)/(n(2)+2) of the refractive index of the solvent was varied over the range R(n) = 0.08-0.14, by changing the pressure of CO2 or CF3H between 90 and 300 bar at the temperature 308 K. For all the carotenoids studied here, a significant hypsochromic shift of ca. 20-30 nm was observed in supercritical fluids as compared to that in nonpolar liquids. The spectral shifts in supercritical fluids were compared with those in liquids and showed a clear linear dependence on the medium polarizability. The temperature-dependent shift of the absorption maxima was less significant. Interestingly, there was almost no difference in the energetic position of the absorption maxima in supercritical CO2 and CF3H at a given R(n) value. This is in contrast to previous extrapolations from studies in liquids at larger R(n) values, which yielded different slopes of the R(n)-dependent spectral shifts for polar and nonpolar solvents toward the gas-phase limit of R(n) = 0. The current experimental results in the gas-to-liquid range show that the polarity of the solvent has only a minor influence on the 1(1)Ag--> 1(1)B(u)(+) transition energy in the region of low R(n). We also obtain more reliable extrapolations of this 0-0 transition energy to the gas-phase limit v(0-0)(gas-phase) approximate to (23 000 +/- 120) cm(-1) for beta-carotene
Density dependence of optical Kerr lens signals in neat N2O and CO2
No full textOptical Kerr lens spectroscopy has been employed to study the reorientation dynamics of N2O and CO2 at the reduced densities 0.8-2.0 and different temperatures in a newly constructed high pressure cell. The transient birefringence induced in the medium by an ultra-short (110 fs) non-resonant pump beam at 400 nm was monitored by a second, time-delayed non-resonant probe beam at 800 nm. The transient change of the probe beam diameter was detected in the far field using a small-area photodiode. Measurements employing parallel and perpendicular polarization allowed us to extract the nuclear and electronic components of the Kerr effect, where the latter one was by far weaker for both CO2 and NO. Accurate time constants for the reorientation dynamics of the CO2 and N2O molecules were extracted from the tail of the nuclear Kerr response. In the case of CO2 the time constants tau(reorientation) were between 2 10 and 280 fs and in good agreement with previous data from NMR and OHD-OKE experiments as well as MID simulations. Reorientation time constants for NO were in the range between 200 and 340 fs, and thus very similar to CO2. In the density range studied, increases with increasing viscosity and decreasing temperature, in agreement with Stokes-Einstein-Debye diffusive models. (c) 2006 Elsevier B.V. All rights reserved
Excited-state dynamics of 12 '-apo-beta-caroten-12 '-al and 8 '-apo-beta-caroten-8 '-al in supercritical CO2, N2O, and CF3H.
Full text linkThe ultrafast excited-state dynamics of the two carbonyl carotenoids 12'-apo-beta-caroten-12'-al (12'C) and 8'-apo-beta-caroten-8'-al (8'C) have been investigated in supercritical (sc) fluids by femtosecond transient absorption spectroscopy. CO2, N2O, and CF3H were employed as solvent media over the pressure range 85-300 bar and at the temperatures 308 and 323 K. The carotenoids were excited to the S-2 state at 390 nm, and the subsequent dynamics were probed at different wavelengths in the UV-vis (390, 545, 580, 600, and 650 nm) and near IR (780 nm) regions. Stimulated emission in the near IR signaled the presence of a state with intramolecular charge transfer character (S-1/ICT). For 12'C in scCO(2) and scN(2)O, the internal conversion (IC) time constant iota(1) for the S-1/ICT -> S-0 transition showed no systematic pressure dependence and yielded an average value of 190 ps. This is slightly smaller than the values in nonpolar organic solvents (ca. 220 ps) found in our previous studies and probably due to the substantial quadrupole moment of the nondipolar CO2 and the small dipole moment of N2O, which might slightly stabilize the S-1/ICT state relative to So. This results in an acceleration of the nonradiative rate in the simple framework of an energy gap law approach. In polar CF3H, a pronounced acceleration of the internal conversion rate was observed with increasing pressure, which can be explained by the polarity increase, as characterized by the parameter Delta f = (epsilon -1)/(epsilon + 2) - (n(2) - 1)/(n(2) + 2). We find scCF(3)H to be the first solvent where the S-1/ICT state of 12'C does not decay in a monoexponential fashion. This is most likely attributed to time-dependent solvation of the S-1/ICT state, vibrational cooling, or conformational relaxation processes in 12'C. In addition, we studied the dynamics of the longer conjugated species 8'C, where the decays of all transients in scCO(2) and scCF(3)H could be described well by monoexponential fits, in good agreement with previous results in organic solvents. Anisotropy decays from polarization spectroscopy of the 12'C species provided orientational relaxation time constants which were increasing with viscosity. The values in scCO(2) were extrapolated to a free rotor time of 4.6 ps, which is in good agreement with a value of 5.2 ps calculated on the basis of the rotational constants. We also report on pressure- and temperature-dependent steady-state absorption spectra of the two apocarotenals in scCO(2), scN(2)O, and scCF(3)H. The band position of the So - S2 transition correlates well with solvent polarizability, but-in contrast to our previous study Of C-40 carotenoids-a substantial influence of polarity was also observed. Specifically, we found indications for solvent clustering, resulting in a saturation of the solvent shift at lower densities.Alexander von Humboldt foundation; German Science Foundatio
Excited-state dynamics of 3,3′-dihydroxyisorenieratene and (3R,3′R)-zeaxanthin: Observation of vibrationally hot S0 species
No full textEvidence for an intramolecular charge transfer state in 12'-Apo-β-caroten-12'-al and 8'-Apo-β-caroten-8'-al: Influence of solvent polarity and temperature
No full textThe ultrafast excited-state dynamics of two carbonyl-containing carotenoids, 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al, have been investigated by transient absorption spectroscopy in a systematic variation of solvent polarity and temperature. In most of the experiments, 12'-apo-beta-caroten-12'-al was excited at 430 nm and 8'-apo-beta-caroten-8'-al at 445 or 450 nm via the S-0 -> S-2 (1(1)A(g)(-) -> 1(1)B(u)(+)) transition. The excited-state dynamics were then probed at 860 nm for 12'-apo-beta-caroten-12'-al and at 890 or 900 nm for 8'-apo-beta-caroten-8'-al. The temporal evolution of all transient signals measured in this work can be characterized by an ultrafast decay of the S-2 -> S-N absorption at early times followed by the formation of a stimulated emission (SE) signal, which subsequently decays on a much slower time scale. We assign the SE signal to a low-lying electronic state of the apocarotenals with intramolecular charge-transfer character (ICT -> S-0). This is the first time that the involvement of an ICT state has been detected in the excited-state dynamics of a carbonyl carotenoid in nonpolar solvents such as n-hexane or i-octane. The amplitude ratio of ICT-stimulated emission to S-2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S-1 and ICT states are populated from S-2 through an ultrafast excited-state branching reaction (tau(2) ICT (tau(3) = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT -> S-1. Determinations of tau(1) were carried out for a large set of solvents. Especially in 12'-apo-beta-caroten-12'-al, the final SE decay, assigned to the nonradiative relaxation ICT -> S-0, was strongly dependent on solvent polarity, varying from tau(1) = 200 ps in n-hexane to 6.6 ps in methanol. In the case of 8'-apo-beta-caroten-8'-al, corresponding values were 24.8 and 7.6 ps, respectively. This indicates an increasing stabilization of the ICT state with increasing solvent polarity, resulting in a decreasing ICT-S-0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S-0 -> S-2 absorption band resulted in no change of tau(1) in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S-0 -> S-2 band (480-525 nm) also show an almost constant tau(1) with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau(1) value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT -> S-0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process
Probing the Local Polarity of Alkylammonium Formate Ionic Liquids and Their Mixtures with Water by Using a Carbonyl Carotenoid
No full textUltrafast transient absorption experiments have been carried out to determine the local polarity of three alkylammonium formate (AAF) protic ionic liquids (PILs), methlyammonium formate (MAF), ethylammonium formate (EAF), and n-butylammonium formate (BAF), by using 12'-apo-beta-carotenoic-12'-acid (12'CA) as a molecular probe. MAF is more polar than methanol; EAF and BAF have polarities similar to ethanol and n-butanol, respectively. In general, the AAF PILs follow rather closely the correlation between the S1/intramolecular charge-transfer (ICT) state lifetime and the polarity parameter ?f, which was previously established in organic solvents. This is in contrast to earlier results for the 12'CA probe in imidazolium-based ILs, in which the local polarity determined by the probe was much larger than that for dipolar organic solvents with the same dielectric constant. The systematic variation of the composition of EAF/water mixtures shows no indication of significant deviations from the local to the bulk composition. We also characterized the photophysical properties of the deprotonated form of the 12'CA probe. It exhibits a structured S0?S2 absorption spectrum, which is blueshifted relative to neutral 12'CA. The lifetime of the S1/ICT state of the anion is about 170 ps, and therefore, similar to the lifetime of 12'CA in less polar solvents. The transient S1/ICT spectrum in methanol closely resembles that of nonpolar carotenes. Both observations suggest that the ICT character of the S1 state of the anion is largely suppressed because the shift of electron density toward the negatively charged carboxylate group is not favorable.DF
Exploring 12'-apo-β-carotenoic-12'-acid as an ultrafast polarity probe for ionic liquids.
Full text linkThe ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12'-apo-beta-carotenoic-12'-acid (12'CA) have been used for probing the microscopic environment in various ionic liquids (ILs). The following IL cations were investigated: 1,3-di-n-alkyl-imidazolium featuring different n-alkyl chain lengths and also additional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. These were combined with different anions: [BF4](-), [PF6](-), ethyl sulfate ([EtOSO3](-)), and bis(trifluoromethylsulfonyl)amide ([Tf2N](-)). The probe molecule was excited via the S-0 -> S-2 transition at 425 or 430 nm, and the characteristic stimulated emission decay of the low-lying excited electronic S-1/ICT (intramolecular charge transfer) state of ITCA was monitored in the near IR (850 or 860 nm). Its lifetime tau(1) is sensitive to the micropolarity-induced stabilization of S-1/ICT relative to S-0. The lifetime tau(1) of the S-1/ICT state varies only moderately in all ionic liquids studied here (similar to 40-110 ps), which lies in the range between ethanol (109 ps) and methanol (49 ps). While organic solvents show an excellent correlation of tau(1) with the solvent polarity function Delta f = (epsilon - 1)/(epsilon + 2) - (n(2) - 1)/(n(2) + 2), where epsilon and n are the static dielectric constant and the refractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostatic probe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at the C2 position of 1,3-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostatic stabilization of the probe, resulting in an increase of tau(1). A similar increase is observed upon extension of one of the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased tau(1) probably due to their more delocalized positive charge which cannot interact so favorably with the probe, in contrast to trialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of tau(1) on the IL anion is much weaker, the only notable exception being [EtOSO3](-), where 12'CA experiences a less polar local environment than expected on the basis of extrapolated static dielectric constants. This is possibly due to the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probecation interactions can be also introduced by addition of large amounts of LiClO4 salt to ethanol and diethyl ether. In this case, tau(1) also strongly decreases, indicating an efficient coordination of Li+ cation(s) with the carbonyl oxygen at the negative end of the probe molecule. The S-1/ICT -> S-0 internal conversion of the 12'CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparent activation energy of a few kJ mol(-1), which is expected for energy-dependent nonradiative processes
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