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We present the first example of macroscalar helices co‐assembled from temperature‐responsive carbohydrate‐based bolaamphiphiles (CHO‐Bolas) and 1,4‐benzenediboronic acid (BDBA). The CHO‐Bolas contained hydrophilic glucose or mannose moieties and a hydrophobic coumarin dimer. They showed temperature‐responsive reversible micelle‐to‐vesicle transition (MVT) in aqueous solutions. After the binding of carbohydrate moieties with boronic acids of BDBA in their alkaline solutions, right‐handed helices were formed via the temperature‐driven chirality transfer of d‐glucose or d‐mannose from the molecular to supramolecular level. These helices were co‐assembled by unreacted BDBA, boronate esters (B−O−C bonds) between CHO‐Bolas and BDBA, as well as boroxine anhydrides (B−O−B bonds) of self‐condensed BDBA. After heating at 300 °C under nitrogen, the helices displayed excellent morphological stability. Moreover, they emitted bright blue luminescence caused by strong self‐condensation of BDBA and decomposition of coumarin dimers., Macroscalar, thermally stable helices were formed by co‐assembly of temperature‐responsive carbohydrate‐based bolaamphiphiles and 1,4‐benzenediboronic acids accompanying chirality transfer in alkaline solutions during the temperature decrease from 80 °C to room temperature.

BackgroundGuidelines advise for more than 20 years to use occlusive plastic wraps for temperature management during delivery room care but data on efficacy of different types of wrap are still scarce.MethodsA random sample of seven different types of plastic wrap was tested using prewarmed aluminium blocks.ResultsThe most effective wrap increased the time to cool by 2°C by one-third for the core and by 100% for the surface whereas the least effective wrap led to even faster heat loss compared with no wrap at all. The least effective wrap concerning all capacities tested was made from polyurethane that contains potentially toxic and narcotic monomers. Heat and water retention did not correlate to wrap thickness.DiscussionLarge differences in heat and water retention capacity warrant a careful choice of the type of wrap as some might be counterproductive. Wraps containing polyurethane should not be used.

We report on an ultrafast transient absorption study of all-trans-3,3′-dihydroxyisorenieratene (“DHIR”) and all-trans-(3R,3′R)-zeaxanthin in organic solvents covering the wavelength range 350–770 nm. The lifetime of the S2 state in both carotenoids is 160–170 fs. Upon internal conversion (IC) non-equilibrated S1 molecules are formed which internally relax on a 300–400 fs time scale. The time constant for IC from S1 depends on the type of terminal substituent: Replacement of the two terminal β-ionone rings of zeaxanthin by two aryl rings in DHIR results in an increase from 9.5 to 10.9 ps in THF. This suggests a mild decrease in the effective conjugation length of DHIR. IC to the ground electronic state prepares vibrationally hot S0* molecules which exhibit characteristic bleach and absorption bands. These are typically denoted as “S* features”. Collisional cooling of S0* happens with a time constant of 15 ps. Based on our results and the findings from previous studies for other carotenoids, such as macro-β-carotenes, β-carotenes and long-chain apocarotenals, we conclude that S0* spectral features are ubiquitous in carotenoid photophysics: They are particularly easy to observe in systems with a very short S1 lifetime and a high quantum yield for IC to the ground electronic state.

We investigated the excited state dynamics of selected all-trans C40 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 β-ionone rings and the change of the effective conjugation length of the polyene system: β-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 S1 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 S1 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 S0 → S2 red-edge absorption and a negative signal in the S0 → S2 bleach region. This spectral signature is assigned to highly vibrationally excited molecules in the ground electronic state S0 * which are generated by internal conversion from S1. The S0 * 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.

The system of Cp* 2 ZrCl 2 and dibenzyl magnesium in toluene , which is a model system for initialization in the catalyzed chain growth of styrene using a bifunctional organometallic chain transfer agent, is studied via time‐resolved NMR spectro­scopy. Based on these data, a mechanism of initialization is proposed, which contains a complex interplay of association/dissociation and rearrangement reactions. The experimental concentration profiles of the individual species can be successfully modeled to the proposed kinetic scheme, and parameter estimates for the complex sequence of reactions can be deduced.

Reversible Additions-Fragmentierungs-Kettentransferpolymerisationen besetzen Oberflachen von Werkstoffen oder Partikeln mit genau definierten Polymerketten. Die physikalischen und chemischen Eigenschaften der Oberflache sind so gezielt zu kontrollieren.

The 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 S2 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 (S1/ICT). For 12'C in scCO2 and scN2O, the internal conversion (IC) time constant tau1 for the S1/ICT --> S0 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 N20, which might slightly stabilize the S1/ICT state relative to S0. 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 deltaf = (epsilon - 1)/(epsilon + 2) - (n2 - 1)/(n2 + 2). We find scCF3H to be the first solvent where the S1/ICT state of 12'C does not decay in a monoexponential fashion. This is most likely attributed to time-dependent solvation of the S1/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 scCO2 and scCF3H 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 scCO2 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 scCO2, scN2O, and scCF3H. The band position of the S0 --> S2 transition correlates well with solvent polarizability, but--in contrast to our previous study of C40 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.

The 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 S2 state with different excess energies, the subsequent intramolecular dynamics were investigated at several probe wavelengths covering the S0--S2 and S1/ICT--Sn absorption bands. Time constants tau1 for the internal conversion process S1/ICT--S0 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), tau1 is almost independent of solvent polarity (4-5 ps), whereas a significant reduction of tau1 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, tau1 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 S1-S0 energy differences were estimated from their steady-state fluorescence spectra. In highly polar solvents, the IC to S0 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.

In recent work, we demonstrated that the S* signal of β-carotene observed in transient pump-supercontinuum probe absorption experiments agrees well with the independently measured steady-state difference absorption spectrum of vibrationally hot ground state molecules S0* in solution, recorded at elevated temperatures (Oum et al., Phys. Chem. Chem. Phys., 2010, 12, 8832). Here, we extend our support for this "vibrationally hot ground state model" of S* by experiments for the three terminally aldehyde-substituted carotenes β-apo-12'-carotenal, β-apo-4'-carotenal and 3',4'-didehydro-β,ψ-caroten-16'-al ("torularhodinaldehyde") which were investigated by ultrafast pump-supercontinuum probe spectroscopy in the range 350-770 nm. The apocarotenals feature an increasing conjugation length, resulting in a systematically shorter S1 lifetime of 192, 4.9 and 1.2 ps, respectively, in the solvent n-hexane. Consequently, for torularhodinaldehyde a large population of highly vibrationally excited molecules in the ground electronic state is quickly generated by internal conversion (IC) from S1 already within the first picosecond of relaxation. As a result, a clear S* signal is visible which exhibits the same spectral characteristics as in the aforementioned study of β-carotene: a pronounced S0 → S2 red-edge absorption and a "finger-type" structure in the S0 → S2 bleach region. The cooling process is described in a simplified way by assuming an initially formed vibrationally very hot species S0** which subsequently decays with a time constant of 3.4 ps to form a still hot S0* species which relaxes with a time constant of 10.5 ps to form S0 molecules at 298 K. β-Apo-4'-carotenal behaves in a quite similar way. Here, a single vibrationally hot S0* species is sufficient in the kinetic modeling procedure. S0* relaxes with a time constant of 12.1 ps to form cold S0. Finally, no S0* features are visible for β-apo-12'-carotenal. In that case, the S1 → S0 IC process is expected to be roughly 20 times slower than S0* relaxation. As a result, no spectral features of S0* can be found, because there is no chance that a detectable concentration of vibrationally hot molecules is accumulated. peerReviewed

Intra- and intermolecular relaxation pathways of 12′-apo- β -carotenoic-12′-acid, an apocarotenoid with terminal carbonyl substitution, were identified after photoexcitation using ultrafast pump-supercontinuum probe spectroscopy in the 350–750 nm range. The apocarotenoid was dissolved in 1- n -hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)amide and excited to the S 2 state at 482 nm. Internal conversion occurred within 170 fs to a single S 1 /ICT state with intramolecular charge transfer character. This state exhibits characteristic excited state absorption band-shifts due to time-dependent solvation, which appears to be a general feature of carbonyl apocarotenoids with polarity-dependent S 1 /ICT lifetimes. We also demonstrate that higher pump beam energies open a route for the clean preparation of long-lived radical cations by resonant one-color two-photon excitation using S 2 as the intermediate state. The radical cations are easily identified by a characteristic absorption peak in the region 700–750 nm and an absorption tail towards shorter wavelengths.

The so-called S* state has been suggested to play an important role in the photophysics of beta-carotene and other carotenoids in solution and photosynthetic light-harvesting complexes, yet its origin has remained elusive. The present experiments employing temperature-dependent steady-state absorption spectroscopy and ultrafast pump-supercontinuum probe (PSCP) transient absorption measurements of beta-carotene in solution demonstrate that the spectral features of S* are due to vibrationally excited molecules in the ground electronic state S(0). Characteristic spectral signatures, such as a highly structured bleach below 500 nm and absorption in the range 500-660 nm result from the superposition of hot S(0) absorption ("S(0)*") on top of the ground-state bleach of room-temperature molecules. Appearance and disappearance of the S(0)* molecules can be completely described by a global kinetic analysis employing time-dependent species-associated spectra without the need to invoke the population of an intermediate electronically excited state.

The 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 S0 --S2 (11Ag- --11Bu+) 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 S2 --SN 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 --S0). 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 S2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S1 and ICT states are populated from S2 through an ultrafast excited-state branching reaction (tau2120 fs). Delayed formation of a part of the stimulated emission is due to the transition S1 --ICT (tau3 = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT --S1. Determinations of tau1 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 --S0, was strongly dependent on solvent polarity, varying from tau1 = 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-S0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S0 --S2 absorption band resulted in no change of tau1 in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S0 --S2 band (480-525 nm) also show an almost constant tau1 with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau1 value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT --S0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process.

Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13'-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an "S* state". Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties. peerReviewed