Tuning of switching properties and excited-state dynamics of fulgides by structural modifications

The ultrafast photo-induced dynamics of the E-isomers of four selected photochromic fulgides with distinct structural motifs have been elucidated by femtosecond broadband transient absorption spectroscopy in n-hexane as solvent. E→C and E→Z isomerisations, respectively, with time constants of ～0.12 ± 0.02 ps and ～0.34 ± 0.03 ps taking place in parallel were found for derivatives with a methyl substituent at the central hexatriene (HT) unit. In contrast, fulgides with increased steric constraints by an iso-propyl substituent or by intramolecular bridging displayed virtually zero E→Z isomerisation, but instead a desired accelerated and more efficient ring closure in a reaction time of only ～50 ± 10 fs. Both photoisomerisations appear to follow excited-state pathways with distinctive conical intersections. For the ring closure, direct barrierless pathways with steep downhill gradients are likely. Furthermore, the results indicate conformer-specific reactions, with ring closure exclusively by the E(α) conformer and E→Z isomerisation predominantly by the E(β) conformer, because the E(α)→Z channel is unfavoured by the faster and kinetically more competitive E(α)→C reaction. DFT calculations of the equilibrium structures showed that the sterically demanding groups at the HT unit shift the conformer equilibria towards the E(α) conformers. At the same time, they appear to cause a favourable pre-orientation of the furyl unit that accelerates the conrotatory ring closure in the E(α)→C reaction. Benzo-annulation of the furyl unit has little effect on the observed dynamics. Overall, the results demonstrate how the excited-state dynamics and thereby the photoswitching properties of fulgides can be successfully tuned and improved by structural modifications at the chromophores.     

Electronic and steric effects on the photo-induced C→E ring-opening of structurally modified furylfulgides

The ultrafast C→E ring-opening reactions of four selectively modified furylfulgides have been studied by means of ultrafast broadband transient absorption spectroscopy after femtosecond laser excitation at λ = 500 nm. A large difference in the dynamics was found in the case of benzannulation at the furyl moiety as an example for an electronic effect by extension of the conjugated π-electron system compared to furylfulgides carrying sterically different alkyl substituents at the central cyclohexadiene (CHD) ring. The measured very similar spectro-temporal absorption maps for the furylfulgides with a methyl or isopropyl group at the CHD ring or an intramolecular alkyl bridge from the CHD to the furyl moiety showed two distinctive excited-state absorptions with slightly different decay times. The first time constant (τ(1) = 0.39-0.57 ps) was assigned to the rapid departure of the excited wavepacket from the Franck-Condon region. The slightly longer second decay time of τ(2) = 0.66-0.92 ps, depending on the compound, was attributed to the electronic deactivation and ring-opening through a conical intersection to the S(0) state. In contrast, the benzannulation at the furyl moiety was found to lead to a bi-phasic excited-state decay with τ(2) = 4.7 ps and a much slower additional contribution of τ(3) = 17.4 ps, ≈25 times longer compared to the normal furylfulgides. The drastic change is attributed to a trapping of excited molecules in a local potential energy minimum en route to the conical intersection.     

Superior Z→E and E→Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.     

Composite hydrogels with temperature sensitive heterogeneities: influence of gel matrix on the volume phase transition of embedded poly-(N-isopropylacrylamide) microgels

The thermo-responsive behaviour of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in covalently cross-linked non-temperature-sensitive polyacrylamide (PAam) hydrogel matrixes with different compositions was investigated by using small angle neutron scattering (SANS). The composition of the composite hydrogel was varied by (a) increasing the cross-linker and acrylamide concentration leading to strong hydrogel matrixes and (b) by increasing the microgel concentration to obtain composite gels with an internal structure. Additionally we synthesized composite hydrogels by using γ-irradiation as initiation for the polymerisation. This leads to the formation of chemical bonds between the PNiPAM microgels and the surrounding polyacrylamide matrix. Thus it is possible to synthesize hydrogels without an additional cross-linker, as well as pure particle networks. Some samples were prepared at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or totally collapsed microgels during the incorporation step. The volume phase transition of microgels is not influenced by a hydrogel matrix with high acrylamide concentration independent of the preparation temperature. However, an increased cross-linker concentration leads to a corset like constraint on microgel swelling. Microgels, which are embedded in the collapsed state (at 50 °C), are not able to swell upon cooling, whereas microgels embedded in the swollen state can collapse upon heating. For samples with an increased microgel concentration, the close microgel packing was disturbed by the formation of the polyacrylamide matrix. The hydrogel matrix squeezes the microgels together and leads to partial aggregation. The experiments demonstrate how composite hydrogels with stimuli-sensitive heterogeneities can be prepared such that the full responsiveness of the embedded microgels is retained while the macroscopic dimensions of the gel are not affected by the volume phase transition of the microgels.     

Prediction of steady-state viscous and elastic properties of polyolefin melts in shear and elongation

The linear and nonlinear steady-state viscosities and elastic compliances were measured in shear and elongational flows for two low-density polyethylenes, a linear polypropylene, and two metallocene catalyzed polyethylenes (one linear and one long-chain branched) by Wolff et al. (Rheol Acta 49:95?C103, 2010) and Resch (dissertation, 2010). Comprehensive data of this type are rarely found in the literature, and comprehensive modeling of both viscous and elastic effects is even rarer. In this contribution, the reliability of a modeling approach proposed by Laun (J Rheol 30(3):459?C501, 1986) and based on the damping function concept is tested. The strain hardening seen for the long-chain branched polymers and its absence in the case of the linear polymer, the stronger decrease of the tensile compliance in comparison to the shear compliance with increasing stress, as well as the extended linear-viscoelastic regime of the shear viscosity in contrast to the shear compliance are correctly modeled. While the modeling of the nonlinear response in shear can be achieved with only one material parameter for most of the polymers considered here, the nonlinear modeling in elongation is achieved with two parameters. The same parameter values are shown to describe viscous as well as elastic properties of the melts, and thus the relations of Laun can be used to test the consistency of viscosity and compliance measurements.     

Concentration Inequalities for Bounded Functionals via Log-Sobolev-Type Inequalities

Journal of Theoretical Probability - In this paper, we prove multilevel concentration inequalities for bounded functionals $$f = f(X_1, \ldots , X_n)$$ of random variables $$X_1, \ldots , X_n$$...     

Soft X‐ray absorption spectroscopy and resonant inelastic X‐ray scattering spectroscopy below 100 eV: probing first‐row transition‐metal M‐edges in chemical complexes

X‐ray absorption and scattering spectroscopies involving the 3d transition‐metal K‐ and L‐edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M‐edges, which are below 100 eV. Synchrotron‐based X‐ray sources can have higher energy resolution at M‐edges. M‐edge X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) could therefore provide complementary information to K‐ and L‐edge spectroscopies. In this study, M_2,3‐edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M_2,3‐edge RIXS on NiO, NiF₂ and two other covalent complexes have been performed and different d–d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M‐edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high‐sensitivity and high‐resolution superconducting tunnel junction X‐ray detectors below 100 eV is also illustrated and discussed.     

Particle Dynamics in an Electrohydrodynamic Flow Field investigated with a two-component laser-doppler velocimeter

A laser-Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40-μm particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady-state velocity for a 1.40-μm particle, provided there is a saturation charge of at least 90f%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (superposition of moving gas ions and neutral gas molecules) on the particle transport, characterized by the dimensionless electrohydrodynamic number N_EHD. A comparison between six different electrohydrodynamic states revealed that N_EHD ≈? 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for N_EHD values > 1 the stochastic particle motion is chiefly determined by the nonsteady-state character of the negative corona, for N_EHD values < 1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These finding might be helpful in adjusting the operating conditions in electrical precipitators for and optimized particle separation.