Dihydroazulene-Azobenzene-Dihydroazulene Triad Photoswitches

Photoswitch triads comprising two dihydroazulene (DHA) units in conjugation with a central trans-azobenzene (AZB) unit were prepared in stepwise protocols starting from meta- and para-disubstituted azobenzenes. The para-connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. In contrast, for the meta-connected triad, all three photochromes could be photoisomerized to generate an isomer with two vinylheptafulvene (VHF) units and a cis-azobenzene unit. Ultrafast spectroscopy of the photoisomerizations revealed a fast DHA-to-VHF photoisomerization and a slower trans-to-cis AZB photoisomerization. This meta triad underwent thermal VHF-to-DHA back-conversion with a similar rate of all VHFs, independent of the identity of the neighboring units, and in parallel thermal cis-to-trans AZB conversion. The experimental observations were supported by computation (excitation spectra and orbital analysis of the transitions).     

High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C₈H₁₇PD₃][NTf₂] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD₃ group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD₃ deuterons. A transition enthalpy of about 12 kJ mol⁻¹ from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P−D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD₃ group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.     

Sequence‐Controlled Polymers via Simultaneous Living Anionic Copolymerization of Competing Monomers

Natural macromolecules, i.e., sequence‐controlled polymers, build the basis for life. In synthetic macromolecular chemistry, reliable tools for the formation of sequence‐controlled macromolecules are rare. A robust and efficient chain‐growth approach based on the simultaneous living anionic polymerization of sulfonamide‐activated aziridines for sequence control of up to five competing monomers resulting in gradient copolymers is presented. The simultaneous azaanionic copolymerization is monitored by real‐time ¹H NMR spectroscopy for each monomer at any time during the reaction. The monomer sequence can be adjusted by the monomer reactivity, depending on the electron‐withdrawing effect by the sulfonamide (nosyl‐, brosyl‐, tosyl‐, mesyl‐, busyl) groups. This method offers unique opportunities for sequence control by competing copolymerization: a step forward to well‐engineered synthetic polymers with defined microstructures.

     

High-efficiency liquid chromatography–mass spectrometry separations with 50 mm, 250 mm,and 1 m long polymer-based monolithic capillary columns for the characterization of complex proteolytic digests

In this study, high-efficiency LC–MS/MS separations of complex proteolytic digests are demonstrated using 50 mm, 250 mm, and 1 m long poly(styrene-co-divinylbenzene) monolithic capillary columns. The chromatographic performance of the 50 and 250 mm monoliths was compared at the same gradient steepness for gradient durations between 5 and 150 min. The maximum peak capacity of 400 obtained with a 50 mm column, increased to 485 when using the 250 mm long column and scaling the gradient duration according column length. With a 5-fold increase in column length only a 20% increase in peak capacity was observed, which could be explained by the larger macropore size of the 250 mm long monolith. When taking into account the total analysis time, including the dwell time, gradient time and column equilibration time, the 50 mm long monolith yielded better peptide separations than the 250 mm long monolithic column for gradient times below 80 min (n_c = 370). For more demanding separation the 250 mm long monolith provided the highest peak production rate and consequently higher sequence coverage. For the analysis of a proteolytic digest of Escherichia coli proteins a monolithic capillary column of 1 m in length was used, yielding a peak capacity of 1038 when applying a 600 min gradient.     

Separation of styrene and ethylbenzene on metal-organic frameworks: analogous structures with different adsorption mechanisms

The metal-organic frameworks MIL-47 (V(IV)O{O(2)C-C(6)H(4)-CO(2)}) and MIL-53(Al) (Al(III)(OH)·{O(2)C-C(6)H(4)-CO(2)}) are capable of separating ethylbenzene and styrene. Both materials adsorb up to 20-24 wt % of both compounds. Despite the fact that they have identical building schemes, the reason for preferential adsorption of styrene compared to ethylbenzene is very different for the two frameworks. For MIL-47, diffraction experiments reveal that styrene is packed inside the pores in a unique, pairwise fashion, resulting in separation factors as high as 4 in favor of styrene. These separation factors are independent of the total amount of adsorbate offered. This is due to co-adsorption of ethylbenzene in the space left available between the packed styrene pairs. The separation is of a non-enthalpic nature. On MIL-53, the origin of the preferential adsorption of styrene is related to differences in enthalpy of adsorption, which are based on different degrees of framework relaxation. The proposed adsorption mechanisms are in line with the influence of temperature on the separation factors derived from pulse chromatography: separation factors are independent of temperature for MIL-47 but vary with temperature for MIL-53. Finally, MIL-53 is also capable of removing typical impurities like o-xylene or toluene from styrene-ethylbenzene mixtures.