Tuning Dispersity by Photoinduced Atom Transfer Radical Polymerisation: Monomodal Distributions with ppm Copper Concentration

Dispersity significantly affects the properties of polymers. However, current methods for controlling the polymer dispersity are limited to bimodal molecular weight distributions, adulterated polymer chains, or low end‐group fidelity and rely on feeding reagents, flow‐based, or multicomponent systems. To overcome these limitations, we report a simple batch system whereby photoinduced atom transfer radical polymerisation is exploited as a convenient and versatile technique to control dispersity of both homopolymers and block copolymers. By varying the concentration of the copper complex, a wide range of monomodal molecular weight distributions can be obtained (?=1.05–1.75). In all cases, high end‐group fidelity was confirmed by MALDI‐ToF‐MS and exemplified by efficient block copolymer formation (monomodal, ?=1.1–1.5). Importantly, our approach utilises ppm levels of copper (as low as 4 ppm), can be tolerant to oxygen and exhibits perfect temporal control, representing a major step forward in tuning polymer dispersity for various applications.     

Gorenstein semigroup algebras of weighted trees and ordered points on the projective line

We determine exactly which semigroup algebras of weighted trees are Gorenstein. These algebras arise as toric degenerations of projective coordinate rings of moduli of weighted points on the projective line. As a corollary, we find exactly when these families of algebras are Gorenstein as well.     

Hydrophobization of phosphorylated cellulosic fibers

Due to their flame retardant behavior, phosphorylated cellulosic fibers could be interesting candidates for use in the composite material field. However, because of the phosphate groups, the fiber network is highly charged and hydrophilic reducing its compatibility towards synthetic resins. An effective hydrophobization method for phosphorylated cellulosic fibers was therefore developed in order to enhance their hydrophobic behavior. The best results were obtained with a straightforward addition of tosylated fatty alcohols. The influence of the carbon chain length on the reaction efficiency, the thermal degradation and the hydrophobic behavior are reported. The success of the alkylation reaction was confirmed by FTIR analyses and the degree of substitution by elemental analysis. Contact angle with water of more than 100° were obtained after alkylation. The cellulosic samples were furthermore characterized by means of SEM, fiber length distribution, NMR spectroscopy and thermo gravimetric analysis.     

Violuric acid monohydrate: a second polymorph with more extensive hydrogen bonding

Crystals of a second polymorph of violuric acid monohydrate [systematic name: pyrimidine‐2,4,5,6(1H,3H)‐tetrone monohydrate], C₄H₃N₃O₄·H₂O, have higher density and a more extensive hydrogen‐bonding arrangement than the previously reported polymorph. Violuric acid and water molecules form essentially planar hydrogen‐bonded sheets, which are stacked in an offset …ABCABC… repeat pattern involving no ring‐stacking interactions.     

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ_2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ_2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. −5.35 eV making them relatively electron rich in spite of the presence of two B⁻–N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

Ladder-type heterocycles encompassing two B⁻–N⁺ dative bonds possess intense green to red emission, large 2PA cross-sections and superb photostability.     

Ligand K-edge X-ray absorption spectroscopy of [Fe4S4]1+,2+,3+ clusters: changes in bonding and electronic relaxation upon redox

Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.