Synthesis of pure blue emissive poly(2,7‐carbazole)s anchored by electron donor pendant

Three novel poly(2,7‐carbazole)s having hole injection and transporting pendent moieties of carbazole and triphenylamine at the N‐position were synthesized for achieving pure blue electroluminescence. The N‐pendants in the polymers correspond to N‐phenylcarbazol‐2‐yl ( P1 ), N,N‐diphenylamino‐N‐phenylcarabazol‐2‐yl ( P2) , and 4‐phenyl having a hydrocarbon chain with a triphenylamine terminal ( P3 ), respectively. Electronic, optical, and electroluminescence properties of these polymers were compared with those of a poly(2,7‐carbazole) directly connected with triphenylamine at the N‐position ( P0 ) having an aggregation‐induced emissive property. The photoluminescence (PL) spectra suggested that they could emit in the region of blue light in the film state. Especially, P2 that has the fixed and large diphenylaminocarbazolyl pendant showed a deep‐blue fluorescence with CIE(x, y) = (0.15, 0.07). The P0 , P2 , and P3 based light emitting diode devices showed maximum electroluminescence wavelengths in the range of 430–450 nm. The P2 device showed pure blue emission (CIE[x, y] = [0.18, 0.16]), high luminance (1130 cd/m²) and current density (628 mA/cm²) at 8 V, whereas low‐energy emissions around 500–600 nm were emerged at higher than 9 V. The P0 and P3 devices also showed a blue electroluminescence in the range of 8–11 V, but their luminance and efficiency were low. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2526–2534     

Photoactive Polyoxometalate/DASA Covalent Hybrids for Photopolymerization in the Visible Range

The synthesis of TBA-DASA-POM-DASA , the first photoactive covalent hybrid polyoxometalate (POM) incorporating a donor–acceptor Stenhouse adduct (DASA) reverse photochrome, is presented. It has been evidenced that in solution the equilibrium between the colorless cyclopentenone and the highly colored triene conformers is strongly dependent not only on the nature of the solvent but also the countercations, allowing to tune its optical properties. This complex has been further associated to photochromic spironaphtoxazine cations, resulting in a material which can be activated by two distinct optical stimuli. Moreover, when combined with N-methyldiethanolamine, TBA-DASA-POM-DASA constitutes a performing photoinitiating system for polyethylene glycol diacrylate polymerization and under visible light irradiation, a promising result in a domain scarcely developed in POM chemistry.     

A screw dislocation near a coated non-elliptical inhomogeneity with internal uniform stresses

We consider the anti-plane shear deformation of a three-phase inhomogeneity-coating-matrix composite containing a coated non-elliptical inhomogeneity whose surrounding matrix is subjected to the action of a screw dislocation and uniform remote anti-plane shear stresses. Our objective is to establish conditions under which the inhomogeneity maintains an internal uniform stress field. Our analysis, which is based on a carefully chosen conformal mapping function, clearly indicates that such an internal uniform stress distribution can be achieved independently of the action of the screw dislocation, which influences the shape of the inhomogeneity depending on its proximity to the dislocation. In fact, we find that when the screw dislocation is located far from the coated inhomogeneity, the corresponding material interfaces become two confocal ellipses as reported previously in the literature. A simple criterion for the convergence of the series in the conformal mapping function is established.     

Hopf bifurcation in a conceptual climate model with ice–albedo and precipitation–temperature feedbacks

In this paper we analyse a dynamical system based on the so-called KCG (Källén, Crafoord, Ghil) conceptual climate model. This model describes an evolution of the globally averaged temperature and the average extent of the ice sheets. In the nondimensional form the equations can be simplified facilitating the subsequent analysis. We consider the limiting case of a stationary snow line for which the phase plane can be completely analysed and the type of each critical point can be determined. One of them can exhibit the Hopf bifurcation and we find sufficient conditions for its existence. Those, in turn, have a straightforward physical meaning and indicate that the model predicts internal oscillations of the climate. Using the typical real-world values of appearing parameters we conclude that the obtained results are in the same ballpark as the conditions on our planet during the quaternary ice ages. Our analysis is a rigorous justification of a generalization of some previous results by KCG and other authors.     

Self-assembled monolayers for interface engineering in polymer solar cells

Polymer solar cell (PSC) has been developed vastly in the past decade due to the advantages of low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, semitransparency, and solution processing. The performance and lifetime of PSCs are highly dependent on the properties of both active materials and their interfaces. The combination of the versatility of organic chemistry and the multitude of well-understood ligand–metal interactions allows self-assembled monolayers (SAMs) of organic molecules to direct control over the electronic and chemical properties at the inorganic–organic interfaces. Thus, SAMs are an attractive pathway to reconcile interfaces with tunable interface properties in PSCs. Hence, this review describes the application of SAMs in PSCs at different interfaces. First, SAMs as alternatives of traditional transporting materials to reduce the barrier at indium tin oxide (ITO)/active layer interface due to the ability of tuning work function of ITO electrode are discussed. Second, the modifications of metal oxide by SAMs to control the electrical contacts at transporting layer/active layer interface are described. Third, tailoring the properties of the donor/acceptor interface by SAMs to improve the performance of PSCs are summarized. Finally, perspectives and challenges are pointed out for developing highly stable and high-performance PSCs by applying SAMs.     

How To Make Nitroaromatic Compounds Glow: Next-Generation Large X-Shaped,Centrosymmetric Diketopyrrolopyrroles

Red-emissive π-expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three-step strategy involving the preparation of diketopyrrolopyrrole followed by N-arylation and subsequent intramolecular palladium-catalyzed direct arylation. Comprehensive spectroscopic assays combined with first-principles calculations corroborated that both N-arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.     

Nanoviscosity effect on the spin chemistry of an electron donor/Pt-complex /electron acceptor triad - classical and quantum kinetics interpretation

The magnetic-field dependent charge recombination kinetics upon ns-laser flash photolysis of a novel triarylamine/cyclometalated platinum complex/napthalenediimide triad (DPtA) have been measured in tetrahydrofuran (THF) and polytetrahydrofuran (pTHF) to study the spinchemical effect of a large increase of the macroviscosity of the solvent by a factor of about 1000. The magnetic-field dependence of the decay kinetics of the charge separated state has been accurately reproduced by a classical kinetic model, wherein the rate constants of transitions between spin substates of different Zeeman energy are represented by a single magnetic-field dependent rate constant k_±. In THF, the magnetic-field dependence of k_± is given by a double Lorentzian function which, in a double log plot, shows two characteristic steps that can be consistently assigned to the magnetic field inhibition of the coherent and incoherent i.e. relaxational spin mixing mechanism. In pTHF, the magnetic field effect on k_± is a single step function decreasing to its limiting value at about 5 times lower fields than in THF. As shown by a quantum theoretical simulation, in this solvent coherent and incoherent processes are contributing equally at all fields. The nanoviscosity of pTHF appears to be much smaller than its macroviscosity.     

Fuel and thermal load flexibility of a MILD burner

The strategy to stabilize distributed combustion regimes adopting cyclonic flow fields has been proven to be challenging. In fact, the establishment of a toroidal flow field within a combustion chamber may ensure the recirculation of mass and sensible enthalpy required to simultaneously dilute the fresh reactants and increase the temperature above the autoignition one. The combination of reactants dilution and preheating may greatly increase system energy efficiency and lower pollutants production producing very peculiar combustion regime (named MILD Combustion). At the same time this strategy can be compromised if the sensible enthalpy is not high enough to promote the auto-ignition process of diluted mixtures. This may happen either because of an inefficient recirculation system or due to a too low calorific fuel. The paper aims at exploiting the performance of a small-size cyclonic burner for a conventional fuel (CH₄) and a low calorific fuel (synthetic biogas) through the characterization of the process stabilization and pollutant emissions as a function of the mixture equivalence ratio and the nominal thermal power of the inlet mixture (from 2 to 10?kW), with the aim of identifying the optimal operating condition of the system. Results suggest that the system has to be exercised with mixtures with compositions slightly under the stoichiometric conditions and in a well identified temperature range to minimize both NO_x and CO emission. The burner can be easily exercised also with low calorific fuels for higher thermal powers according to the low LHV. However, it results that an efficient recirculation of the exhausts produces a robust MILD combustion condition also when low calorific fuels are used.     

Visible-Light-Mediated Charge Transfer Enables C−C Bond Formation with Traceless Acceptor Groups

The development and application of traceless acceptor groups in photochemical C−C bond formation is described. This strategy was enabled by the photoexcitation of electron donor–acceptor (EDA) complexes with visible light. The traceless acceptors, which were readily prepared from amino acid and peptide feedstocks, could be used to alkylate a wide range of heteroarene and enamine donors under metal- and peroxide-free conditions. The crucial role of the EDA complexes in the mechanism of these reactions was explored through combined experimental and computational studies.     

Ultra-Weak Metal−Metal Bonding: Is There a Beryllium-Beryllium Triple Bond?

Metal-metal triple bonds featuring s-block element have not been reported until now. Only Be−Be double bonds between have been predicted theoretically based on the intuitive electron donation from four s¹ type electron-donating ligands. Herein, we theoretically predicted a novel species featuring a Be−Be triple bond in the Li₆Be₂ molecule. The molecule was found to be thermodynamically stable. The presence of the triple bond was confirmed by adaptive natural density partitioning (AdNDP), electron localization function (ELF), and atoms in molecules (AIM) analyses. Moreover, the mechanical strength of the Be−Be triple bond was analyzed by using compliance matrix, pointing towards its ultra-weak nature.