Comparison of Phosphorescent Pt(II) Complexes with C^N^N vs N^N^N Chelators and Caffeine-Based NHC-co-ligands†

In this work, we explored coordination compounds featuring caffeine-based carbene co-ligands and tridentate dianionic pincer luminophores derived from 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine (N), as well as from 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine (C), bearing either Ad (adamantyl) or tBu (tertiary butyl) substituents. The new 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine-based ligand precursors along with four Pt(II) complexes, namely Pt(C-tBu), Pt(C-Ad), Pt(N-tBu) and Pt(N-Ad) were characterized. Further on, the influence of the different substituents at the chelating luminophores and of the caffeine-based NHC-co-ligand on the photophysical properties (including photoluminescence quantum yields (Φ_L), excited-state lifetimes (τ), radiative (k_r), and non-radiative (k_nr) deactivation rate constants) was assessed in fluid solutions at room temperature (RT) and in frozen glassy matrices at 77 K. All four luminophores perform equivalently well within the experimental uncertainty. In deoxygenated fluid solutions at RT, photoluminescence quantum yields reaching up to 24 ± 2% and excited-state lifetimes of around 12 μs were found. The generally long excited-state lifetimes and only minor blue shift upon cooling to 77 K along with mostly well-resolved vibrational progressions point to metal-perturbed ligand-centered excited states. Notably, the yield of the complexation reaction in case of Pt(C-tBu) and Pt(C-Ad) was almost two times higher compared to Pt(N-tBu) and Pt(N-Ad). Cyclometallation is not an essential feature to achieve high photoluminescence quantum yields, but it can improve the synthetic efficiency. In summary, it can be observed that coordination chemical concepts based on natural products can lead to stable phosphorescent species with interesting excited-state properties.     

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher-Order Cycloaddition Reactions

The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect.     

Supersymmetric WZ-Poisson Sigma Model and Twisted Generalized Complex Geometry

It has been shown recently that extended supersymmetry in twisted first-order sigma models is related to twisted generalized complex geometry in the target. In the general case there are additional algebraic and differential conditions relating the twisted generalized complex structure and the geometrical data defining the model. We study in the Hamiltonian formalism the case of vanishing metric, which is the supersymmetric version of the WZ-Poisson sigma model. We prove that the compatibility conditions reduce to an algebraic equation, which represents a considerable simplification with respect to the general case. We also show that this algebraic condition has a very natural geometrical interpretation. In the derivation of these results the notion of contravariant connections on twisted Poisson manifolds turns out to be very useful.     

Passive and active vibration isolation under isolator-structure interaction: application to vertical excitations

This work studies the influence of a vibration isolator on the response of a flexible base structure. Two strategies are compared: passive and active vibration isolation (PVI, AVI). Although the multiple advantages of AVI over PVI techniques are well known, their effect in the base structure has not to date been compared. This interaction has an important role in the performance of the general control system, especially when the vibration isolation system is not the only system on the base structure or when there are multiple isolators working simultaneously on it. In addition, the structural serviceability of the base structure can also be affected. The analysis of the vibration isolation problem is made from a wide perspective, including the effect that isolator has on the base structure. Hence assuming the base structure is a non-rigid system. The effect of the isolation system on the base response is studied for an extensive range of base structures, thus showing different possible scenarios. The influence is quantified by comparing the peak magnitude response of the base when both passive and active vibration isolation techniques are used. The theoretical results have been corroborated by undertaking experimental tests on a full-scale laboratory structure.

     

Ketone–Alcohol Hydrogen‐Transfer Equilibria: Is the Biooxidation of Halohydrins Blocked?

To ensure the quasi‐irreversibility of the oxidation of alcohols coupled with the reduction of ketones in a hydrogen‐transfer (HT) fashion, stoichiometric amounts of α‐halo carbonyl compounds have been employed as hydrogen acceptors. The reason that these substrates lead to quasi‐quantitative conversions has been tacitly attributed to both thermodynamic and kinetic effects. To provide a clear rationale for this behavior, we investigate herein the redox equilibrium of a selected series of ketones and 2‐propanol by undertaking a study that combines experimental and theoretical approaches. First, the activity of the (R)‐specific alcohol dehydrogenase from Lactobacillus brevis (LBADH) with these substrates was studied. The docking of acetophenone/(R)‐1‐phenyethanol and α‐chloroacetophenone/(S)‐2‐chloro‐1‐phenylethanol in the active site of the enzyme confirms that there seems to be no structural reason for the lack of reactivity of halohydrins. This assumption is confirmed by the fact that the corresponding aluminum‐catalyzed Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reactions afford similar conversions to those obtained with LBADH, showing that the observed reactivity is independent of the catalyst employed. While the initial rates of the enzymatic reductions and the IR ν(C?O) values contradict the general belief that electron‐withdrawing groups increase the electrophilicity of the carbonyl group, the calculated ΔG values of the isodesmic redox transformations of these series of ketones/alcohols with 2‐propanol/acetone support the thermodynamic control of the reaction. As a result, a general method to predict the degree of conversion obtained in the HT‐reduction process of a given ketone based on the IR absorption band of the carbonyl group is proposed, and a strategy to achieve the HT oxidation of halohydrins is also shown.     

Black hole radiance, short distances, and TeV gravity

Using a derivation of black hole radiance in terms of two-point functions one can provide a quantitative estimate of the contribution of short distances to the spectrum. Thermality is preserved for black holes with kappalp<1. However, deviations from the Planckian spectrum can be found for mini black holes in TeV gravity scenarios, even before reaching the Planck phase.     

Some modified Adams-Bashforth methods based upon the weighted Hermite quadrature rules

In this paper, we first introduce a modification of linear multistep methods, which contain, in particular, the modified Adams-Bashforth methods for solving initial-value problems. The improved method is achieved by applying the Hermite quadrature rule instead of the Newton-Cotes quadrature formulas with equidistant nodes. The related coefficients of the method are then represented explicitly, the local error is given, and the order of the method is determined. If a numerical method is consistent and stable, then it is necessarily convergent. Moreover, a weighted type of the new method is introduced and proposed for solving a special case of the Cauchy problem for singular differential equations. Finally, several numerical examples and graphical representations are also given and compared.