Impact of concentration self-quenching on the charge generation yield of fullerene based donor-bridge-acceptor compounds in the solid state

A fullerene based Donor-Bridge-Acceptor (DBA) compound, incorporating a π-extended tetrathiafulvalene electron donor, is investigated with respect to its photophysics in solution versus solid state. Solid films of neat DBA are compared with blend films where the DBA compound is diluted in the inert, low dielectric, polymer poly(styrene). It is found that the moderate intermolecular electronic coupling and donor-acceptor separation (22 ?) in this case leads to the generation of more dissociated, intermolecular charges than a mixture of the donor and acceptor reference compounds. However, the increased intermolecular interactions in the solid state lead to the excited state of the fullerene suffering from concentration self-quenching. This is found to severely affect the charge generation yield in solid films. The impact of competing intra and intermolecular interactions in the solid state upon the film photophysics is analysed in terms of a kinetic model which includes both the effects of concentration self-quenching and the impact of film composition upon the dielectric stabilisation of charge separated states. We conclude that both concentration self-quenching and dielectric stabilisation are critical in determining the photophysics of the blend films, and discuss strategies based upon our observations to enhance the charge photogeneration properties of organic films and photovoltaic devices based upon DBA compounds.     

Rhodamine 6G and 800 J-heteroaggregates with enhanced acceptor luminescence (HEAL) adsorbed in transparent SiO2 GLAD thin films

An enhanced fluorescent emission in the near infrared is observed when the Rhodamine 800 (Rh800) and 6G (Rh6G) dyes are coadsorbed in porous SiO(2) optical thin films prepared by glancing angle deposition (GLAD). This unusual behavior is not observed in solution and it has been ascribed to the formation of a new type of J-heteroaggregates with enhanced acceptor luminescence (HEAL). This article describes in detail and explains the main features of this new phenomenology previously referred in a short communication [J. R. Sánchez-Valencia, J. Toudert, L. González-García, A. R. González-Elipe and A. Barranco, Chem. Commun., 2010, 46, 4372-4374]. It is found that the efficiency and characteristics of the energy transfer process are dependent on the Rh6G/Rh800 concentration ratio which can be easily controlled by varying the pH of the solutions used for the infiltration of the molecules or by thermal treatments. A simple model has been proposed to account for the observed enhanced acceptor luminescence in which the heteroaggregates order themselves according to a "head to tail" configuration due to the geometrical constrains imposed by the SiO(2) porous matrix thin film. The thermal stability of the dye molecules within the films and basic optical (absorption and fluorescence) principles of the HEAL process are also described.     

Convergence rate of McCormick relaxations

Theory for the convergence order of the convex relaxations by McCormick (Math Program 10(1):147–175, 1976) for factorable functions is developed. Convergence rules are established for the addition, multiplication and composition operations. The convergence order is considered both in terms of pointwise convergence and of convergence in the Hausdorff metric. The convergence order of the composite function depends on the convergence order of the relaxations of the factors. No improvement in the order of convergence compared to that of the underlying bound calculation, e.g., via interval extensions, can be guaranteed unless the relaxations of the factors have pointwise convergence of high order. The McCormick relaxations are compared with the αBB relaxations by Floudas and coworkers (J Chem Phys, 1992, J Glob Optim, 1995, 1996), which guarantee quadratic convergence. Illustrative and numerical examples are given.     

The reactivity of N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxo-pyrimidinyl)-L-histidine towards silver(I) and zinc(II) ions

Reactions of N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)-L-histidine with Ag^I and Zn^II ions in 1/1 and 4/1 ligand-to-metal molar ratios, in aqueous media at 25 °C and 0.1 M KNO₃ and KCl ionic strength, respectively, were followed by potentiometric techniques in order to determine formation equilibria for the complex species. The spectral curves, as a function of ligand-to-metal molar ratio at established pH values, were studied and four solid phases of stoichiometry: AgL·4H₂O, Ag(HL)NO₃·1.5H₂O, AgL(OH)₂K₂·H₂O and Zn(HL)₂Cl₂·7H₂O were isolated and characterised by analytical, spectroscopic and thermal measurements.     

Olefin epoxidations in the ionic liquid [C4mim][PF6] catalysed by oxodiperoxomolybdenum species in situ generated from molybdenum trioxide and urea–hydrogen peroxide: The synthesis and molecular structure of [Mo(O)(O2)2(4-MepyO)2]·H2O

Commercially available molybdenum(VI) compounds, including molybdenum trioxide, were successfully employed as catalyst precursors in the epoxidation of olefins with urea–hydrogen peroxide adduct (UHP) in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [C₄mim][PF₆]. After oxidation, the corresponding epoxides were isolated by extraction with diethyl ether. Additionally the ionic liquid–catalyst mixture was recycled and reused in further catalytic cycles. The catalytic species is assumed to be an oxodiperoxomolybdenum species which forms in situ. A representative complex of this type was thus isolated and characterised. Reaction of excess 4-methylpyridine-1-oxide (4-MepyO) with MoO₃ dissolved in aqueous hydrogen peroxide afforded [Mo(O)(O₂)₂(4-MepyO)₂]·H₂O (1) as yellow crystals. Compound 1, an active epoxidation catalyst, was subsequently characterised and its structure determined by X-ray crystallography.     

The relaxation of a reconstructed metallic surface and its influence on the EFG acting on a substitutional PAC probe

The structure of the 1×2 reconstructed Au(110) surface, calculated using equivalent crystal theory, is studied. We perform point-charge calculations to estimate the electric field gradient (EFG) acting on a substitutional¹¹¹In probe deposited in that surface. The relaxation breaks the local symmetry producing an asymmetric EFG but the calculated asymmetry is lower than the experimental one. Comparing the computed orientations of the principal axes of the EFG to the experimental values we conclude that In probes prefer to substitute gold into the high coordinated sites at the bottom of the channels.     

Amperometric PMMA-microchip with integrated gold working electrode for enzyme assays

The use of a gold film in-channel detector combined with a poly(methyl methacrylate) (PMMA) CE microchip has been tested for alkaline phosphatase (AP) enzymatic assays. Tris-borate or Tris-Gly (pH 9.0, 50 mmol L^–1) buffer solutions were appropriate as running buffer. Signals for three common AP products: -naphthol, p-nitrophenol, and ascorbic acid, were obtained. They were reproducible (RSD 4.4% for six successive electropherograms corresponding to 5 mmol L^–1 -naphthol solution) and the response was dependent on concentration (linear relationship for ascorbic acid solutions between 5 and 20 mmol L^–1 concentration). Use of an end-channel gold film electrode was also investigated. If one of the reagents (substrate or enzyme) is included in the running buffer, two different types of enzymatic assay are feasible in less than 3 min.     

A theoretical study of the bonding capabilities of the zinc-zinc double bond

The theoretical knowledge about the zinc-zinc bond has been recently expanded after the proposal of a zinc-zinc double bond in several [Zn₂(L)₄] compounds (Angew. Chem. Int. Ed. 2017 , 56, 10151-10155). Prompted by these results, we have selected the [Zn₂(CO)₄] species, isolobally related to ethylene, and theoretically investigated the possible η²-Zn₂-coordination to several first-row transition metal fragments. The [Zn₂(CO)₄] coordination to the metal fragment produces an elongation of the dizinc bond and a concomitant pyramidalization of the [Zn(CO)₂] unit. These structural parameters are indicative of π-backdonation from the metal to the coordinated dizinc moiety, as occurred with ethylene ligand. A quantum theory of atoms in molecules study of the Zn Zn bond shows a decrease of ρ_BCP, ∇²ρ_BCP ∫_Zn∩Znρ and delocalization indexes δ(Zn,Zn), relative to corresponding values in the parent [Zn₂(CO)₄] molecule. The Zn Zn and M Zn bonds in these [(η²-Zn₂(CO)₄)M(L)_n] complexes can be described as shared interactions with an important covalent component where the Zn Zn bond is preserved, albeit weakened, upon coordination.     

The crystal and molecular structures of cyclohexanone thiosemicarbazone

Cyclohexanone thiosemicarbazone (Hchtsc) crystallizes in the triclinic crystal system with space group P (No. 2) and the following unit cell parameters: a = 6.2989(2), b = 7.9730(3), and c = 9.4118(2) Å = 79.607(3), = 85.519(2), and = 73.50(2)° V = 445.60(2) ų, Z = 2. The lengths of the bonds C(1)–S, C(1)–N(1), C(1)–N(2), and N(2)–N(3) suggest electron delocalization in all four. The S atom is trans to N(3), and this E configuration is stabilized by intramolecular hydrogen bonding between N(3) and the N(1)H₂ group. Two intermolecular hydrogen bonds involving the S atom and the N(1)–H(1b) and N(2)–H(2) groups give rise to a polymeric chain of molecule pairs.