Photonic switching of photoinduced electron transfer in a dihydropyrene-porphyrin-fullerene molecular triad

Photonic control of photoinduced electron transfer has been demonstrated in a dimethyldihydropyrene (DHP) porphyrin (P) fullerene (C(60)) molecular triad. In the DHP-P-C(60) form of the triad, excitation of the porphyrin moiety is followed by photoinduced electron transfer to give a DHP-P(*)(+)-C(60)(*)(-) charge-separated state, which evolves by a charge shift reaction to DHP(*)(+)-P-C(60)(*)(-). This final state has a lifetime of 2 micros and is formed in an overall yield of 94%. Visible (>or=300 nm) irradiation of the triad leads to photoisomerization of the DHP moiety to the cyclophanediene (CPD). Excitation of the porphyrin moiety of CPD-P-C(60) produces a short-lived (<10 ns) CPD-P(*)(+)-C(60)(*)(-) state, but charge shift to the CPD moiety does not occur, due to the relatively high oxidation potential of the CPD group. Long-lived charge separation is not observed. Irradiation of CPD-P-C(60) with UV (254 nm) light converts the triad back to the DHP form. Thermal interconversion of the DHP and CPD forms is very slow, photochemical cycling is facile, and in the absence of oxygen, many cycles may be performed without substantial degradation. Thus, light is used to switch long-lived photoinduced charge separation on or off. The principles demonstrated by the triad may be useful for the design of molecule-based optoelectronic systems.     

A 3000 K laboratory emission spectrum of water

An emission spectrum of hot water with a temperature of about 3000 K is obtained using an oxy-acetylene torch. This spectrum contains a very large number of transitions. The spectrum, along with previous cooler laboratory emission spectra and an absorption spectrum recorded from a sunspot, is analyzed in the 500-2000 cm(-1) region. Use of a calculated variational linelist for water allows significant progress to be made on assigning transitions involving highly excited vibrational and rotational states. In particular emission from rotationally excited states up to J=42 and vibrational levels with up to eight quanta of bending motion are assigned.     

Nano-Diamond Synthesis in Strong Magnetic Field

Diamond is one of the five known allotropes of carbon. Diamonds are important due to their esthetic beauty and excellently remarkable properties. Single crystalline diamond has been synthesized by catalytic processes under high temperature and high pressure. Polycrystalline diamond has been prepared by various activated hot filament and plasma chemical vapor deposition methods. Industrial synthesis of diamonds has been limited by the extreme synthetic conditions, purity, crystallinity, size and high cost. Synthetic advancement for future industrial production will require better understanding and exploitation of the mechanism of formation. Current mechanistic uncertainty arises out of limited consideration of atomic scale dynamics. A new comprehensive model by Little addresses atomic scale electronic processes with nonclassical significance of the intermediary states. This new nonclassical, electronic mechanistic perspective identifies the creation, stabilization and condensation of carbon and metal intermediates for pathways from carbon precursors thru solvated carbon and carbides to sp³ carbon condensation as diamond. In order to test these mechanistic ideas and exploit the impact for better diamond synthesis, catalytic carbon condensation has been explored in strong magnetic field in an effort to influence various radicals and high spin metal and carbon intermediates along the pathway for more efficient diamond condensation. In this effort, static magnetic fields in excess of 15 T are observed to direct these carbon and metal intermediary microstates to promote nano-diamond nucleation and growth at atmospheric pressure and temperature of 900°C. The observed nano-diamond formation is consistent with the predicted faster kinetics due to lower potential energies of intermediates along pathways to diamond. The predicted stability of nano-diamond relative to nano-graphite also accounts for the observed nano-diamond in this work. This novel use of static magnetic field for diamond synthesis is compared with advancements since the first synthetics bulk diamond formation by scientists at both ASEA (Sweden) and GE Research Laboratory, Schenectady, NY, 50 years ago. On the basis of this predicted, invented, and intrinsic magnetic influence on the dynamics of carbon condensation and the promise for faster, feasible single crystal diamond formation, this discovery of the use of strong magnetic field for diamond production is asserted a major advancement during this 50-year period since the first successful synthesis.     

Complex formation followed by internal electron transfer: The reaction between cysteine and iron(III)

Summary A kinetic study of the anaerobic oxidation of cysteine (H₂ L) by iron(III) has been performed over thepH-range 2.5 to 12 by use of a stopped-flow high speed spectrophotometric method. Reaction is always preceded by complex formation. Three such reactive complex species have been characterized spectrophotometrically: FeL ⁺ (_max=614 nm, =2 820 M^–1cm^–1); Fe(OH)L (_max=503 nm; shoulder at 575 nm, =1 640 M^–1cm^–1); Fe(OH)L ₂ ^2– (_max=545 nm; shoulder at 445 nm, =3 175 M^–1 cm^–1). Formation constants have been evaluated from the kinetic data: Fe³⁺+L ^2– FeL ⁺: logK ₁ ^M =13.70±0.05; Fe(OH)²⁺+L ^2– Fe(OH)L: logK ₁ ^MOH =10.75±0.02; Fe(OH)L+L ^2– Fe(OH)L ₂ ^2– ; logK ₂ ^MOH =4.76±0.02. Furthermore the hydrolysis constant for iron(III) was also obtained: Fe(OH)²⁺+H⁺ Fe _aq ³⁺ : logK ^FeOH=2.82±0.02). Formation of the mono-cysteine complexes, FeL ⁺ and Fe(OH)L, is via initial reaction of Fe(OH)²⁺ with H₂ L (k=1.14·10⁴M^–1s^–1), the final product depending on thepH. FeL ⁺ (blue) formed at lowpH decomposes following protonation with a second-order rate constant of 1.08·10⁵M^–1s^–1. Fe(OH)L (purple) decomposes with an apparent third order rate constant ofk=3.52·10⁹M^–2s^–1 via 2 Fe(OH)L+H⁺ products, which implies that the actual (bimolecular) reaction involves initial dimer formation. Finally, Fe(OH)L ₂ ^2– (purple) is remarkably stable and requires the presence of Fe(OH)L for electron transfer. A rate constant of 8.36·10³M^–1s^–1 for the reaction between Fe(OH)L and Fe(OH)L ₂ ^2– is evaluated.Dedicated to Prof. Dr. mult. Viktor Gutmann on the occasion of his 70th birthday     

A DFT study of the thermal, orbital symmetry forbidden, cyclophanediene to dihydropyrene electrocyclic reaction. Predictions to improve the dimethyldihydropyrene photoswitches

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.     

A theoretical and experimental scale of aromaticity. The first nucleus-independent chemical shifts (NICS) study of the dimethyldihydropyrene nucleus

Nucleus-independent chemical shift (NICS) values were calculated at several locations for a series of dimethyldihydropyrenes (DDPs). These NICS values were used to assess the relative aromaticities of the dimethyldihydropyrene nucleus (DDPN) of these DDPs and to construct a NICS scale of aromaticity. The NICS and experimentally determined relative aromaticities of these DDPNs are in complete agreement, verifying that NICS can be used not only to classify a compound as aromatic but also to determine the degrees of aromaticity of structurally related systems.     

Energy levels of one-dimensional systems satisfying the minimal length uncertainty relation

The standard approach to calculating the energy levels for quantum systems satisfying the minimal length uncertainty relation is to solve an eigenvalue problem involving a fourth- or higher-order differential equation in quasiposition space. It is shown that the problem can be reformulated so that the energy levels of these systems can be obtained by solving only a second-order quasiposition eigenvalue equation. Through this formulation the energy levels are calculated for the following potentials: particle in a box, harmonic oscillator, Pöschl–Teller well, Gaussian well, and double-Gaussian well. For the particle in a box, the second-order quasiposition eigenvalue equation is a second-order differential equation with constant coefficients. For the harmonic oscillator, Pöschl–Teller well, Gaussian well, and double-Gaussian well, a method that involves using Wronskians has been used to solve the second-order quasiposition eigenvalue equation. It is observed for all of these quantum systems that the introduction of a nonzero minimal length uncertainty induces a positive shift in the energy levels. It is shown that the calculation of energy levels in systems satisfying the minimal length uncertainty relation is not limited to a small number of problems like particle in a box and the harmonic oscillator but can be extended to a wider class of problems involving potentials such as the Pöschl–Teller and Gaussian wells.     

Phenoxythiazoline (FTz)-Cobalt(II) Precatalysts Enable C(sp2)–C(sp3) Bond-Formation for Key Intermediates in the Synthesis of Toll-like Receptor 7/8 Antagonists**

Evaluation of the relative rates of the cobalt-catalyzed C(sp²)–C(sp³) Suzuki–Miyaura cross-coupling between the neopentylglycol ester of 4-fluorophenylboronic acid and N-Boc-4-bromopiperidine established that smaller N-alkyl substituents on the phenoxyimine (FI) supporting ligand accelerated the overall rate of the reaction. This trend inspired the design of optimal cobalt catalysts with phenoxyoxazoline (FOx) and phenoxythiazoline (FTz) ligands. An air-stable cobalt(II) precatalyst, (FTz)CoBr(py)₃ was synthesized and applied to the cross-coupling of an indole-5-boronic ester nucleophile with a piperidine-4-bromide electrophile that is relevant to the synthesis of reported toll-like receptor (TLR) 7/8 antagonist molecules including afimetoran. Addition of excess KOMe⋅B(OⁱPr)₃ improved catalyst lifetime due to attenuation of alkoxide basicity that otherwise resulted in demetallation of the FI chelate. A first-order dependence on the cobalt precatalyst and a saturation regime in nucleophile were observed, supporting turnover-limiting transmetalation and the origin of the observed trends in N-imine substitution.     

Screening for cocrystallization tendency: the role of intermolecular interactions

Pharmaceutical cocrystals have rapidly emerged as a new class of API solids with great promise and advantages. Much work has been focused on exploring the crystal engineering and design strategies that facilitate formation of cocrystals of APIs and ligands/cocrystal formers. However, fewer attempts have been made to understand the equilibrium phase behavior and phase transition kinetics of the cocrystallizing solutions. This limited knowledge on the solution physical chemistry often leads to difficulty in screening for potential molecular pairs of API and ligand that form cocrystals effectively. In this study, the long-time self-diffusivities measured using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) are used to characterize the particle interactions in solutions for pharmaceutical cocrystallizing systems. For the pairs of API and ligand that produce cocrystals, the heteromeric attractions between API and ligand are found to be stronger than the homomeric attractions between API molecules and between ligand molecules, suggesting that an energetically favorable condition is induced for the formation of cocrystals. To the best of our knowledge, this is the first report of using the pair contribution of the self-diffusivity as a screening tool for cocrystal formation.     

Viscous solvent colloidal system for direct visualization of suspension structure, dynamics and rheology

We introduce a model colloid system comprised of particles dispersed in a viscous solvent that can be applied to 3D direct visualization studies of suspension structure, dynamics and rheology. The colloids are poly(methyl methacrylate) (PMMA) spheres sterically stabilized by a copolymer of poly(diphenyl-dimethyl) (DPDM) siloxane that matches the refractive index of PMMA. The monodisperse particles, synthesized with mean diameter varying from 0.7 to 1.1 microm, are stably dispersed in a DPDM siloxane solvent, with viscosity varying from 2.2 to 4.3 Pa s at 20 degrees C. As opposed to other classes of PMMA colloids dispersed in organic solvents, this system displays minimal charge interactions. At room temperature, pair potential interactions (measured by extrapolation of pair correlation functions to infinite dilution) are well modeled by a generalized Lennard-Jones alpha-2alpha potential (alpha=10) with dimensionless interaction energy, epsilon/k(B)T=0.6. We use the DPDM-PMMA colloidal system in conjunction with confocal microscopy studies to measure: (i) the radial distribution function in 3D at dilute concentrations and (ii) the colloid self-diffusivity in 3D at dilute concentrations. Both measurements, neither previously reported in uncharged systems, are facilitated by the slow, viscous dynamics of the system. We also show that the viscosity and particle size of the system are such that the high-volume fraction shear thickening transition can be accessed at shear rates amenable to direct visualization.