Stochastic model of muon diffusion in the presence of traps: Nonsecular effects on spin depolarization

A stochastic model is presented for calculating the depolarization of the positive muon when the latter undergoes jump diffusion in a solid in the presence of trapping impurities. The theory, restricted to low concentration of traps, is applicable to relaxation studies in both transverse and zero magnetic fields. In the transverse geometry, the dipolar interaction between the μ⁺ and the surrounding nuclear spins can be treated classically, and the analysis is simpler. The results obtained are shown to be identical to those derived before in a ‘two-state’ model, widely used recently in interpreting various experimental studies on trap-limited diffusion of light interstitials (μ⁺,H, etc.) in solids. The zero field technique is more versatile but is also more complicated to analyze as it involves nonsecular terms in the dipolar interaction. Assuming that the local dipolar fields are isotropic with their magnitudes distributed in a Gaussian manner, tractable results can be obtained for the Laplace transform of the zero field relaxation function. The latter needs to be inverted in order to facilitate comparison with experimental data, which are normally recorded in the time space. A scheme to handle this numerical problem is described and results are presented using realistic parameters.     

Phenalenyl-based neutral radical molecular conductors: substituent effects on solid-state structures and properties

We report the preparation, crystallization, and solid-state characterization of cycloheptyl and cyclooctyl-substituted spirobiphenalenyl radicals and the corresponding sigma-dimer of the cyclooctyl derivative. The crystal structure shows that the cycloheptyl radical (9) is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-chain structure, whereas the cyclooctyl variant exists both as pi-dimer 10 and sigma-dimer 10d. The neutral radical 9 shows the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility, chip approximately 4.5x10(-4) emu/mol and is assigned a resonating valence bond (RVB) ground state. We highlight the relationship between the magnetic properties of the Heisenberg antiferromagnet and the RVB ground state in 1-D and further elucidate the electronic structure of this new class of compounds. Magnetic susceptibility measurements show that 10 is a diamagnetic pi-dimer, whereas 10d is a diamagnetic sigma-dimer. Extended Hückel calculations for 9 indicate that the solid is a one-dimensional organic metal with a bandwidth of about 0.4 eV. Pressed pellet conductivity measurements indicate values of sigmaRT=1.5x10(-3) S/cm for compound 9 and sigmaRT=1.0x10(-6) S/cm for compound 10. The structural results and transport properties are discussed in the light of extended Hückel theory band structure calculations and DFT investigations of the electronic structure of related compounds.     

Thieno–Pyrrole‐Fused BODIPY Intermediate as a Platform to Multifunctional NIR Agents

We report the synthesis, photophysical and electrochemical properties, and in vivo fluorescence imaging of a series of new thieno–pyrrole‐fused near‐infrared (NIR) BODIPY agents by using a versatile intermediate as a building block. The versatile thieno–pyrrole‐fused BODIPY intermediate was rationally designed to bear bromo‐substituents and absorb in the mid‐red region (635 nm) to act as an organic electrophile for the development of NIR multifunctional agents. The use of subsequent palladium‐catalyzed and nucleophilic substitution reactions afforded highly conjugated NIR BODIPYs. The novel BODIPYs exhibit long‐wavelength absorptions in the NIR region (650–840 nm). The agents produce sharp fluorescence bands, and most of them display respectable quantum yields of fluorescence (0.05–0.87) useful for biomedical imaging, as demonstrated by in vivo imaging with SBDPiR740 . Interestingly, a number of agents in the series that are non‐halogenated were reactive to O₂ at the triplet photo‐excited state coupled with a favorable redox potential and decent fluorescence, and hence could be potential candidates for use as photosensitizers in fluorescence‐guided photodynamic therapy. Furthermore, the synthetic approach allows further functionalization of the highly conjugated NIR BODIPYs to tune the excited states (PET, ICT) and to conjugate targeting moieties for enhanced biological applications.     

Cobalt(III), nickel(II) and ruthenium(II) complexes of 1,10-phenanthroline family of ligands: DNA binding and photocleavage studies

DNA binding and photocleavage characteristics of a series of mixed-ligand complexes of the type [M(phen)₂LL]ⁿ⁺ (where M = Co(III), Ni(II) or Ru(II), LL = 1,10-phenanthroline (phen), phenanthroline-dione (phen-dione) or dipyridophenazine (dppz) andn = 3 or 2) have been investigated in detail. Various physico-chemical and biochemical techniques including UV/Visible, fluorescence and viscometric titration, thermal denaturation, and differential pulse voltammetry have been employed to probe the details of DNA binding by these complexes; intrinsic binding constants (K _b) have been estimated under a similar set of experimental conditions. Analysis of the results suggests that intercalative ability of the coordinated ligands varies as dppz>phen>phen-dione in this series of complexes. While the Co(II) and Ru(II) complexes investigated in this study effect photocleavage of the supercoiled pBR 322 DNA, the corresponding Ni(II) complexes are found to be inactive under similar experimental conditions. Results of detailed investigations carried out inquiring into the mechanistic aspects of DNA photocleavage by [Co(phen)₂(dppz)]³⁺ have also been reported.     

Understanding the micro structure of Berea Sandstone by the simultaneous use of micro-computed tomography (micro-CT) and focused ion beam-scanning electron microscopy (FIB-SEM)

Berea sandstone is the building block for reservoirs containing precious hydrocarbon fuel. In this study, we comprehensively reveal the microstructure of Berea sandstone, which is often treated as a porous material with interconnected micro-pores of 2-5 μm. This has been possible due to the combined application of micro-computed tomography (CT) and focused ion beam (FIB)-scanning electron microscopy (SEM) on a Berea sample. While the use of micro-CT images are common for geological materials, the clubbing and comparison of tomography on Berea with state-of-the-art microstructure imaging techniques like FIB-SEM reveals some unforeseen features of Berea microstructure. In particular, for the first time FIB-SEM has been used to understand the micro-structure of reservoir rock material like Berea sandstone. By using these characterization tools, we are able to show that the micro-pores (less than 30 μm) are absent below the solid material matrix, and that it has small interconnected pores (30-40 μm) and large crater-like voids (100-250 μm) throughout the bulk material. Three-dimensional pore space reconstructions have been prepared from the CT images. Accordingly, characterization of Berea sandstone specimen is performed by calculation of pore-structure volumes and determination of porosity values.     

Photoinduced charge separation in three-layer supramolecular nanohybrids: fullerene-porphyrin-SWCNT

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene-porphyrin-SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π-π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H(2)), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-π and π-π interactions between the MP(alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor-acceptor interactions between them in the ground and excited states. The donor-acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)(4) in two-layered nanohybrids (MP(alkyl)(4)/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)(4) ((1)MP*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) × 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP˙(+)(alkyl)(4)/SWCNT˙(-) and/or MP˙(-)(alkyl)(4)/SWCNT˙(+) for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via(1)ZnP* in C(60)Im→ZnP(alkyl)(4)/SWCNT is observed to form C(60)˙(-)Im→ZnP˙(+)(alkyl)(4)/SWCNT and C(60)˙(-)Im→ZnP(alkyl)(4)/SWCNT˙(+) charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k(CR)) in the range of (6-10) × 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.