Topological effects of a rigid chiral spacer on the electronic interactions in donor-acceptor ensembles

Two triads (donor-spacer-acceptor), exTTF-BN-C60 (6) and ZnP-BN-C60 (7), in which electron donors (i.e., exTTF or ZnP) are covalently linked to C60 through a chiral binaphthyl bridge (BN), have been prepared in a multistep synthetic procedure starting from a highly soluble enantiomerically pure binaphthyl building block (1). Unlike other oligomeric bridges, with binaphthyl bridges, the conjugation between the donor and the acceptor units is broken and geometric conformational changes are facilitated. Consequently, distances and electronic interactions between the donor and C60 are drastically changed. Both donor-spacer-acceptor (D-s-A) systems (i.e., 6 and 7) exhibit redox processes that correspond to all three constituent electroactive units, namely, donor, BN, and C60. Appreciable differences were, however, observed when comparing triad 6, in which no significant exTTF-C60 interactions were noted, with D-s-A 7, whose geometry favors donor-acceptor and pi-pi interactions that result in ZnP-C60 electronic communication. This through-space interaction is, for example, reflected in the redox potentials. Excited-state studies, carried out by fluorescence and transient absorption spectroscopy, also support through-space rather than through-bond interactions. Although both triads form the corresponding radical-ion pair, that is, exTTF*+-BN-C60*- and ZnP*+-BN-C60*-, dramatic differences were found in their lifetimes: 165 micros and 730 ns, respectively.     

In situ powder diffraction study of belite sulfoaluminate clinkering

Belite sulfoaluminate (BSA) cements have been proposed as environmentally friendly building materials, as their production may release up to 35% less CO₂ into the atmosphere when compared with ordinary Portland cement fabrication. However, their formation mechanism has not been studied in detail so far. Here, an in situ high‐temperature high‐resolution synchrotron X‐ray powder diffraction study is reported. Two types of BSA clinkers have been characterized, both containing 50–60 wt% C₂S and 20–30 wt% C₄A₃ as main phases. One type is iron‐rich and a second type (with different phase assemblage) is aluminium‐rich. Furthermore, the C₂S phase reacts slowly with water, thus activation of this compound is desirable in order to enhance the mechanical strength development of the resulting cements. To do so, iron‐rich BSA clinkers have been doped with minor amounts of B₂O₃ and Na₂O to promote stabilization of α‐forms of C₂S, which are more reactive with water. The decarbonated raw materials were loaded into Pt tubes and heated to between 973 K and 1673 K, and patterns were collected using a high‐energy synchrotron beam of wavelength λ = 0.30 Å. The thermal stability of Klein's salt in these clinkers has been clarified. Several reactions have been followed: formation and decomposition of Klein's salt, melting of aluminates and ferrite, and polymorphic transformations of dicalcium silicate: ‐C₂S →α‐C₂S. Changes in mineralogical phase assemblages at a given temperature owing to the addition of minor amounts of selected elements have also been determined.     

Design of two-photon molecular tandem architectures for solar cells by ab initio theory

An extensive database of spectroscopic properties of molecules from ab initio calculations is used to design molecular complexes for use in tandem solar cells that convert two photons into a single electron–hole pair, thereby increasing the output voltage while covering a wider spectral range. Three different architectures are considered: the first two involve a complex consisting of two dye molecules with appropriately matched frontier orbitals, connected by a molecular diode. Optimized combinations of dye molecules are determined by taking advantage of our computational database of the structural and energetic properties of several thousand porphyrin dyes. The third design is a molecular analogy of the intermediate band solar cell, and involves a single dye molecule with strong intersystem crossing to ensure a long lifetime of the intermediate state. Based on the calculated energy levels and molecular orbitals, energy diagrams are presented for the individual steps in the operation of such tandem solar cells. We find that theoretical open circuit voltages of up to 1.8 V can be achieved using these tandem designs. Questions about the practical implementation of prototypical devices, such as the synthesis of the tandem molecules and potential loss mechanisms, are addressed.     

Phthalocyanine-pyrene conjugates: a powerful approach toward carbon nanotube solar cells

In the present work, a new family of pyrene (Py)-substituted phthalocyanines (Pcs), i.e., ZnPc-Py and H(2)Pc-Py, were designed, synthesized, and probed in light of their spectroscopic properties as well as their interactions with single-wall carbon nanotubes (SWNTs). The pyrene units provide the means for non-covalent functionalization of SWNTs via π-π interactions. Such a versatile approach ensures that the electronic properties of SWNTs are not impacted by the chemical modification of the carbon skeleton. The characterization of ZnPc-Py/SWNT and H(2)Pc-Py/SWNT has been performed in suspension and in thin films by means of different spectroscopic and photoelectrochemical techniques. Transient absorption experiments reveal photoinduced electron transfer between the photoactive components. ZnPc-Py/SWNT and H(2)Pc-Py/SWNT have been integrated into photoactive electrodes, revealing stable and reproducible photocurrents with monochromatic internal photoconversion efficiency values for H(2)Pc-Py/SWNT as large as 15 and 23% without and with an applied bias of +0.1 V.     

Tandem enyne metathesis-Diels-Alder reaction for construction of natural product frameworks

Enynes connected through aromatic rings are used as substrates for metathesis reactions. The reactivity of three ruthenium carbene complexes is compared. The resulting 1,3-dienes are suitable precursors of polycyclic structures via a Diels-Alder process. Some domino RCM-Diels-Alder reactions are performed, suggesting a possible beneficial effect of the ruthenium catalyst in the cycloaddition process. Other examples require Lewis acid cocatalyst. When applied to aromatic ynamines or enamines, a new synthesis of vinylindoles is achieved. Monitorization of several metathesis reactions with NMR shows the different behavior for ruthenium catalysts. New carbenic species are detected in some reactions with an important dependence on the solvent used.     

Synthesis and Structure Elucidation of 1-Aryl-substituted Tetrahydropyridone Derivatives

A series of 1-aryl-1,4,5,6-tetrahydro-4(1H)-pyridones having substituents in 2,3- and 2,3,5-positions was prepared from N-aryl-β-alanines and ethyl acetoacetate or 2,4-pentanedione. Twelve tentative biologically active compounds were identified by the combination of ¹H, ¹³C and ¹H/¹³C NMR spectroscopy. The extensive interest has been focused on the influence of substituents as well as on the number, attachment position, and the nature of the substituents. The unknown shielding of the heterocyclic ring on the aromatic carbon atoms was determined and the averaged chemical shift increments were successfully used for the assignment of the aromatic moiety of the studied compounds. The presence of two chiral elements in compounds 16, 17, 20–21 resulted in the mixture of diastereomers and double sets of the resonances in NMR spectra. __________ Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 874–881, June 2005.     

Synthesis and investigation of some 1,4-disubstituted 2-pyrrolidinones

A series of condensation products of 1-aryl-4-hydrazinecarbonyl-2-pyrrolidinones with acetone, 2,4-pentanedione, and aromatic aldehydes was obtained and identified by the combination of IR, mass and ¹H, ¹³C NMR spectroscopy. The results of their structural studies by NMR spectroscopy are provided. It was ascertained that the presence of the NH group determines the existence of the mixtures of the Z/E-isomers of compounds under study. The availability of Z-isomer as a sterically favorable one was also verified by computer molecular modeling. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1336–1345, September, 2006.