On Doi–Naganuma and Shimura liftings

The Ramanujan Journal - In this paper, we extend the Doi–Naganuma lifting to higher levels by following the methods of Zagier and Kohnen. We prove that there is a Hecke-equivariant linear map...     

Solvent‐free and nonisocyanate melt transurethane reaction for aliphatic polyurethanes and mechanistic aspects

A novel melt transurethane polycondensation route for polyurethanes under solvent‐free and nonisocyanate condition was developed for soluble and thermally stable aliphatic or aromatic polyurethanes. The new transurethane process was investigated for A + B, A‐A + B, and A‐A + B‐B (A‐urethane and B‐hydroxyl) ‐type condensation reactions, and also monomers bearing primary and secondary urethane or hydroxyl functionalities. The transurethane process was confirmed by ¹H and ¹³C NMR, and molecular weight of the polymers were obtained as M_n = 10–15 × 10³ and M_w = 15–45 × 10³ g/mol. The mechanistic aspects of the melt transurethane process and role of the catalyst were investigated using model reactions, ¹H NMR, and MALDI‐TOF‐MS. The model reactions indicated the occurrence of 97% reaction in the presence of catalyst, whereas its absence gave only less than 2% of the product. The polymer samples were subjected for end‐group analysis using MALDI‐TOF‐MS, which confirms the Ti‐catalyst mediated nonisocyanate pathway in the melt transurethane process. Almost all the polyurethanes were stable up to 280 °C, and the T_g of the polyurethanes can be easily fine‐tuned from ?30 to 120 °C by using appropriate diols in the melt transurethane process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2445–2458, 2008     

Herringbone and Helical Self-Assembly of π-Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self-organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self-assembly in π-conjugated molecules based on custom-designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single-crystal X-ray structures were resolved for these OPV synthons and the existence of long-range multiple-arm CH/π interactions was revealed in the crystal lattices. Alignment of these π-conjugated OPVs in the solid state was found to be crucial in producing either right-handed herringbone packing in the crystal or left-handed helices in the liquid-crystalline mesophase. Pitch- and roll-angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self-assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π-conjugated materials.     

Control of pi-stacking for highly emissive poly(p-phenylenevinylene)s: synthesis and photoluminescence of new tricyclodecane substituted bulky poly(p-phenylenevinylene)s and its copolymers

In the present work, we have demonstrated a facile approach to increase the luminescence of the poly (p-phenylenevinylene)s via controlling the molecular aggregates induced by pi-stacking. We have synthesized new bulky tricyclodecane (TCD) substituted PPVs: poly(2-methoxy-5-tricyclodecanemethyleneoxy-1,4-phenylenevinylene) (MTCD-PPV), poly(bis-2,5-tricyclodecanemethyleneoxy-1,4-phenylenevinylene) (BTCD-PPV), and a series of symmetrically substituted bulky PPV copolymers (P-1-P-7) covering the entire composition range from 0 to 100 mol %. The structures of the monomers and polymers were confirmed by 1H NMR and FTIR, and the molecular weights were determined by gel permeation chromatography. The composition analysis by NMR revealed that the bulky monomer was highly reactive and the incorporation of bulky units in MEH-PPV increased irrespective of the feed ratio. The polymers possess good solubility, high molecular weights, good thermal stability, and so forth. The molecular weights of the PPV copolymers were also significantly affected by the bulky substitution: the higher the incorporation of bulky units, the lower the molecular weight. The absorption and emission studies revealed that there was no influence on the MEH-PPV by TCD substitution in solution whereas in the solid state the photoluminescence intensity of PPV increased more than 10 times. The luminescence increase in PPV was observed throughout the entire bulk and was not confined to any particular domain in the polymer. The bulky PPV copolymers showed that both the luminescence intensity (in film) and quantum yields (in solution) increased with an increase in the extent of BTCD incorporation in the MEH-PPV and attained a maximum for 50% BTCD. The TCD unit has thus proved to be an efficient bulky susbstituent for PPV as it controls the pi-stack-induced molecular aggregates in the polymer chains by increasing the interchain distances. The new bulky PPV copolymers are highly soluble, thermally stable, and highly luminescent besides being economically cheap compared to the other materials reported so far for the bulkier approach in pi-conjugated materials.     

Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification

Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at − 10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC₆ at − 10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB.     

Cyclopolymerization Initiated by Peroxydisulfate Ion and Metal Ion Catalysis

The kinetics of cyclopolymerization of N,N' -methylenebisacrylamide (MBA) was carried out using peroxydisulfate ion as initiator. The rate of polymerization, R_p, was found to be proportional to [M] and [S₂O₈ ^2-]^1/2 for the uncatalyzed system. A cyclopolymerization mechanism was found to fit the experimental results. The k₂₁/k_t ^1/2 value for MBA polymerization was evaluated. Evidence in favor of the cyclopolymerization mechanism is provided. The addition of Ag ion enhances R_p.     

Selective one-pot synthesis of Z-iodoallylic iodides from propargyl alcohols

A selective one-pot procedure was developed for the production of Z-iodoallylic iodides from the corresponding propargyl alcohols.     

Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ _‖) to increase by a factor of 10⁷ or more relative to that in undoped samples; σ _‖ attains a value of 10⁻²S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ _‖/σ _⊥ ≥ 10¹⁰, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.