Effect of surfactants on the fluorescence spectra of water-soluble MEHPPV derivatives having grafted polyelectrolyte chains

Poly(2-methoxy-5-[2'-ethylhexyoxy]-1,4-phenylenevinylene) (MEHPPV) derivatives with polyacrylic acid (PAA) chains grafted onto their backbone were found to be water soluble, and they exhibited a dramatic increase in their fluorescence intensity in the presence of a variety of surfactants, even at concentrations far below their critical micelle concentrations (CMC). This increase was accompanied by a blue-shift in the emission maximum. These observations are rationalized based on the postulate that the backbone conformation of the conjugated polymer is modulated upon interaction of the surfactant molecules with the polyelectrolytic tethers, which in turn results in a significant depletion of intra-chain interchromophore interactions that are known to cause red-shifted emission bands with significantly lower emission yields.

Propargyloxycarbonyl as a protecting group for the side chains of serine, threonine and tyrosine

Propargyloxycarbonyl group is used as a protecting group for the hydroxyl groups of serine, threonine and tyrosine. The propargyloxycarbonyl derivatives of these hydroxy amino acids are stable to acidic and basic reagents commonly employed in peptide synthesis. The deprotection of the O-Poc derivatives using tetrathiomolybdate does not affect commonly used protecting groups such as N-Boc, N-Cbz, N-Fmoc, methyl and benzyl esters. The di-and tripeptides synthesized using O-Poc derivatives of serine, threonine and tyrosine are stable, isolable compounds and give the hydroxy peptides in good yields when treated with tetrathiomolybdate.

Structural synthesis by transformation of binary chains Strukturelle synthese durch umwandlung binaerer ketten

The paper presents a general method of structural synthesis which can be used to derive all possible simple- and multiple-jointed chains of positive, zero or negative degree-of-freedom. In this method all possible chains with N links and F degrees-of-freedom are derived by the transformation of the corresponding “binary chains” with N binary links and F degrees-of-freedom. The method is illustrated by applying to the case of chains with degrees-of-freedom 1,2,0 and −1.

Band to correlated crossover in alternating Hubbard and Pariser-Parr-Pople chains: Nature of the lowest singlet excitation of conjugated polymers

The evolution with increasing Coulomb correlations of a semiconductor to a magnetic insulator is related to an excited-state crossover in pi-electron models for conjugated polymers. We associate strong fluorescence with a lowest singlet excitation S1 that is dipole allowed, on the band side, while S1 becomes two-photon allowed on the correlated side. S1/S2 crossovers in Hubbard, Pariser-Parr-Pople, or other chains with electron-hole symmetry and alternating transfer integral t(1 +/- delta) are based on exact results at delta=0 and 1, on molecular exciton theory at large delta, and on oligomer calculations up to twelve sites.

On the Milito-Cruz adaptive control scheme for Markov chains

Milito and Cruz have introduced a novel adaptive control scheme for finite Markov chains when a finite parametrized family of possible transition matrices is available. The scheme involves the minimization of a composite functional of the observed history of the process incorporating both control and estimation aspects. We prove the a.s. optimality of a similar scheme when the state space is countable and the parameter space a compact subset ofR.

Lattice embedding and equilibrium geometry of metal-halogen chains in the two-band extended Hubbard model

Two-band extended Hubbard model studies show that the shift in optical gap of the metal-halogen (MX) chain upon embedding in a crystalline environment depends upon alternation in the site-diagonal electron-lattice interaction parameter (epsilon(M)) and the strength of electron-electron interactions at the metal site (U(M)). The equilibrium geometry studies on isolated chains show that the MX chains tend to distort for alternating epsilon(M) and small U(M) values.

Quantum-confinement effects on the ordering of the lowest-lying excited states in conjugated chains

The symmetrized density-matrix renormalization-group approach is applied within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond alternation delta) to study the ordering of the lowest one-photon (1(1)B(u)(-)) and two-photon (2(1)A(g)(+)) states in one-dimensional conjugated systems with chain lengths N up to N = 80 sites. Three different types of crossovers are studied, as a function of U/t, delta, and N. The ''U crossover'' emphasizes the larger ionic character of the 2A(g) state compared to the lowest triplet excitation. The ''delta crossover'' shows strong dependence on both N and U/t. the ''N crossover'' illustrates the more localized nature of the 2A(g) excitation relative to the 1B(u) excitation at intermediate correlation strengths.

Regular versus alternating (FeS4)(n) chains: Magnetism in KFeS2 and CsFeS2

We report crystal magnetic susceptibility results of two S = 1/2 one-dimensional Heisenberg antiferromagnets, KFeS2 and CsFeS2. Both compounds consist of (FeS4)(n) chains with an average Fe-Fe distance of 2.7 Angstrom. In KFeS2, all intrachain Fe-Fe distances are identical. Its magnetic susceptibility is typical of a regular antiferromagnetic chain with spin-spin exchange parameter J = -440.7 K. In CsFeS2, however, the Fe-Fe distances alternate between 2.61 and 2.82 Angstrom. This is reflected in its magnetic susceptibility, which could be fitted with J = -640 K, and the degree of alternation, alpha = 0.3. These compounds form a unique pair, and allow for a convenient experimental comparison of the magnetic properties of regular versus alternating Heisenberg chains.

Anomalous Behavior of Linear Hydrocarbon Chains in Water-DMSO Binary Mixture at Low DMSO Concentration

We find that at a mole fraction 0.05 of DMSO (x(DMSO) = 0.05) in aqueous solution, a linear hydrocarbon chain of intermediate length (n = 30-40) adopts the most stable collapsed conformation. In pure water, the same chain exhibits an intermittent oscillation between the collapsed and the extended coiled conformations. Even when the mole fraction of DMSO in the bulk is 0.05, the concentration of the same in the first hydration layer around the hydrocarbon of chain length 30 (n = 30) is as large as 17%. Formation of such hydrophobic environment around the hydrocarbon chain may be viewed as the reason for the collapsed conformation gaining additional stability. We find a second anomalous behavior to emerge near x(DMSO) = 0.15, due to a chain-like aggregation of the methyl groups of DMSO in water that lowers the relative concentration of the DMSO molecules in the hydration layer. We further find that as the concentration of DMSO is gradually increased, it progressively attains the extended coiled structure as the stable conformation. Although Flory-Huggins theory (for binary mixture solvent) fails to predict the anomaly at x(DMSO) = 0.05, it seems to capture the essence of the anomaly at 0.15.

Polymorphism in Opto-Electronic Materials with a Benzothiazole-fluorene Core: A Consequence of High Conformational Flexibility of pi-Conjugated Backbone and Alkyl Side Chains

Fluorene and its derivatives are well-known organic semiconducting materials in the field of opto-electronic devices because of their charge transport properties. Three new organic semiconducting materials, namely, 2,2'-((9,9-butyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C4; 2,2'-((octyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C8; and 2,2'-((9,9-dodecayl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C12 with a benzothiazole-fluorene backbone, were synthesized and characterized for their photophysical properties. A phenomenon of concomitant polymorphism has been investigated in the first two derivatives (C4 and C8) and has been analyzed systematically in terms of the packing characteristics involving pi ... pi interactions. The conformational flexibility of the pi-conjugated 2,2'-(fluorene-2,7-diyl)bis(4,1 phenylene)bisbenzod]thiazole backbone coupled with orientational freedom of the terminal alkyl chains were found to be the key factors responsible for these polymorphic modifications. Attempts to grow suitable crystals for single crystal X-ray diffraction of compound C12 were unsuccessful.

Nematogens with more flexible chains than aromatic rings in the core

Mesogens containing four rings in the main core can accommodate one terminal and two nearby lateral chains on each outside aromatic ring. These compounds containing six chains present an enantiotropic nematic range which is influenced by the rigidity of the links. The conformational behaviour of the first methyleneoxy group within the chains was investigated by one and two dimensional C-13 NMR. The sign of the jump in chemical shifts when entering the nematic phase indicates the folding of each lateral branch. Dipolar oscillations during cross-polarization contact provide the values of the bond order parameter. The two First lateral fragments do not behave in the same way, demonstrating the influence of the fragment along which the chain is back: folded.