Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [Fe^III(qsal-I)₂]NTf₂ (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current–voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling. Temperature-dependent X-ray absorption spectroscopy and density functional theory confirm the SCO complex retains its SCO functionality on the surface implying that van der Waals molecule—electrode interfaces provide a good trade-off between junction stability while retaining SCO switching capability. These results provide new insights and may aid in the design of other types of molecular devices based on SCO compounds.

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction.     

Probing the limits of rate acceleration mediated by hydrogen bonds

[Structure: see text] A simple receptor and substrate are used to probe the relationship between transition-state charge and the level of rate acceleration that can be created by stabilizing the transition state through hydrogen bonding. Pericyclic reactions are accelerated less than 2-fold by the receptor, whereas a conjugate addition reaction is accelerated more than 30-fold. Therefore, substrate polarization by hydrogen bonding would only appear to be effective for reactions that generate significant charge at the transition state.     

The effect of microtacticity on the glass temperature of poly[α-methyl styrene]

The effect of micrestructure on the glass temperature Tg of poly (α-methyl styrene) has been investigated. It has been observed that T_g increases as the syndiotacticity of the molecule increases-thus T_g = 446 ± 2°K for polymer with 67% syndiotactic placements, while a value of T_g = 453 ± 2°K is found for a sample with 95% syndiotactic placements. Coefficients of expansion for the liquid and glassy states have been calculated and from these a chain stiffness parameter δε/k has been estimated using the Gibbs-DiMarzio theory. The result indicates that the flex energy of the α-methyl styrene chain is 20% larger than for the polystyrene chain.     

Relaxation processes in the glassy state: Molecular aspects

Molecular relaxations in the glassy state of amorphous polymers are discussed, and related to the energy requirements of the various possible molecular motions involved under two broad headings: those occurring in groups pendant to the main chain and those occurring in the main chain units themselves. A variety of motions can result in relaxations in polymers with alkane side chains, whereas only rotation (or partial rotation) is possible for pendant phenyl rings. Relaxations in polymers with cycloalkane rings in the side chain can originate within the ring itself or from motion of the ring as a unit. Restricted movements of various types of backbone chain are discussed, and possible alternatives to the postulated crank-shaft motions in polymethylene chains are presented.     

Interpolymer complexation in hydrolysed poly(styrene-co-maleic anhydride)/poly(styrene-co-4-vinylpyridine)

Complexation between hydrolysed poly(styrene-co-maleic anhydride) (HSMA) copolymers containing 28% and 50% maleic anhydride and a poly(styrene-co-4-vinylpyridine), St4VP32 copolymer with 32% of 4-vinylpyridine content has been investigated. Formation of interpolymer complexes from 1,4-dioxane solutions is observed, over the entire composition range and the stoichiometry of these complexes has been determined from elemental analysis.Quantitative FTIR study of the system HSMA50/StV4Py32 shows that the ideal complex composition leads to 2:1 unit mole ratio of interacting component. FTIR results are in good agreement with DSC and TGA ones, since this complex composition gives the maximum value of the glass transition temperature and the best thermic stability.For the systems investigated, the T_g versus composition curve do not follow any of the commonly accepted models proposed for polymer blends. A new model proposed by Cowie [Cowie JMG, Garay MT, Lath D, McEwen IJ. Br Poly J 1989;21:81] is used to fit the T_g data and found to reproduce the experimental results more closely.     

Physical ageing in poly(vinyl acetate) 1. Enthalpy relaxation

The enthalpy changes ΔH_∞ between a poly(vinyl acetate) glass formed by rapid cooling and the corresponding fully relaxed glass have been estimated at four temperatures below the glass transition. The values obtained were different to those expected by extrapolating liquid behavior below the glass transition and were found to agree well with the predictions of a simple expression for the combined main chain conformational and free volume contributions to enthalpy. Conformational contributions from the side chain alone were also considered but were not required to obtain agreement with experiment. It can be concluded that the side chains remained mobile below the glass transition and do not contribute to the heat capacity discontinuity at T_g. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1107–1116, 1997     

Shedding light upon macromolecular behaviour: Luminescence studies of polymers

In this paper the use of two time-resolved luminescence techniques in the study of polymer behaviour is illustrated. Time-resolved anisotropy measurements, TRAMS, have been used to study macromolecular mobilities both in solution and bulk phases. The first use of TRAMS involving phosphorescent labels in the study of synthetic polymers is reported and the potential of the technique for future applications in polymer science is discussed. Time-resolved energy transfer has been used to study polymer compatibility and interdiffusion in blends of PS-PMMA. The data offer clear evidence of the fact that the local concentration of chain termini in phase-separated systems is enriched in the interphase regions compared to that in the two bulk phases of the blend.     

Relative electron inelastic mean free paths for diamond and graphite at 8 keV and intrinsic contributions to the energy-loss

We used hard X-ray photoelectron spectroscopy (HAXPES) with 8 keV X-rays to investigate the 1s emission of carbon. We recorded spectra extending from the peak of the C 1s electrons (“elastic” line) to electrons with up to 110 eV energy-loss. Using two samples side by side, we could compare the inelastic mean free paths (IMFPs) of the electrons of almost 8 keV in diamond and graphite and find them to be practically identical despite about 50% difference in densities. Published extrapolations of their IMFP calculations at lower energies are in good agreement with this result. We show that information from the almost structureless region of overlapping multiple extrinsic energy-losses can be used to quantify the fraction of photoelectrons experiencing intrinsic energy-losses (those due to the sudden creation of the hole). We find that this fraction is 58% of the primary excited C 1s electrons for diamond and is practically the same for graphite. This is at first sight an unexpected result since hole-screening should differ in a semimetal from that in an insulator. The observation can be accounted for by dynamic screening in contrast to static screening.