A wave function based ab initio non-equilibrium Green's function approach to charge transport

We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Green's function formalism. In addition we present an extension so as to include effects of the two-particle propagator. Our procedure is demonstrated for a dithiolbenzene molecule between silver electrodes. The full current-voltage characteristic is calculated. Specific conclusions for the contribution of correlation and two-particle effects are derived. The latter are found to contribute about 5% to the current. The order of magnitude of the current coincides with experiments.Comment: 21 pages, 3 figure

New fluorescent perylene bisimide indicators—a platform for broadband pH optodes

Asymmetric perylene bisimide (PBI) dyes are prepared and are shown to be suitable for the preparation of fluorescence chemosensors for pH. They carry one amino-functional substituent which introduces pH sensitivity via photoinduced electron transfer (PET) while the other one increases solubility. The luminescence quantum yields for the new indicators exceed 75% in the protonated form. The new indicators are non-covalently entrapped in polyurethane hydrogel D4 and poly(hydroxyalkylmethacrylates). Several PET functions including aliphatic and aromatic amino groups were successfully used to tune the dynamic range of the sensor. Because of their virtually identical spectral properties, various PBIs with selected PET functions can easily be integrated into a single sensor with enlarged dynamic range (over 4 pH units). PBIs with two different substitution patterns in the bay position are investigated and possess variable spectral properties. Compared with their tetrachloro analogues, tetra-tert-butyl-substituted PBIs yield more long-wave excitable sensors which feature excellent photostability. Cross-sensitivity to ionic strength was found to be negligible. The practical applicability of the sensors may be compromised by the long response times (especially in case of tetra-tert-butyl-substituted PBIs)

From a local Green's function to molecular charge transport

Introduction As of very recently, the trend in miniaturization of electronic devices indicates a possibility to ultimately arrive at electronically active elements being constituted by just a single molecule [1, 2]. Specifically it has been demonstrated in break-junction experiments, that a single organic molecule might be positioned between two electrodes so as to yield what has become known as a molecular junction [3, 4]. Under certain circumstances such a system allows for electron transfer from the one electrode to the other. Based on these experiments two far-reaching conclusions have been ventured, namely that molecular junctions might lead the way to ultimate one-electron switches, and secondly, that such a switch behaviour can be triggered by various rather different physical and chemical circumstances. However, at present experiments are still notoriously difficult and hard to interpret, thus shifting weight to theoretical considerations which are expected to both create a f

Syntheses of dialkenes and diepoxides, and the influence of their structural parameters on liquid crystalline properties

Liquid crystalline dialkenes and diepoxides having different structural parameters were prepared and characterized. The compounds contained two or three aromatic rings in the mesogenic core. The base structure for the compounds containing two aromatic rings is the 4-(omega-oxiranylalkoxy) benzoic acid 4-(omega-oxiranylalkoxy) phenyl ester and for the compounds containing three aromatic rings the 4-{[4-(omega-oxiranylalkoxy) benzoyl]oxy}phenyl 4-(omega-oxiranylalkoxy)benzoate. The hydroquinone moiety is either unsubstituted or contains polar nitrile or fluorine groups as substituents. The flexible spacers consist of different numbers of methylene groups terminated with double bonds or epoxide groups. Some of the diepoxides undergo thermal polymerization of the epoxide group during the determination of the transition temperatures, and therefore the dialkenes were also investigated because of their almost analogous phase behaviour, but with higher thermal stability. Most of the compounds studied show liquid crystalline behaviour. A systematic study is presented of the influence of structural parameters such as the number of aromatic rings in the mesogenic core, substituents and length of the flexible side chain, on the transition temperatures. The diepoxides substituted with polar nitrile or fluorine groups are suitable precursors for ordered network polymers

Specific interactions in miscible poly(p-vinylphenol)/poly(N-methyl-3-piperidinemethyl methacrylate) blends

10.1002/1521-3935(20010101)202:1<31Macromolecular Chemistry and Physics202131-35MCHP