摘要:

Reaction dynamics for electric field‐induced electron transfer have been studied for two systems, a spiro‐switch molecular system first proposed by Aviram as a possible building block for a molecular device, and a mixed‐valence di‐&mgr;‐oxo bridged manganese complex selected as a candidate for detecting field‐dependent crystal structures. The electric field dependence of the energetics of the molecular switch points to the key role played by the dipole moment of the precursor state. This finding is supported by the results obtained for the mixed‐valence complex, which however exhibits a markedly different behavior. In this case the polarizability of the complex is responsible for drastically different electric field dependent energetics. The switching field, the field at wich the electron transfer rate is largest, is predicted to be significantly higher for the manganese complex than for the spiro‐switch. We conclude that low‐field switching as desired for molecular device will require charge separation large enough for polarizability effects to be negligible, but not so large that there would be no overlap between the donor and acceptor states.

ISSN:0094-243X    

DOI:10.1063/1.42670

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The geometries of both the small and large (in the infinite chain limit) Brooker cations C2n−1H2n+3N+2are studied using a &pgr;‐electron Hamiltonian designed to reproduce AM1 results. Ions with ≤15 CH units are predicted to be symmetrical (C2vwith a soliton defect at the chain centre; larger ions are predicted to be planar (Cs) but have a localized defect at one end of the molecular and a very low energy transition state. While the Csstructures could be useful in molecular switches, here we concentrate on the usefullness of the C2&ngr;structures as molecular wires. The Brooker ion C2&ngr;structures appear qualitatively differently from the analogous polyene anions for chain lengths less than 40 CH units because the end effects in the Brooker ions are stronger and of longer range than in the polyene ions. For chain lengths greater than this, the C2&ngr;structures of both the polyene ions and the Brooker ions resembles those of negative‐defect and positive‐defect trans‐polyacetylene, respectively, owing to the onset of Peierls distortion. All of the unit positive charge is shown to reside on the central region of the C2&ngr;structures of large Brooker ions. The band gap in the infinite C2&ngr;Brooker ion is shown to be about half that of polyacetylene, and the lowest singlet absorption frequency is shown to be non‐zero. The rate constant for electron transfer through symmetrical Brooker ion bridges is predicted to be larger than that through the related even‐membered bridges; symmetry breaking is predicted to inhibit direct electron transfer and to facilitate soliton transport.

ISSN:0094-243X    

DOI:10.1063/1.42664

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The chemistry and physics associated with the development of conducting organic materials during the past decade has resulted in a great deal of speculation regarding their possible role as molecular wires in molecular switching. In typical conducting polymers, such as polyacetylene, polythiophene or poly[arylene vinylene], polaronic or bipolaronic charge states are delocalized over 3–6 repeats units (18–36 atoms) as determined by a variety of techniques. In this paper we will explore how these delocalized charge states can be extended and stabilized, and whether there seems to be limits to further delocalization. If inherent limitations are structure related, then more consideration must be given as to how molecular wires need to be visualized.

ISSN:0094-243X    

DOI:10.1063/1.42662

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

When Cu phthalocyanine molecules are chemically bound to graphite surfaces, the tunneling currents of a scanning tunneling microscope becomes much more unsymmetrical than with pristine graphite surface. An explanation of this rectification, in terms of the energy levels of the molecules and the unsymmetrical location of the molecules between the tip and substrate, is offered. The method used for chemical bonding may have wide application for the attachment of molecules to graphite surfaces.

ISSN:0094-243X    

DOI:10.1063/1.42669

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Bispyridinium conjugated polyenes of different lengths and charges have been synthesized. Since they combine the features of carotenoids and of viologens, they have been termed caroviologens. Such molecules, possessing an extended conjugated chain fitted with polar electroactive endgroups, and having a length sufficient to span a lipid membrane could function as transmembrane electron channels, i.e., as molecular wires.

ISSN:0094-243X    

DOI:10.1063/1.42678

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A physical system consisting of a lipid monolayer surrounded by a set of electrodes is defined and proposed as a plausible example of a molecular circuit for information processing. The lipid monolayer is characterized as a cellular array of electric dipoles, where each dipole lying on the monolayer plane represents the head of a lipid molecule. Each dipole experiences the effects of the electric field generated by both the surrounding dipoles and the electrodes. The electrodes’ contribution to the electric field is calculated through a proper discretization of their charge densities. According to this model the dipoles of the cellular array play the role of neurodes whose state is described by the dipole orientations on the plane, and a cellular automation can be defined once the orientation values have been properly discretized. The electrodes act as input‐ and output‐signal sources. it is shown that the simple system considered here, i.e., a two‐port system, is characterized by a nonlinear voltage transfer curve, with a switching threshold associated with a state transition of the cellular array. These results can be considered as a first step toward of more realistic structures useful in information processing, e.g., molecular cellular automata and neutral networks.

ISSN:0094-243X    

DOI:10.1063/1.42676

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The control of electron‐transfer processes is a major challenge in the design of molecular devices. Novel conjugared oligomers and polymers are presented in which photo‐ and redoxactive building blocks are linked by different spacers. It is also possible by chemical transformations to extend or interrupt &pgr;‐conjugation and to switch from localized to delocalized electronic states. By a proper choice of the spacer units the title systems are tailored for electron transfer.

ISSN:0094-243X    

DOI:10.1063/1.42647

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Described is the synthesis of two spiro core segments of orthogonally arranged conducting systems and the adhesion of oligomeric branching units to the core segments. The final systems are to have a potentially conducting chain fused perpendicularly to a second potentially conducting chain via a sigma bonded network. These systems may be suitable for incorporation into future molecular electronic devices. One of the core segments synthesized is based on a thiophene conducting group and it is formed by a zirconium‐promoted bis(bicyclization) of bis(diyne) systems. The second is a phenylene‐based system that is derived from fluorene. Terminal bromide groups provide the linkage points for further extensions of the chains via palladium‐catalyzed cross coupling methods. Four thiophene trimers are added in a single operation to spiro‐fused cores to afford the target molecules. A spiro‐fused thiophene‐based monomer system is converted to a spiro‐fused heptamer that is 25 A˚ long. The synthesis of a mixed phenylene‐thiphene system is described that provides a spiro‐fuled octamer that is 30 A˚ long. In each case, alkyl substituents on the thiophenes afford soluble materials. Trimethylsilyl end groups flank each orthogonally fused system. Organopalladium‐ and organonickel‐catalyzed procedures are used extensively for the synthesis of the orthogonally fused compounds.

ISSN:0094-243X    

DOI:10.1063/1.42648

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The continuing quest for an organic unimolecular rectifier is reviewed. Langmuir‐Blodgett (LB) films of D‐&sgr;‐A molecules (D=one‐electron donor, &sgr;=carbamate, A=one‐electron acceptor) have been prepared, and electrical current measurements through monolayers have been measured, both by macroscopic means and by scanning tunneling microscopy. A definitive experimental proof of the 1974 Aviram‐Ratner Ansatz still eludes us.

ISSN:0094-243X    

DOI:10.1063/1.42649

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Poly(2‐ethylaniline) chains were synthesized within the three‐dimensional channels of dehydrated zeolites X and Y. The Na‐ and acidic forms of the zeolite hosts were loaded with the monomer from hexane solution, which was subsequently oxidized with aqueous (NH4)2S2O8. The steric demands for the polymerization are illustrted by the inability of the polymer to form in the one‐dimensional channel system of mordenite. Since protons are not necessary for the polymerization to occur, the polymer is formed in the acid form of the zeolites as well as in the Na‐forms. Polymerization occurs also at the outer surfaces of the zeolite crystals. The external polymer can be removed with chloroform after it was converted to the soluble base form. The intrazeolite polymer chain length peaks at about 0.1 &mgr;m (determined from gel permeation chromatography), and is substantially greater than that obtained in bulk polymerizations.

ISSN:0094-243X    

DOI:10.1063/1.42650

出版商:AIP    

年代:1992

数据来源: AIP