Molecular electronics is, relatively speaking, a young field. Even so, there have been many significant advances and a much greater understanding of the types of materials that will be useful in molecular electronics, and their properties. The purpose of this review is to provide a broad basis for understanding the areas where new advances might arise, and to provide introduction to the subdisciplines of molecular electronics. This review is divided into two major parts; an historical examination of the development of conventional electronics, which should provide some understanding of the push towards molecular electronics. The problems associated with continuing to shrink conventional systems are presented, along with references to some of the efforts to solve them. This section is followed by an in-depth look at the most important research into the types of behaviors that molecular systems have been found to display.

An internal or external electric field F can drive the chemical structure, bond order alternation, and electronic structure of linear polymethine dyes from a neutral, bond-alternated, polyene-like structure, through a cyanine-like structure, and ultimately to a zwitterionic (charge-separated) bond-alternated structure. As the structure evolves under the influence of F, the linear polarizability alpha, the first hyperpolarizability beta, and the second hyperpolarizability gamma are seen to be derivatives, with respect to F, of their next lower order polarization (for alpha) or polarizability (for beta and gamma). These derivative relations provide a unified picture of the dependence of the polarizability and hyperpolarizabilities on the structure in linear polymethine dyes. In addition, they allow for predictions of structure-property relations of higher order hyperpolarizabilities.

Contents 1. Introduction 4367 2. Elimination Lithography 4369 2.1. STM Modification under Ambient Conditions 4370 2.2. STM Modification in UHV 4372 2.3. Modification with AFM (Mechanical and Electrical) 4374 2.4. Scanning Probe Lithography with SECM 4380 2.5. Scanning Probe Lithography with NSOM 4380 3. Addition Lithography 4384 3.1. Historical Precedent for Addition Lithography 4384 3.2. Fundamentals of DPN 4384 3.3. Variation of the Ink and Substrate 4388 3.4. DPN Structures as Templates 4391 3.5. Mechanistic Aspects underlying DPN 4393 4. Substitution Lithography 4397 4.1. Substitution Initiated by Removal of SAM under Elevated Bias 4398 4.2. Substitution Initiated by Removal of SAM via Mechanical Desorption 4401 4.3. Substitution Followed by ex-Situ Addition to Patterns 4404 4.4. Substitution via Tip-Induced Terminal Group Modification 4406 5. Conclusions and Outlook 4413 6. Acknowledgments 4414 7. Glossary 4414 8. References 4415

The solvent dependence of the second hyperpolarizability, gamma, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured gamma is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in non-polar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, gamma can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.

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Two series of redox-active, iron−sulfur core dendrimers of the general structure (nBu4N)2[Fe4S4(S-Dend)4] (Dend = dendrons of generations 1 through 4) were prepared. Heterogeneous electron-transfer rate constants indicated that the rigid series of dendrimers were more effective at attenuating the rate of electron transfer than were the flexible series of dendrimers. These results were rationalized using computationally derived models which indicated an offset and mobile iron−sulfur core in the flexible series of molecules and a more central and relatively immobile iron−sulfur core in the rigid series of molecules. Further consideration of these data indicated that, while the dendrimers containing rigid ligands had better encapsulated redox cores for a given molecular weight, these molecules had higher electron-transfer rates for a given molecular radius.

A computational method was devised to explore the relationship of charge separation, geometry, molecular dipole moment (p), polarizability (a), and hyperpolarizabilities (,B, y) in conjugated organic molecules. We show that bond-length alternation (the average difference in length between single and double bonds in the molecule) is a key structurally observable parameter that can be correlated with hyperpolarizabilities and is thus relevant to the optimization of molecules and materials. By using this method, the relationship of bond-length alternation, it, a, 3, and y for linear conjugated molecules is ilustrated, and those molecules with maximized a, 3, and y are described. The first coefficient, a, is the linear polarizability and is related to the first derivative of the dipole moment with respect to E. The higher-order terms (3 and y are referred to as the first and second hyperpolarizabilities. The polarization associated with these higher-order coefficients is responsible for second-and third-order NLO effects. In this paper, we discuss a computational method that predicts structural features ofmolecules that have maximized a, B, and 'yfor a given molecular length. Throughout, we will emphasize what we believe to be a key experimentally accessible structural parameter, bond-length alternation, and illustrate the correlation between it and the NLO response.Prototypical organic chromophores for second-order nonlinear optics contain a polarizable r-electron system and donor and acceptor groups to create an asymmetric polarizability in the molecule. It has been recently hypothesized that there is an optimal combination of donor/acceptor strengths that will maximize 8 (1 (Fig. la). Thus, the aromatic ground state impedes electronic polarization in an applied field and effectively reduces the donor and acceptor strengths of a given pair connected by an aromatic bridge. The visible absorption maximum and extinction coefficients of merocyanines ( Fig. 1 b and c) are sensitive to the dielectric properties of the surrounding medium. Although solvatochromic behavior is usually interpreted solely as a change in the electronic distribution at a fixed nuclear geometry (4), for merocyanines it has been shown that the molecular geometry also undergoes significant changes as well (5)(6)(7)(8)(9)(10)(11)

The phenomenon of negative differential resistance (NDR) is potentially very useful in molecular electronics device schemes. Here, it is shown that NDR can be observed in self-assembled monolayers composed of electroactive thiols on gold. Furthermore, these monolayers can be patterned using a scanning probe lithography technique described earlier to form a basis for potential molecular electronic device construction.