Molecular wire compounds

In this chapter, the electrochemistry of Langmuir-Blodgett (LB) films of functionalized molecules including molecular wire compounds, phthalocyanines, artificial lipids, proteins and fullerene derivatives, and carbon nanotubes (CNTs) is described. 

We have reviewed a recent progress on the electrochemistry of LB films and related films of functionalized molecules including molecular wire compounds, phthalocyanines, artificial lipids, proteins and fullerene derivatives, and carbon nanotubes. Ultrathin films with ordered structures on substrates should be promising nanomaterials for future nanoelectronic and molecular-information transducers with controllable and fine structures at the molecular level. The construction of such artificial nanomolecular devices is a great challenge in the near future. 

Polymer and chain formation is another property of chalcogen-nitrogen compounds that distinguishes them from their oxygen analogues. In addition to the unique, superconducting poly(sulfur nitride) (SN) (1.24) (Section 14.2), a variety of poly(thiazyl) chains such as RS5N4R (1.25) (Section 14.3) have been characterized. Interest in these chains stems from their possible use as models for the behaviour of (SN) and as components in molecular materials, e.g., as molecular wires. 

Heteroaryl groups are present in the 4-pyridyl-ethynyl compounds81,99,100 and their extended variants with C=C-C6H4-C=C-pyr-4 ligands.81,99 These compounds can be quaternized with Mel or coordinated to a metal complex at their terminal pyridyl functions to give species addressed as molecular rods or molecular wires.81,99 Similar reactions were carried out with 3-phenanthrolinyl-ethynyl complexes,62 and special examples are also known with R based on calixarenes.51... 

A rapid and versatile covalent assembly technique starting from DEE oligomers has provided the 11.9 nm long hexadecameric poly(triacetylene) rod 20.1481 With its linearly conjugated 16 double and 32 triple bonds spanning in-between the terminal silicon atoms, compound 20 is currently the longest linear, fully Jt-conjugated molecular wire without aromatic repeat units in the backbone. 

The use of molecular wires and devices for electronics applications is destined to occur. The ability to control molecular structures at the subnanometer scale is obvious throughout chemical synthesis. These are the same techniques that have been optimized over the last 50 years for the synthesis and modification of compounds for pharmaceutical, dye, petroleum, and fine chemical indus-... 

Before examining the electrochemical properties of this class of compounds (we will limit the discussion to homonuclear derivatives), it must be clear that the technological application of molecular wires belongs to solid-state chemistry. Nevertheless, since the main target of such new molecules is to conduct electricity, it seems useful to ascertain preliminarily their intrinsic ability towards intramolecular electron mobility by electrochemical investigations in solution, i.e. in the absence of intermolecular interactions. 

Compounds 159-161 could be used as models for redox-active molecular wires <2003EJO3534>. Photovoltaic cell measurements showed 325-327 to be p-type semiconductors and 22 to be an n-type semiconductor <1994BCJ2017>. 

The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. Przibilla, K.J. Lehn, J.-M., unpublished data). 

Wardman P, Dennis MF, Everett SA, Patel KB, Stratford MRL, Tracy M (2003) Radicals from one-electron reduction of nitro compounds, aromatic N-oxides and quinones the kinetic basis for hypoxia-selective, bioreductive drugs. Biochem Soc Symp 61 171-194 Warman JM, de Haas MP, Hummel A, van Lith D, VerberneJB, Loman H (1980) A pulse radiolysis conductivity study of frozen aqueous solutions of DNA. Int J Radiat Biol 38 459-459 Warman JM, de Haas MP, Rupprecht A (1996) DNA a molecular wire Chem Phys Lett 249 319-322 Warters RL, Lyons BW (1992) Variation in radiation-induced formation of DNA double-strand breaks as a function of chromatin structure. Radiat Res 130 309-318 Warters RL, Hofer KG, Harris CR, Smith JM (1977) Radionuclide toxicity in cultured mammalian cells Elucidation of the primary site of radiation damage. Curr Top Radiat Res Q 12 389-407 Weiland B, Huttermann J (1998) Free radicals from X-irradiated, dry and hydrated lyophilized DNA as studies by electron spin resonance spectroscopy analysis of spectral components between 77 K and room temperature. Int J Radiat Biol 74 341-358 Weinfeld M, Soderlind K-JM (1991) 32P-Postlabeling detection of radiation-induced DNA-damage identification and estimation of thymine glycols and phosphoglycolate termini. Biochemistry 30 1091-1097... 

Due to the high degree of conjugation, oh go(thiophene)s advanced as attractive candidates for molecular bridges. For instance, Sato et al. [34] constructed hexyl-sexithiophene and methoxy-terthiophene derivatives bearing two terminal ferrocenyl groups. These served as model compounds for molecular wires. In the hexyl-sexithiophene derivative, the resultant oxidized states spread over both the ferrocene and the sexithiophene moieties. Similarly, in the methoxy-terthiophene derivative, the oxidized species spreads over the entire molecule containing the terthiophene and the other ferrocene moiety. In both cases, CT between the terminal units is inferred as it is mediated via the oh go(thiophene)s. 

The reaction of thioxanthone with various 3-thienyllithium compounds is the initial step in the synthesis of the diols such as 602 from which the bis(thioxanthylium) dication 603 is obtained. This species functions as a reversible redox pair with its reduction product, the hexaarylethane, creating an electrochromic system in which electron transfer brings about bond making and bond breaking. These oligomers 604 may be considered to be a new class of molecular wires (Scheme 238) <2004OL2523>. 

Molecular wires, 208 211 in which two metal centers are connected by an unsupported C3 chain, as in compound 191, have been generated by addition of BF3 to binuclear bridged (l-alkynyl)carbene complex lw (Scheme 80).27 29,212 Similar compounds were derived from tungsten, rhenium, and iron moieties. A binuclear complex 192 containing a C5 bridge has been also obtained 27... 

A three-level switching device has been demonstrated in which photochromic properties are used to control electrical properties, and vice versa. Such a system has been realized in the form of thiophene bisphenol [90, 91]. Conversion of the open (8a) to the closed (8b) form of the thiophene was achieved by absorption of 312 nm light, and revered by absorption of 600 nm light. The bisphenol oxidation occurs at +0.735 V (vs. SCE), forming the closed-ring bisquinone, compound 8c. This species has large absorptions at 400 and 534 nm. The optical properties of the quinone phenol couple have previously been used in a bianthrone-based system [87]. The bisquinone (8c) cannot be converted to the open thiophene, and locks the system in the closed form. The thiophene has also been incorporated as a component in two-level molecular switches [99, 128] and switchable molecular wires [30]. 

It should be recalled, however, that even an interaction of a few cm (which cannot be noticed in spectroscopic experiments) may be sufficient to cause intercomponent energy transfer or electron transfer processes. As already mentioned, the nature and length of the bridging ligand can contribute strongly to the rate of the photoinduced processes. Many compounds have been labeled wire molecules, but in most cases the wire-type behavior could not be observed. However, one should first define what is a molecular wire and what are the expectations for such a system. 

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