Molecular conductors organic

Keywords Cation-radical salts, Molecular conductors, Organic superconductors, Organometallic anions, Tetrakis(trifluoromethyl)metallates, Tetrathiafulvalene... 

Carrier generators in molecular conductors have been associated for a long time to a partial charge transfer between the HOMO (or LUMO) electronic band and other chemical species. These systems are known as two-component molecular conductors. Tetrathiofulvalene derivatives are versatile systems for the formation of molecular organic conductors due to their electron donor capacity by transferring one u-electron from the HOMO orbital, and to their planar shape that promotes their stacking as a consequence of the n-n orbital overlap. The electronic properties of these salts are essentially determined by the packing pattern of the donor molecules which, in turn, depends on the counter-ion. 

Since the discovery of the first organic semiconductor perylene-bromine complex in 1954 [1], a large number of molecular conductors, including more than 100 molecular superconductors, have been prepared. Conducting molecular materials are characterized by the following features ... 

Compared with the conducting anion radical salts of metal complexes, the number of molecular conductors based on cationic metal complexes is still limited. Donor type complexes M(dddt)2 (M = Ni, Pd, Pt Fig. 1) are the most studied system. The M(dddt)2 molecule is a metal complex analogue of the organic donor BEDTTTF. Formally, the central C=C bond of BEDT-TTF is substituted by a metal ion. The HOMO and LUMO of the M(dddt)2 molecule are very similar in orbital character to those of the M(dmit)2 molecule. In addition, the HOMO of the M(dddt)2 molecule is also very similar to that of BEDT-TTF. More than ten cation radical salts of M(dddt)2 with a cation (monovalent) anion ratio of 2 1 or 3 2 are reported [7]. A few of them exhibit metallic behavior down to low temperatures. The HOMO-LUMO band inversion can also occur in the donor system depending on the degree of dimerization. In contrast to the acceptor system, however, the HOMO-LUMO band inversion in the donor system leads a LUMO band with the one-dimensional character to the conduction band. 

Tomonaga-Luttinger liquid state [14-16], unconventional superconductivity [17], etc. These molecular conductors once used to be called organic metals, but nowadays this terminology has become obsolete in order to avoid possible confusion with organometallies. ... 

The first edition of this textbook was widely praised. In this new edition the authors have taken the opportunity to bring the work completely up to date by the addition of new material on mesoporous materials, fullerenes, molecular magnets, organic conductors and high-temperature superconductors. All of the chapters have been revised with new additional sections. 

The field of organic conductors has been extensively reviewed (B-77MI1300, 78ANY25, B-78MI11300, 78MI11302, 79ACR79) and no attempt is made here to describe the synthesis and properties of known materials. Molecular conductors of the doped polymer type have been excluded since these are neither intrinsic conductors nor heterocyclic. A review of the principal requirements for the molecular and crystal structures as currently understood is presented as an introduction to the field of organic conductors. 

Table 11. Comparison of mean free paths for some organic and inorganic molecular conductors...

It is convenient to divide the synthesis of organic molecular conductors into two basic processes (1) the synthesis of donor and acceptor molecules, and (2) the oxidation and reduction of the donors and acceptors to radical cations and radical anions. The chapter is organized along these lines. Donor and acceptor synthesis is discussed first, followed by a brief coverage of the formation and growth of radical cation- and radical anion-based crystals. 

The electrocrystallization technique has provided the most general method for the synthesis of high-quality organic molecular conductors and has given rise to the majority of organic superconductors. In an electrocrystallization experiment, a donor or an acceptor is oxidized or reduced electrochemically to form radical cations or radical anions. Crystal formation takes place at the working electrode when the radical cations/anions combine with suitable counterions that are furnished by the supporting electrolyte. 

The optical properties of conducting polymers are important to the development of an understanding of the basic electronic structure of the material. These and other problems were described in various books and review papers [90-93]. Raman spectroscopy is also an ideal tool for predicting many important electronic properties of molecular materials, organic conductors, and superconductors as well as for understanding their different physical properties, since it is a nondestructive tool, which can be used in situ and with spatial resolution as good as 1 xm. 

The salts of the tetramethylselenafulvalene (TMTSF) molecule [5] are considered as prototype materials for one-dimensional molecular conductors. The overlap between organic stacks along the transverse b direction... 

Molecular conductors at the first stage were single component systems. For example, the conduction band in KCP is a one-dimensional dz band. TTF-TCNQ has a HOMO band of TTF and a LUMO band of TCNQ, but both of them are one-dimensional pure -ir bands. Recently, however, increasing number of interesting systems which have two bands with different characters near the Fermi level have been reported for example, the DCNQI-Cu salt with tt and (itinerant) d bands, Pd(dmit)2 salts with a two-dimensional HOMO band and a one-dimensional LUMO band, the organic superconductor (TMET-STF)2BF4... 

Interest in organic conductors focused initially on the powerful it-acceptor TCNQ, 2.2 -(cyclohexadiene-1.4-diylidene)bispropanedinitrile, prepared by the DuPont group Their best conductor was MP-TCNQ, the first organic metal . MP-TCNQ was subsequently studied in detail by Heeger and coworkers whose application of solid-state techniques to organic systems has been central to the characterization of even better molecular conductors p, polymeric conductors... 

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