Perylene-TCNQ

Infrared spectroscopy of the 3 1 compound showed two types of perylene molecules, one pair of molecules associated with TCNQ and one free molecule [17]. The charge distribution was clarified by UV and near infrared spectroscopy [20]. For both perylene-TCNQ compounds a charge-transfer band was identified and also the monomer TCNQ spectrum. The typical bands of (TCNQ)J are not observed. For Per3TCNQ the UV spectrum clearly shows the existence of neutral perylene. Thus the suggestion from the crystalline structure that the flanking perylene molecules are neutral seems confirmed. Therefore, given the existence of the anion TCNQ, each of the two other perylene species share one positive charge. EPR lines are narrow for both compounds 0.61 G and 0.97 G for the 3 1 and 1 1 compounds respectively at room temperature, and 0.67 G and 0.84 G at 77 K [17]. 

Figure 8.71 Skeletal representations of face-to-face stacking in the X-ray crystal structures of some typical charge transfer complex co-crystals (a) naphthalene-TCNE, (b) skatole-trinitrobenzene, (c) perylene-fluoroanil (d) anthracene-trinitrobenzene and (e) TCNQ-TMPD.

There is a very short list of organic semiconductors with reported thin-film field-effect mobilities greater than 1 cm2 V-1 s h These include pentacene, sexithio-phene [5a], and anthradithiophene [17]. If we extend this list to include single crystal and n-type materials, we can add perylene [18], rubrene [19], copper phtha-locyanine (CuPc) [20], tetracyanoquinodimethane (TCNQ) [21], and dithiophene-tetrathiofulvalene (DT-TTF) [22] - still a short list. 

Fig. 3. Donor-acceptor projections for [Pd(bqd)2][TCNQ] and [perylene][Ni(tfd)2] (inset) showing the unsymmetrical molecular overlaps resulting from the slipped stacking arrangement (Refs. 35, 33 (inset))...

Fig. 1. Molecules that form conducting crystals described in this article. The left column contains the donors perylene (Per), tetrathiafulvalene (TTF), hexamethyl-enetetraselenafulvalene (HMTSF), tetramethyltetraselenafulvane (TMTSF), and bis(ethylenedithio)-TTF (BEDT-TTF or ET ). To the right are the acceptor molecules tetracyanoquinodimethane (TCNQ) and tetracyanonaphthalene (TNAP).

In the above radical-cation salts, the crystal contains partially oxidized donors, while the electroneutrality is achieved by the presence of closed shell anions. The structural requirements necessary for electrical conductivity in solid salts can also be met upon mixing of donors and acceptors in the resulting charge-transfer (CT) complexes both the donor and acceptor exist in a partially oxidized and reduced state, respectively. Famous examples are the conducting CT complexes formed upon mixing of perylene (112) [323. 324] and iodine or of tetrathiafulvalene (TTF, 119) as donor and 7,7,8,8-tetracyanoquinodimethane (TCNQ, 120) as acceptor [325-327] the crucial structural finding for the... 

In fact, perylene has been used to produce many cation-radical salts with simple inorganic monoanions (Br, 13 C104", or PF5" and AsF5"), 5 and charge-transfer salts with organic acceptors such as TCNQ and per-fluoroanil. Electrical conductivities at room temperature up to 1400 S.cm have been measured in some cases. On the other hand, perylene salts with magnetic anions such as FeX4 (X" = Cl, Br) and... 

Figure 13. Reflectance spectra of radical-cation salts in comparison with the one-dimensional conductors KCP and TTF TCNQ. A FA2(PF6)o.74(AsF6)o.26> B Pyi2(SbF6)7, and C Pe(PF6)L1(CH2Cl2)o.8 (where Pe is perylene).

Volume 1 contains 15 chapters on the recent developments in charge-transfer salts, fullerenes, photoconductors and molecular conductors. The topics covered include electron acceptor molecules, photo-induced intermolecular electron transfer systems, perylene based conductors, tetrachalcogenafulvalenes, metal 1,2-dichaIcogenoIenes and their conductive salts, conductive hetero-TCNQs (tetracyano-p-quinodi-methane derivatives), molecular metals and superconductors based on transition metal complexes. 

There are a large number of strong CT complexes or radical-ion salts. Many organic compounds can be oxidised to radical cations by a suitable partner (e.g. perylene) or reduced to radical anions (e.g. TCNQ ".) Figure 9.12 shows a variety of such molecules. The complexes can consist of two organic molecules, as in the case of TTF-TCNQ. One of the partners can however also be an inorganic ion, e.g. Cu+ in (2,5-Dimethyl-DCNQI)2 Cu or also Ag", Li+, Cs+, Na", K+, Rb+ (see Sect. 9.5) or PF, as in the radical-cation salt (fluoranthene)J PF (see Sect. 9.6). 

Fig. 3 gives the structures of (i) six classes of good one-electron donors [TTF and friends HMTTF and friends TTT and friends BEDT-TTF perylene, and MPc (metal phthalocyanines)] and (ii) three classes of good one-electron acceptors (TCNQ, TNAP, and DCNQI). All these donors and acceptors are flat, or almost perfectly flat, molecules this requirement seems almost self-evident if one wants to form a tight crystal lattice with good intermolecular overlap. It is conceivable that this requirement could be relaxed, but a good example of this has not been found. It is known that there are small non-planarities (e.g. in BEDT-TTF) but it is not clear whether this is critical for the solid-state properties. 

Two different complexes of perylene with 7,7, 8,8 -tetracyanoquinodimethane (TCNQ), Per-TCNQ and PeriTCNQ, have been synthesised, the first by slow evaporation of a mixed solution of the donor and acceptor [16] and the second by a diffusion procedure [17]. Although these compounds are poor semiconductors, some details about the structure and oxidation states will be briefly discussed as they appear in other complexes for which more than one stoichiometry was found. 

Figure 2.5. View along a of the crystal structure of Per3(TCNQ), showing the stacking with the repetition motive one TCNQ sandwiched between two perylene species. This stack is flanked by extra perylene molecules. (Reproduced by permission of International Union of Crystallography, from ref. 19.)...

Figure 2.6. Overlap of donor and acceptor molecules for several perylene complexes with organic molecules, (a) TCNQ (tetracyanoquinodimethane) in Per3(TCNQ). (Reproduced by permission of International Union of Crystallography, from ref. 19) in the 1 1 complex the overlap is slightly twisted relative to this one. (b) Fluoranil. (Reproduced by permission of International Union of Crystallography, from ref. 23). (c) PMDA (pyxomellitic dianhydride). (Reproduced by permission of The American Chemical Society, from ref 24) (d) HCBD (hexacyanobutadiene). (Reproduced by permission of Elsevier Science, from ref 26). (e) TCNE (tetracyanoethylene). (Reproduced by permission of International Union of Crystallography, from ref 25.)...

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