Neutral Metal 1,2-dithiolenes: Preparations, Properties and Possible Applications of Unsymmetrical in Comparison to the Symmetrical

This paper is an overview concerning the preparations and properties as well as possible applications of neutral (one component) metal 1,2-dithiolenes (and selenium analogues). The structural, chemical, electrochemical, optical and electrical behavior of these complexes depend strongly on the nature of ligand and/or the metal. The results of unsymmetrical in comparison to those of symmetrical complexes related to the properties of materials in the solid state are primarily discussed. The optical absorption spectra exhibit strong bands in the near IR spectral region ca. 700 to ca. 1950 nm. X-ray crystal structure solutions show that the complexes usually have square-planar geometry with S–S and/or M–S contacts. Some of them behave as semiconductors or conductors (metals) and are stable in air. The cyclic voltammograms at negative potentials are different from the corresponding potentials of tetrathiafulvalenes (TTFs). As a consequence, the LUMO bands occur at much lower levels than those of TTFs. Consequently, electrical measurements under conditions of field effect transistors exhibit n-type or ambipolar behavior. Illumination of materials with high power lasers exhibits non-linear optical behavior. These properties enable metal 1,2-dithiolene complexes to be classified as promising candidates for optical and electronic applications, (e.g., saturable absorbers, ambipolar inverters).

It should be noted that the choice of the oxidizing reagent (iii) or the electrooxidation conditions to obtain the neutral compounds is based on redox potentials of the corresponding anionic complexes (see below section 5).
With this method, the neutral complexes [Au(dpedt) 2 ] [79] and Cu(L11) 2 [109] have been prepared, recently.In most cases, the deprotection of the ligand precursor is made with MeONa in N 2 atmosphere and the addition of NiCl 2 at low temperature [22].In some cases, e.g., when the ligand is dmit, dmio and etdt, cation deficient (1 > x > 0) complexes are obtained, instead of neutral ones (see [6,12]).Also, the anionic salts, precursors of neutral complexes, could be obtained from several starting materials (e.g., CS 2 , CSe 2 , vinylene trithiocarbonate) through the dianionic complexes of Zn or Hg (see [6,9,12,16,30,130,131] and references therein) as in the procedures of Scheme 2. Scheme 2. Procedures for syntheses of M 1,2-DTs from several starting materials.These Zn (or Hg) based anionic complexes react with NiCl 2 , PtCl 2 (PhCN) 2 , etc. to give the corresponding mono and/or dianionic complexes of Ni, Pt, etc. [9,12,16].Also, using the compounds of Figure 7 and selenium analogues, as starting materials, the preparation of (neutral) M 1,2-DTs is possible (see [9,14,36,[48][49][50]52,63,64]).The old method, using 1,2-diketones (or benzoins) as starting materials, was applied for the preparation of a number of neutral complexes.The first step of the reactions is the conversion of diones to dithions and/or to the phosphorus thioesters with P 4 S 10 or Lawesson's reagent.The next step of the reactions is the hydrolysis and then the formation of neutral complexes using metal carbonyl or other reactive salts, e.g., NiCl 2 , PdCl 2 (PhCN) 2 .The following Scheme 4 gives an example of the preparation of a series of complexes from the corresponding 1,2-diketones [44,60,65,68,89,98,101,110,111].The unsymmetrical neutral complexes could be prepared by a ligand exchange reaction, which is affected by refluxing two different neutral complexes as in the (reversible) reaction of Scheme 5 (see [4,14]), or by refluxing a neutral and a dianionic or two equivalent monoanionic complexes, followed by oxidation.The unsymmetrical is isolated from the mixture as a second fraction of liquid column chromatography.This means that the retardation factor of the unsymmetrical is in between the factors of the corresponding symmetrical as in the cases of unsymmetrical and symmetrical TTFs [9,12,16].The rate of reaction depends on the solvent, the temperature and the nature of R and R'.A number of unsymmetrical complexes of Ni, such as [Ni(dddt) [14] have been prepared by this method.

Scheme 5. Example of a ligand exchange reaction.
A similar kind of reaction by double substitution of an anionic salt with a cationic at ca. 50 °C has been applied to the preparation of unsymmetrical complexes, as in the following Scheme 6.

S
Scheme 6. Reaction of an anionic and a cationic salt.
By this method a number of the so called push-pull complexes, such as ] and similarly with mnt instead of dmit have been prepared [80,95].
Also, neutral unsymmetrical complexes could be prepared by the so called cross-coupling method, in which ketones are used as starting materials and the neutral complexes were obtained through the corresponding anionic complexes, according to the procedure of the following Scheme 7.

Scheme 7.
Cross-coupling procedure for preparation of unsymmetrical complexes.
As in the case of the ligand exchange method, the unsymmetrical complexes are isolated by liquid column chromatography.In some cases the compounds SM1-SM6 and Se analogues as well as, the Zn dianionic compounds (Schemes 2 and 3) instead of 2, have been used to give 3.The advantage of the method is that the reactions take place at room or lower temperatures, and usually the yields are larger than those of the ligand exchange method.A number of neutral complexes, with simple ligands and extended TTF-dithiolato ligands have been prepared by this method: These are [Ni(edt) [64] and [Ni(dpedt)pddt)] [94].Neutral unsymmetrical M 1,2-DTs complexes could be obtained as main products of the reaction of Scheme 8 (see [106] and references therein).Scheme 8. Some alternative procedures for the preparation of unsymmetrical complexes.
Generally, the cross-coupling method of Scheme 7 gave the unsymmetrical complexes in moderate or low yield.However, a slight modification of the method, in which the deprotection of ligands takes place with NaBH 4 , gave the unsymmetrical complexes obtained in better yield, according to the procedure of Scheme 9 ([12,16,77,101].Scheme 9. A modified procedure of cross coupling method. In these preparations, the starting materials of the types SM1-SM6 could be used instead of 2a'-2c' or 3a'-3c'.The complexes [Ni(dpedt)(dmit)], [Ni(dpedt)(dddt)] and [Ni(dpedt)(dt)] were prepared by this method ( [9,48,49] and work in progress).Generally speaking, from a number N = 100 ligands of Tables 1-3, one can expect N(N + 1)/2 binary combinations, namely 5050 symmetrical and unsymmetrical complexes for each metal, by applying several methods of preparation.

Growth and Morphology of Crystals
Single crystals suitable for X-ray structure solutions and electrical measurements have been obtained from solutions in organic solvents.It has been found that the vapor diffusion method, using CS 2 as solvent, and the dilute diffusion method [77,115] using hexane and CS 2 as solvents give good crystals of symmetrical and unsymmetrical M 1,2-DTs [36,50,63,64,79,84].Also, crystals of neutral complexes with an extended TTF-dithiolato ligand have been obtained by electroxidation of the corresponding monoanionic or/and the dianionic complexes.(see [22,23,43,45,46]).
The crystals have a thin needle or thin platelet morphology.From a large number of crystals, only a few of them have been suitable for X-ray structure solution and/or conductivity measurements.

Instrumentation and Background
The optical absorption (OA), crystal structure (CS), electrical (EL), and electrochemical (ELC) data, described here, were obtained from measurements performed with commercial instruments, the accuracy of which was considered good enough.Consequently, the results obtained by several groups could be compared.The reported conductivity values have been obtained from measurements with the two and/or four-terminal method.
The electrical parameters (e.g., the mobility) under conditions of field-effect transistor (FET) were obtained from two-terminal measurements using the devices of Figure 8.Each of them consists of a gate (G) electrode (e.g., n + -Si), a thin dielectric layer (e.g., SiO 2 , parylene), an organic semiconductor (SM) layer (e.g., TTF, M 1,2-DT), a source (S) electrode and a drain (D) electrode (e.g., Al, Ag, TTF• TCNQ, Au) on a substrate.Figure 8 shows two types of electrical connections: with bottom-gate, bottom-contact configuration, suitable for polycrystalline deposits (Figure 8a) and with a top-gate, top-contact configuration suitable for single crystals (Figure 8b).[124,128,129,136].
The system looks like a gate/insulator/channel capacitor, where the channel is the 2-dimensional layer of semiconductor between the S and D electrodes and insulator.When the gate is biased, a current flowing between the S and D electrodes is measured as a function of length (L) and the width (W) of the channel and other parameters concerning the system.The data were used for characterization the system as electron current (n-type system) or hole current (p-type system) and ambipolar (n-type and p-type).
The NLO response of the complexes has been obtained from measurements on homemade apparatus using the Z-scan technique, which allows simultaneous determination of the sign and the magnitudes as both the real (i.e., Reχ (3) ) and the imaginary (i.e., Imχ (3) ) parts of the third-order nonlinear susceptibility χ (3) of the material in one single measurement with lasers operating in a wavelength of the visible (e.g., 532 nm) or near-IR (e.g., 1064 nm) region [35,48,99,100,111].
The magnitude of the molecular first hyperpolarizabilities of several unsymmetrical complexes was determined from electric-field induced second harmonic (EFISH) generation experiments usually at 1.9 µm [80,95,106].

Structural Properties
In the most neutral M 1,2-DTs, the ligand forms a strictly square-planar arrangement, with all S-M-S bond angles very close to 90°, but with variation in the value of intermolecular S-S contacts.The neutral complexes may exist as planar monomers or form dimers with either metal-metal bonds or metal-sulfur (selenium) bonds.Most of M 1,2-DTs were found to be crystallized in centrosymmetric space group and, up to now, only three complexes have been found to be crystallized in non centrosymmetric space group (see [7,10,15,17,20,63,64,69,94,97]).The data from selected M 1,2-DTs are described here, starting from crystals with weak intermolecular interactions.
The symmetrical complex α-[Ni(dpedt) 2 ], for example is crystallized in the centrosymmetric space group P2 1/n , where Ni Both interstacking and intrastacking S-S intermolecular distances are ca.4.5 Å, i.e., much larger than the sum of van der Waal's radii (3.7 Å), while in (Bu 4 N)[M(dmit) 2 ] these distances are 3.59-3.72Å (see [20] and references therein).This means that the complex has an almost 3D structure, with weak S-S interactions.The long axis of the needle shaped crystals is almost parallel to the a-axis.In the samples they have not been found in other habits.Some complexes of the type , and similar compounds of Au with ligands containing long chain groups [89,110]    The neutral [Ni(dmit) 2 ], studied years ago (see [20,69] and references therein), which is crystallized in plate-like crystals of monoclinic space group P2 1/a , show that the molecules stack along the [010] direction, making an angle with the normal to the molecule main plane of 48°.Within the stack, the Ni-Ni spacing is 5.302 Å and the plane to plane distance 3.562 Å.In contrast to that observed in the structure of (Bu 4 N)[Ni(dmit) 2 ], (see [20,69] and references therein), in the structure of [Ni(dmit) 2 ] there are short S-S contacts between adjacent stacks involving the thioacetone sulfur atom (ca.3.58 Å) [69].The complex [Ni(etoddt) 2 ] (Figure 11) [100] crystallizes in the triclinic space group Pl, and as in the case of [Au(dpedt) 2 ] [79] there are no S-S contacts smaller than the sum of the van der Waals radii.[36,64].Figure 12 shows a perspective view of the [Ni(pddt)(dmio)] structure and a photography of the crystal.In a layer almost parallel to the ab-plane there are S-S intermolecular contacts of 3.49 and 3.66 Å.In the other directions the distances are larger.In [Ni(tmedt)(dddt)] and [Ni(tmedt)(dmit)] these contacts are 3.59 and 3.70-3.73Å, respectively.The intermolecular contacts form a quasi-two-dimensional (q-2D) network.It was found that the largest surface of the crystal is parallel to the crystallographic ab-plane.[36] with permission of the Verlag der Zeitschrift für Naturforschung, and photograph of crystal [77].
The complex [Ni(dmeds)(dmit)] crystallizes in the triclinic space group P1 13 shows that the partial labeled plot of complex showing the intermolecular contacts [50].The structure consists of centrosymmetric dimers [7] with the inversion center sitting on the center of the   [63,64,94].Figure 14 shows the stacking of [Pd(dpedt)(dddt)] molecules in a column along the c-axis.However, there is no pseudocentro-symmetrical arrangement of molecules [20].It was found that the needle axis of the crystal is the c-axis.The shortest S-S distance in [Pd(dpedt)(dddt)] is 3.714 Å.The intermolecular contacts form a q-1D network.In  Reproduced from [63] with permission of the Verlag der Zeitschrift für Naturforschung, and photograph of crystal [77].
In [Ni(dt)(dmit)] and similar complexes with one extended TTF dithiolate ligand there are no crystallographic data.The IR vibrational spectra of [Ni(dt)(dmit)] indicate a dithioglyoxal structure (large HC-CH bond) in the dt ligand of this complex, rather than an olefinic structure (short HC-CH bonds) as in the edt ligand of [Ni(edt) 2 ] [49], and consequently, a push/pull behaviour in [Ni(dt)(dmit)] and similar compounds [48,89].It is suggested that the molecular structure has a resonance form similar to that of the push/pull complexes [95].
In the structure of [Ni(edo)(dmit)], the almost planar molecules are repeated in band to tail fashion along the c-axis.The average bond lengths of Ni-S, C-S, and C-C are 2.15, 1.70 and 1.39 Å, respectively.In the structure of [Ni(edo)(mnt)], the molecules are arranged alternately along the c-axis.The average bond lengths of Ni-S, C-S, and C-C are 2.15, 1.62 and 1.32 Å, respectively.In the crystal structure of Cu(dmdt) 2 , the molecules are not planar [25].The ligands have an arrangement similar to that of κ-phase TTFs based organic conductors and superconductors [12].
The complex [Ni(tmdt) 2 ], which is based on the extended TTF dithiolate ligand, tmdt, crystallizes into a very simple and compact structure, the tri form a close packed structure.The S-S contacts are 3.44-3.75Å.The structural features suggest that the complex is a three-dimensional (3D) anisotropic solid [46] Briefly, the examples of structures given herein and those described elsewhere (see  and references therein) show the existence of a variety of three and low dimensional networks formed from short or large M-S, S-S, etc. contacts.The nature of the individual molecules plays an important role in the unique intermolecular interactions.From the crystallographic data and the transfer integrals the electronic band structure could be calculated [18,20,51].Consequently, it is expected that there is also a wide variety in behavior, concerning electronic (see [7,18]) and other (see [2][3][4][6][7][8][9][10][11][12][13][14][15]17,) properties of M 1,2-DTs, in the solid state.

Optical Properties
The optical properties of M 1,2-DTs in the form of single crystals, polycrystalline pellets, thin film (or deposits), suspensions, composites and solutions have been reported in a number of papers.It was observed, years ago, that the optical absorption (OA) spectra of solutions of monoanionic M 1,2-DTs (M = Ni, Pd, Pt), which are paramagnetic compounds, exhibit strong bands, which span the range 700 to ca 1900 nm, depending on the nature of the metal, the ligand and the solvent (see for examples [10,18,34,63,70]).The bands have been interpretated as the HOMO-LUMO transitions or as ligand-to-ligand (LL) charge transfer (CT) transitions [34,95].These bands and the bands of the isoelectronic neutral Au 1,2-DTs have an unsymmetrical shape.After a Gaussian decovolution it has been found that the bands are dominated by two transitions: one intense at low frequency is assigned to 1b 1u →2b 2g (x-polarized) and one medium to 1a 1u →2b 2g (y-polarized), as well as some weaker ones [18,34].The experimental values of transitions, i.e., the band position and intensity of OA bands, have been found to be in reasonable agreement with the theoretically calculated ones [18,34].The splitting of the low frequency band is more discreet in the OA (and reflectance) spectra of the complexes in the solid state (see for example [29] and refs.[9][10][11] cited therein).
Figure 15 shows the polarized reflectance spectra of (Bu 4 N)[Ni(dmit) 2 ] with the wavevector of the light parallel (R ║ ) and perpendicular (R ┴ ) to the needle axis of the crystal, as well as the OA spectra of the material in CS 2 solution and a suspension in CCl 4 .The reflectance spectra show anisotropic behavior due to the low dimensional structure of the material [20,120].In parallel polarization the band is split into two sub-bands at 1100 and 1350 nm.The reflectance spectrum of a polycrystalline pellet shows the same bands, while the spectra of a suspension of the material in CCl 4 and that of the solution in CS 2 show narrow bands at ca. 1204 nm, with an unsymmetrical shape.Different results have been obtained from cation deficient complexes.The reflectance spectra of polycrystalline pellets of (Bu 4 N) x [Ni(dmit) 2 ] and (Bu 4 N) x [Pt(dmit) 2 ], obtained after oxidation of monoanionic salts with iodine or bromine [6,12,47], are shown in Figure 16.In these cation-deficient complexes (which behave as mixed-valence complexes) with x = 0.25 and x = 0.2, respectively, the low frequency band-position, -intensity, and -shape depend on the x-value.When x→0, the (neutral) complex exhibits a narrow band, blue shifted from that of monoanionic (x = 1), while for intermediate values of x, the complex exhibits a new band at lower frequencies.The reflectance spectra of Figure 16 seem to be the superposition of those of a mixture of cation-deficient and neutral complexes.Reproduced from [54] with permission from Elsevier.
The OA spectra of neutral complexes exhibit strong bands, the position of which depends strongly on the nature of the metal, the ligand the solvent, etc.The OA band positions (and intensities) of unsymmetrical complexes (AB) in solutions occur in between those of the corresponding symmetrical complexes (AA, BB).The characteristic OA wavelength (maximum or onset) is given by the equation: (1) Figure 17 shows the experimentally observed spectra of [Ni(edt) 2 ], [Ni(edt)(dddt)] and [Ni(dddt) 2 ] solutions in MeOH [27].The observed OA band of the unsymmetrical [Ni(edt)(dddt)] occurs at 864 nm while, the calculated from those of symmetrical and Equation 1 was found at 867 nm.
The OA band position of neutral [Ni(dmit) 2 ] in solution has been calculated from the positions of the corresponding bands of some symmetrical Ni(L) 2 and unsymmetrical [Ni(L)(dmit)] complexes using Equation 1.It has been found to be ca.1070 nm in CS 2 and 1043 nm in CH 3 CN.The calculated value for [Ni(dmio) 2 ] is 932 nm in CS 2 .The OA spectra of some Au-based complexes and the OA band position of Pd-based complexes in CS 2 are shown in Figure 18.It can be seen that the Au-materials exhibit wide transparent spectral regions, i.e., from ca. 500 nm to ca. 1500 nm.It was found that the spectra of [Au(tmedt)(dddt)] and [Au(tmedt) 2 ] in CS 2 exhibit OA bands at ca. 1732 and 1500 nm, respectively.Generally, the OA bands of neutral Au 1,2-DTs occur at lower frequencies, even, than those of monoanionic salts of Ni, Pd and Pt, which are isoelectronic (see Figures 17 and 18 for examples) [63,70].The OA and reflectance bands of single crystals of [Au(bdt) 2 ] occur close to 2000 nm [29].[63,70].
The OA spectra of unsymmetrical M 1,2-DTs with a push-pull behavior, such as M(Bz 2 pipdt)(mnt) [95,106], exhibit broad bands at ca. 751 (660sh), 651 (560sh) and 685 nm in DMF for M = Ni, Pd, and Pt, respectively, with low to medium molar absorption coefficient and negative solvatochromism [106].The OA spectra occur between the spectra of the corresponding symmetrical (if they exist), but do not obey Equation 1. Similar results have been obtained from In solution four peaks are present, and that at 737 nm may be tentatively assigned to the HOMO-LUMO transition.This band shows negative solvochromism and occurs at higher energy, when compared to the one in the corresponding Ni compound (910 and 965 nm respectively, in CHCl 3 ).The position, intensity and shape of the bands in the solid state (single crystals, polycrystalline pellets, thin deposits, etc.) depend on the structural feature of the complex.If the S-S contacts, for example, are weak as in [Au(dpedt) 2 ], the position of the OA band of a thin film occurs close to that of the solutions, while, if the S-S contacts are strong as in the [Au(dddt) 2 ] and [Au(bdt) 2 ] [29], the OA bands occur at lower frequencies.
As in the OA spectra of charge transfer (CT) and mixed valence (MV) compounds (see [117,120]), the OA onsets or the E g values of single component species (i), including small particles of M 1,2-DTs could be obtained from the following Equation 2, where ΔE is the HOMO/LUMO gap, (t 2 ) i and (t 1 ) i the corresponding intermolecular transfer integrals.These integrals could be obtained from band structure calculations based on crystallographic data.
Figure 19 shows the schematic presentation of bands and gap (E g ) i formation from a one dimensional array of neutral molecules with HOMO/LUMO (H, L) gap of ΔE, where (t 1 ) i and (t 2 ) i are the corresponding transfer integrals, for any species (i) concerning bulk (large t) and small particles (small t).If the structure results from dimers, the energy gap is lower than the corresponding levels of dimers [7,21,51].Figure 20 shows the OA spectra of thin deposits of [Au(dpedt) 2 ], [Au(dpedt)(dddt)] and [Au(tmedt) 2 ] obtained by rubbing the complexes on quartz plates.Because of the weak intermolecular interactions in the solid state [63,79], the bands occur almost at the same positions as those of the solutions, but are broader, and the fine structure is not clear.The same OA spectra have been obtained from suspensions of the complex in CCl 4 or toluene and from composites of the complex in a polymethylmethacrylate matrix.However, the spectra of films, obtained by a spin-coating technique, showed a fine structure [79], which could be attributed to several molecular transitions [34].However, in the cases of [Ni(etdt)  [48] the differences between the spectra of deposits and those of solutions are larger, because of the stronger intermolecular interactions in the solid state.Figure 21 shows the OA spectra of [Ni(etdt) 2 ] thin deposits before (a) and after (b) rubbing as well as the spectrum of a solution in CS 2 (c), for comparison.Figure 22 shows the spectra of thin deposits of [Ni(dmdt)(dmio)] and [Ni(dmstfdt)(dmio)].It can be seen again that after extensive rubbing, the bands occur close to those of the solutions, which indicates weak interactions [134].4).The reflectance spectrum of [Pd(Me 2 pipdt)(dmit)], except for the main peak at ca. 737 nm, shows an additional peak which could be attributed to an intermolecular transition involving the dmit moieties interacting through short S-S intermolecular contacts [88].Table 4 gives the OA band positions and onsets in a number of selected M 1,2-DTs in solutions (CS 2 , CH 2 Cl 2 ) and in the solid state, which give information on the LUMO-HOMO gap and the energy gap of material in the solid state (E op g = E op onset ), respectively.One can see that this last parameter varies from 0.62 to ca. 1 eV.The values of OA onsets were obtained from the OA spectra of thin deposits.This means that the corresponding values from bulk materials (i.e., single crystals) should be smaller.On the other hand, the values obtained from very thin deposits (and suspensions) will be larger and close to those of solutions.In these cases the OA band positions of unsymmetrical and corresponding symmetrical obey Equation 1.In the push-pull complexes the E g -value is ca.1.5 eV (830 nm), while in complexes with extended-TTF dithiolato ligands this is small (<< 0.6 eV).
The spectra of  show that the complexes have significant absorption maximum near the telecommunication laser wavelengths (i.e., 1300 and 1550 nm).This is a requirement, which dyes have to meet to be usable as optical filters, saturable absorbers, Q-switches and mode-lockers (see for example [10,24,31,76,121]).Because of the wide transparent range, the band position, shape and intensity as well as the stability in air, illumination of solutions of complexes with high power lasers exhibit some other significant non-linear optical (NLO) properties [20,35,48,58,63,64,67,80,94,95,99,100,103,106,111].For example, using solutions of symmetrical or unsymmetrical M 1,2-DTs the second hyperpolarizability γ has been observed to be as large as 10 −27 esu [35].Also, from solutions of unsymmetrical complexes of the type push-pull, a large value in the (negative) second polarizability (β) has been observed [106].However, for similar observations concerning second harmonic generation from solid materials there is no information.Although, there are known complexes with noncentrosymmetrical structures [63,64,94], in these structures the molecules do not have a pseudo-centrosymmetric arrangement [20].

Chemical and Electrochemical Properties
The chemical and electrochemical reactivities, mainly of homoleptic M 1,2-DTs, reported before 2004, have been summarized in [8,19,24].For results concerning new ligands and structures see .Also, for a wide range of electrochemical aspects of molecular solids see [122].Examples, concerning chemical reactivity, are ligand exchange reactions as well as ligand addition and substitution reactions.Methods of preparations of neutral and unsymmetrical complexes, based on these kinds of reactions, have been already described in section 2. The selection of reactions as well as the selection of oxidizing, or reducing agents for the preparation of stable (in air) complexes are related to the electrochemical (redox properties) of the materials.The cyclic voltametry (CV) measurements of M 1,2-DTs provide useful electrochemical parameters.These are the half wave redox potentials, defined as E 1/2 (i) = [Eox(i) + Ered(i)]/2, where Eox(i) and Ered(i) are the oxidation and reduction potentials of several redox couples (i = 2−/1−, 1−/0, etc.), respectively.These parameters play an important role in the formation, stability and other properties of M 1,2-DTs and their salts.Figure 24 shows a schematic presentation of a voltammogram of a M 1,2-DT and that of a donor molecule, e.g., of a TTF compound, for comparison.One can see that there are some common features at their positive potentials, which lead to the formation of cationic salts in both cases [16,21].The couple E 1/2 (0/x+) (0 < x ≤ 1) is rare [79,95].Instead of which, an irreversible wave E(0/x+) is observed However, in the negative potentials there are considerable differences: the couples concerning 1−/0 (or 2−/1−) states are absent or rare in the voltammograms of donor molecules [12,16].These couples are observed in the voltammograms of acceptors, e.g., of TCNQ (0.17, −0.37 V vs. SCE), and TCNQ(CN) 2 (1.31, 0.51 V vs. SCE) [122][123][124].In the literature, it can be found that there are a wide variety of redox values obtained from M 1,2-DTs under different conditions and electrochemical techniques.To help compare studies employing different reference electrode-couples, the redox potential values are converted versus a common reference electrode, the saturated calomel electrode (SCE), which is approximately equivalent to the Ag/AgCl electrode, according to the following relationship [19]: Ag/AgCl (in satur.KCl solution) = SCE (in satur.KCl solution) + 0.045 = Fc/Fc + (in 0.2 M LiClO 4 /MeCN) + 0.352 = Ag/Ag + + 0.604 = NHE − 0.196 (V).
The conversion factors depend on the solvent and the supporting electrolyte [19,42,77].Tables 5-8 give the E 1/2 (2−/1−), E 1/2 (1−/0) and E(0/x+) (in V versus Ag/AgCl) as well as other electrochemical and OA data observed at room temperature from a number of selected M 1,2-DTs and selenium analogues.The difference E(0/x+) − E 1/2 (1−/0) ≈ E onset ox − E onset rd = ΔE is a measure of the electronic energy gap of the materials in solutions or suspensions [45,46,63,64,77,79].One can see that the results from electrochemical measurements (of solutions, E g elc ) are in agreement with the optical absorption E onset opt of solutions in CS 2 , while there is a large deviation from the data in the solid state (OA max and E onset opt of deposits or pellets), as well as from the electrical measurements (E g el ) (see below).This is a consequence of the variety of intermolecular interactions arising from the S-S contacts, in the solid state.Namely, the difference is larger in cases of complexes with strong S-S interactions (Table 8).From these data, the HOMO and LUMO energy levels could be calculated via the following equations (see [47][48][49][50]63,64,79] and references therein).These are applied when the E onset ox and E onset rd are given versus Ag/AgCl as reference electrode.If one of the E onset is observed by CV, this value and the difference E onset ox − E onset rd ≈ E onset op obtained from the OA spectra of solutions, suspensions or very thin films could be used for the calculation of HOMO and LUMO energy values.However, HOMO energy values could be estimated from ultraviolet photoelectron spectra and the LUMO energy values from the inverse photoelectron spectra, in the solid state [125].The electrochemical parameters of unsymmetrical M 1,2-DTs (AB) occur in between those of the corresponding symmetrical (AA, BB) as in the case of the OA parameters.If the couples are reversible, the redox values obey the following equation as in the case of TTFs [16].
The calculated values of several unsymmetrical complexes are given in Tables 5-8 in comparison to the observed ones.It can be seen that the values of LUMO and HOMO levels vary from ca. 4.25 to 4.85 and from ca. 5.17 to 5.68 eV, respectively.It has been demonstrated that these compounds are stable in air (H 2 O + O 2 ), because the LUMO value is larger than 4 eV (see [47] and references therein).This finding is almost equivalent with the suggestion that the species (such as neutral M 1,2-DTs) with E 1/2 < 0.00 V (vs.SCE) are susceptible to air oxidation in solution (H 2 O + O 2 ), while species with E 1/2 > 0.00 V are stable in air.A number of neutral M 1,2-DTs were prepared by oxidation of the corresponding anionic complexes.In these reactions and others similarly involving oxidation, proper selection of the oxidizing agent is crucial.However, the knowledge of the redox values of the species could be a guide to the synthesis of new compounds [14,15,[48][49][50]64,77]. For example, if the species has E 1/2 < 0.2 V the oxidation can be obtained with I 2 , otherwise, a stronger oxidant should be used [19].Also, if a monoanionic M 1,2-DT has two reversible redox processes between the dianions and neutral complexes at ca. −0.7 and 0 V (vs.Ag/AgCl) as in [Ni(dddt) 2 ] further oxidation is associated with the formation of partially oxidized compounds {M 1,2DT] x+ .If these values are −0.23 and 0.26 V, as in [Ni(dmit) 2 ], the oxidation gives cation deficient salts such as [Ni(dmit) 2 ] x− .In the cases of complexes with extended TTF dithioleto ligands these values are ca.−0.4 and 0 V, respectively, the oxidation gives neutral complexes.In other words, the knowledge of redox values is a guide for fabrication of several stable (in air) electronic and optoelectronic devices (see below).
Recently, the results of electrical measurements on M 1,2-DTs under the conditions of field effect transistor (FET) have been reported [37,39,47,56,57,65,93,113].These results are discussed here and compared with those obtained from donors (e.g., TTFs) or acceptors (e.g., TCNQ) [51,66,126].Mainly, electrical circuits like those shown in Figure 8 suitable for measurements on thin deposits or single crystals of M 1,2-DTs have been used.The observed currents are due to the semiconductor-insulator interface which is a thin (2-dimensional) layer in the semiconductor size, the channel of the semiconductor; the rest of the material is inert and does not contribute drastically to this kind of current.As an example, the process of measurement on [Ni(dpedt)(dmit)] thin film is described [47].Si wafers of (p-type) as a gate electrode, with 200 nm SiO 2 layer at the gate electrode, gold source and drain electrodes were defined in a two terminal bottom contact configuration (Figure 8a), with channel width (W) of 10 mm and length (L) of 10 µm.A 10 nm titanium was used as an adhesion interlayer for the gold on SiO 2 .The SiO 2 was treated with the primar hexamethyldisilazane prior to semiconductor deposition, in order to passivate its surface.Films of [Ni(dpedt)(dmit)] were then drop cast on top from a solution in CH 2 Cl 2 .Under appropriate biasing conditions strong hole and electron accumulation has been observed.In particular, for negative drain (V D ) and gate (V G ) voltages, accumulation of holes is evident, while for positive V D and V G , electron accumulation has been observed.Figure 28a shows the output current-voltage characteristics and Figure 28b shows the transfer characteristics at various drain voltages on a channel of [Ni(dpedt)(dmit)] [47].From the curves and the Equations 7 and 8, the hole and electron field effect mobilities (µ h , µ e ) have been calculated.
Where C i is the capacitance per unit area of the insulator layer (given by C i = εε ο /d, with d the thickness of the layer, ε is the dielectric constant of the insulator and ε ο the permittivity in vacuum).
From the calculations the maximum hole and electron mobilities have been found to be 1 × 10 −4 and 3 × 10 −4 cm 2 /Vs, respectively.The on-off current ratio was calculated from the data of the transfer characteristics and Equation 9 assuming that the semiconductor film of thickness t has a uniform (bulk) conductivity, σ, and uniform mobility, μ, the ratio is of the order 10 2 -10 3 .
For linear regime (7) For the saturated regime (8) Reproduced with permission from [47] by permission of the American Institute of Physics.
Measurements on (NO) x [Ni(dmit) 2 ] (1 > x ≥ 0) films showed that the behaviour is of n-type in any x value [25].When x = 1 (monoanionic) the mobility (μ e ) is small 2.4 × 10 −5 , as in most cases of M 1,2-DTs.However after doping [25] (oxidation) with iodine, the material became cation deficient (1 > x > 0) or neutral (x = 0) as was discussed above for (Bu 4 N) x [Ni(dmit) 2 ] (see [6]).In this last case (1 > x > 0) the mobility in a FET device found a much larger μ e = 0.18 cm 2 /Vs [25].The results of measurements from the reported M 1,2-DTs are summarized in Table 10.Similar measurements have been made also on single crystals of M 1,2-DTs with electrical circuit connections as those of Figure 8b (see [77,79,105,[135][136][137][138][139] and references therein]).Figure 29      Figure 30 shows the obtained characteristics.In this case, the material (No. 1) behaves as an n-type semiconductor with a mobility of 0.02 cm 2 /Vs and I on /I off of 1.4.On recrystallization of compounds the mobility values were increased.Figure 31  It can be seen that, when TTF-TCNQ electrodes are used (work function = 4.6 eV) [135] the complex [Ni(tmedt)(dddt)] exhibits n-type behaviour with a mobility μ e = 0.045 cm 2 /Vs, while using Au electrodes (work function = 4.7 eV) the complex exhibits ambipolar behaviour with μ e = μ h = 0.02 cm 2 /Vs, because of the decreasing hole barrier (see below).Also the crystal of [Ni(dpedt)(dddt)] with TTF-TCNQ electrodes exhibits ambipolar behaviour (Figure 31) with mobility values μ e = 1 × 10 −4 and μ h = 3 × 10 −4 cm 2 /Vs; and I on /I off of 4. These low values of I on /I off are attributed to small band gap values or to mid gap levels [65] of materials, in the solid state.The results on a single crystal are summarized in Table 10.However, experiments performed on crystals in other directions, i.e., perpedicular to the needle axes of the crystals are needed.The results on single crystals of Ni 1,2-DTs are similar to those reported for organic single crystals with p-type behaviour [23,24], as well as with n-type behaviour [135,139].These kind of diagrams could be useful guides for selection of appropriate components in order to obtain an optimum value of the mobility in FET circuits.[7,51,66,107,[127][128][129].
In each case of metal-semiconductor and semiconductor-insulator combinations, in order to obtain high performance FETs, alignment of LUMO and/or HOMO levels with the work function of the metal (electrode) is necessary.For example, in the case of a TTF derivative (which is a donor molecule) with a HOMO-LUMO gap of ca.2.65 eV (Figure 32a) the HOMO level (4.85 eV below the vacuum level) could be in alignment with the work function of an Au electrode (which at ambient conditions is contaminated with O and C) and has a value ca.4.7 eV (Figure 32b) [125].In other words, in Au-TTF couple the hole barrier injection is small (Φ h = 0.15 eV) and holes can be injected from the HOMO level of the semiconductor to the Au electrode (p-type channel).For these kinds of compounds (donors) the electron injection barrier is large (Φ e = 2.5 eV) and injection of electrons from the Fermi level is not possible with Au electrodes.It could be possible with Sm electrodes (Φ e = 0.5 eV), but this system is not stable in air [47].On the other hand, in TCNQ and other acceptor molecules entrapped with dicyanomethylene groups, the LUMO level occurs close to 4.3-4.4eV and the HOMO level at 5.48-5.56eV below the vacuum level, with a gap ≥1.1 eV on Au exhibit hole mobility μ h = 0.014 cm 2 /Vs [126].However, in the case of M 1,2-DTs the HOMO-LUMO gap is smaller (ca.0.7 eV) and the work function of Au could be aligned with both HOMO and LUMO levels (Figure 32c-e).The Φ h and Φ e are small and holes (p-type channel) or electrons (n-type channel) or both of them (ambipolar) could be injected.In some cases, the alignment is controlled by changing the material-semiconductor interface barrier (Δ), which leads to the change of Φ h and Φ e of FET [127][128][129].It has been demonstrated that self-assembled monolayers (SAMs), e.g., alkanethiols and perfluorinated alkanethiols have opposite dipoles and could be used to, respectively, decrease and increase the work function of metals (i.e., shift the vacuum energy levels as in Figure 32c,d).This method (of SAMs) could be applied in the case of clean Au, Mo and other electrodes in which the work function is close to 5.0 eV [125].Also, the additional edge groups in the ligands of M 1,2-DTs play more or less a role similar to that of SAMs.Moreover, during recent years, measurements under condition of FET have been performed on CT complexes of the type TTF-TCNQ [66,135,136].However, according to our knowledge, there is no information for similar measurements on CT of the type TTF-M 1,2-DTs [15].However, it is expected there is an alignment of the energy gap levels of these kinds of CT complexes with the work function of Au and other metals, as is illustrated in Figure 33.One can see that both Φ e and Φ h values could be different from those of the corresponding M 1,2-DTs.Of course, for dimerized M 1,2-DTs the diagrams are different.Although, the mobility observed up to now from FETs based on M 1,2-DTs is small in comparison to that of FETs on porous silicon (μ h = 1 cm 2 /Vs), it has been demonstrated that the ambipolar semiconductors [Ni(dpedt)(dmit)] and [Ni(L10a) 2 ] (with μ e = μ h ≈ 10 −3 cm 2 /Vs) could be used for fabrication inverters, which are the main components of complementary-like circuits [37,47].Figure 34 shows the quasi static transfer curves obtained by integrating two FETs based on [Ni(dpedt)(dmit)] (inset of Figure 33), for V IN and V DD being positively biased.Similar curves have been obtained for V IN and V DD being negatively biased.

Conclusions
This paper is an overview, of the preparations and properties of neutral metal 1,2-dithiolenes (and selenium analogues).It has been shown that, using CS 2 , CSe 2 , 1,3-dithio-2 thiones, vinylene trithiocarbonate, 1,2 diketones, etc. as starting materials, a plethora of anionic metal 1,2-dithiolenes (and selenium analogues) could be prepared by chemical and electrochemical methods.From the anionic complexes the corresponding neutral complexes could be obtained by chemical oxidation and/or electro-oxidation.In a large number of them, the HOMO-levels are close to 4.4 eV and LUMO-levels close to 5.4 eV, forming a gap of ca. 1 eV.These complexes are stable (in air) semiconductors and found to be useful candidate materials for further investigation with possible applications.Their solutions, suspensions or thin deposits exhibit strong OA bands in the near-IR spectral region and have NLO properties.They can be used as saturable absorbers for telecommunication wavelength lasers (e.g., 1550 nm).Channels in FETs, made from these kinds of materials, in single crystal or thin film forms, exhibit n-type (electron) or ambipolar (electron and hole) mobilities.Some of them could be used for fabrication of inverters for complementary-like circuits.It is expected that in the near future new tailored systems, based on M-1,2 DTs with optimum properties, suitable for applications will be synthesized.

Figure 4 .
Figure 4. General molecular formula of TTF compounds.

Figure 7 .Scheme 3 .
Figure 7. Molecular formulas of a variety of starting materials.

Figure 8 .
Figure 8. Electrical connections and circuits for measurements under conditions of field-effect transistor (FET): (a) The bottom-gate, bottom-contact configuration (suitable e.g., for polycrystalline semiconducting films) and (b) the top-gate, top-contact configuration (suitable e.g., for single crystal semiconductors)[124,128,129,136].

Figure 9 .
Figure 9. Packing diagram of a crystal of [Au(dpedt) 2 ].Reproduced from [79] with permission from Elsevier, and photograph of a crystal.

Figure 10 .
Figure 10.Crystal structure of [Ni(edo) 2 ].Reproduced from [14] with permission of the Royal Society of Chemistry.
Ni 2 Se 2 core.The closest Ni-S and Ni-Se bond lengths in the coordination sphere are ca.2.2 and 2.3 Å respectively, while the longer Ni-Se bond (2.56 Å) is responsible for the formation of the dimers.The closest intramolecular Ni-Ni distance is ca.3.06 Å.It was found that the largest surface of the rectangular crystals is almost parallel to the crystallographic ab-plane.There are S-S and S-Se intermolecular contacts of 3.594 and 3.560 Å, respectively, slightly smaller than the sums of van der Waals radii (3.70 and 3.82 Å, respectively), which give rise to the formation of layers almost parallel to the ab-plane.In other directions the distances are larger.This indicates a quasi two dimensional behavior of the material.

Figure 21 .Figure 22 .
Figure 21.OA spectra of thin deposits of [Ni(etdt) 2 ] before (a) and after (b) rubbing on a quartz plate and the OA band of a solution in CS 2 , for comparison (c) [77].

Figure 26 .
Figure 26.Plots of resistivity versus the inverse temperature for a single crystal of Ni(pddt)(dmio) with current approximately parallel (||) and perpendicular ( ┴ ) to the ab-plane.Reproduced from[36] by permission of the Verlag der Zeitschrift für Naturforschung.

Figure 27 .
Figure 27.Plots of resistivity versus the inverse temperature for a single crystal of [Ni(dmeds)(dmit)] with current approximately parallel (||) and perpendicular ( ┴ ) to the ab-plane, before (I) and after (II) applying a number of heating/cooling cycles.Reproduced from [36] by permission of the Verlag der Zeitschrift für Naturforschung.

Figure 28 .
Figure 28.Output current-voltage characteristics (a) and transfer characteristics at various drain voltages (b) obtained from an ambipolar channel based on [Ni(dpedt)(dmit)].Reproduced with permission from[47] by permission of the American Institute of Physics.
Figure30shows the obtained characteristics.In this case, the material (No. 1) behaves as an n-type semiconductor with a mobility of 0.02 cm 2 /Vs and I on /I off of 1.4.On recrystallization of compounds the mobility values were increased.Figure31shows the obtained characteristics from a single crystal (No. 2) of recrystallized [Ni(tmedt)(dddt)] and from a single crystal of [Ni(dpedt)(dddt)].It can be seen that, when TTF-TCNQ electrodes are used (work function = 4.6 eV)[135] the complex [Ni(tmedt)(dddt)] exhibits n-type behaviour with a mobility μ e = 0.045 cm 2 /Vs, while using Au electrodes (work function = 4.7 eV) the complex exhibits ambipolar behaviour with μ e = μ h = 0.02 cm 2 /Vs, because of the decreasing hole barrier (see below).Also the crystal of [Ni(dpedt)(dddt)] with TTF-TCNQ electrodes exhibits ambipolar behaviour (Figure31) with mobility values μ e = 1 × 10 −4 and μ h = 3 × 10 −4 cm 2 /Vs; and I on /I off of 4. These low values of I on /I off are attributed to small band gap values or to mid gap levels[65] of materials, in the solid state.The results on a single crystal are summarized in Table10.However, experiments performed on crystals in other directions, i.e., perpedicular to the needle axes of the crystals are needed.The results on single crystals of Ni 1,2-DTs are similar to those reported for organic single crystals with p-type behaviour[23,24], as well as with n-type behaviour[135,139].

Figure 33 .
Figure 33.Energy level diagram for a possible alignment of Au with the electronic levels of CT of the type TTF-M 1,2-DT [7,51,66,107,127-129].
OA band position of thin deposits obtained by drop-casting (a); grinding rubbing (b) and spin-coating (c) techniques.Optical onset values from the spectra of solutions in CS 2 are given in parenthses.
* Observed vs. Ag/AgCl.Calculated values from equation E onset (AB) = [E onset (AA) + E onset (BB)]/2 as well as the corresponding E LUMO and E HOMO values are given in parentheses; ** E onset op observed from thin deposits.rd ) 1 eV, considering x = 1, are given in brackets.

Table 7 .
[63,79]ted E LUMO and E HOMO values and the corresponding electrochemical and optical absorption data[63,79].The E LUMO values were calculated from the reduction onset values (−E LUMO= 4.4 + E onset rd ).The E HOMO values were calculated from the oxidation onset values (−E HOMO = 4.4 + E onset ox ).Also, E HOMO values calculated from electrochemical and OA data (E HOMO = E LUMO − E onset * opt) are given in brackets.