TTF (tetrathiafulvalene), shown in Figure 1.7, is one of the most studied molecules of the last 40 years [14, 15] (cf. Figure 1.8), because of its physical and chemical properties.
It was first studied as a component of the first organic superconductor [16] and more recently for some of its chemical and photochemical properties. Indeed, it is a very well known luminescence quencher and electron donor.
Figure 1.7: Tetrathiafulvalene (TTF)
The studies and the knowledge thus gained about TTF play an important role in numerous research fields, such as solid state physics, photochemistry and photophysics, chemistry and biology.
Figure 1.8: Graph representing the number of publications on and with TTF between 1920 and 2010
!
!
1.3.2. General aspects 1.3.2.1. History
Investigations on TTF began some 80 years ago with a report by Hurtley and Smiles.[17,
18] However, articles on TTF and TTF derivatives really began to appear regularly around 1970,[14] when Wudl et al. found that TTF was an “unusually stable organic radical cation”. Moreover, TTF in its neutral form is an orange organic molecule and readily loses electrons in the presence of oxidizing agents (or by electrochemistry) to form the purple radical cation,[14] and then a yellow dication. Both cationic species are aromatic in the Hückel sense and are quite stable. So, TTF has good electron donor properties.
Figure 1.9: Oxidation of TTF gives rise to stable cationic species[18]
In 1972, the electrical conductivity of TTF was established, also by Wudl et al.,[20] and in 1973, the first “organic metal” TTF-TCNQ (tetracyanoquinodimethane) was discovered.[16] TCNQ is an electron acceptor and gives a black crystal when precipitated with TTF. A partial charge transfer is observed between TTF and TCNQ, resulting in electrical conductance several orders of magnitude higher than for other organic compounds. The band structure of TTF-TCNQ showed some features usually found in metals, so it was called an “organic metal”.
1.3.2.2. Structure and chemical bonding
Since the discovery of TTF, a lot of investigations and studies were performed on its synthesis in order to make efficient derivatives for better superconductors, !-donors or sensors. Despite the large number of studies on TTF, very few concerned its structure.
Indeed, TTF was assumed to have a D2h symmetry, that is, to be planar. In 1994, a gas phase electron diffraction study showed that the planar structure was not the best according the experiments but that a non-planar boat structure fits better.[21]
In fact, it was realized that TTF is a very flexible molecule and can have different conformations depending of the type of interactions it encounters. In 1999, Katan published an article discussing the structure of TTF as a function of its oxidation state.[22] It appears that in principle the planar confirmation should be more stable than the effectively encountered boat-like conformation (C2v). However, the difference in energy between the planar and the boat structure is quite small: 0.02 eV for DFT calculations at the LDA level and 0.04eV with a BP gradient correction.[22] The results of these calculations are summarized in Table 1.1 (cf. also Figure 1.10). This small energy
intermolecular interactions, TTF can adopt a planar or a boat conformation. Moreover, according to the calculations, the TTF radical cation seems to be much more stable in the planar conformation.
Figure 1.10: Definition of the atomic numbering for the atoms of the TTF molecule[22]
Table 1.1: Calculated Bond Lengths (Å) and Angles (deg) for the TTF Molecule in Comparison with Gas Phase Electron Diffraction Resultsa. This table is taken from [22].
a & corresponds to the dihedral angle between SCS and SCCSplanes.
b Spin-polarized calculations lead to the same values.
!
A model for electron transfer processes from a TTF molecule in a charge transfer complex was obtained by ab-initio calculations.[22] The results of the first principles calculations for a planar monomer in different oxidation state are given in Table 1.2.
Figure 1.11: Isodensity representation of the HOMO of the TTF molecule.[22]
Table 1.2: PAW-LDA Calculated Total Energy and Energy of the HOMO State for Different Total or Spin-Up and Spin-Down Occupations of the HOMO State(s)a. This table is taken from [22] .
a Deduced values for the Coulomb repulsion U and the spin-splitting parameter J.
All energies are given in eV.
The ionization energy of TTF can be estimated by two different ways: by the difference in total energy for the neutral molecule and the radical cation
Eionization = Etot(TTF+ ) – Etot(TTF),
or by the mean values of the HOMO energies for TTF and TTF+ . The same result is found for both methods, that is, 6.3 eV.[22] This value is in good agreement with those obtained by photoelectron spectroscopy in the gas phase (6.7 eV [23]).
TTF being well known for its ability to donate electrons, reduction and oxidation are
provides a direct measure of the Gibb#s free energy of adding or removing electrons from TTF. Experimentally, TTF has two oxidation waves: E11/2
= 0.34 V for the couple TTF/TTF+ and E21/2
= 0.78 V for the couple TTF+/TTF2+, vs Ag/AgCl in acetonitrile.
Moreover, a lot of TTF based compounds are studied in solution and it is known that the solvent has a big influence on some processes as demonstrated by Marcus in his treatment of electron transfer reactions.[24-26] Ab-initio molecular dynamic can give an idea of the solvent effect for electron transfer in TTF [27] and is illustrated with acetonitrile in Figure 1.12. For TTF in the neutral form, a preferential orientation of the acetonitrile methyl group toward the sulphur atom with the nitrogen pointing away from it was observed.[27] This preference is reduced near the carbon atoms. In contrast, the average orientation of acetonitrile is reversed upon oxidation of TTF (the nitrogen is turned toward the sulphur atom). This is consistent with the fact that the largest amount of spin density and therefore electron density is located near the sulphur atoms.
Figure 1.12: «Snapshots of TTF in solution […], with only the closest solvent molecules visualized. The spin density (upper panels) and the difference electron density (lower panels, see text for the definition) are shown at contour levels of +0.002 (green) and '0.002 (pink) au; i.e., green implies an increase of electron density in the reduced state. The difference density illustrates the electronic polarization induced by the cation, in particular of the C'H bonds and first solvation shell. For each solute, the molecular configuration in the upper panel is identical to the configuration in the lower panel. However, the TTF configuration is taken from a trajectory of the neutral molecule […]. The images are generated with VMD». Figure taken from [27].
1.3.3. TTF as an intramolecular !-electron donor
!
TTF and its derivatives have been extensively studied for more than 35 years as !-electron donors in intermolecular charge transfer materials. The most famous example for that is given by the number of studies on TTF-TCNQ over the past three decades.[16,
28-33]
The majority of the applications of TTF are determined by its donor abilities, which are the result of the high-lying HOMO. Nevertheless, the potential of TTF as donor in an intramolecular sense has only recently been developed. Indeed, as Bryce mentioned in one of his reviews,[34] TTF has received attention as a donor moiety in intramolecular charge transfer (ICT) systems principally only over the last decade because of the synthetic challenge of obtaining functionalized TTFs in reasonable quantities.[35]
Progress of the last ten years in the branches of TTF chemistry has changed this situation and TTF can now be synthesized in 20 g batches from readily available starting materials.[34] Fanhaenal et al have reviewed different synthetic methods [36-38]
which are summarized in the following scheme shown in Figure 1.13:[36]
Figure 1.13: Scheme of synthetic methods for construction of the TTF skeleton.
This is taken from [36].
TTF as an electron donor evokes the topic of intramolecular donor-acceptor (D-A) molecules. This includes different fields like molecular electronic devices,[39, 40] organic metals,[41] chromophores for dyes[42] and non linear optics,[43, 44] and also excited state energy and electron transfer processes,[45, 46] and theoretical aspects of charge transport at the molecular level, conjugation and aromaticity.[47] Different applications are summarized in Figure 1.14.
Figure 1.14: Different fields of application. This Figure is taken from [35].
The concept of D-A molecules is linked with electron transfer processes. They depend principally on the energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels and the orbital interactions. An enhanced redox activity is observed for donor and acceptor molecules upon photo-excitation. Indeed, donor molecule D becomes D* when excited and has an electron transferred to a higher-lying LUMO. As a result it becomes a better reducing agent. In the same way, an acceptor A* is a stronger oxidizing agent than A because the hole created by photo-excitation in the ground state HOMO increases the electron affinity of the excited state.[48]
TCNQ, gave rise to the idea of extending the !-conjugated system of TTF in order to decrease the Coulomb repulsion between the charges on the molecules in the solid state and thus to improve the intermolecular interactions.
The first !-extended derivative of TTF, dibenzo-TTF, was, in fact, synthesized 50 years before its parent molecule.[17] However, its electron-donor properties were studied much later. The benzene rings have an electron withdrawing influence, which increases the oxidation potential by 250 mV compared to TTF.[49] Numerous homologues of dibenzo-TTF were then synthesized, including polymers.
Wudl et. al. presented an exhaustive review on that subject in 2004.[50] The authors notice that molecules with low HOMO/LUMO gap are of particular importance due to their ability to easily donate or accept an electron. Some molecules, called amphoteric compounds, can even act as an electron donor and as an electron acceptor at the same time, forming stable redox states within the same molecule. Molecules with a controlled HOMO/LUMO gap are the prime targets for electronic applications at the single molecule level. Two strategies can be applied in order to obtain a lower HOMO/LUMO gap:[50] (i) by extending the !-conjugation in the molecule and (ii) by the construction of covalent D-A compounds (D being a !-electron donor and A a !-electron acceptor) in which the HOMO and the LUMO can be tuned relatively independently of each other.
The first method was successful in the design of !-conjugated polymers, fullerene materials, etc. The second method gave rise to various TTF-A derivatives.
The major problem for the synthesis of TTF-spacer-A compounds is the need to prevent the formation of a stable intermolecular CT complex between the TTF and the acceptor moieties, prior to their covalent coupling. Three conceptually different ways of preventing this unwanted interaction have been explored and reported by Bryce:[34]
• “A TTF derivative can be linked to a group that is either a weak electron acceptor (e.g., a quinone) or not an acceptor at all (e.g., a pyridine moiety), which is subsequently converted into a stronger acceptor (TCNQ, or pyridinium cation, respectively). The conversion of a TTF-quinol derivative into its corresponding TTF-quinone has been demonstrated on numerous occasions […] but in spite of several attempts, there appears to be no example of a TTF-quinone being converted into the corresponding TCNQ or DCNQI (N,N-dicyanoquinodiimide) derivative although TTF-aldehydes have been converted cleanly into the
corresponding dicyanomethylene derivatives […]. The converse disconnection is also possible: an acceptor unit is linked to a 1,3-dithiole-2-thione unit (very weak donor), which is then converted into a TTF group, by standard coupling methodology […].
• Lithiated TTF can be substituted by an electrophilic reagent, which, after covalent bond formation, is transformed into an acceptor group in situ […].
• Steric hindrance in either the TTF or acceptor unit (or both) can be exploited to prevent their close !-! association, thereby allowing pendant substituent groups to couple […]”.[34]
In 1994, Jorgensen and co-workers [19] reported the principal landmarks in TTF chemistry. It is always evolving and always involving a lot of different disciplines in science, such as chemistry, biology or physics, depending upon the area of application, which is very vast.
Table 1.3: Landmarks in TTF chemistry. Table taken from [19].
1926: The first TTF derivative, dibenzo-TTF was synthesized as part of a general study of five-membered ring systems[17, 18]
1965 : Deprotonation of 1,3-dithiolium salts afforded TTF derivatives for the first time[51]
1970: First observation of parent TTF. TTF is found to form a stable purple radical cation on reaction with chlorine gas[14]
1973: First observation of metallic conductivity in an organic solid
(TTF)(TCNQ)[16]. TCNQ = tetracyanoquinodimethane. Conductivity: 500
!-1cm-1
1980: Superconductivity observed in a TTF derivative at 1.4 K:
tetramethyltetraselenafulvalene hexafluorophosphated ([TMTSF]2PF6)[52]
1980: TTF was derived extensively in the search for organic (super-) conductors[19]
1985: Macrocyclic TTF-based systems investigated with the aim of making molecular devices, sensor, switches and shuttles[19]
Initially, TTF and its derivatives were prepared in order to develop electrically
active supramolecular systems, and chemical sensors and redox-switchable ligands have been prepared with TTF containing rotaxanes and catetanes.[19] TTF has also an important role in molecular electronics. Indeed, TTF containing D-"-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. TTF can also have non linear optic (NLO) responses in second and third harmonic generation and a good thermal stability.[53] Segura and co-workers have summarized the reviews on TTF up to 2001, thus giving further examples for the many fields of applications of TTF.
Table 1.4: Recent reviews on specific aspects of TTF. Table taken from [53].
Main author Title
M. R. Bryce Recent progress on conducting CA salts[54]
V.
Khodorkovsky Molecular design of organic conductors[41]
M. R. Bryce Increasing dimensionality in the solid state[55]
G. Schukat TTF chemistry[38]
J. Garín Reactivity of TTF and TSeF[56]
K. B. Simonsen Functionalisation of TTF[57]
K. B. Simonsen Macrocyclic and Supramolecular TTF chemistry[62]
M. B. Nielsen Two- and three-dimensional TTF macrocycles[63]
E. Coronado Hybrid polyoxometalates-TTF materials[64]
J. Roncali Linearly !-extended TTF derivatives[65]
P. Day Molecular magnetic semiconductors, metals and superconductors[66]
M. R. Bryce TTF as !-donors in intramolecular CT-materials[34]
M. B. Nielsen Tetrathiafulvalenes as building blocks in supramolecular chemistry[67]
M. R. Bryce Functionalised tetrathiafulvalenes: new applications as versatile
!-electron systems in materials chemistry[68]
One of the key reasons for the synthesis of TTF containing D-A systems was the search for new charge-transfer materials with a well-defined ratio of D and A molecules. Some of these systems are discussed in detail below.
1.3.4. TTF-"-A
1.3.4.1. TCNQ acceptors
Aviram and Ratner proposed for the first time that TTF could be considered as an electron donor moiety for ICT in D-"-A molecules, where D is an organic one-electron donor, " is a covalent and non-conjugated bridge and A is an organic one-electron acceptor.[39] TTF and TCNQ were known to act as donor and acceptor units. Aviram and Ratner proposed a compound TTF-"-TCNQ, 1 (cf. Figure 1.15), never synthesized, but source of inspiration for the first TTF-"-A compound to be studied experimentally, 2[69]
(cf. Figure 1.16). The powder EPR spectrum of 2 at room temperature showed a broad signal from a ground state biradical.
1 Figure 1.15: Scheme of molecule 1, from [39].
2 Figure 1.16: Scheme of molecule 2, from [69].
Compound 3 (cf. Figure 1.17) was reported at round about the same time.[70] The cyclic voltammogram (cf. Figure 1.18) of this compound exhibits two reversible one-electron oxidation waves, typical of TTF, and one reversible two-electron wave, corresponding to the reduction of TCNAQ. Unlike compound 2, compound 3 is neutral in its ground state.
CT in compound 3 can be readily assigned in its UV-vis absorption spectrum. Indeed, it shows, in addition to the absorption bands of TCNAQ and TTF, a weak and broad band centred around 450 nm, assigned to ICT band (cf. Figure 1.19).
3 Figure 1.17: Scheme of molecule 3, from [70].
Figure 1.18: Cyclic voltammogram of compound 3 (in MeCN solution, vs. Ag/AgCl, Pt working electrodes, electrolyte Bu4N+ClO4- at 20°C). Taken from [70].
Figure 1.19: UV-vis spectrum of compound 3 in MeCN. The inset shows an expansion of the ICT band observed in the 420 to 600 nm region (from [70]).
1.3.4.2. Pyridinium and bipyridinium acceptors
Pyridinium cations and bipyridinium dications have been studied as acceptor groups in TTF-"-A systems. It is known that the pyridinium cation is a strong acceptor.[71]
Molecule 4, represented in Figure 1.20, was synthesized by Becker and co-workers by methylation of the corresponding pyridine system. The pyridinium moiety is thus covalently linked to the TTF unit, making the charge transfer possible due to the proximity of the donor and acceptor groups. A weak ICT absorption band (Figure 1.21) is found for compound 4, centred at ca. 665 nm. The solution of 4 was stable without light, but even with the beam of the spectrophotometer, the ICT process is triggered and a strong absorption band centred at ca. 675 nm, characteristic of the TTF+" radical cation appeared.[72]
4 Figure 1.20: Scheme of molecule 4[73]
Figure 1.21: UV-vis spectra for molecule 4 in acetone: i) a freshly prepared solution, and consecutive runs after ii) 1h, iii) 2h, iv) 3h[72]
The triad, A-"-TTF-"-A 5 (cf. Figure 1.22)[74] has also been studied. Inspired by this work, Becher and co-workers[75] have obtained a prototype thermally controlled TTF based molecular switch. Equilibrium between 6 and 6! (cf. Figure 1.23) shows a CT absorption band at 785 nm (cf. Figure 1.24), corresponding to the TTF unit being inserted into the cyclophane cavity.[76] The reflux of the solution for 45 min causes the disappearance of this band. So, by refluxing, the decomplexed form 6! is obtained. The complexed form 6 is re-established after 20 hours at room temperature and the CT absorption reappears.
5 Figure 1.22: Scheme of compound 5[74]
6 6!
Figure 1. 23: Scheme of compound 6 and its decomplexed form 6!.[76]
Figure 1. 24: a) UV-vis spectrum of the initially decomplexed 6! in MeCN at i) 0h, ii) 3h and iii) 19h. b) Variation of the maximum absorbance ((max = 785 nm) of 6! with time.[76]
1.3.4.3. A fluorescent acceptor
D-A molecules with TTF as electron donor can also be used for redox-fluorescence switch molecules, as reported by Zhang et al.[77] (Figure 1.25). In this case, the acceptor unit fluoresces (here, anthracene). But, because it is linked to TTF, only a weak fluorescence can be observed before oxidation of the molecule. After oxidation, TTF becomes TTF+" and loses its ability to donate an electron and thus quench the fluorescence. As a result, a fluorescence increase can be observed. When reduced back to the neutral form, TTF quenches the fluorescence of the acceptor again. Zhang et al. show that this quenching is indeed due to a photo-induced electron transfer reaction. The molecule was oxidised both by Fe(ClO4)3 (see Figure 1.27) as well as electrochemistry. Spectroelectrochemistry (see Figure 1.26 for the cyclic voltammogram and Figure 1.27 for the corresponding emission spectra) also served to demonstrate reversibility of the fluorescent redox switch.[77]
7
Figure 1.25: Scheme of molecule 7, and of the redox switch fluorescence, taken from [77].
Figure 1.26: Cyclic voltammogram of 7 (scanning rate 50 mV/s) with Pt wires as working and counter electrodes, Ag wire as a reference electrode, and n-Bu4PF6 as a supporting electrolyte. Figure taken from [77].
Figure 1.27: (A) Fluorescence spectra of 7 (5.8x10-5M) in THF in the presence of different amounts of Fe(ClO4)3. (B) Absorption spectra of 7 (5.8x10-5M) in THF in the presence of different amounts of Fe(ClO4)3. Figure taken from [77].
Figure 1.28: (A) Fluorescence spectra of the solution of 7 in THF (4.94x10-5M) containing n-Bu4NPF6 (27.8 mM) after applying an oxidation potential of 0.7V (vs Ag wire). (B) Fluorescence spectra of the solution of 7 that has been oxidized electrochemically for 3 min after applying potential of 0.2 V (vs Ag wire). Figure taken from [77].
1.3.5. TTF-!-A and TTF-A 1.3.5.1. Quinone acceptors
Dumur and co-workers reported TTF-benzoquinone diad (8) and triad (9).[78] The compounds showed pronounced amphotericity in cyclic voltammetry (Table 1.5). A
weak and broad intramolecular CT band (around 900 nm for 8, see Figure 1.29) was observed in these compounds.
8 9
10
Figure 1.29: Scheme of compounds 8, 9[78] and 10.[79]
Figure 1.30: Evolution of the UV'vis'NIR spectrum during reduction of D'A 8, at different times of reaction. The inset is the enlargement of the vis'NIR part. Figure taken from [78].
Table 1.5: Summary of the oxidation and reduction potentials of compounds 8 and 9
Thioindigo possesses a similar electron affinity to that of benzoquinone. It was attached to TTF by Aqad and co-workers (10).[79] This dyad has a long wavelength absorption band at 760 nm, which can be attributed to an intramolecular charge transfer transition (ICT).
1.3.5.2. Fullerene acceptors
!
Llacay and co-workers[80] and Boulle and co-workers[81] and have performed studies on the series of TTFs attached to C60: 11, 13, 14 and 15. Electronic spectra of 13 and 14 indicated weak inter- or intramolecular interactions between the TTF and C60 moieties
Llacay and co-workers[80] and Boulle and co-workers[81] and have performed studies on the series of TTFs attached to C60: 11, 13, 14 and 15. Electronic spectra of 13 and 14 indicated weak inter- or intramolecular interactions between the TTF and C60 moieties