The preparation of the TCNQ derivatives carrying nitroxide substituents (3a–3c) was carried out by condensing 2,5-bis(2-hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane 2 with 4-carboxy-TEMPO (2,2,6,6-tetramethylpiperidin-1-oxy), 3-carboxy-PROXYL (2,2,5,5-tetramethylpirrolidin-1-oxy), or 3-carboxy-PO (2,2,5,5-tetramethyl-3-pyrrolin-1-oxy) and DMAP (4-dimethylaminopyridine)/DCC (N,N'-dicyclohexylcarbodiimide). The yields were rather low mainly because of the difficulty of the tedious purification by column chromatography and recrystallization.

In order to evaluate the electron-accepting and donating properties of the derivatives, CV measurements of 3a-3c were performed and the data are summarized in Table 1 together with those of 2 as a reference.

It is apparent from the first and second reduction potentials of 3a–3c and the reference compound 2 that there is almost no influence of the radical substituents to the potentials, resulting in no significant difference in their electron-accepting properties. On the contrary, the first oxidation potentials due to the nitroxide groups are somewhat different, reflecting the different donating ability of the radical groups. On the whole, the co-existence of accepting and donating ability in each molecule is apparent from the CV data, enabling occurrence of some CT interaction between them.

The X-ray analyses of the TCNQ derivatives 3a–3c were performed by using each single crystal grown from an appropriate solvent, and the crystal data are summarized in the Experimental Section.

As shown in the crystal structure of the TEMPO-substituted derivative 3a (Figure 1), there are two molecules in a unit cell and very close contacts are clearly observed between the centre of the TCNQ core and a couple of the oxygen atoms of nitroxide groups of the outer two molecules with the same distance of 2.906 Å for both sides. The distance of the close contacts is comparable to that observed for a black-colored spin-carrying benzoquinone (2.796 Å), in which a distinct CT band is observed in the diffuse reflectance UV-VIS spectrum in the solid state [5]. Thus, the formation of a single-component CT crystal is apparent for 3a to reflect the black color of the crystal. Even with the stronger acceptor property of 3a than that of 1a (E₁^RED = −0.40 V vs. SCE in CH₃CN), the distance of the contacts is quite similar to that for 1a (2.901 Å), suggesting the limitation of short contacts would be around 2.9 Å.

A similar structural feature is observed for the crystal structure of the PROXYL-substituted derivative 3b to that of 3a as shown in Figure 2 (upper), in which the oxygen-to-TCNQ contacts amount to 2.902 Å to afford a single-component CT crystal also in this case. The weaker donor property of the PROXYL group than that of the TEMPO group does not affect the distance of the short contacts. On the contrary, a rather different contacting feature is found for the PO-substituted derivative 3c to that of 3a and 3b (Figure 2b). Namely, a couple of short contacts are seen between the oxygen atoms of outer nitroxide groups and the carbon atoms of a couple of cyano groups, as shown in Figure 2 (lower) with the distances of 2.886 Å shorter than those of 3a and 3b. With this unique contacting feature, the derivative 3c consequently provides another single-component CT crystal.

The magnetic measurements of the solid samples were carried out by using a SQUID susceptometer in the temperature range from 2 K to 300 K and the data for 3a–3c are summarized in Table 2.

All of the radical compounds show paramagnetic Curie-Weiss behavior with small antiferromagnetic interactions between the spins. As no close contact is observed in their crystal structures other than those described above, the weak antiferromagnetic interactions observed are probably due to those between the spin centers mediated by a couple of TCNQ cores. The smaller Curie constants for 3b and 3c than the theoretical value are considered to be due to the contaminated monoradicals, which were quite difficult to separate by usual purification methods.

Commercial grade 2,5-bis(2-hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane 2 4-carboxy-TEMPO, 3-carboxy-PROXYL, and 3-carboxy-PO [11] were used for the reactions described in the text.

Melting points of the solid samples are uncorrected. The data of FAB-MS spectra were obtained by using m-nitrobenzylalcohol as matrix and appropriate polyethylene glycol samples as internal standard. Magnetic susceptibility measurements were carried out on a SQUID susceptometer using ca. 10 mg for each powdered sample. Crystal structures of all compounds were determined at room temperature. X-ray diffraction data were recorded using a CCD area detector on a diffractometer and crystal structures were solved by the direct method. The refinements were made by the full-matrix least squares methods. Anisotropic temperature factors were used for the non-hydrogen atoms and the hydrogen atoms were included in the final calculation. All the calculations were performed using the Texan crystallographic software package. Crystal data of 3a, 3b, 3c and their TTF complexes have been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

To a stirred mixture of 2 (0.81 g, 2.5 mmol), 4-carboxy-TEMPO (1.00 g, 5.0 mmol) in dichloromethane (5 mL) was added DCC (1.03 g, 5.0 mmol) and DMAP (0.61 g, 5.0 mmol) at ambient temperature. After stirring for 3d, the reaction mixture was subjected without concentration to column chromatography on silica gel by using ethylacetate as an eluent. After repeated column chromatography, the solid product was recrystallized from n-hexane-dichloromethane (1:1) to give black blocks (0.24 g, 14%). mp: 215–220 °C; FAB-MS (m/z): 689 (M + 1); Crystal data: C₃₆H₄₂N₆O₈, FW = 686.76, monoclinic (P2₁/n), a = 9.143(3), b = 19.352(5), c = 10.894(3) Å, β = 106.285(5)°, V = 1850.3(9) ų, T = 293 K, Z = 2, Dc = 1.233 mg·m⁻³, R = 0.0854, wR = 0.2980 [4166 reflections and 226 parameters with I > 2σ(I)].

In a similar manner, 3b and 3c were obtained albeit with low yields (5% for 3b and 4% for 3c) and their data are as follows; 3b: mp: 220–225 °C; FAB-MS (m/z): 661 (M + 1); Crystal data: C₃₆H₄₄Cl₄N₆O₈ (2CH₂Cl₂ molecules are incorporated), FW = 830.59, monoclinic (P2₁/n), a = 12.890(4), b = 9.700(3), c = 16.890(5) Å, β = 102.920(5)°, V = 2058.3(9) ų, T = 293 K, Z = 2, Dc = 1.340 Mg·m⁻³, R = 0.0607, wR = 0.1906 [4586 reflections and 244 parameters with I > 2σ(I)]. 3c: mp: 210–213 °C; FAB-MS (m/z): 657 (M + 1); Crystal data: C₄₀H₄₂N₆O₈ (C₆H₆ molecule is incorporated), FW=734.81, monoclinic (P2₁/c), a = 12.000(4), b = 6.7200(19), c = 24.400(7) Å, β = 92.210(6)°, V = 1966.2(10) ų, T = 293 K, Z = 2, Dc = 1.241 mg·m⁻³, R = 0.0635, wR = 0.1763 [4438 reflections and 244 parameters with I > 2σ(I)].