Preparation, Crystal Structure, and Properties of a New Semiconducting Material Bearing Tetrathiafulvalene-Pyridine
Lejia Wang , Ning Xiao , Qing Sun , Liangjun Shen , Xiaofang Li , Xunwen Xiao
The organic-inorganic hybrid supramolecular frameworks are mostly constructed by metal-ligands coordination, for coordination-driven self-assembly method is an easy procedure to implement under the mild conditions and can afford micro- or nanometer-sized materials[1]. In the past decades, crystal engineering of supramolecular architectures has attracted much attention, especially on the electrical conducting nano/micro crystals, for conductive nanostructures show adjustable properties with diversity and flexibility[2~4]. The organic ligand with proper geometry and functionality is critical for directing the structure and properties of the organic-inorganic hybrid supramolecular frameworks. As a result, the rational assemblies of π-conjugated molecules have been extremely investigated[5, 6]. As a famous donor molecule, tetrathiafulvalene (TTF) is a π-conjugated system which has been exploited in materials and supramolecular sciences[6~10]. Hence, incorporating TTF ligand to a hybrid framework is an effective way to get the conducting materials. Shao et al.[11] reported a bent donor molecule which assembles along a specified direction and can form micro/nanocrystals by electro-crystallization without any template.However, this method has a drawback of a large-scale synthesis resulting from the electro-synthesis. Previously, Wu et al. reported a donor molecule in which a TTF motif and one pyridyl group (py) linked with a π-conjugated bridge could form semiconducting neutral polymer microstructure by coordinative self-assembly method[12]. Thus, donor-acceptor coordination-driven assembly method could provide an efficient way to get micro- or nanometer-sized materials[13~15].
Recently, we and other groups both show much interest in a type of pyridyl-functionalized TTF compounds, in which the pyridyl group is directly attached to the TTF core without any spacer part. Our previous work reported a TTF-py type donor molecule, EDT-TTF-4-py (L1) (Fig. 1), which could form microstructures by the coordinative self-assembly method[13, 14]. Although TTF is a well-known donor molecule that can form many molecular conductors, the conductivity of charge transfer salts based on this type of TTF-py is very low due to the neutral state of TTF motif[16, 17]. Recently, we reported a silver (I) complexed with the ligand L1, [Ag(L1)2]3(CF3SO3-)4·2H2O, showing a semiconducting behavior, however, we could not get the micro crystal of [Ag(L1)2]3(CF3SO3-)4·2H2O[18]. As a consecutive work, Cu(ClO4)2 is selected as coordination ion to induce charge transfer from TTF-py to copper ions in a complex. We obtained prospective results and got a new copper coordination complex [CuI(L1)4](ClO4)2. Two types of crystals (micro and bulk crystals) were formed when Cu(ClO4)2 was reacted with L1 under different reaction conditions. To the best of our knowledge, it is reported for the first time that the ligand L1 was oxidized and coordinated with copper ion.
In this paper, a Cu (I) coordination complex [CuI(L1)4](ClO4)2(Fig. 1) was successfully synthesized with micro and bulk crystals forms. Herein, the preparation, structural analyses, and conductivity property of this complex with micro and bulk crystals forms are described.
All reagents were of analytical grade and used as received. EDT-TTF-4-py (L1) was synthesized according to the procedures previously reported[13].The SEM images and EDX results were obtained on a Hitachi S-4300. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C analyzer. The powder XRD results were performed by using a Brucker D8 X-ray diffractometer with graphite-monochromator Cu-Kα radiation (λ=0.15418nm).
Cu(ClO4)2 (0.05mmol) in CH3CN (5mL) was layered over a solution of L1 (0.05mmol) in dichloromethane (5V). The mixed solution was stirred for 30 min at room temperature, and a purple precipitate was found. [CuI(L1)4](ClO4)2: microcrystal, 31.1 mg, Yield: 50.8%. Elemental analysis calcd (%) for C52H36Cl2CuN4O8S24: C 35.72, H 2.07, N 3.20; Found (%): C 35.82, H 2.11, N 3.15.
Single crystals of complex [CuI(L1)4](ClO4)2 suitable for X-ray diffraction analysis were obtained by slow diffusion. Cu(ClO4)2 (0.01mmol) in CH3CN (5mL) was layered over a solution of L1 (0.02mmol) in dichloromethane (5mL) at 10℃. The reaction mixture was left to stand undisturbed for 2 weeks. Shiny purple crystals were isolated by filtration, washed with hexane, and air-dried at room temperature. [CuI(L1)4](ClO4)2: bulk crystal, 2.4 mg, Yield: 19.6%.
X-ray diffraction data were collected on a Bruker SMART CCD diffractometer at room temperature with a graphite monochromated Mo Kα radiation (λ=0.71070Å). The crystal structure was solved by direct methods within SHELXS-97[19], and refined by full matrix least-squares methods on F2 by means of SHELXL-97. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were introduced at calculated positions. Final refinement was performed with modification of the structure factors for contribution of the disordered solvent electron densities using the SQUEEZE option of PLATON[20]. Details of crystal data, data collections, and structure refinement are summarized in Tab. 1. All data (except structure factors) have been deposited with the Cambridge Crystal-lographic Data Centre as supplementary publications CCDC 1864958 for [CuI(L1)4](ClO4)2 in this paper. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (deposit@ccdc.cam.ac.uk).
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| Parameters | [CuI(L1)4](ClO4)2 |
| emperical formula | C52H36Cl2Cu N4 O8S24 |
| Formula wt | 1748.73 |
| T/K | 296(2) |
| wavelength /Å | 0.71073 |
| crystal system | Orthorhombic |
| space group | 'Pban' |
| a /Å | 34.667(17) |
| b /Å | 7.9087(4) |
| c/Å | 14.5451(8) |
| V/Å3 | 3987.9(4) |
| Z | 2 |
| Dc /(g·cm-3) | 1.456 |
| abs coeff /mm-1 | 1.016 |
| F(000) | 1774 |
| limiting indices | -40≤h≤40 -9≤k≤8 -16≤l≤17 |
| reflns collected | 3519 |
| indep reflns | 2995 |
| abs correction | 0.7~0.9 |
| data/restraints/param | 3544/192/284 |
| GOF on F2 | 1.127 |
| Rindices (all data) | R1=0.1213 wR2=0.2798 |
Electrical conductivity of the bulk crystal of [CuI(L1)4](ClO4)2 was measured with a single crystal in the temperature region of 20~298 K. Four gold electrodes (15 μØ) were contacted with a gold paste in parallel with the largest plane of the needle crystals. The Keithley 4200-SCS semiconductor characterization system was used to measure I-V characteristics of the micro crystal of [CuI(L1)4](ClO4)2 based on SiC, and these data were recorded using a computer controlled by the Labview program.
The UV-Vis spectra of L1 upon addition of Cu(ClO4)2 were measured, and the results were plotted in Fig. 2. All measurements were carried out in a CH2Cl2/CH3CN (1:1 in volume) mixed solvent. Upon addition of Cu(ClO4)2 to the L1 solution, a new broad absorption band with λmax at ca. 860nm and 445nm could be observed, corresponding to the absorption of radical cation (TTF·+)[18]. After addition of 1.0 equiv of Cu(ClO4)2, the intensity of the new band reached the maximum values. These results mean that L1 could be oxidized to radical cation (TTF·+) by Cu(ClO4)2.
The solvent diffusion method was used to form the hybrid microstructure. In a typical experiment, Cu(ClO4)2 solutions in CH3CN (5mL) were added to the donor molecule L1 solution in dichloromethane (5mL) at room temperature, then precipitates were formed. Note that a slow diffusion method could afford bulk crystals of [CuI(L1)]4(ClO4)2, with typical sizes of 0.30mm×0.24mm×0.11mm. X-ray singlecrystal diffraction was carried out for both micro and bulk crystal.
The X-ray analysis of [CuI(L1)]4(ClO4)2 reveals that the asymmetric unit is composed of one-quarter Cu atom, one ligand L1 and half ClO4- for charge balance. The molecular structure of [CuI(L1)]4(ClO4)2with the atom-numbering scheme is depicted in Fig. 3a. The Cu ion shows a slight distorted square coordination geometry consisting of four N atoms from four L1 ligands with the same Cu—N distances of 2.000(6)Å and bond angles of 89.06(4)° and 91.6(4)°. The complex contains four L1 ligands, one Cu atom and two ClO4- anions, suggesting the formula of the salt should be [CuI(L1)]4(ClO4)2. The TTF core in complex is almost planar with the mean deviation from plane being 0.044(9)Å. The pyridine group is twist 30.47(5)° from the mean plane of TTF, larger than that in the neutral state (the pyridine group is twist 17.9(4)° from the TTF plane[18]). The center C＝C double bond length in L1 is 1.372(11)Å, suggesting that L1 is in an oxidation valence. From the valence balance, we can estimate that the charge of the TTF moiety should be +1/4. L1 is arranged in a head-to-tail configuration in which the pyridine rings stick out of the donor stack column. The cation [(L1)4-Cu]2+ is constructed by forming one-dimensional chain with S…S [3.585Å(S2…S4)] noncovalent contacts along c axis. As shown in Fig. 3b, a 1D copper supramolecular chain is formed by S…S noncovalent contacts. The adjacent Cu ions are separated by a distance of 7.909Å, indicating no overlap between the neighboring [(L1)4-Cu]2+ cation along b axis. The molecules of complex are stacked with b axis, and there are many S…S noncovalent contacts in the ac plane. There are S…S noncovalent contacts [3.426Å (S2…S6) and 3.469Å (S2…S4)] existing between neighbouring chains. The ClO4- anions are linked to the [(L1)4-Cu]2+ cation by C—H…O (2.469, 2.536, 2.610, 2.695Å) weak interactions. Interestingly, through these two copper supramolecular chains, a 3D microporous supramolecular network was formed and a 1D open channel (8.05 × 7.67Å) along b axis was generated (as shown in Fig. 3c).
Unfortunately, the structure of micro crystal could not be solved. And the crystalline phase purity of the microcrystals was investigated by comparing with the experimental XRD results and the simulated patterns obtained from the single-crystal data of [CuI(L1)]4(ClO4)2 (as shown in Fig. 4). The major peaks in the experimental case could match well with the corresponding simulated patterns, which also could indicate the purity phase of the composite microcrystals[20].
As shown in Fig. 5a, SEM images of microcrystals of the complex showed a needle-like shape. The diameters of microcrystal are about 500 nanometers, but the lengths can reach tens of microns. Energy-dispersive X-ray (EDX) spectroscopy and elemental analysis were employed to measure the chemical composition of the microctructre-type complexes (Fig. 5b). In the EDX profile of the micro-crystal, the peaks of C, S, O, N, Cu and Cl are clearly identified. As a result, we could make a conclusion that the microcrystal of [CuI(L1)]4(ClO4)2 is composed of L1 and CuClO4. Elemental analysis results also prove that each complex has one CuI and four L1 ligands.
The electrical conductivity measured on the single crystals of [CuI(L1)]4(ClO4)2 was 9.1 × 10-2 S·cm-1 at room temperature. Upon cooling, the electrical resistivity increases very slowly from 300 to 50 K and increases sharply when the temperature is further lowered (Fig. 6a). The complex showed a semiconducting behavior, and the activation energy was estimated about 50 meV. The semiconducting behavior may be come from the one-dimensional uniform array of the TTF+0.25 along the c axis. Meanwhile, we measured the electrical conductivity of the microcrystals of the complex [CuI(L1)]4(ClO4)2 by using a two-probe technique at room temperature. As shown in Fig. 6b, in the range of -2~+2 V, the characteristic (I versus V) curve of the microcrystals shows a linear behavior, suggesting that the contact resistance is too small to achieve the threshold voltage in the I-V curve. Moreover, the conductivity of the microcrystal calculated from the I-V curves of the devices is on the order of 10-2 S·cm-1, and the result is in line with that measured for bulk crystals.
In conclusion, we successfully synthesized a copper complex [CuI(L1)]4(ClO4)2 (L1=EDT-TTF-4-py). And we also got two forms of crystal when Cu(ClO4)2 was reacted with L1 at different conditions. Moreover, the X-ray (EDX) spectroscopy and elemental analysis illustrated that the chemical composition in these two types are the same. More interestingly, the complex shows a semiconductor behavior with σrt=9.1×10-2 S·cm-1 and an activation energy of Ea=50 meV. The molecule structures of the complex show a 3D microporous framework which is constructed through two supramolecular 1D chain. The result indicated that coordination bond is an effective tool in constructing conductive microporous framework. Synthesis of other microporous network with other metal ions and L1 is in progress.
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Figure 1 Illustration of the process of forming [CuI(L1)4](ClO4)2 microporous supramolecular network
Figure 2 Changes of the UV-Vis absorption of L1 (1.0×10-5mol/L) in CH2Cl2/CH3CN upon the addition of increasing equiv of Cu2+
Figure 3 (a) ORTEP drawing of [CuI(L1)]4(ClO4)2 with 50% probability ellipsoids views; (b) Molecular packing structure along the baxis, showing the short S…S (organic dotted line) and C-H…O (red dotted line) contacts; (c) Open channels of the complex, along b axis
Figure 4 XRD pattern for microcrystals of [CuI(L1)]4(ClO4)2 (black solid line), and the simulated pattern obtained from the single-crystal data (gray dash line)
Figure 5 SEM Characterization of microcrystals of [CuI(L1)]4(ClO4)2 (A) image; (B) EDX spectrum
Figure 6 (a) Temperature dependence of normalized resistivity (ρ/ρrt) of [CuI(L1)]4(ClO4)2; (B) I-V characteristics of the [CuI(L1)]4(ClO4)2 microcrystal
Table 1. Crystal data and structure refinement for [CuI(L1)4](ClO4)2
| Parameters | [CuI(L1)4](ClO4)2 |
| emperical formula | C52H36Cl2Cu N4 O8S24 |
| Formula wt | 1748.73 |
| T/K | 296(2) |
| wavelength /Å | 0.71073 |
| crystal system | Orthorhombic |
| space group | 'Pban' |
| a /Å | 34.667(17) |
| b /Å | 7.9087(4) |
| c/Å | 14.5451(8) |
| V/Å3 | 3987.9(4) |
| Z | 2 |
| Dc /(g·cm-3) | 1.456 |
| abs coeff /mm-1 | 1.016 |
| F(000) | 1774 |
| limiting indices | -40≤h≤40 -9≤k≤8 -16≤l≤17 |
| reflns collected | 3519 |
| indep reflns | 2995 |
| abs correction | 0.7~0.9 |
| data/restraints/param | 3544/192/284 |
| GOF on F2 | 1.127 |
| Rindices (all data) | R1=0.1213 wR2=0.2798 |
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