Synthesis, Structures and Properties of Molecular Conductors Based on Bis-fused Donors Composed of (thio)pyran-4- Ylidene-1,3-dithiole and Tetraselenafulvalene

Bis-fused donors composed of (thio)pyran-4-ylidene-1,3-dithiole and tetraselenafulvalene (1a, 2a) and their bis(methylthio) derivatives (1b, 2b) were synthesized. Cyclic voltamograms of all the donors consisted of four pairs of one-electron redox waves, and it was suggested that a positive charge of 1 + • and 2 + • distributed mainly on the (thio)pyran-4-ylidene-1,3-dithiole moiety. X-ray structure analysis revealed that (1b)PF 6 (C 6 H 5 Cl) 0.5 and (2b)PF 6 (C 6 H 5 Cl) formed one-dimensional conducting stacks in which the donors were dimerized or tetramerized. In those salts, intramolecular charge disproportionation of the donors was suggested by X-ray structure analysis and density functional theory (DFT) calculation with UB3LYP/6-31G(d) basis function. A tight-binding band calculation suggested that these materials were band insulators. All the donors gave highly conducting TCNQ (7,7,8,8-tetracyanoquinodimethane) complexes and I 3 − salts (σ rt = 0.3–19 S cm −1 on a compressed pellet) with very low activation energies of 0.017–0.040 eV, while single crystals of (1b)PF 6 (C 6 H 5 Cl) 0.5 and (2b)PF 6 (C 6 H 5 Cl) exhibited semiconductive behavior with large activation energies (E a = 0.16–0.22 eV).


Electrochemical Properties
Electrochemical properties of new donors were investigated by cyclic voltammetry.Their redox potentials are summarized in Table 1.Cyclic voltamograms of all the new donors consisted of four pairs of one-electron redox waves.The first redox potentials (E 1 ) of 1 and 2 did not depend on the substituent on the TSF moiety, while the E 1 values varied on the chalcogen atom in the chalcogenopyran-2-ylidene moiety.In addition, the values of 1a and 1b were lower by 0.05-0.06V than that of ST-STP (0.00 V).These results indicated that a positive charge formed by the first oxidation distributed on the 2-(chalcogenopyran-4-ylidene)-1,3-dithiole moiety mainly, and TSF moiety preferably acted as a selenium-based substituent.The second redox potential of 1a was higher than that of 1b, suggesting that the TSF moiety contributed to the second oxidation.These results suggested that intramolecular charge disproportionantion might occur in the conducting solids of 1 and 2. The theoretical calculations of 1a and 2a were carried out using a density functional theory (DFT) B3LYP method with the 6-31G(d) basis set. Figure 2 shows the HOMOs of 1a and 2a.They distribute mainly to the chalcogenopyran-2-ylidene moiety and it is consistent with their redox property.

Molecular and Crystal Structures of a Neutral Molecule TM-PDS-STP (1b)
Single crystals of TM-PDS-STP (1b) were obtained by slow evaporation of the 1b solution containing carbon disulfide.Figure 3 shows ORTEP drawings of TM-PDS-STP.The molecule is almost planar except for the carbon atoms in methylthio groups, which are projected over inclined upper direction.As shown in Figure 4, the molecules formed pseudo stack along the a-axis and are arranged in a so-called θ-type fashion.The interplanar distance along the pseudo stack is 3.54 Å.There is short Se• • • Se contact (3.624(1) Å) shorter than sum of van der Waals radii (Se• • • Se = 3.80 Å) [10].

Preparation, Crystal and Band Structures of Radical Cation Salts
Radical cation salts of TM-PDS-STP (1b) and TM-TPDS-STP (2b) were grown by an electrocrystallization technique at a controlled current method [11] from 0.2 to 1.0 μA in the presence of the corresponding tetra-n-butylammonium salts as the supporting electrolyte in chlorobenzene (C 6 H 5 Cl) containing 5% ethanol at 35 °C.Crystallographic data for (TM-PDS-STP)PF 6 (C 6 H 5 Cl) 0.5 and (TM-TPDS-STP)PF 6 (C 6 H 5 Cl) are summarized in Table 3 (Experimental section).All the salts crystallize in triclinic system, space group P 1.

(TM-PDS
One donor molecule and anion were crystallographically independent, and were located on a general position.The PF 6 − anion showed disorder based on uni-axial rotation.The chlorobenzene was crystallographically unique and its chlorine atoms showed disorder at two positions.Moreover, the center of inversion exists in the benzene ring of the chlorobenzene, therefore, extreme disorder was observed.The donors are dimerized in the stack, and were overlapped in a head-to-tail manner with interplanar distances of 3.54 Å for a1 and 3.55 Å for a2, respectively.The slip distance along the molecular long axis for a1 (1.18 Å) is smaller than that for a2 (4.72 Å) as shown in Figure 7. Extended Hückel method is usually used for the calculation of the overlap integrals.This method does not contain Coulomb terms, that is, it is unsuitable when the calculation is carried out for the present charged species.The unrestricted DFT method using B3LYP functional with 6-31G(d) basis set was used for the charged donor (TM-PDS-STP) •+ and the frontier molecular orbitals were summarized in Supplementary Materials (Figure S1).The SOMO of (TM-PDS-STP) •+ distributes mainly to the TSF skeleton, while the HOMO-1 is dominantly localized on the (pyran-4-ylidene)-1,3-dithiole unit.The energy level of the SOMO (−7.66 eV) is lower than that of the HOMO-1 (−7.46 eV), suggesting that the utilization of the HOMO-1 could be favorable for the calculation of overlap integrals of this salt.There is a difference in the bond length of C=C bond, which connects pyran ring and 1,3-dithiole ring, between the neutral state of TM-PDS-STP (1.35(1) Å) and the cation radical state of TM-PDS-STP (1.40(2) Å).In contrast, the bond length of the central C=C in TSF moiety is almost the same between neutral (1.34(1) Å) and cation radical (1.33(1) Å) state.These results indicate the distribution of positive charge on the (pyran-4-ylidene)-1,3-dithiole unit.Therefore, the overlap integrals were calculated by considering the HOMO-1 of the DFT calculation.The overlap integrals a1 and a2 are 16.0 and 7.3 × 10 −3 and the donors are electronically dimerized along the stacking direction (Figure 8).Although the side-by-side interaction was prohibited owing to the presence of the anion and the solvent, the values of the overlap integrals p1, p2, and p3 along the molecular long axis showed considerable magnitude due to the interactions among pyran units and thiomethyl groups.A tightbinding band calculation suggested that the salt is a band insulator.11).The anions and chlorobenzene molecules were located between the donor molecules along the molecular short axis of TM-TPDS-STP, and they prevented side-by-side interaction between the donor molecules.The donor molecules A and B in this salt stacked in a head-to-tail manner with the interplanar distances 3.52 Å and 3.50 Å for the intermolecular interactions A-A (p1) and B-B (p3), and in a head-to-head manner with the interplanar distance 3.56 Å for A-B (p2) as shown in Figure 11.The slip distances of p1, p2, and p3 are 4.59, 1.75, and 1.95 Å, respectively.
In (TM-TPDS-STP)PF 6 (C 6 H 5 Cl), the DFT calculations at the UB3LYP/6-31G(d) level suggested that the HOMO-1 of Molecule A and B distributed to the (thiopyran-4-ylidene)-1,3-dithiole unit, and it is the same situation with the TM-PDS-STP salt as described above (Figure S2).Consequently, the overlap integrals were calculated by considering the HOMO-1 of the DFT calculation.The donors of the intermolecular interaction p2 stacked with head-to-head manner and the (thiopyran-4-ylidene)-1,3dithiole units interacted effectively with each other (Figures 11 and 12).Since the HOMO-1 distributed on the (thiopyran-4-ylidene)-1,3-dithiole unit of TM-TPDS-STP, the overlap integral p2 (20.2 × 10 −3 ) is larger than those of p1 and p3 (15.7 and 12.6 × 10 −3 ), in which the donors stacked in a head-to-tail manner.The differences among the overlap integrals p1, p2, and p3 led to the electronic tetramerization along the stacking direction.Since the thiopyran units of the interaction c1 were adjacent to each other, the overlap integrals c1 along the molecular long axis was relatively large.It was found that (TM-TPDS-STP)PF 6 (C 6 H 5 Cl) was a band insulator due to the electronic tetramerization and the D:A = 1:1 composition.

Electrical Properties of the Conducting Materials
Electrical properties of conducting materials based on 1 and 2 obtained so far are summarized in Table 2.The TCNQ complexes and the I 3 − salts were prepared by mixing of the donors with TCNQ or ).This result is consistent with those of X-ray structure analysis and band calculation of the PF 6 − salts, that is, they are band insulators due to dimerized (tetramerized) stack of donors with fully ionized oxidation states.measured against Ag/Ag + and converted to vs. Fc/Fc + .Electrical conductivity measurements were achieved using a Fuso-HECS 994 conductivity instrument in the temperature range 80-300 K. Electrical contacts were achieved with gold paste.

General Procedure for Preparation of TCNQ Complexes and I 3 − Salts
Hot solutions of donor molecule and TCNQ or tetra-n-butylammonium triiodide in chlorobenzene were mixed, and the resultant precipitate was collected by filtration.The TCNQ complex was washed with carbon disulfide and acetonitrile, and dried in vacuo.The I 3 − salts were washed with carbon disulfide and methanol, and dried in vacuo.The stoichiometry of TCNQ complexes and the I 3 − salts was determined by elemental analysis.

General Procedure for Preparation of Cation Radical Salts
Cation radical salts of 1b and 2b were prepared by electrochemical oxidation in chlorobenzene (5% ethanol, v/v, 18 mL) at a controlled current from 0.2 up to 1.0 μA in the presence of the corresponding tetra-n-butylammonium salts at 50 °C for 1-3 weeks.The crystals obtained were washed with ethanol and were air-dried at room temperature.

Electronic Band Calculations
From the results of the crystal structure analysis, intermolecular overlap integrals were calculated by considering in the HOMO-1 of donor molecules obtained by the result of density functional theory (DFT) calculation at the UB3LYP/6-31G(d) level [14] using Gaussian 09 program package [15].The electronic band dispersions and Fermi surfaces were calculated using the intermolecular transfer integrals under the tight-binding approximation [16].

Conclusions
Four kinds of new bis-fused donors composed of (thio)pyran-4-ylidene-1,3-dithiole and tetraselenafulvalene (1a, 2a) and their bis(methylthio) derivatives tetraselenafulvalene (1b, 2b) were successfully synthesized.The results of cyclic voltammetry indicated that a positive charge of 1 +• and 2 +• distributed mainly on the (thio)pyran-4-ylidene-1,3-dithiole moiety, probably due to more powerful donating ability of (thio)pyran-4-ylidene-1,3-dithiole than tetraselenafulvalene. Occurrence of such an intramolecular charge disproportionation in the radical cation salts (1b)PF 6 (C 6 H 5 Cl) 0.5 and (2b)PF 6 (C 6 H 5 Cl) was also suggested by the results of X-ray structure analysis and DFT calculation.The expected observation of "windmill" type structure was not observed, probably due to intramolecular charge disproportionation of the donors as well as 1:1 composition of the donor to anion.High conductivity found in the TCNQ complexes and the I 3 -salts (σ rt = 0.3-19 S cm −1 on a compressed pellet) with very low activation energies (0.017-0.040 eV) brings high expectation that 1 and 2 are promising donor components for development of new molecular conductors.We are currently investigating synthesis of the analogues of 1 and 2, in which selenium atoms are exchanged or added to the other positions of sulfurs, as well as preparation of molecular conductors of 1 and 2 with different anions.

Figure 1 .
Figure 1.Molecular structures of BDT-TTP derivatives and their analogues.

Figure 3 .
Figure 3. ORTEP drawings of TM-PDS-STP.(a) Top view and (b) side view.Displacement ellipsoids are drawn at the 50% probability level.

Figure 4 .
Figure 4. (a) Crystal structure of TM-PDS-STP viewed along the a-axis.Blue broken lines represent short Se• • • Se contacts shorter than sum of van der Waals radii; (b) Molecular arrangement of TM-PDS-STP viewed along the molecular long axis.
Figure 5 shows the ORTEP drawings of TM-PDS-STP molecule in (TM-PDS-STP)PF 6 (C 6 H 5 Cl) 0.5 .The donor molecule was almost planar; the deviations from the leastsquare plane are less than 0.45(2) Å except for the hydrogen atoms.The largest deviation (0.45(2) Å) is observed at the C14 position of the methylthio group.Both the methylthio groups were located the molecular plane, and projected outside of the molecular long axis in the plane composed of the donor skeleton.The crystal structure of (TM-PDS-STP)PF 6 (C 6 H 5 Cl) 0.5 is shown in Figure6.The molecules formed a face-to-face stack along the a-axis.The side-by-side interstack interactions were inhibited by the anions and disordered solvent molecules, which were located in the cavity formed by the donor molecules with large methylthio groups.

Figure 7 .
Figure 7. Molecular overlap modes for a1 and a2 of (TM-PDS-STP)PF 6 (C 6 H 5 Cl) 0.5 .The atoms and bonds of the depth molecules are faded out.

Bu 4
NI 3 in hot chlorobenzene.Compressed pellets of all the TCNQ complexes and the I 3 − salts showed relatively high conductivity of σ rt = 10 −1 -10 1 S cm −1 .It is noted that electrical conductivity of the TCNQ complexes and the I 3 − salt of TPDS-STP reached up to 20 S cm −1 at room temperature even the measurement of conductivity was carried out on a compressed pellet.Although resistivity of all the materials exhibited semiconductive temperature dependence, activation energies (E a ) of TCNQ complexes of 1b, 2a, b and I 3 − salts of 1a and 2a were very small (0.017-0.040 eV).Empirically, a single crystal of these materials is expected to show metallic conducting behavior.On the other hand, room temperature conductivity of the PF 6 − and AsF 6 − salts of TM-PDS-STP and TM-TPDS-STP were not so high (σ rt = 10 −3 -10 −2 S cm −1

Table 2 .
Electrical properties of conducting materials based on 1 and 2.
Cl)) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.