Synthesis and Electrochemical and Computational Analysis of Two

Article pubs.acs.org/JPCC

Synthesis and Electrochemical and Computational Analysis of Two New Families of Thiophene-Carbonyl Molecules Weidong Zhou, Kenneth Hernández-Burgos, Stephen E. Burkhardt, Hualei Qian, and Héctor D. Abruña* Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *

ABSTRACT: Two oligomeric families of materials containing thiophene-carbonyl and thiophene-dicarbonyl units have been synthesized and characterized with emphasis on structural, electrochemical, and electronic properties. Three thiophenecarbonyl derivatives containing 1, 2, and 3 carbonyl groups exhibited one, two, and three one-electron reversible reduction processes, respectively. On the other hand, three thiophenedicarbonyl derivatives exhibited two, four, and six reversible reduction processes, respectively. The electrochemical stability of these materials derives from the existence of the various anionic species, monoanion, dianion, trianion, and so forth, respectively, in the form of enolate anions. Single crystals and density functional theory models of typical structures were obtained and compared to investigate the electronic properties of the two series of molecules. The exhibited electrochemical characters endowed these structures with a potential application prospect in the field of electrical energy storage.

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INTRODUCTION As conductive materials, polythiophene and oligothiophene derivatives1 have garnered much interest in recent years due to their attractive physical properties and excellent performance as electroactive materials in applications such as organic fieldeffect transistors,2 organic light-emitting diodes,3 photovoltaic devices,4 and Li ion batteries (LIBs).5 Oligothiophene derivatives often serve as important model compounds for understanding the fundamental properties of conjugated polythiophenes due to their monodispersity, and hence, a more direct correlation between structure and properties, compared to their polymeric counterparts, can be established. Polythiophene derivatives generally display broad redox peaks in cyclic voltammograms making it difficult to investigate their electrochemical processes in detail. On the other hand, the corresponding oligomers exhibit much narrower and welldefined redox peaks and are thus expected to undergo much better defined changes during the redox processes making them attractive candidates for the investigation of their electrochemical behavior. As a result, a large number of oligothiophene derivatives have been proposed and studied for the past decades in order to better understand molecular and electronic structures,1−4 although the synthesis of discrete oligomers is often more difficult and time-consuming than the synthesis of conjugated polymers. Carbonyl-modified thiophene derivatives, typical organic donor−acceptor structures, have been employed to lower the band gap of conjugated polymers and enhance the overlap of the polymer’s absorption spectrum with solar emission, so as to improve the efficiency of solar cells.4,6 In addition, carbonyl aromatic derivatives have also been shown to be promising candidates as new cathode materials for LIBs because the redox process of the carbonyls © XXXX American Chemical Society

can reversibly yield oxygen anions, which allows for the Li ions to interact and dissociate reversibly with them.7−10 The abovementioned practical applications of these materials require chemical and electrochemical reversibility and stability of the redox processes involved, making the investigation of their electrochemical properties important and indispensable. Inspired by recent contributions in oligomer design,11−13 we have taken interest in new materials that contain thiophene and carbonyl moieties to provide an opportunity for developing new potential electrical energy storage materials. Considering the electrochemical complexity of polymers, we currently seek to understand the reversibility of redox reactions in short conjugated oligomers so as to develop novel high-performance materials employing polythiophenes and their derivatives. In this study, two families of carbonyl-containing molecules have been synthesized (shown in Scheme 1) and characterized using electrochemical and computational strategies to gain a deep understanding of their redox processes. The electrochemical behaviors of these two series of hybrid molecules were investigated in acetonitrile solution using cyclic voltammetry (CV). Single crystal structures, density functional theory (DFT) models, and absorption data were also obtained and compared to better reveal the electronic structure of these molecules. Such a fundamental evaluation is a necessary first step in the development of novel materials that could potentially be incorporated into electrical energy storage applications. Received: October 24, 2012 Revised: March 4, 2013

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dx.doi.org/10.1021/jp310555b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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diethynylthiophene and 2-iodothiophene. Oxidation of compound 4 with DMSO and I2 produced compound D-2 in a yield of 80%.16,17 2,5-Dibromothiophene was debrominated with n-BuLi and then coupled with oxalyl chloride in the presence of CuCl and LiCl giving compound 5 as a yellow powder.18,19 A subsequent Sonogashira coupling with ethynyltrimethylsilane produced compound 6 as a yellow powder, which was then deprotected with K2CO3 to give compound 7. Compound 8 was prepared from compound 7 and 2iodothiophene as a red powder through a Sonogashira coupling reaction. The resulting oxidation reaction with DMSO and I2 provided the target molecule D-3 as a slightly yellow powder. Electrochemical Properties. CV was conducted at room temperature in a three-compartment cell using a glassy carbon electrode (GCE) (5.0 mm diameter) as the working electrode, a Pt wire as the counter electrode, and a silver/silver ion (Ag/ Ag+) as the reference electrode. All experiments were carried out in 0.1 M tetrabutyl-ammonium perchlorate (TBAP) in CH3CN solutions and thoroughly purged with nitrogen gas during experiments. The CV profiles of both series compounds are shown in Figures 1 and 2, respectively. The electrochemical data are collected in Table 1, which presents the formal potentials En (taken as the average of the anodic and cathodic peaks) for the sequential (n = order) reduction processes. As shown in Figure 1, one, two, and three well-defined redox couples were clearly observed for C-1, C-2, and C-3 and could be assigned to the reduction of the carbonyl units to the radical anion, dianion, and triradical anion, respectively. All of these processes were chemically reversible, and the corresponding first reduction peaks of the thiophene carbonyl family were observed at formal potentials of −1.87, −1.50, and −1.28 V relative to Ag/Ag+. That is, the formal potential of the first reduction peak became more positive from C-1 to C-3, indicating that the reduction process became easier with an increase in the number of carbonyl units in the molecules. In a previous report,15b C-3 was shown to exhibit two reversible peaks and a third, irreversible reduction, in DMF at an amalgamated Au working electrode. While speculative on our part, we believe that, at such an electrode, the three-electron reduction product could

Scheme 1. Structures of Compounds C-1, C-2, C-3, D-1, D2, and D-3

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RESULTS AND DISCUSSION Synthesis. The synthesis routes of oligomers C-2, C-3, D-2, and D-3 are depicted in Schemes 2 and 3. Compounds C-1 and D-1 were synthesized according to previous literature.14 A new synthetic strategy for C-2 and C-3 was developed here so as to preclude the need for the highly toxic and complex mercuric reagents used in a previously reported method.15 C-2 was obtained through the coupling reaction of 2-(tributylstannyl)thiophene and thiophene-2,5-dicarbonyl dichloride converted from thiophene-2,5-dicarboxylic acid. 5-Bromothiophene-2carbaldehyde was reacted with 5-bromothiophen-2-lithium at −78 °C followed by oxidation with pyridinium chlorochromate (PCC) to give compound 1 in 45% overall yield. Treatment of compound 1 with ethylene glycol and p-toluenesulfonic acid as the catalyst in anhydrous toluene gave compound 2 in 63% yield. Compound 3 was obtained in 60% yield by reaction of compound 2 with thiophene-2-carbaldehyde in the presence of n-BuLi followed by oxidation with PCC. The resulting deprotecting reaction with trifluoroacetic acid gave the target molecule C-3. As shown in Scheme 3, compound 4 was prepared through a Sonogashira coupling reaction from 2,5Scheme 2. Synthesis Routes for Compounds C-2 and C-3

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Scheme 3. Synthesis Routes for Compounds D-2 and D-3

Figure 2. Cyclic voltammograms at a scan rate of 20 mV/s recorded in CH3CN for 0.5 mM (a) D-1, (b) D-2, and (c) D-3 with Ag/Ag+ as the reference electrode.

Figure 1. Cyclic voltammograms at a scan rate of 20 mV/s recorded in CH3CN for 0.5 mM (a) C-1, (b) C-2, and (c) C-3 with Ag/Ag+ as the reference electrode.

Table 1. Formal Potentials for Thiophene Carbonyls En (n = 1−5) vs Ag/Ag+ (V)

have precipitated onto the electrode surface, rendering the process irreversible. As shown in Figure 2, two, three, and five redox couples were clearly observed for D-1, D-2, and D-3, respectively, and could be similarly attributed to the corresponding reduction of the dicarbonyl units to radical anions. All of these processes were again chemically reversible, and the corresponding formal potentials of the first reduction peaks were observed at −1.31, −0.98, −0.83 V, respectively. This first reduction potential represents a positive shift of about 400 mV when compared with that of the C-1 to C-3 series and indicates that the carbonyl groups became easier to be reduced when present as

C-1 C-2 C-3 D-1 D-2 D-3

E1

E2

−1.87 −1.5 −1.28 −1.31 −0.98 −0.83

−1.88 −1.61 −1.91 −1.44 −1.23

E3

E4

E5

−2.09

−2.35

−2.27 −2.25 −1.57

the form of dicarbonyl units in D-1 to D-3. This is readily understandable when we consider the structures of these two families of molecules. The number of carbonyl units in D-1, DC



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2, and D-3 is twice that in C-1, C-2, and C-3, respectively. Given that carbonyl groups are electron withdrawing in nature, the expectation would be that dicarbonyls should be easier to be reduced as was indeed clearly observed. As shown in the cyclic voltammogram of D-2, it is clear that the current intensity of the peak at −2.25 V was twice of the other two peaks, indicating it is likely a two-electron reduction process. A similar behavior was observed for D-3, where the current peak at −2.09 V was about twice the other four peaks, suggesting a two-electron reduction process. Considering the fact that C-2 and C-3 show two and three single-electron redox processes, it is clear that all the carbonyl units in both families of thiophenecarbonyl and thiophene-dicarbonyl exhibit a reversible redox process, which suggests that these structures could contribute all carbonyl units for future electrical energy storage applications. As can be ascertained from Table 1, the difference in potential between consecutive redox states increased from 330 to 660 mV in C-3. In the three redox processes associated with this tricarbonyl compound, a dianion C-32− (see Scheme 4) is

Figure 3. Cyclic voltammograms for 0.5 mM C-2 on a bare GCE in a 0.1 M TBAP/CH3CN solution at different scan rates. Inset presents 1

the ip−v /2 plots obtained from the cyclic voltammograms.

Scheme 4. Two-Electron Electrochemical Reduction Processes of C-3 and D-2

Figure 4. Cyclic voltammograms for 0.5 mM D-1 on a bare GCE in a 0.1 M TBAP/CH3CN solution at different scan rates. Inset presents 1

the ip−v /2 plots obtained from the cyclic voltammograms.

formed after reduction by two electrons. Such a species is analogous to a quinone dianion, which is generally stable. Such stability, typically associated with delocalization effects, renders further reduction unfavorable and thus the large increase in the difference between the second and third reduction processes. An analogous effect was observed for D-2, where the peak separation between consecutive peaks rose from 460 to 810 mV. (see Scheme 4). To further characterize the electrochemistry of the carbonyl units in these two series of molecules, the cyclic voltammograms of C-2 and D-1 were studied and compared in detail, since they both have two carbonyl units. The main panels of Figures 3 and 4 present cyclic voltammograms at different scan rates for C-2 and D-1. The peak potential separations for the redox couples were all less than 70 mV and did not change for sweep rates from 10 up to 200 mV/s, indicating that the reactions are all one-electron processes and electrochemically 1 reversible under these conditions. Plots of ip versus v /2 (insets in Figures 3 and 4) were linear suggesting that the processes are mass transport controlled and the reduced species are chemically stable on this time scale. The first formal potentials of C-2 and D-1 were −1.50 and −1.31 V, respectively. The 190 mV potential difference is a clear indication that D-1 is easier to be reduced than C-2. On the other hand, the formal potentials for the second reduction peak of C-2 and D-1 were −1.88 and −1.91 V, respectively. These values are much closer to each other and indicate that the radical anions C-2−· and D-1−·

exhibit similar affinities toward the second electron. These observations are also consistent with the chemical structures of C-2 and D-1. As shown in Scheme 5, C-2 contains two carbonyl groups separated by a thiophene unit. The thiophene unit, as an electron donor, can delocalize its electrons to the carbonyl unit via π-electron interactions and lower the reduction potential of the carbonyl units. On the other hand, two carbonyl units are connected directly in D-1 with no thiophene bridge so that the carbonyl can be more easily reduced, resulting in the relatively positive first reduction potential. Single Crystal Structures. The single-crystal X-ray structures of C-2, D-1, and D-2 along with side views of each molecule are shown in Figure 5. Single crystals of C-2, D-1, and D-2 suitable for diffraction analysis were grown via slow diffusion of hexane into CH2Cl2 solutions. Efforts to grow single crystals of C-3 and D-3 failed because of their poor solubility. C-2 shows a twisted structure with three thiophene units on the same side as the carbonyl groups. Interestingly, the single-crystal structure of D-1 exhibited a geometrically coplanar conformation and an anticonformation of the two thiophene units. As for D-2, thiophene rings 1 and 2 also adopt an anticonformation as shown in Figure 5c. On the other hand, thiophene ring 3 rotates around the C12−C13 bond and adopts two distinct conformations, syn and trans, in the solid state. D



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Scheme 5. Reaction Scheme for Electrochemical Reduction of C-2 and D-1 to Give the Radical Anions and Dianions

Figure 5. Single-crystal structures and side views of molecules (a) C-2, (b) D-1, and (c) D-2. Intramolecular S---O distances are indicated by dotted lines with an error limit of 0.002 Å.

carbon−carbon bond to adopt various conformations. In D-1, two thiophene units adopt an anticonformation in a coplanar structure, and a short S---O distance of 2.760 Å was observed as well. Even for D-2, the S---O distances were all less than 2.98 Å, which are again clearly shorter than the sum of the van der Waals radii of sulfur and oxygen, indicating the existence of self-

Interatomic distances in a single crystal generally reflect the extent of interactions. In C-2, the S1---O1, O1---S2, S2---O2, and O2---S3 distances were 2.892, 2.889, 2.974, and 2.953 Å, respectively. These values are shorter than the sum of the van der Waals radii of sulfur and oxygen (1.85 Å + 1.50 Å = 3.35 Å), although the thiophene units can rotate freely around the E



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rigidification in both series of molecules. It has previously been observed that, in poly-3,4-ethylenedioxythiophene (PEDOT), the distance between the sulfur atom in thiophene to the neighboring oxygen atom is less than the sum of the van der Waals radii of the two atoms.20−23 Two theories have been put forth to explain these features: S---O noncovalent intramolecular interactions and π-electron interactions.23−25 The driving force here could be associated with the π-electron interactions, since the thiophene-carbonyl structure is a typical electronic donor−acceptor system and the π electrons favor delocalization over the entire molecule. D-1 adopts a coplanar structure, which maximizes the electronic delocalization and lowers the energy. In D-2, the anticonformation of thiophene rings 1 and 2 could also be attributed to the delocalization of π electrons as in D-1. As shown in the single-crystal structure of D-2, thiophene rings 1 and 2 and carbonyl C11---O3 are quasicoplanar, which facilitates π-electron delocalization. In addition, it is clear that thiophene ring 3 and carbonyl C12---O4 are also coplanar in both conformations, which would benefit from the π-electron delocalization and consequently lower the energy. As for the reason of thiophene ring 3 adopting two distinct conformations, it could be attributed to a break of conjugation around the C11−C12 bond in long linear D-2. On the other hand, if S---O attractive interactions were at play, the interaction strengths and energy levels in the two conformations should be different because the S3---O3 and S3---O4 distances are different. That is, the molecule should only adopt one fixed conformation if S---O attractive interactions were really operating here. The C−C bond lengths comparisons between the singlecrystal structures and DFT lower energy conformations are shown in Figure 6. As can be observed, these values are similar but not exactly the same. These discrepancies can be attributed, at least in part, to the fact that the calculations were performed with the conductor polarizable continuum model (CPCM)26,27 of acetonitrile as a solvent and the single crystals are all in the solid state. Among the DFT lower energy conformations, C-2 shows a twisted structure with three thiophene units on the same side of the carbonyl groups, which is in good agreement with the single-crystal structure shown in Figure 5a. As for D-1 and D-2, the DFT lowest energy conformations show some differences relative to the single crystals. From the DFT calculations, the single-crystal structures of D-1 and D-2 have energies of 0.029 and 0.158 eV above the lowest energy level, respectively. Comparison of the two conformations of D-2 in the single crystal yielded an energy difference of only 10 meV, which means that the two conformations are isoenergetic. A scheme that summarizes all the DFT conformational studies is shown in the Supporting Information. To further study the structures of these molecules and validate the calculation results, the UV−vis absorption spectra of both series of molecules were measured in CH3CN solution (1 × 10−5 M) and as solid films. As shown in Table 2, the maximum absorption wavelengths (λmax) of C-1, C-2, and C-3 on the films did not exhibit significant shifts when compared to results in CH3CN solution. These features suggest that C-1, C2, and C-3 adopt similar conformations both in solution and in the solid state. On the other hand, the λmax‑film of D-1, D-2, and D-3 films on quartz exhibited significant redshifts of about 52, 41, and 35 nm when compared with results of λmax‑sol in CH3CN solution. In addition, the λmax‑sol of D-1 was 20 nm less than those of C-1 and C-2, which is different from the results of single crystals where D-1 exhibited a longer conjugation than

Figure 6. C−C bond length comparison of DFT models (a) C-2, (b) D-1, and (c) D-2 with single-crystal structures.

Table 2. UV−Vis Absorption Data for the Two Families of Compounds in CH3CN Solution (λmax‑sol, 1 × 10−5 M) and as Solid Films on Quartz Plates (λmax‑film) compd

λmax‑sol (nm)

λmax‑film (nm)

C-1 C-2 C-3 D-1 D-2 D-3

328.8 328.9 341.9 308.8 335.2 346.3

337.2 339.3 352.1 360.6 376.5 381.2

C-2. These data suggest that the series of D-1, D-2, and D-3 adopts different structures in CH3CN solutions and in the solid state. In solution, the thiophene units in D-1, D-2, and D-3 are free to rotate around the carbon−carbon single bond, while in solid films, D-1, D-2, and D-3 all tend to adopt coplanar and conjugated structures to facilitate the electronic delocalization restricting the rotation of thiophene units. All of these data can F



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also explain the differences between DFT results and single crystals, where the single-crystal structures of D-1 and D-2 have a little higher energy than the DFT lowest energies models simulated in CH3CN. Extensive calculations on both families of thiophene-carbonyl materials were carried out to reveal the underlying electronic properties. The two series of structures were first optimized in planar, all-anti conformations, using the Universal Force Field28 (UFF) implemented in the Avogadro 1.0.1 software program.29 All calculations were performed using Gaussian0930 at the DFT level.31 B3LYP32,33 was used to further optimize the structures and obtain the minimum energy for the specific conformation. The 6-31+G(p,d) basis set was employed to achieve better accuracy in treating anionic compounds. The CPCM26,27 for an acetonitrile solution was adopted with a solvent cavity using UFF radii, which places a sphere around each solute atom (including hydrogen atoms). The calculated first reduction potentials of each molecule are shown in Figure S3 (Supporting Information). Those values were in good agreement with experimental results, indicating that, with this method, a good description of ground-state properties for neutral and radical anion species could be obtained. This clearly validates our approach of using this method to obtain structural information for both families of molecules. In order to have a better understanding of the structural changes in the molecule during the reduction process, the bond deformations during the reduction process were studied in the two families of molecules. The use of the structural analysis reported by Moro et al.,34,35 where a change in bond length was induced by the addition of one electron, helped us understand the structural changes in the molecule during the reduction process. With this analysis, we were able to gain a better understanding of the most probable place for the electron during the reduction process. Figure 7 presents the bond deformation induced by the presence of one electron in C-2 and D-1.34,36 In Figure 7a (for C-2), a symmetrical deformation in C−C bonds 5 and 9 with an increase of 0.05 Å was observed with respect to the neutral species. The spin density (SD) surface, which shows the unpaired spin in the radical anion species, is located in the central thiophene ring and the carbonyl groups. In Figure 7b, D-1, a deformation in C−C bond 5 between the two carbonyl units with an increase of 0.09 Å, is observed versus the neutral species, which shows the unpaired electron delocalization to be in the dicarbonyl units.25 In the Supporting Information, SD surfaces and bond deformation plots for all the other four molecules are provided. These features allow us to explain the delocalization of the electron during the reduction process (Scheme 5 and Figure 7). Upon comparing Scheme 5 with the SD structures and bond deformation plots, one can appreciate that the reduction processes are all located in the carbonyl units, which is in agreement with the electrochemistry results presented above.

Figure 7. SD surfaces and bond length deformation plots of the oneelectron reduction for molecules C-2 and D-1.

exhibited more positive reduction potentials. In addition, the molecules containing the dicarbonyl units tend to adopt a planar structure in the solid state due to π-electron delocalization between adjacent thiophene and carbonyl units. A comprehensive comparison of single-crystal X-ray structures, DFT models, and absorption spectra offered reasonable explanations for the spectral differences observed between the solid state and solution. We believe that the introduction of additional carbonyl units in the molecular structure will give rise to more positive potentials, making these materials attractive candidates for electrical energy storage applications.

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CONCLUSION Two families of thiophene-carbonyl structures have been synthesized and characterized by electrochemistry, singlecrystal structures, as well as computation. Both the CV (electrochemical) experiments and theoretical calculations suggested that all the thiophene-carbonyl structures can experience reversible electrochemical reduction processes and that the molecules become easier to reduce with an increase in the number of carbonyl units. Relative to the single-carbonylcontaining compounds, the dicarbonyl-containing compounds

EXPERIMENTAL SECTION General Methods. All reagents were obtained from commercial suppliers and used as received unless otherwise noted. Column chromatography was performed on silica gel (160−200 mesh) and thin layer chromatography was performed on precoated silica gel plates. NMR spectra were recorded on Bruker Avance DPS-300 and Bruker Avance DPS500 spectrometers. All single crystals were grown via slow diffusion of hexane into CH2Cl2 solutions at room temperature.

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Compound C-2. Thiophene-2,5-dicarboxylic acid (0.85 g, 5 mmol) was added to sulfurous dichloride (10 mL) at room temperature. The mixture was refluxed for 5 h and then concentrated under reduced pressure. The white thiophene-2,5dicarbonyl dichloride and 2-(tributylstannyl)thiophene (3.7 g, 10 mmol) were added to 200 mL of toluene at room temperature. The reaction mixture was refluxed for 10 h under a nitrogen atmosphere and then concentrated under reduced pressure. The resulting solid was subjected to successive column chromatography over silica gel (CH2Cl2/n-C6H14, 2/1) to give a white compound C-2 1.08 g (71%). 1H NMR (300 MHz, CDCl3): 7.97(d, 2H, J = 3.9 Hz), 7.91(s, 2H), 7.77(d, 2H, J = 4.5 Hz), 7.23(t, 2H, J = 4.1 Hz). 13C NMR (75 MHz, DMSO): 178.97, 147.76, 142.31, 137.29, 135.94, 134.56, 130.07. MS (EI): 303. Elemental analysis (C14H8O2S3): C, 55.24; H, 2.65; S, 31.60; found C, 55.22; H, 2.66; S, 31.55. Compound 1. n-BuLi(6.5 mL, 1.5 mol/L) was added to a solution of 2,5-dibromothiophene (2.4 g, 10 mmol) in anhydrous THF at −78 °C under a nitrogen atmosphere. The mixture was stirred for 2 h, and 5-bromothiophene-2carbaldehyde (1.9 g, 10 mmol) was added dropwise at −78 °C. The mixture was stirred for 30 min after the temperature was raised to room temperature. Then, 30 mL of water was added, and the aqueous phase was extracted with CH2Cl2 (3 × 50 mL). The CH2Cl2 extracts were combined and dried with anhydrous Na2SO4. The solvent was removed under reduced pressure to afford a white solid, which was used without further purification. The white solid was dissolved in 100 mL of CH2Cl2/THF (1/1), and pyridinium chlorochromate (PCC) (2 g) was added in one portion. The dark brown suspension was stirred at room temperature overnight. The reaction mixture was poured into 200 mL of H2O, and the organic phase was separated. The aqueous phase was extracted with CH2Cl2, and the solvent was then removed under reduced pressure. Purification was accomplished by column chromatography on silica to afford a white solid 1.6 g (45%). 1H NMR (300 MHz, CDCl3): 7.59(d, 2H, J = 4.05 Hz), 7.14(d, 2H, J = 4.05 Hz).13C NMR (75 MHz, CDCl3): 176.61, 143.97, 133.64, 131.62, 123.21. MS (EI): 349. Elemental analysis (C9H4Br2OS2): C, 30.70; H, 1.15; S, 18.22; found C, 30.77; H, 1.19; S, 18.19. Compound 2. To a toluene solution of compound 1(1.5 g, 4.3 mmol) and ethylene glycol (15 mL) in a flask with a Dean− Stark trap, p-toluenesulfonic acid (20 mg) was added. The solution was heated to reflux and stirred for 36 h in an atmosphere of nitrogen. The reaction mixture was washed with potassium carbonate solution, and the organic layer was dried over anhydrous Na2SO4. The crude product was purified by column chromatography with CH2Cl2/hexane (2/1, v/v) and gave compound 2 (1.0 g) as an oil in a yield of 63%. 1H NMR (300 MHz, CDCl3): 6.91(d, 2H, J = 3.2 Hz), 6.80(d, 2H, J = 3.2 Hz), 4.13(s, 4H). 13C NMR (75 MHz, CDCl3): 146.05, 130.07, 127.54, 114.24, 105.62, 65.93. MS (EI): 393. Elemental analysis (C11H8Br2O2S2): C, 33.35; H, 2.04; S, 16.19; found C, 33.39; H, 2.09; S, 16.07. Compound 3. To a stirred solution of compound 2 (0.8 g, 2 mmol) in 100 mL of anhydrous THF at −78 °C, n-BuLi (2.7 mL, 1.5 mol/L) was added under a nitrogen flow. The reaction mixture was stirred for 1 h, and then, thiophene-2-carbaldehyde (448 mg, 4 mmol) was added to the solution. Gray solids precipitated immediately. The solution was washed with water and CH2Cl2 (3 × 50 mL), and the organic layer was dried over anhydrous Na2SO4 and filtered. The filtrate was reduced in volume to obtain a white solid that was used for the next

reaction without further purification. The crude product was dissolved in 100 mL of anhydrous CH2Cl2/THF (1/1). PCC (1 g) was added in one portion, and the dark brown suspension was stirred at room temperature for 8 h. The reaction mixture was then poured into 200 mL of H2O and washed with CH2Cl2 (3 × 50 mL). The organic layer was dried over anhydrous Na2SO4, concentrated, and purified by column chromatography with CH2Cl2. The white product, compound 3, was obtained with a yield of 60% (0.55g). 1H NMR (300 MHz, CDCl3): 7.89(d, 2H, J = 3.75 Hz), 7.76(d, 2H, J = 3.94 Hz), 7.70(d, 2H, J = 5.02 Hz), 7.18(m, 4H), 4.23(s, 4H). 13C NMR (75 MHz, CDCl3): 179.17, 152.87, 143.55, 142.91, 134.32, 133.83, 133.33, 128.59, 127.47, 105.69, 66.49. MS (EI): 457. Elemental analysis (C21H14O4S4): C, 55.00; H, 3.08; S, 27.97; found C, 55.09; H, 3.12; S, 27.91. Compound C-3. TFA (1 mL) was added to a solution of compound 3 (0.5 g) in CH2Cl2, which was stirred at room temperature for 30 min. Gray solids precipitated gradually. The mixture was filtered under reduced pressure to give gray powders, which were further purified by recrystallization from hot chloroform. The pure compound C-3 was obtained as gray needlelike crystals. 1H NMR (300 MHz, DMSO): 8.21(m, 4H), 8.15(m, 4H), 7.36(t, 2H, J = 4.4 Hz). 13C NMR (125 MHz, DMSO, 363K): 178.18, 178.09, 148.27, 146.47, 141.93, 136.58, 135.31, 134.85, 133.89, 129.46. MS (EI): 413. Elemental analysis calcd (%) for (C19H10O3S4): C, 55.05; H, 2.43; S, 30.94; found C, 55.09; H, 2.41; S, 30.89. Compound 4. 2,5-Diethynylthiophene (0.65 g, 5 mmol) and 2-iodothiophene(2.1 g, 10 mmol) were dissolved in 100 mL of anhydrous THF/Et3N (1/1) under a nitrogen flow. Pd(PPh3)4 (20 mg) and CuI (20 mg) were added at room temperature. The above mixture was stirred for 8 h, and the solvent was evaporated off. The residue solid was purified by column chromatography with CH2Cl2/hexane (1/5, v/v) to give compound 4 (1.2 g, 81%) as a gray solid. 1H NMR (300 MHz, CDCl3): 7.32(m, 4H), 7.15(s, 2H), 7.02(m, 2H). 13C NMR (75 MHz, CDCl3): 132.98, 132.46, 128.53, 127.72, 124.96, 122.94, 88.02, 86.33. MS (EI): 295. Elemental analysis (C16H8S3): C, 64.83; H, 2.72; S, 32.45; found C, 64.91; H, 2.79; S, 32.39. Compound D-2. Iodine (500 mg, 2 mmol) was added to a solution of compound 4 (600 mg, 2 mmol) in 20 mL of DMSO. The mixture was stirred at 155 °C for 10 h. The solution was poured into 1% aq Na2S2O3 (50 mL) and was filtered under reduced pressure. The residue solid was purified by column chromatography with CH2Cl2 to give compound D2 as a slightly yellow powder in a yield of 80%. 1H NMR (300 MHz, CDCl3): 8.14(dd, J = 4.9 Hz, J = 1.05 Hz, 2H), 8.08(s, 2H), 7.89(dd, J = 4.9 Hz, J = 1.05 Hz, 2H), 7.24(m, 2H). 13C NMR (75 MHz, CDCl3): 182.74, 181.39, 145.68, 138.85, 138.26, 138.16, 136.62, 129.32. MS (EI): 359. Elemental analysis (C16H8O4S3): C, 53.32; H, 2.24; S, 26.69; found C, 53.38; H, 2.27; S, 26.65. Compound 5. n-BuLi (6.5 mL, 1.5 mol/L) was added to a solution of 2,5-dibromothiophene (2.4 g, 10 mmol) in anhydrous THF at −78 °C under a nitrogen atmosphere. The mixture was stirred for 2 h at −78 °C. Anhydrous LiCl (0.84 g, 20 mmol) was added at room temperature to a suspension of CuCl (0.98 g, 10 mmol) in THF under nitrogen. The resulting mixture was stirred for 1 h until it became homogeneous and cooled to 0 °C. A solution of 2-bromo-5lithium-thiophene was quickly added to the solution of CuCl·2LiCl followed by addition of oxalyl chloride (0.63 g, 5 H



Article

mmol). The mixture was stirred at 0 °C for 20 min, quenched with saturated aqueous NH4Cl, and extracted with CH2Cl2. The organic extracts were dried over Na2SO4 and concentrated under vacuum. The residue solid was purified by silica gel chromatography with eluent of CH2Cl2/n-C6H14 (3/1) leading to compound 5 as a yellow solid. 1H NMR (300 MHz, CDCl3): 7.89(d, 2H, J = 4.17 Hz), 7.18(d, 2H, J = 4.17 Hz). 13C NMR (75 MHz, CDCl3): 179.75, 138.78, 138.15, 132.08, 128.50. MS (EI): 377. Elemental analysis (C10H4Br2O2S2): C, 31.60; H, 1.06; S, 16.87; found C, 31.65; H, 1.09; S, 16.81. Compound 6. Ethynyltrimethylsilane (1.5 g, 15 mmol) was added to Et3N solution (1.8 g, 5 mmol) under nitrogen at room temperature. Then, Pd(PPh3)4 (20 mg) and CuI (20 mg) were added quickly. After 16 h, the solution was washed with CH2Cl2 (3 × 80 mL) and H2O (3 × 50 mL), the organic layer was dried over anhydrous Na2SO4 and filtered, and the filtrate was reduced in volume to obtain a white solid. Purification was accomplished by column chromatography on silica with CH2Cl2/n-C6H14 (3/1) to give compound 6. 1H NMR (300 MHz, CDCl3): 7.97(d, 2H, J = 4.1 Hz), 7.23(d, 2H, J = 4.1 Hz), 0.27(s, 18H). 13C NMR (75 MHz, CDCl3): 180.84, 138.15, 137.43, 135.70, 133.61, 105.78, 96.89, 0.005. MS (EI): 413. Elemental analysis (C20H22O2S2Si2): C, 57.93; H, 5.35; S, 15.46; found C, 57.96; H, 5.39; S, 15.40. Compound 7. To a solution of compound 6 in THF/ MeOH (1/1) was added anhydrous K2CO3 under nitrogen at room temperature. After stirring for 4 h, the reaction mixture was then washed with CH2Cl2 (3 × 80 mL) and H2O (3 × 50 mL). The organic layer was dried over anhydrous Na2SO4 and concentrated under vacuum. The residue solid was purified through column chromatography to give brown compound 7. 1 H NMR (300 MHz, CDCl3): 8.01(d, 2H, J = 4.1 Hz), 7.30(d, 2H, J = 4.1 Hz), 3.62(s, 2H). 13C NMR (75 MHz, CDCl3): 180.68, 138.42, 137.41, 134.47, 134.19, 86.79, 76.52. MS (EI): 269. Elemental analysis (C14H6O2S2): C, 62.20; H, 2.24; S, 23.72; found C, 62.27; H, 2.29; S, 23.67. Compound 8. To a stirred solution of compound 7 (800 mg, 3 mmol) and 2-iodothiophene (1.36 g, 6.5 mmol) were added Pd (PPh3)4 (20 mg) and CuI (20 mg) under a nitrogen flow. After stirring for 8 h, the solution was then washed with CH2Cl2 (3 × 80 mL) and H2O (3 × 50 mL). The organic layer was dried over anhydrous Na2SO4 and filtered, and the filtrate was reduced in volume to obtain a solid mixture. Purification was accomplished by column chromatography on silica with CH2Cl2/n-C6H14 (3/1) to give red compound 8. 1H NMR (300 MHz, CDCl3): 8.05(d, 2H, J = 3.9 Hz), 7.41(m, 2H), 7.38(m, 2H), 7.29(d, 2H, J = 3.9 Hz), 7.07(m, 2H). 13C NMR (75 MHz, CDCl3): 180.83, 138.37, 137.76, 135.64, 133.81, 132.99, 129.51, 127.87, 122.16, 92.34, 86.32. MS (EI): 433. Elemental analysis (C22H10O2S4): C, 60.80; H, 2.32; S, 29.51; found C, 60.91; H, 2.35; S, 29.39. Compound D-3. A mixture of compound 8 (86 mg, 0.2 mmol) and I2 (50 mg, 0.2 mmol) in 10 mL of DMSO was heated at 140 °C for 10 h. The solution was then poured into 1% aq Na2S2O3 solution (100 mL). A yellow powder, which precipitated in the solution, was filtered and washed with water and MeOH. After drying in vacuum, a slightly yellow powder was obtained in a yield of 85%. 1H NMR (300 MHz, DMSO): 8.31(d, 2H, J = 4.9 Hz), 8.17(m, 4H), 8.10(d, 2H, J = 4.2 Hz), 7.36(t, 2H, J = 4.4 Hz). 13C NMR (125 MHz, DMSO, 363 K): 182.93, 181.52, 180.95, 145.11, 144.45, 139.68, 138.36, 137.75, 137.11, 136.70, 129.64. MS (EI): 497. Elemental analysis calcd

(%) for (C22H10O6S4): C, 53.00; H, 2.02; S, 25.73; found C, 53.07; H, 2.05; S, 25.69.

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ASSOCIATED CONTENT

S Supporting Information *

DFT calculated energy levels for different conformations of all six molecules; bond length deformation of the one-electron reduction for molecules C-1, C-3, D-2, and D-3; and crystallographic information for compounds C-2, D-1, and D2 as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (607) 255-4720; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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