Synthesis, Crystal Structure, and Electropolymerization of 1,4-Di([2,2′-bithiophen]-3-yl)buta-1,3-diyne

Abstract

For their great structural versatility, thiophene-based π-conjugated systems have been widely exploited in the preparation of low band gap materials. Here, we report the synthesis of a highly conjugated tetrathiophene system, namely 1,4-di([2,2′-bithiophen]-3-yl)buta-1,3-diyne (1), that presents two bithiophene units connected at position 3 by a butadiynylene spacer. Single-crystal X-ray diffraction (SC-XRD) analysis elucidated the structure of 1, confirming the planarity of the molecule. The molecule was then electropolymerized onto the surface of a gold-coated piezoelectric quartz crystal, showing a high reactivity that is ascribable to the extended conjugation. The frontier molecular orbital energies of 1 were obtained via DFT optimization performed on the crystal structure-derived molecular geometry. Finally, DFT was also used to estimate the polymer band gap.

1. Introduction

Thiophene-based conjugated systems are one of the most extensively investigated classes of organic semiconductors [1,2,3,4] due to their potential industrial applications as active materials in devices such as field-effect transistors (OFETs) [5,6], organic light-emitting diodes (OLEDs) [7,8], photovoltaic cells (OPVs) [9], and sensors [10]. Furthermore, thiophene-containing systems are easily electropolymerized thanks to their extended conjugation, providing materials that show both plastic-like (elasticity, low density, low molecular weight, and flexibility) and metal-like (conductivity) properties [11,12].

Both the molecular and crystal structure strongly affect the properties of π-conjugated molecules and the related functional materials, thereby warranting structure–property investigations [13,14,15,16,17,18]. With regard to this, organic π-conjugated systems containing 1,3-butadiynylene spacers between heteroaromatic rings have been presented [19,20,21]. The presence of the triple bonds in covalent motifs yields materials with good thermal and environmental stability and, significantly, with no steric distortion between adjacent aromatic rings [22]. This, in turn, allows for a reduction in the energy gap, thus achieving near-infrared (NIR) absorbers (of relevance to OPVs) [23] and emitters (of relevance to OLEDs) [24]. Therefore, the design and synthesis of new π-conjugated molecules with tailored optoelectronic properties is highly desired.

Here, we present the synthesis and the electropolymerization of a highly conjugated tetrathiophene system, i.e., 1,4-di([2,2′-bithiophen]-3-yl)buta-1,3-diyne (1, Figure 1), where two bithiophene units are linked together at position 3 through two ethynyl bonds. The solid-state structure of 1, obtained via single-crystal X-ray diffraction (SC-XRD), confirmed the planarity of the molecule. Computational studies were carried out to calculate the frontier molecular orbital energies and to estimate the polymer band gap.

2. Materials and Methods

Glassware was flame-dried to remove adsorbed water. Reactions were conducted under an argon atmosphere using anhydrous solvents. All commercial reagents were used without further purification. Flash column chromatography was conducted using silica gel 60 (Fluka 230–400 mesh or Merck 70–230 mesh). ¹H and ¹³C NMR spectra were recorded using a Bruker AVANCE 300 (300 MHz) spectrometer at 298 K (Bruker Corporation, Billerica, MA, USA). Chemical shifts (δ) are reported in ppm relative to the residual proton resonances of the NMR solvents. High-resolution ESI-LTQ Orbitrap MS analyses were performed on a Mass Spectrometer LTQ ORBITRAP XL Thermo (Thermo Fisher Scientific Inc., Waltham, MA, USA). Attenuated Total Reflectance Fourier-Transformed Infrared spectra (ATR-FTIR) were acquired on a Perkin Elmer FT-IR Spectrum Two (Perkin Elmer, Waltham, MA, USA), equipped with a diamond crystal. The PXRD analyses were performed using Ni-filtered Cu Kα radiation on a Rigaku SmartLab XE diffractometer equipped with a HyPix-3000 detector (Rigaku, Tokyo, Japan).

3-Ethynyl-2,2′-bithiophene was prepared according to the procedure reported in the literature [25].

Preparation of 1,4-di([2,2′-bithiophen]-3-yl)buta-1,3-diyne (1). In a Schlenk tube, 3-ethynyl-2,2′-bithiophene (0.090 g, 0.45 mmol) was dissolved in THF (5 mL), and N,N-diisopropylethylamine (DIPEA, 0.093 mL, 0.54 mmol) was added. The mixture was de-gassed (3 × freeze–pump–thaw cycles), and PdCl₂(PPh₃)₂ (0.006 g, 9 μmol) and CuI (0.002 g, 9 μmol) were added, followed by ethyl bromoacetate (0.030 mL, 0.27 mmol). The reaction mixture was stirred at room temperature for 24 h. The solvent was removed under a reduced pressure and the brown residue was recovered with CH₂Cl₂ and was passed through a short plug of silica. Flash column chromatography (silica gel, hexane/ethyl acetate 95:5) afforded product 1 as a bright yellow solid, which was further purified via recrystallization from a CH₂Cl₂/hexane mixture (0.053 g, 0.14 mmol). Yield: 62%. ¹H NMR (CDCl₃, 300 MHz): δ (ppm) = 7.57 (dd, 2H, J₁ = 3.7 Hz, J₂ = 1.2 Hz, ArH_3′); 7.35 (dd, 2H, J₁ = 5.1 Hz, J₂ = 1.3 Hz, ArH_5′); 7.16–7.06 (m, 6H, ArH₅ + ArH₄ + ArH_4′). ¹³C NMR (CDCl₃, 75 MHz):δ = 135.9; 135.6; 131.6; 127.5; 125.9; 125.9; 122.9; 115.8; 78.8; 78.7. ESI-FT-Orbitrap-MS: calculated for C₂₀H₁₁S₄ [M + H]⁺ m/z = 378.9738, found m/z = 378.9744.

Electropolymerization of 1. The electropolymerization and subsequent characterization of the polymeric coatings were conducted utilizing a computerized Autolab PGSTAT 20 potentiostat (Ecochemie, Utrecht, The Netherlands), operated via the Ecochemie GPES 4.9 software. The polymer was synthesized using cyclic voltammetry (CV) within a 5 mL customized Teflon cell, using a gold-coated piezoelectric quartz crystal (PQC) as the working electrode, a silver/silver chloride pseudoreference as the reference electrode, and a platinum rod as the counter electrode. The silver pseudoreference electrode was calibrated using ferrocene as a redox probe, resulting in an E_1/2 value of 0.402 V. This value was calculated as the mean of the cathodic and anodic peak potentials observed in CV scans. With regard to the PQC units, 10 MHz AT-cut piezoelectric quartz crystals, coated using gold electrodes on both sides, were exploited. The crystal diameter and thickness were 13.9 mm and 160 μm, respectively, whereas the diameter of the gold electrodes was 6.0 mm. Monomer 1 was electropolymerized on the PQC starting from 5 × 10⁻⁴ M solutions, prepared in a 1:5 mixture of acetonitrile and dichloromethane. Tetrabutylammonium hexafluorophosphate was used as the supporting electrolyte, at a concentration of 0.1 M. The potential window for the electropolymerization process was set between −1 and +1.25 V, with a scan rate of 0.1 V/s. The frequency signal was acquired simultaneously with the current output in order to monitor the growth of the polymer in the EQCM mode. This approach has enabled a standard thickness to be maintained throughout the electropolymerization process, with a cutoff frequency shift of −2 kHz, corresponding to 8 × 10⁻⁶ g of a polymeric film, as predicted by the Kanazawa–Gordon equation (see Supplementary Materials). With regard to the time required for the electropolymerization process, it can be observed that the growth of the polymeric film took a total of 675 s, which is equivalent to just over 11 min. This was determined by considering the forward and backward scans of the 15 CV cycles in the potential range between −1 and +1.25 V and a scan rate of 0.1 V/s.

The obtained polymeric film was characterized under the aforementioned experimental conditions, using a monomer-free solution containing only the supporting electrolyte.

3. Results

3.1. Synthesis of Tetrathiophene 1

Compound 1 was synthesized via the one-step palladium-catalyzed homocoupling of the corresponding terminal alkyne [26], as reported in Scheme 1. The homocoupling reaction of 3-ethynyl-2,2′-bithiophene, prepared following a previously reported procedure [25], was performed in THF at room temperature, in the presence of PdCl₂(PPh₃)₂ and CuI as catalysts (0.02 eq. each). Ethyl bromoacetate (0.6 eq.) was used to initiate the coupling reaction through the formation of a PdBr(enolate) intermediate via the oxidative addition of a Pd(0) species [27]. Compound 1 was obtained in good yield after column chromatography and was fully characterized using nuclear magnetic resonance (NMR) spectroscopy (Figures S1–S3) and ESI-FT-Orbitrap mass spectrometry (MS). Crystals of 1 were obtained via the slow evaporation of a dichloromethane/hexane solution as yellow thin plates of micrometric dimensions.

3.2. Solid-State Crystal Structure

The crystal structure of 1 was determined using a single-crystal X-ray diffraction experiment. The information about crystal data, data collection, and refinement is reported in Table S1. The compound crystallizes in the monoclinic space group P2₁/c. The title molecules are situated on a center of inversion, so that their asymmetric unit comprises half of the molecule (Figure 2).

The synthesis and crystallographic data of an isomer of 1, namely 1,4-bis(2,2′-bithiophen-5-yl)-1,3-butadiyne, in which the 1,3-butadiyne moiety connecting the two bithiophene fragments is bonded in position 5 of the ring instead of in position 3, has been reported [28]. Molecule 1 is essentially planar, and the dihedral angle between the mean planes passing through the two thiophene rings C1–C4/S1 and C5–C8/S2 is 8.47(8)°.

The bond distances are all in a normal range for thiophene/alkyne compounds of this type [14,29,30]; in particular, the presence of a triple bond between atoms C9 and C10 is evidenced by a value of 1.199(5) Å; C3–C9 and C10–C10ⁱ are of 1.415(5) and 1.384(5) Å, respectively, indicating that some delocalization is present in the linear chain C3–C9–C10–C10ⁱ–C9ⁱ–C3ⁱ, which connects the two 2,2′-bithiophene units (i = 1 − x, −y, 1 − z).

The crystal packing is driven by two sets of C-H∙∙∙π interactions, as follows: (i) supramolecular contacts of the type C1-H1∙∙∙Cg1ⁱⁱ (where Cg1 is the centroid of the ring C5–C8/S2 and ii = x, 3/2 − y, ½ + z) [see Figure 3a; H1∙∙∙Cg1, 2.94 Å; C1∙∙∙Cg1, 3.583(4) Å; C1-H1∙∙∙Cg1, 126°]; (ii) weak, symmetry-related C6-H6∙∙∙π (C7)ⁱⁱⁱ (iii = −x, 1/2 + y, 1/2 − z) and C7∙∙∙H6-C6^iv (iv = −x, −1/2 + y, 1/2 − z) interactions [see Figure 3b; H6∙∙∙C7, 2.82 Å; C6∙∙∙C7, 3.693(6) Å; C6-H6∙∙∙C7, 152°].

3.3. Electropolymerization Experiments

Compound 1 was electropolymerized on the gold surface of a piezoelectric quartz crystal, yielding a compact reddish polymeric film [4,31,32]. The potentiodynamic electropolymerization, monitored via the simultaneous acquisition of current and frequency outputs, showed the high reactivity of 1, if compared to parent 2,2′-bithiophene derivatives β-functionalized with electron-donor moieties [33]. On the other hand, the oxidation potential of 1 (≈1.1 V vs. Ag/AgCl pseudoreference) resulted in a higher value with respect to similar ethynyl-bridged polythiophenes functionalized in the alpha positions, with respect to the sulfur heteroatom, of the thiophene ring [21]. Conversely, the β-bridging of the molecule keeps the reactivity of the α-positions of both bithiophene moieties, allowing them to behave as standalone dimeric building blocks, but improving the stability of the polymer, thanks to the highly conjugated network provided by multiple ethynyl bridges connecting the polythiophene chains [34]. Cyclic voltammograms (CVs) and the frequencygram reported in Figure 4 show a highly regular cycle-by-cycle growth, associated with the conducting behavior of the polymer confined within a narrow potential window (0.75–1.25 V).

The polymer film obtained using electrochemical polymerization was found to be insoluble in common laboratory solvents and was characterized using ATR-FTIR spectroscopy (Figure S4). The signal at about 2100 cm⁻¹ can be ascribed to the triple bond C≡C stretching of the alkyne functions. The bands at 1664 cm⁻¹ correspond to the C=C asymmetric stretching vibrations of the thiophene ring, while the vibration bands observed at 1223 cm⁻¹ and 1052 cm⁻¹ are attributed to C-H bending and in-plane deformation. The amorphous nature of the polymer was confirmed by the absence of signals in the powder X-ray diffraction pattern (Figure S5). The electrochemical properties of the electrosynthesized polymer were assessed using voltammetric characterization that was carried out in a monomer-free solution, containing the same supporting electrolyte used for the electropolymerization. The voltammetric and frequencymetric patterns, acquired in the same potential range used for the electrodeposition of the polymer confirmed the high stability of the system, documented by quick π-doping/un-doping processes not involving significant mass loss of the polymeric coating, as evidenced by the flat baseline of the frequency response recorded between the subsequent doping peaks (Figure 5).

The high electrochemical stability of the polymer was also supported by further CV experiments that were carried out by extending the potential range explored, namely by increasing the anodic limit up to 1.45 volts, without this involving over-oxidation processes associated with significant variations in the voltammetric pattern (See Figure S4).

3.4. HOMO–LUMO DFT Calculations

The DFT quantum-chemical calculations for compound 1 were performed at the B3LYP/6-311G(d,p) level [35], as implemented in GAUSSIAN09 [36]. The DFT structure optimization was performed based on the X-ray geometry and the theoretical values of the bond lengths and bond angles were consistent with the experimental ones. The DFT study shows that the HOMO and LUMO are localized in the plane of the molecule, as can be seen by their plots shown in Figure 6, and both show a predominant π character, mainly deriving from the contribution of the Pz atomic orbitals of the carbon atoms belonging to the bithiophene moiety. The frontier molecular orbital energies, E_HOMO and E_LUMO, are −5.51 eV and −2.09 eV, respectively, so that the HOMO–LUMO gap is 3.42 eV.

To theoretically estimate the band gap of the polymer based on 1, an iterative approach was employed [21,37]. The cross-linked structure was simplified taking into consideration a linear growth of the polymer chains by reacting the 5′ positions of the monomers. The geometries of the oligomers formed by 2 to 5 repeating units were optimized via DFT calculations at the B3LYP/6-311G(d,p) level of theory (the model of the dimer is shown as an example in Figure S7). For each oligomer, the frontier molecular orbital energies and the relative E_HOMO-E_LUMO gap were then calculated. Finally, the gap values were plotted versus 1/n (where n is the number of repeating units in the oligomer) and the data were subjected to both a linear and a quadratic interpolation. The theoretical band gap for the polymer (n = ∞) is the intercept of the resulting functions with the y-axis. This value is of 2.15 or 2.37 eV for the linear and the quadratic interpolation, respectively (Figure 7). As expected, in both cases, it is smaller than the gap of the monomer and is in good agreement with data from the literature [21,34].

4. Conclusions

In conclusion, we reported the synthesis and characterization of a new highly conjugated multi-thiophene monomer, 1. XRD analysis confirmed the planarity of the molecule. Tetrathiophene 1 was electropolymerized onto the surface of a gold-coated piezoelectric quartz crystal, showing a high reactivity that is ascribable to the extended conjugation provided by the alkyne functionalities connecting the thiophene rings. DFT optimization, based on the crystal structure-derived molecular geometry, was used to obtain the frontier molecular orbital energies of compound 1 and to estimate the polymer band gap.