Synergistic effects of halogen bond and π–π interactions in thiophene-based building blocks

Abstract

Although recognized as a significant force in crystal engineering, halogen bonding (XB) has been scarcely investigated in “bottom-up” approaches towards organic electronics. We report, herein, the use of a thiophene-based building block, pyridyl-thiophene (Pyr-T), to achieve an assembly driven by XB and π–π stacking interactions with iodopentafluorobenzene (IPFB). Spectroscopic and thermal analysis of the co-crystal provide indirect evidence of the assembly. The combined effects of XB and π–π stacking are confirmed experimentally via X-ray crystallography. Density functional theory (DFT) computations support experimental observations. The results of the study speak to the use of halogen bond driven self-assembly in organic electronic device application.

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Introduction

Organic semiconducting devices have received enormous attention as low cost and flexible alternatives to silicon-based technologies.^1,2 Studies within the field of organic electronics have emerged linking its progression towards “bottom-up” approaches. The basis of these studies is aimed at understanding the molecular assembly of complex π-conjugated systems as a means to increase device efficiency.^3–6 It has been demonstrated that the synergistic action of non-covalent interactions such as hydrogen bonding (HB) and π–π stacking in thiophene-based donors can induce well-defined solid state packing between molecules.^7,8 The strategic design of these π-conjugated oligomers, equipped with self-guided domains, can increase overall device performance and stability.

Although recognized as a significant force in improving the packing of many halogen-containing organic compounds,^9–12 halogen bonding (XB) has been scarcely investigated within “bottom-up” approaches towards organic devices.^13–16 Halogen bonds, analogues of hydrogen bonds, are non-covalent interactions between Lewis acidic halogen atoms and electron-pair-donating heteroatoms.¹⁷ The phenomenon is explained by the presence of a region of positive electrostatic potential (i.e., σ-hole) on the outermost surface of the halogen atom.¹⁸ This area of electron deficiency affords a highly directional interaction with electron donating heteroatoms.

Such an effect can be greatly beneficial in organizing π-conjugated molecules that are potentially relevant to organic electronics applications.

As a result, well-defined assemblies can be engineered by exploiting the interactions between XB and π-stacking in thiophene-based building blocks.

Herein, we investigate the halogen bond driven assembly between pyridyl thiophene (Pyr-T)—a π-conjugated building block equipped with a self-assembling domain—and iodopentafluorobenzene (IPFB) (Fig. 1). The truncated version of a pyridyl-capped oligothiophene (e.g., Pyr-T) is employed to provide a fundamental understanding and design guidelines for the synthesis and characterization of more complex molecular assemblies. It was chosen as the XB-acceptor (i.e., pyridyl N as the electron donor) because it can be easily integrated into the oligomeric framework of any π-conjugated system via palladium-catalyzed cross coupling. The XB-donor, IPFB, was selected as the electron acceptor as the fluoro substituents provide an inductive effect which increases σ-hole bonding.^19–21 The presence of XB and π–π stacking was confirmed experimentally via X-ray crystallography, NMR, and Raman spectroscopy. Further analysis via density functional theory (DFT) computations supports experimental observations and reveals that XB and π-stacking provide the dominant energetics in the observed assembly.

Fig. 1 Top image: X-ray structural analysis of the complex of halogen bond donor and acceptor. Selected bond distances [Å] between nitrogen and halogen is 2.76 Å (red dotted line). Bottom image: Molecular structure of Pyr-T and IPFB.

Experimental section

General remarks

Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise specified. ¹H and ¹³C NMR spectra were recorded on a Bruker Avance-300 (300 [75] MHz), Bruker Avance DRX-500 (500 [125] MHz) spectrometer and are reported in ppm using the solvent as an internal standard (CDCl₃ at 7.26 ppm). Additional synthetic details, Raman spectroscopy data, summary of theoretical calculations, structural figures, TGA plots, and X-ray crystallographic tables containing bond distances and angles can be found in the ESI.†

X-ray crystallography

Crystal evaluation and data collection were performed on a Bruker APEX II diffractometer with Mo Kα (λ = 0.71073 Å) radiation. Reflections were indexed by an automated indexing routine built into the APEX II program suite. The solution and refinement was carried out in Olex² version 1.2 using the program SHELXTL. Non-hydrogen atoms were refined with anisotropic thermal parameters while hydrogen atoms were introduced at calculated positions based on their carrier/parent atoms. Crystal data and structure refinement parameters for all compounds are given in Table S1.† The single crystal X-ray structure of the co-crystal CCDC number is 1410980.†

Crystal data. Crystal data for C₁₅H₇F₅INS (FW = 455.18 g mol⁻¹): mono-clinic, space group P2₁/n (no. 14), a = 8.7694(5) Å, b = 7.5097(4) Å, c = 22.9574(13) Å, β = 91.757(3)°, V = 1511.16(15) ų, Z = 4, T = 100(2) K, Mo-Kα = 0.71073 Å, μ = 2.307 mm⁻¹, D_calc = 2.001 g cm⁻³, 26 339 reflections measured (3.54° ≤ 2Θ ≤ 52.98°), 3130 unique (R_int = 0.0266), which were used in all calculations. The final R₁ was 0.0158 (I > 2σ(I)) and wR₂ was 0.0377 (all data).

Theoretical methods

The Gaussian 09 software package was employed to calculate the energies of interaction and vibrational frequencies as well as the corresponding Raman intensities. The B3LYP^22–24 and M06-2X²⁵ global hybrid density functional method was used in conjunction with the following triple-zeta basis sets: 6-311+G(2df,2pd), def2-TZVPD, and aug-cc-pVTZ.

Raman spectroscopy

Raman spectra of the monomer and co-crystal were obtained by using methods similar to those previously reported.²⁶ A LabView-controlled Jobin-Yvon Raman or HG2-S Raman spectrometer was employed. A 647.1 nm line of a SpectraPhysics Kr⁺/Ar⁺ laser was used as the excitation source. A photomultiplier tube was used as the detector for the setup. The solid crystals were placed onto a microscope slide and positioned on the microscope stage in the path of the laser. The crystals were scanned from 20–2000 cm⁻¹ four times at a speed of 2 cm⁻¹ per second. The experimental data gathered from Raman spectroscopic analysis was graphed using the IGOR Pro graphing program.

Thermogravimetric analysis

Measurements were performed on Seiko Instruments TG/DTA 6200 (platinum pan, room temperature to 600 °C, ramp rate of 20 °C min⁻¹ under nitrogen atmosphere) and analysed on TG/DTA Highway Conversion Software.

Results and discussion

Pyr-T was synthesized according to a previously published procedure.²⁷ Experimental studies to identify the signature of XB began with solution-based ¹H and ¹³C NMR (Fig. S1 and S2†). NMR spectra of chemical shifts are commonly employed to afford insight into the strength of the bonds.

Spectra of a 1 : 1 mixture of IPFB and Pyr-T are taken in a halogen bond promoting solvent (i.e., 150 mM; toluene-d₈) and compared to a neat solution of Pyr-T.²⁸ As the lone pairs on the pyridyl nitrogen interact with the region of positive electrostatic potential on the iodine in IPFB, the electronegativity of the nitrogen decreases. The peaks corresponding to the proton and carbon nuclei ortho (toluene-d₈: δ_H 8.63 ppm, δ_C 151.3 ppm) to the nitrogen atom become more shielded. However, chemical shifts with changes less than 1 ppm indicate that weak intermolecular interactions between the solvent and solute are screening the effects of XB.

Co-crystals of Pyr-T and IPFB were prepared at a 1 : 1 ratio by dissolving Pyr-T in tetrahydrofuran with IPFB in borosilicate glass vials. The resulting mixture was ultrasonicated for 10 minutes. The open vials were contained in a secondary vial containing n-hexane. The solvent was allowed to evaporate at −20 °C for 14 days until the formation of crystals. Confirmation of co-crystallization was observed through a ∼26 °C difference in melting point between the co-crystal (66 °C) and the Pyr-T monomer (92 °C).^29,30 Single crystal X-ray analysis provides detail about the 1 : 1 assembly of Pyr-T and IPFB (Fig. 2). Antiparallel arrangement of the donor–acceptor pairs in the crystal are indicated by the monoclinic space group, P2₁/n. This arrangement is preferential as the high molecular dipole moment of each molecule is cancelled and electrostatic repulsion is avoided. In analysing the solid state assembly, the strength of the halogen bond is associated with the distance and angle between the donor and acceptor atoms. Stronger halogen bonds can possess a nearly linear geometry between the electron donor and carbon-halogen acceptor (e.g., N⋯X–C) and distances 20–30% shorter than the sum of the corresponding van der Waals radii of the complexing atoms.^31,32 The N⋯X–C angle in the co-crystal is 178.8°. The N⋯I distance is 2.76 Å, which corresponds to a 22% shortening relative to the total van der Waals radii of nitrogen and iodine.

Fig. 2 Fragment of the packing diagram along the a-axis conveying the presence of both XB and π-interactions.

Observed among the two end-to-end XB interactions are important lateral intermolecular contacts between adjacent molecules. When viewed along the a-axis (Fig. 2), adjacent molecules are in contact through C–H⋯F interactions with an average distance of 2.70 Å. The negative charge distribution surrounding the fluorine atom affords this hydrogen–fluorine relationship. The interaction occurs between the meta hydrogens of the pyridyl ring and the ortho fluorines of the neighboring iodopentafluorobenzene. Pyr-T molecules are arranged in an edge-to-face orientation with opposing molecular dipoles. The centroid to centroid distance between the fluoroaromatic and thiophene ring of Pyr-T is approximately 3.68 Å, indicative of π–π stacking. The interaction is due to the interplay of electron deficiency in the fluorinated ring and electron-richness of the thiophene. In addition, the fluoroaromatic rings are arranged in an offset π–π stacked orientation in which the distance between the two centroids is 4.43 Å. These additional intermolecular interactions contribute to the stability of the assembly.

DFT computations were employed to investigate the relative strengths of the interactions forming the assembly. Analysis began with optimization of the Pyr-T monomer. According to B3LYP^22–24 and M06-2X²⁵ computations with sufficiently flexible triple-zeta basis sets (6-311+G(2df,2pd),³³ def2-TZVPD³⁴ and aug-cc-pVTZ^35–37), the torsional angle of the carbon–sulfur bond relative to the pyridine ring-plane is roughly ±20° (Fig. S3†) indicating that an isolated Pyr-T monomer may be slightly non-planar. However, the perfectly planar structure is essentially isoenergetic (within 0.2 kcal mol⁻¹). Even though this planar structure is a transition state according to all DFT computations, the non-covalent contacts present in the crystal structure can certainly overcome this small energy difference and stabilize the planar conformation of Pyr-T. Consequently, the 1 : 1 assembly (or dimer) between IPFB and Pyr-T was examined in both a quasi co-planar orientation with C₁ symmetry and perpendicular arrangement with C_s symmetry (Fig. S4†). The electronic energies of these two optimized dimer structures differ by less than 0.3 kcal mol⁻¹. The higher-energy perpendicular C_s structure is a transition state for rotation about the XB, which indicates that it is essentially a barrierless process. The M06-2X interaction energies of the nearest-neighbor pairwise contacts (Fig. S5†) observed in the crystal structure are summarized in Table S1.† These results clearly show that the XB interaction and π–π stacking between IPFB and Pyr-T molecules are quite significant and of similar magnitude (roughly 8 ± 1 kcal mol⁻¹). The interaction energies for the other contacts between IPFB and Pyr-T are far smaller (ca. 1 kcal mol⁻¹). The only other appreciable interactions arise from π–π stacking between neighboring IPFB units (ca. 6 kcal mol⁻¹) and a face-to-edge contact between two Pyr-T fragments (ca. 4 kcal mol⁻¹). The results strongly suggest XB and π–π stacking provide the dominant forces driving the assembly of this system.

Additional evidence of XB is confirmed via Raman spectroscopy.³⁸ Analysis of the experimental Raman spectra of the Pyr-T monomer and the co-crystal was complicated by the presence of an emissive background. The emission is attributed to intramolecular charge transfer between the thiophene donor portion of the molecule and acceptor group, pyridine.³⁹ Typically, supramolecular assemblies resulting from XB afford blue shifts in the bands of the Lewis base and red shifts for the halogen containing molecule.⁴⁰ Comparison of the experimental Raman spectra indicated small blue shifts in the modes of the Pyr-T in the co-crystal (Table S2, Fig. S6†).

Collectively, experimental Raman features agree very well with theoretical predictions. Comparisons of experimental and theoretical Raman spectra of Pyr-T in the co-crystal using additional and different levels of theory are included in the ESI (Fig. S7–S12†). Prominent features in the experimental spectrum include the stretch of the bond between the pyridyl and thiophene groups at 330 cm⁻¹ and the concurrent ring-breathing mode of both pyridyl and thiophene at 703 cm⁻¹. Additional features of interest include the pyridyl ring breathing mode at 1000 cm⁻¹ and the carbon–carbon double bond stretches of thiophene and pyridyl present at 1428 cm⁻¹ and 1598 cm⁻¹, respectively. The magnitude of the vibrational energy shifts are difficult to attribute directly to XB interactions. Despite this difference, many experimental characteristics are recovered by theory (Fig. 3).

Fig. 3 Comparison of the experimental (top) and theoretical using the M06-2X/aug-cc-pVDZ method and basis set combination (bottom) Raman spectra of Pyr-T within the co-crystal.

Relevant towards further applications in electronic devices is the thermal stability of the XB assembly. The thermal properties of the co-crystal compared to the monomer were examined using themogravimetric analysis (TGA). The thermal curve (Fig. S7†) for the co-crystal reveals a dual step decomposition pattern in which the initial decomposition is within 3 °C of that observed for Pyr-T. The co-crystal decomposes at 120 and 183 °C. The weight loss is higher than the decomposition temperature of the pure monomer. It is believed to be due to dissociation and possible sublimation of the Pyr-T prior to decomposition of the co-crystal as continuous weight losses above 100 °C were observed, which indicate the absence of solvent molecules in the crystal lattice.

Conclusions

In summary, we describe the use of a thiophene-based building block to achieve a supramolecular assembly driven by XB. Spectroscopic and thermal analysis affords evidence of XB between of Pyr-T and IPFB. X-ray crystallography and theoretical data of the co-crystal indicates that intermolecular interactions—specifically XB and π-stacking—contribute to the formation of the assembly. The results of the study speak to the use of XB molecules in organic electronic device application. Incorporation of the truncated model into more complex oligomers is currently underway.

Acknowledgements

D. L. W. appreciates financial support of this work from Oak Ridge Associated Universities through the Ralph E. Powe Award and the National Science Foundation, MRI; CHE-1338056. The computational work is supported by the Mississippi Center for Supercomputing Research and the National Science Foundation under Grant Numbers CHE-1338056 (G. S. T.). G. S. T and N. I. H. acknowledge NSF EPSCoR support under grant no. EPS-0903787 and N. I. H also acknowledges CHE-0955550 for support of the spectroscopic work. J. W. J. and D. L. W. thank the University of Mississippi for Laboratory Start-up Funds. Special appreciation to Dr Amal Dass and colleagues for initial crystallographic data.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1410980. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra16680b

‡ The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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