Benzoselenadiazole Containing Donor–Acceptor–Donor Small

Article pubs.acs.org/crystal

Benzoselenadiazole Containing Donor−Acceptor−Donor Small Molecules: Nonbonding Interactions, Packing Patterns, and Optoelectronic Properties Palas Baran Pati and Sanjio S. Zade* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mohanpur 741252, India S Supporting Information *

ABSTRACT: Herein, we describe the fine-tuning of intermolecular Se···N interaction in benzoselenadiazole (BDS) derivatives to form head-to-head dimers in the solid state. The structures and photophysical properties of phenyl-, thiophene-, and selenophenecapped BDS (1−3, respectively) are studied here. Because of the presence of the strong intramolecular Se···N interaction, selenophene-capped BDS 3 showed syn arrangement of two capped selenophene rings, whereas two thiophene rings in 2 showed an anti orientation. Compounds 1 and 2 showed the tendency to form head-to-head dimers in the solid state through the intermolecular Se···N interactions. In contrast to compounds 1 and 2, compound 3 does not form a dimer in the solid state and, instead, shows strong intramolecular Se···N interactions. The tendency to form dimers largely depends on the nonbonding interactions and the steric effect of capped rings.

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INTRODUCTION π-Conjugated systems comprising five-membered heterocyclic aromatic rings (furan, thiophene, and selenophene) have received considerable attention for their applications in organic electronics such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs).1 Compared to the interest in the conjugated polymers, the growing interest in the small conjugated molecules originates from their easier synthetic strategies, well-defined molecular structures, easy purification methods, better processability, and reproducibility. A small change in the molecular architecture of small conjugated molecules significantly changes the optical and electrochemical properties.2 Therefore, small conjugated molecules with a high degree of conjugation have been synthesized and studied for their structure−property correlation. The structure−property correlation in the small conjugated molecules is helpful for understanding the properties of the related polymers. In 1992, the donor−acceptor (D−A) concept3 was introduced; this concept has been extensively used to design low-band gap conjugated polymers. D−A−D type small molecules containing benzooxadiazole (BDO),4 benzothiadiazole (BDT),5 and benzoselenadiazole (BDS)6 as acceptors were used in many organic electronic devices. These D−A−D small molecules afforded high hole and electron mobility in FET devices.7 The solid state packing in BDT- or BDS-containing molecules strongly depends on intra- and intermolecular S···N and Se···N interactions. Bunz et al. have shown that © 2014 American Chemical Society

acenothiadiazole molecules dimerize in a head-to-head manner in the solid state via S···N interaction.8a Via the replacement of sulfur with the more polarizable and electropositive selenium, N···chalcogen interactions became more pronounced with the formation of a highly coplanar dimer. 8b In general, chalcogenadiazoles show a stronger tendency to form headto-head dimers with the heavier chalcogen atoms (Se and Te).9 Thus, being a better supramolecular motif, Se···N noncovalent interaction can be effectively exploited for the formation of supramolecular self-assemblies.8,10 Singh et al. have shown the formation of interesting metal−organic supramolecular assemblies based on selenadiazolopyridine via Se···N interactions.11 Herein, we report the synthesis, structures, and photophysical properties of phenyl-, thiophene-, and selenophenecapped benzoselenadiazoles (1−3, respectively). In phenyl- and thiophene-capped BDS (1 and 2, respectively), the Se···N interactions facilitate the formation of head-to-head pairwise dimers in the solid state. Selenophene-capped BDS (3) does not show dimer formation and, instead, shows strong intramolecular Se···N interactions. Therefore, packing in compound 3 is mostly dominated by Se···π and H···π interactions. Compounds 1−3 showed strong green, orange, and red emission, respectively, in benzene solutions upon being excited at their corresponding absorption maxima. Received: December 9, 2013 Revised: February 10, 2014 Published: February 13, 2014 1695

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Scheme 1. Synthesis of 1 and 3a

a (i) Phenylboronic acid, K2CO3, Pd(PPh3)4, THF/water, 80 °C, 16 h, 82%; (ii) tributyl(2-selenophenyl)stannane, Pd(PPh3)4, toluene, reflux for 12 h, 85%.

Figure 1. Absorption (solid lines) and emission (dashed lines) spectra of 1−3 of (a) benzene solutions and (b) thin films on an ITO-coated glass slide.

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120 mesh) for purification. A green solid powder (265 mg, 82%) was obtained as the pure product eluting by a hexane/DCM (9:1) solvent mixture: mp 177 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.90−7.93 (m, 4H), 7.73 (s, 2H), 7.50−7.54 (m, 4H), 7.44−7.46 (m, 2H); 13C NMR (100 MHz, DMSO-d6) δ 128.02, 128.04, 128.2, 128.4, 134.1, 137.7, 158.7; 77Se NMR (95 MHz, DMSO-d6) δ 1521.2; HRMS with TOF mass analyzer (ESI) m/z calcd for C18H12N2Se [M+] 336.0166, found 336.0170. Synthesis of 2. Compound 2 was synthesized as a red powder by following the method reported in the literature:14 yield 72%; mp 127 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.22−7.25 (m, 2H), 7.72−7.75 (m, 2H), 7.99 (s, 2H), 8.10−8.12 (m, 2H); 13C NMR (100 MHz, DMSO-d6) δ 126.0, 127.1, 127.8, 128.3, 129.0, 139.6, 157.5; 77Se NMR (95 MHz, DMSO-d6) δ 1518.6; HRMS with TOF mass analyzer (ESI) m/z calcd for C14H8N2 S2Se [M+] 347.9294, found 347.9290. Synthesis of 3. In an oven-dried 50 mL three-neck round-bottom flask, 4,7-dibromoselenadiazole (340 mg, 1 mmol), tributyl(2selenophenyl)stannane (3 mmol), and 30 mL of dry toluene were added. The reaction mixture was purged with nitrogen for 15 min. Pd(PPh3)4 (55 mg, 0.05 mmol) was added to the reaction mixture. The reaction mixture was refluxed for 12 h. After the mixture had cooled to room temperature (rt), the solvent was removed and the crude residue was directly loaded on a silica gel column (60−120 mesh). A dark-red powder (374 mg, 85%) was obtained as the pure product: mp 167 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.44−7.47 (m, 2H), 8.09 (s, 2H), 8.22 (m, 2H), 8.39 (m, 2H); 13C NMR (100 MHz, DMSO-d6) δ 124.3, 127.8, 128.1, 129.7, 135.8, 143.3, 156.8; 77 Se NMR (95 MHz, DMSO-d6) δ 641.4, 1516.6; HRMS with TOF mass analyzer (ESI) m/z calcd for C14H8N2Se3 [M+] 443.8183, found 443.8190. Crystal Structure Solution and Refinement. Crystal data were collected at rt on a Bruker KAPPA APEX II CCD Duo instrument with graphite-monochromated Mo Kα radiation (0.71073 Å). The intensity data were processed using Bruker’s suite of data processing programs (SAINT), and absorption corrections were applied using SADABS.16 Crystal structures were determined by direct methods using SHELXS-97, and the data were refined by full matrix least-

EXPERIMENTAL SECTION

General Method. All reactions were performed under a nitrogen atmosphere to maintain the dry condition. Dry toluene and tetrahydrofuran (THF) were distilled from sodium/benzophenone prior to use. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra of the compounds were recorded on a 400 MHz spectrometer in DMSO-d6, and chemical shifts (δ) are reported in parts per million relative to tetramethylsilane as the internal standard. 77Se NMR spectra were recorded on a Bruker 500 MHz spectrometer in DMSOd6. 77Se NMR chemical shifts are reported using Ph2Se2 as the external standard with chemical shifts of 470 ppm with respect to Me2Se. Thus, the values are reported with respect to Me2Se. Electrochemical studies were conducted using a platinum (Pt) disk electrode as the working electrode, a platinum wire as the counter electrode, and a AgCl-coated Ag wire as the reference electrode. A nonaqueous Ag/Ag+ wire was prepared by dipping a silver wire in a solution of FeCl3 and HCl. Pt disk electrodes were polished with alumina, washed with water and acetone, and dried with nitrogen gas before being used to remove any incipient oxygen. Synthesis. 4,7-Dibromobenzoselenadiazole12 and tributyl(2selenophenyl)stannane13 were prepared by following previously reported methods. Compound 2 was prepared by Stille coupling between 4,7-dibromobenzoselenadiazole and tributyl(2-thiaphenyl)stannane in a 72% yield.14 Suzuki coupling between 4,7-dibromobenzoselenadiazole and phenylboronic acid afforded compound 1 in an 82% yield (Scheme 1). Stille coupling between tributyl(2selenophenyl)stannane and 2,7-dibromobenzosenaladiazole in toluene solvent afforded 3 in an 85% yield (Scheme 1).15 Synthesis of 1. In a 50 mL three-neck round-bottom flask, 4,7dibromobenzoselenadiazole (340 mg, 1 mmol), potassium carbonate (1.5 g, 10 mmol), and 2-phenylboronic acid (300 mg, 2.5 mmol) were mixed in water (5 mL) and THF (10 mL). The reaction mixture was purged with nitrogen for 15 min. Pd(PPh3)4 (55 mg, 0.05 mmol) was added to the reaction mixture. The reaction mixture was stirred overnight at 80 °C. The solvent was removed under reduced pressure, and the crude product was directly loaded on a silica gel column (60− 1696

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Figure 2. Photographs of (a) 1, (b) 2, and (c) 3 in benzene solutions under a 365 nm UV lamp. (d) CV of compounds 1−3 measured in DCM containing 0.1 M TBAPC with a scan rate of 100 mV/s with nonaqueous Ag/Ag+ as the reference electrode and a Pt wire as the counter electrode.

Figure 3. ORTEP representations of (a) 1, (b) 2, and (c) 3. Thermal ellipsoids are drawn at a 50% probability level. squares refinement on F2 with anisotropic displacement parameters for non-H atoms, using SHELXL-97.17 Figures are drawn from X-seed version 2.0.18 Calculation. DFT calculations were conducted at the B3LYP/6311+G(d,p) level using Gaussian 09.19

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spectrum of 1 is significantly blue-shifted compared to that of compounds 2 and 3. Emission spectra of compounds 1−3 were recorded in benzene via excitation at the corresponding λmax of lowerenergy bands. Compounds 1−3 exhibit emission maxima at 505, 597, and 625 nm with Stokes shifts of 97, 119, and 125 nm, respectively. Compound 1 shows green emission, whereas 2 and 3 show orange and red emission, respectively (Figure 2). Fluorescence quantum yields of compounds 1−3 as benzene solutions are 0.74, 0.58, and 0.40, respectively, with respect to quinine sulfate as a standard. The highest quantum yield of 1 can be attributed to its twisted structure. Because of the presence of heavy atoms (S and Se), compounds 2 and 3 show relatively lower fluorescence quantum yields. To study the absorption and emission behavior in the solid state, the thin films of compounds 1−3 were prepared on an ITO-coated glass plate by spray coating via their chloroform solutions. The thin film absorption spectra of compounds 1−3 show the bathochromic shifts in the absorption spectra with the broadening of the peaks compared to their absorption spectra in benzene solutions. It suggests the presence of stronger intermolecular interactions in the solid state such as π-sacking and van der Waals interactions. The absorption spectrum of the thin film of compound 1 shows an additional broad peak in the

RESULTS AND DISCUSSION

Absorption and Emission Properties in Solution and in the Solid Phase. The absorption and emission spectra of all three compounds as benzene solutions are shown in Figure 1a, and data are summarized in Table S1 of the Supporting Information. The absorption spectra of 1−3 possess a characteristic dual-band nature. The higher-energy bands (318−343 nm) are due to the π → π* transition of the conjugated backbones, and the lower-energy bands (406−500 nm) are assigned to charge transfer transitions (CT, D → A). The λmax of the higher-energy band remains relatively constant for all three compounds (1−3). The lower-energy charge transfer band is red-shifted upon moving from 1 to 3. As selenophene is a stronger donor, compound 3 shows an ICT band at a significantly higher wavelength. Because of the significant deviation of phenyl rings from the plane of the central BDS unit, the charge transfer band in the absorption 1697

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Figure 4. Intermolecular interaction responsible for the formation of the head-to-head dimer in the crystal structure of (a) compund 1 and (b) compound 2. (c) Intermolecular interaction in compound 3 responsible herringbone type packing.

4a,b). In the dimers of 1 and 2, the BDS planes are parallel to each other with offsets of 0.152 and 0.242 Å, respectively. In the packing of 2, dimers form a slip-stack ribbon network. Two adjacent molecules of 2 are stacked in such a way that the thiophene unit (donor) of one of the molecules resides over the BDS unit (acceptor) of another molecule with Se···S (3.666 Å) interaction. The distance between the two adjacent molecular planes and the adjacent ribbon is 3.579 Å, which indicates the presence of π-stacking interactions. Such π-stacking interactions are absent in the crystal packing of 1. Strong Se···π (3.232 and 3.448 Å) and H···π (2.875 and 2.885 Å) interactions are the major interactions that lead to herringbone packing of 3 (Figure 4c). In contrast to compounds 1 and 2, compound 3 does not form head-to-head dimers. Overall, the tendency to form dimers and the strength of intermolecular Se···N interactions in these small molecules depend on the steric effect of capped aromatic rings, intramolecular Se···N/S···N interaction, and the twisting energy of capped aromatic rings from the central BDS ring. Phenyl rings in the structure of compound 1 are twisted significantly from the central BDS unit, which facilitates dimer formation by reducing steric hindrance. Thus, compound 1 showed strong intermolecular Se···N interaction. In compound 2, the capped thiophene rings cannot deviate significantly from the plane of the central BDS unit to maintain efficient conjugation. As a result, compound 2 showed weak intermolecular Se···N interaction. In the case of 3, both nitrogens are engaged in intramolecular interactions, no intermolecular Se···N interactions are observed, and, hence, no head−head dimer formation is observed. In general, BDS has a strong tendency to form head-to-head dimers; however, the strength of dimerization in these D−A−D small molecules having a central BDS unit depends significantly on the capped aromatic rings. Competing factors such as Se···N interaction, planarity, D−A interaction, π-stacking, and the steric effect of end-capped rings play an important role in the overall solid state packing of these D−A−D molecules. Theoretical Approach. In the optimized structure of 1, two benzene rings deviate from the central BDS ring by ∼44°. In the crystal structure of compound 3, two selenophene rings are syn to each other with selenium pointing toward the selenadiazole ring, whereas in the crystal structure of compound 2, thiophene rings are anti to each other. Therefore, compounds 2 and 3 were optimized in both syn and anti forms (Table S3 of the Supporting Information). The syn form of 3 is

region of 500−600 nm. Emission spectra of 1−3 show bathochromic shifts in the thin films compared to that in benzene solutions. Electrochemical Properties. The electrochemical properties of compounds 1−3 were studied in dichloromethane (DCM), and tetrabutylammonium perchlorate (TBAPC) was used as the supporting electrolyte (Figure 2d). Compounds 1− 3 exhibit first oxidation peak potentials at 1.79, 1.19, and 1.11 V, respectively, and reversible reduction peak potentials in the region from −1.14 to −1.40 V. As expected, compound 1 showed a first oxidation potential significantly higher than those of 2 and 3. Because of the more electropositive nature of selenium, compound 3 exhibits the lowest oxidation potential among the three compounds.20 The HOMO energy levels of 1−3 were calculated to be −5.99, −5.56, and −5.46 eV, respectively, from the lower-potential onsets of the first oxidation peaks [Eox(pa)] by using the equation HOMO = −[Eox(pa) − 0.35 + 4.81] eV, where Eferrocene is 0.35 V. 1/2 Structural Analysis of 1−3. Single crystals suitable for Xray analysis were obtained by the slow evaporation method from a hexane/ethyl acetate solvent mixture (8:2) for compounds 1 and 2 and from a methanol/DCM solvent mixture (7:3) for compound 3. ORTEP diagrams of compounds 1−3 are shown in Figure 3. Crystallographic data and structure refinement parameters are listed in Table S2 of the Supporting Information. Two phenyl rings in the crystal structure of compound 1 are twisted by ∼46° from the central BDS unit and are nearly orthogonal with each other. In the crystal structure of 2, thiophene rings are anti to each other. One of the rings with a sulfur atom pointed toward the selenadiazole ring is nearly coplanar with the central BDS ring, and another thiophene ring deviates from planarity by a torsion angle of 15°. In the crystal structure of 3, both selenophene rings are syn to each other and nearly coplanar with the central BDS unit as a result of strong intramolecular Se···N (Se1···N, 2.835 Å; Se3···N, 2.842 Å) interactions. Packing and Intra and Intermolecular Interaction Pattern. Crystal structures of 1 and 2 showed similar twodimensional interlock type packing, whereas the crystal structure of 3 showed herringbone type packing (Figure S1 of the Supporting Information). The crystal structure of 1 and 2 showed head-to-head dimerization with intermolecular Se···N distances of 2.993 and 3.424 Å for 1 and 2, respectively (the sum of van der Waals radii of Se and N is 3.450 Å) (Figure 1698

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more stable by 0.7 kcal/mol than its anti form, whereas the anti form of 2 is more stable than its syn form by 0.5 kcal/mol. Stabilization via strong Se···N nonbonding interaction is responsible for the syn orientation of selenophene rings in compound 3. The intramolecular Se···N distance obtained from the optimized geometry (2.855 Å) is in good agreement with the distance obtained from the crystal structure (2.835 Å). To investigate the Se···N interaction further, we have optimized dimers of 1−3 at the same level of theory (Figure S2 of the Supporting Information). A geometry optimization of a dimer of 3 starting from a coplanar arrangement led to the optimized geometry without intermolecular Se···N interaction that closely matches the arrangement of molecules in the crystal structure of 3 (Figure 4c). In the optimized structures of a dimer of 1 and a dimer of 2, BDS planes are parallel with offsets of 0.44 and 0.70 Å, respectively. Calculated intermolecular Se··· N distances of a dimer of 1 and a dimer of 2 (3.09 and 3.18 Å, respectively) are in agreement roughly with the values obtained from the crystal structures. A dimer of 1 and a dimer of 2 are stabilized by 3.8 and 2.5 kcal/mol, respectively, compared to two monomer units (Table S5 of the Supporting Information).

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CONCLUSIONS

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ASSOCIATED CONTENT

S Supporting Information *

Packing diagrams of 1−3, tabular summary of optical and electronic data, data from DFT calculations, and copies of NMR spectra of the new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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In summary, we report here syntheses, structures, and photophysical and electrochemical properties of phenyl-, thiophene-, and selenophene-capped benzoselenadiazoles (1− 3, respectively). Se···N interactions facilitate the formation of a head-to-head dimer in phenyl- and thiophene-capped BDS (1 and 2, respectively). In the crystal structure of selenophenecapped BDS (3), strong intramolecular Se···N interactions are observed. Therefore, packing in compound 3 is dominated by Se···π and H···π interactions. BDS has a strong tendency to dimerize through Se···N interactions; however, the strength of dimerization strongly depends on the capped aromatic rings. Competing factors such as Se···N interaction, planarity, D−A interaction, π-stacking, and the steric effect of capped aromatic rings play an important role in the overall solid state packing of these D−A−D molecules. By changing the capped aromatic ring, we are able to obtain two primary emissive colors, green (1) and red (3), in benzene solutions. Additionally, compound 2 showed orange emission.

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ACKNOWLEDGMENTS

P.B.P. thanks the University Grant Commission (UGC), India, for a fellowship and Soumik Mandal (IISER Kolkata) for crystallographic help. We acknowledge DRDO, India, for funding. 1699

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dx.doi.org/10.1021/cg401830f | Cryst. Growth Des. 2014, 14, 1695−1700