Synthesis, photophysical, electrochemical and single-crystal x-ray diffraction study of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile

ABSTRACT The optical characteristics, redox properties, thermogravimetric stability and single-crystal X-ray diffraction study of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile are examined using ultraviolet–visible spectrophotometry, cyclic voltammetry, thermal gravimetric analysis–diffraction scanning calorimetry analysis, single-crystal X-ray diffraction and density functional theory calculations. Evidently, the crystal structure of compound 6 is sustained by a number of weak nonconventional intermolecular forces of attraction such as C-H … N, C-H … π donor–acceptor interactions. GRAPHICAL ABSTRACT

Introduction 2,1,3-Benzothiadiazole (BTD) derivatives are outstanding compounds because of their electron-withdrawing properties and have been used as units of electron acceptors for conducting materials. [1] BTD containing compounds are expected to afford well ordered crystal structures because of their highly polarized properties, leading to intermolecular interactions such as heteroatom constants or π-π interactions. [2] BTD derivatives are also known as efficient fluorophores. [3] In addition to that, polymers containing BTD units have been used as luminescent compounds in electroluminescence devices. [4] The highly electron-deficient BTD unit is one of the most popular building blocks in organic electronics. BTD can conjugatively link with an electron-rich molecule to form low bandgap functional polymers or small molecules, and materials prepared in this manner showed usefulness in organic light-emitting diodes (OLEDs), dye-sensitized solar cells (DSSCs), light harvesting, and other optical or electronic functional devices. [5] For molecularly hybridized push-pull type materials, where the alternating arrangement of electron-rich and electron-deficient units along the π-conjugated backbone effectively controls the frontier molecular orbitals, BTD is often the electron-deficient unit of choice. [6] Moreover, to further fine-tune the frontier molecular orbital as well as other important materials parameters such as solubility and crystallinity, chemical modifications of the BTD unit have attracted much interest. [7] Recently, a BTD-based small molecule and its selenium analog showed good efficiency in heterojunction solar cells and photovoltaic cells, respectively. [8] Alternatively, BTDbased small molecules are becoming increasingly popular for devising hybrid solar cells, because of high electron mobility and excellent chemical and physical stability of inorganic semiconductors. [9a,b] Recent studies have demonstrated that formation of self-assembled monolayer of conjugated molecules on the surface of the inorganic semiconductors can lead to the formation of interfacial dipoles, which in turn can improve the work function of substrate materials. [9c,d] These types of small conjugated molecules can act as interfacial modifiers (IMs), which enhance charge injection from metals into organic materials. Yu and coworkers have prepared and examined one of such IMs having cyano-acrylic acid as the anchoring group, and it showed an increase in electron affinity at the polymerinorganic semiconductor interface and formation of dipoles, which are oriented away from the semiconductor surface. [10] In this communication, we report the synthesis and photophysical study of dithienobenzothiadiazole 4, dithienobenzooxadiazole 12, and a new conjugated compound having a cyano-benzylic anchoring group, (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5] thiadiazol-7-yl)thiophen-2-yl)acrylonitrile 6. These compounds were successfully prepared by a multistep synthetic route. All the molecules consisted of donor-acceptor segments. The molecular structure of compound 6 was further confirmed by single-crystal X-ray diffraction study. The thermal and electrochemical properties of synthesized compound 6 were studied.

Single-crystal X-ray diffraction (SCXRD)
The molecular structure for compound 6 was elucidated by SCXRD (Fig. S1). Crystals of compound 6 were obtained from ethyl acetate (solvent). A suitable crystal was selected and data were collected using CuKα (λ ¼ 1.5418 Å) radiation on an Xcalibur-Eos-Gemini diffractometer. The crystal was kept at 293 K during data collection. The structure was solved and refined using Olex2. [12] The structure was solved with the Superflip [13] structure solution program using charge flipping and refined with the ShelXL [14] refinement package using least squares minimization. Table S1 summarizes the crystal structure and refinement data whereas Tables S2 and S3 summarize selected bond lengths and bond angles, respectively.
The X-ray structure of compound 6 clearly suggests that it crystallizes in monoclinic system with space group P2 1 /c. Asymmetric unit of this contains full molecules and there are four such molecules present in the unit cell, as shown in Fig. S2. The unit cell parameters for this are a ¼ 16.9905 (7) (7) and α ¼ 90.00, β ¼ 107.130 (4), and ϒ ¼ 90.00.
Notably, the presence of various polar subunits in the molecular framework of compound 6 induces conformational changes in the molecule that apparently changes the nature and number of donor-acceptors sites. For instance, all the heterocyclic and carbocyclic rings present in compound 6 are not coplanar, making differential dihedral angles with one another; for example, a dihedral angle between the least squares planes drawn through peripheral thiophene ring and adjacent benzothiadiazole ring is estimated as 24.24°, whereas a dihedral angle between the least squares planes drawn through internal thiophene ring and adjacent benzothiadiazole ring is estimated 23.18°, and vice versa. In fact, the crystal structure of compound 6 is sustained by a number of weak nonconventional intermolecular forces of attraction such as C-H … N, C-H … π donor-acceptor interactions as shown in Fig. 1.
Interestingly, C-H … N donor-acceptor interaction limits the growth of molecular packing along the c axis by forming a closed contact. However, C-H … π donor-acceptor interactions arranged the molecules of 6 along the b axis form a zigzag network as shown in Fig. 2.

Photophysical and electrochemical properties of compounds 4, 6, and 12
The photophysical properties of compounds 4, 6, and 12 were studied. The absorption and emission spectra of 4, 6, and 12 were carried out in methanol (Fig. 3). All these compounds possess the dual-band nature; one lower energy band ranges between 250 and 340 nm, which is due to π-π* transition of the conjugated backbones, while a second higher energy band ranges from 400 to 520 nm due to the charge transfer transition between D→A. Compounds 4, 12, and 6 show absorption maxima at 441, 436, and 463 nm with Stokes shifts of 159, 146, and 142. The lower energy charge transfer leads to red shift (bathochromic shift) while going from 4 (441 nm) to 6 (463 nm).
The band-gap calculation for all three compounds (4, 6, and 12) was carried out using density functional theory [15] as well as electronic spectra. As can be seen in Table 1, there is a slight difference in E band-gap (theoretical) and E band-gap (optical) for compounds 4, 6, and 12.  spectra of 4, 6, and 12 (red, black, and blue solid lines) measured in methanol containing 3.33 � 10 À 7 , 3.33 � 10 À 6 , and 6.67 � 10 À 7 mol respectively. Photographs of 4, 6, and 12 in methanol under a 365-nm UV lamp.

Cyclovoltammetry experiment data for compound 6
The electrochemical properties of compound 6 were studied, in which ferrocene was used as an internal standard. According to Fig. 4, cyclic voltammograms exhibit an irreversible oxidation curve with no reduction wave deducted. The highest occupied molecular orbital (HOMO) was then calculated from the oxidation potential (E oxi ), according to the formula of E HOMO ¼ -[(E oxi -E Fc ) þ 4.8 eV], where E Fc denotes the measured oxidation potential of ferrocene and 4.8 eV is the absolute oxidation potential value of ferrocence under vacuum. Calculated value of E HOMO for compound 6 is À 5.86 eV and value of E LUMO is À 3.59 eV, which is calculated from the formula E LUMO ¼ [E HOMO þ E Optical ].

Thermogravimetric analysis data of compound 6
Compounds 4, 6, and 12 showed reasonably good thermal stabilities up to 450 °C. The thermogravimetric analysis (TGA) shows high stability of compound 6 (Fig. 5). The DTA curve shows a phase transition at 172.6 °C, which is also in accordance with the melting point of compound 6 (173-175 °C). It shows thermal decomposition at 429.3 °C as suggested by the DTG curve.  [16] Figure 4. Cyclic voltammogram of compound 6 in anhydrous THF containing 0.1 M tetra butylammoniumhexafluorophosphate as a supporting electrolyte with glassy carbon as working electrode, platinum as counterelectrode, and nonaqueous Ag/AgNO 3 as reference electrode.

Conclusion
In this communication we have reported the synthesis of (Z)-2-phenyl-3-(5-(4-(thiophen-2yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile. The synthesized compound was well characterized by ( 1 H and 13 C) NMR, mass spectra, IR spectral data, and single-crystal X-ray diffraction analysis data. This compound was screened for its band-gap properties using absorption and emission spectral data and cyclic voltammograms. The experimental value of the band gap was compared with the theoretical band gap. Thermal stability was studied using thermogravimmetric analysis. In summary, the synthesized new class of donor-acceptor compound, featuring a benzothiadiazole electron-accepting unit, displays distinct optical, electrochemical, and thermal properties. The experimental data show that compound 6 could be a potential candidate as an interfacial modifier for photovoltaic devices.

Experimental
All the chemicals were reagent grade and used as purchased. Moisture-sensitive reactions were performed under an inert atmosphere of dry nitrogen with dried solvents. Reactions were monitored by thin-layer chromatographic (TLC) analysis using Merck 60 F 254 aluminium-coated plates and the spots were visualized under ultraviolet (UV) light. Column chromatography was carried out on silica gel (60-140 mesh). All melting points were determined using Thiele's tube using paraffin oil and are uncorrected. IR spectra were recorded on a Shimadzu Prestige 21 spectrometer. Mass spectra were recorded on Thermo-Fischer DSQ II GCMS instrument. NMR spectra were recorded on a Bruker Avance-III 400 spectrometer in CDCl 3 . Diffraction data were collected using CuKα (λ ¼ 1.5418 Å) radiation on an Xcalibur, Eos, Gemini diffractometer. CV data were obtained with CH Instruments model of CHI 600C with three electrode (glassy carbon as the working electrode, platinum as the counter electrode, and nonaqueous Ag/AgNO 3 as the reference electrode) cells in anhydrous THF solution containing 0.1 M tetra-n-butylammoniumhexafluorophosphate at a scan rate of 100 mV s À 1 under nitrogen atmosphere. DFT calculations were performed using Gaussian 09 with B3LYP functional and 6-311 G (þþ) basis set. [15] Compounds 2-6 Compounds 2-5 were prepared by the corresponding literature methods. [10] J ¼ 3.6,1.2 Hz,2H). 13 C NMR: δ 125.8,126.0,126.8,127.5,128.0,139.3,152.6. Mass (EI) m/z: 300 (Mþ, 100%), 299 (90%), 81 (15%), 68 (22%).