Three New-CF 3 ,-CN Containing π-Conjugated Heteroaromatic Compounds : Synthesis , Crystal Structure and Photoelectronic Properties

Three new π-conjugated hetero aromatic materials consisting of pyridine 3a, furan 3b, and thiophene 3c have been synthesized by Knoevenagel condensation reaction. These molecules have been characterized by H NMR, EI-MS and single crystal X-ray diffraction analysis. Molecule 3a crystallized under monoclinic system with space group C2/c, molecule 3b crystallized under triclinic system with space group P-1 and the molecule 3c crystalized under triclinic system with space group P-1. The optoelectronic properties of these compounds have been studied. Molecules 3a, 3b and 3c showed strong absorption maxima wavelengths at 300, 419 and 418 nm, respectively. The molar extinction coefficients (ε) of the compounds 3a, 3b and 3c suggested that molecule 3b has better ability to absorb UV light; molecule 3a has better fluorescence intensity than molecule 3b and 3c. Low energy gaps of HOMO and LUMO energy levels of these compounds suggests that these compounds may be a promising new class of lead compounds for developing high performance semiconductor materials. Compounds 3a, 3b and 3c has emissions near to blue light, a slight modification of the structures by extending conjugation may find important applications in optoelectronic devices as blue light emitters in organic light-emitting devices for the development of new generation organic semiconducting materials.


Introduction
π-Conjugated systems are building blocks of organic semiconductors and have been of great interest due to their important applications in organic electronic circuits as semiconducting materials [1]- [7].They have been used as semiconducting materials in organic photovoltaic cells, sensors, organic light-emitting diodes and organic field-effect transistors (OFETs) due to the offering of low-cost, large-area, and flexible electronic devices [8] [9] [10] [11].N-type semiconductor materials are precise components in ambipolar transistors and complementary circuits [12] [13].However, due to their limitations such as low air stability and difficult in design, synthesis these materials are being less explored.It has been documented that air stability and performance of n-type semiconducting material can be improved by substituting electron withdrawing groups such as fluorine, cyano, carbonyl etc. into the n-type semiconducting material [14].It was reported that incorporation of electron withdrawing groups increases the air stability of their anions due to increase in electron affinity of materials, LUMO energy gap will be reduced [15] [16] [17].Jones and co-workers have explored the n-type air stable perylenediimide derivatives by incorporating cyano (CN) and fluorine (F) substituent [18] [19].Recently, we reported n-type transport characteristics of p-phenylenevinylene derivative of 3',3'-(1,4-phenylene)bis{2'-(4''-trifluoromethyl)phenyl}acrylonitorile having two cyano (CN) and two trifluoromethyl (CF 3 ) substituents [20].We also reported the synthesis of 2,5-Difluoro-1,4-phenylene-bis{2- [4-(trifluoromethyl)phenyl]acrylonitrile} and fabricated OFET using this compound exhibited good n-channel OTFT properties with high electron mobility [21].Herein, we report the synthesis, crystal structure and photoelectronic properties of three π-conjugated heteroaromatic materials having pyridine, furan, and thiophene and cyano, fluorine as electron withdrawing substituents.We hope this structural-property study will provide better insight for material chemists in designing more efficient and air resistant organic semiconducting materials, which are strongly desired for next generation optoelectronic application.

Materials and Methods
All reagents were purchased from TCI and Sigma Aldrich and used without further purification.All the products were characterized by 1 H NMR, EI-MS and X-ray diffraction analysis.The NMR spectrum was recorded on a Bruker AMX-500 MHz instrument (Bruker, Yokohama, Japan) at room temperature.Absorption spectrum was measured by using JASCO V-550 UV/VIS Spectrophotometer (JASCO Corporation, Tokyo, Japan) and Fluorescence spectrum was measured by using Hitachi F-2500 Fluorescence Spectrophotometer (Hitachi High-Technologies Corporation, Tokyo, Japan).X-ray data for the compound were collected at room temperature using a Bruker Apex II KY CCD diffractometer with graphite

Synthesis of Compounds 3a, 3b and 3c
In

Single-Crystal X-Ray Analysis and Structure Determination
Crystallographic and structural refinement data for 3a, 3b and 3c summarized in Table 1.Single crystals of 3a, 3b and 3c were easily obtained at room temperature from a solution of dichloromethane using slow diffusion technique.Crystals of 3a, 3b and 3c measuring good dimensions were mounted on a glass fiber.The data were collected on a Bruker APEX II KY CCD diffractometer by using a graphite-monochromatized Mo/Kα radiation (λ = 0.71073 Å) and a nominal crystal-to-area detector distance of ca.83 mm.APEX2 software was used for preliminary determination of the unit cell.Determination of integral intensities [22] and unit cell refinement were performed using SAINT program [23].And, data were corrected for absorption effects with SADABS using the empirical technique [24].The structures were solved by the SHELXS-2014/7 direct method, and subsequent structure refinements were performed using SHELXL-2014/7.

Results and Discussion
The three n-type semiconducting materials 3a, 3b and 3c have been synthesized by Knoevenagel condensation of pyridine dicorboxaldehyde 1a, furan dicorboxaldehyde 1b, thiophene dicorboxaldehyde 1c with 4-(trifluoromethyl) phenyl acrylonitrile 2 in the presence of sodium ethoxide (Scheme 1, Scheme 2).Compounds 3a, 3b and 3c have been characterized with 1 H NMR and EI-MS.Crystal Structure Theory and Applications  Further to confirm the molecular structures of molecules 3a, 3b and 3c, suitable crystals for X-ray diffraction analysis were obtained by the slow evaporation of dichloromethane solution using slow diffusion method.The crystallographic details are summarized in Table 1.The compound 3a crystallizes with monoclinic crystal system, space group C2/c.The molecular structure of 3a was depicted in Figure 1.
The asymmetric unit of the title compound C The compound 3b crystallizes with triclinic crystal system, space group P-1.
The molecular structure and packing diagram of 3b was depicted in Figure 3.The compound 3c crystallizes with triclinic crystal system, space group P-1 with two moleucles and one benzene solvent molecule in the unit cell.The molecular structure and packing diagram of 3c was depicted in Figure 5.
The molecule 3c appears to be perfect planar from the top and side views.
Crystal packing of the molecule 3c was also illustrated in Figure 6.Intermolecular short contacts of the molecule 3c were listed in Table 4.In the crystal contacts between the molecules generated three dimensional molecular networks.
Further, the photophysical properties of molecules 3a, 3b and 3c, the UV-Vis   ing spectra were shown in Figure 7 and Figure 8, respectively.The molecules 3a, 3b and 3c displayed absorption maxima wavelength at 300, 419 and 418 nm, respectively.

Conclusion
Three new -CF3, -CN containing π-conjugated heteroaromatic compounds consisting of pyridine 3a, furan 3b, and thiophene 3c have been synthesized and these compounds have been characterized by 1 H NMR, EI-MS and X-ray diffraction analysis.Molecules 3a, 3b and 3c showed strong absorption maxima wavelengths at 300, 419 and 418 nm, respectively.The molar extinction coefficients

25 H 13 F 6 N 3 ,
contains one-half of the formula unit.The molecular structure appears to be planar from the top view.But, from the side view we can clearly see that three aromatic rings are slightly distorted from the planar geometry.Two aromatic ring planes, A (defined by C7, C8, C9, C10, C11, C12) and B (defined by N1, C1, C2, C3, C2, C1) were slightly leaned and the plane angles between two aromatic rings is 2.58˚ as shown in Figure1.The two cyano groups, which are located in the same direction causes a deviation from planarity of the molecule 3a.Triple bond lengths of the cyano group was 1.145 (2) Å for C6N2.On the other hand, the bond angle consisting cyanoethenyl C-CN triple bond was slightly twisted, the angles is 173.95 (2)˚ for C5-C6-N2.Crystal packing of the molecule 3a was also illustrated in Figure2.Intermolecular short contacts of the molecule 3a were listed in Table2.The molecular packing of the molecule 3a has layer like geometry.In the crystal packing diagram, C-H•••N and C•••C short contacts between the molecules generated three dimensional molecular networks.

Figure 1 .
Figure 1.Top (above) and side (below) views of the molecule 3a and thermal ellipsoids are drawn on 50% probability level.A and B represents phenyl and pyridine rings.

Figure 2 .
Figure 2. Molecular packing of the compound 3a and thermal ellipsoids are drawn on 50% probability level.Blue and pale green ellipsoids represents N and F atoms, respectively.

Figure 3 .
Figure 3. Top (above) and side (below) views of the molecule 3b and thermal ellipsoids are drawn on 50% probability level.A, B and C represents phenyl and furan rings.

Figure 4 .
Figure 4. Molecular packing of the compound 3b and thermal ellipsoids are drawn on 50% probability level.Blue, red and pale green ellipsoids represents N, O and F atoms, respectively.

Figure 5 .
Figure 5. Top (above) and side (below) views of the molecule 3c and thermal ellipsoids are drawn on 50% probability level.A, B and C represents phenyl and thiophene rings.

Figure 6 .
Figure 6.Molecular packing of the compound 3c and thermal ellipsoids are drawn on 50%probability level.Blue, yellow and pale green ellipsoids represents N, S and F atoms, respectively.

Figure 9 .
Figure 9. HOMO and LUMO orbitals in the crystals of the compounds 3a, 3b and 3c.
(ε) of the compounds 3a, 3b and 3c indicated molecule 3b has better ability to absorb UV light, molecule 3a has better fluorescence intensity than molecule 3b and 3c.Low energy gaps of HOMO and LUMO energy levels of these compounds suggest that these compounds may be a promising new class of lead compounds for developing high performance semiconductor materials.Compounds 3a, 3b and 3c has emissions near to blue light, a slight modification of the structures by extending conjugation may find important applications in optoelectronic devices as blue light emitters in organic light-emitting devices.
T. Moriguchi et al.

Table 1 .
Crystal data and structure refinements for 3a, 3b and 3c.
Scheme 2. Schematic representation of synthesis of compound 3b and 3c.