Synthesis and Characterization of Benzodithiophene ( BDT ) Quinoid Compounds as a Potential Compound for n-Type Organic Thin-Film Transistors ( OTFT )

Received: 27 December 2019 Revised: 4th July 2020 Accepted: 17th July 2020 Online: 31st July 2020 Two potential compounds as an n-Type organic thin-film transistor (OTFT) from benzodithiophene (BDT) derivatives have been synthesized and characterized. BDT was chosen as the core because it has 7t -conjugated bonds, rigid structures, and planar. Quinoid structure with end-cap (terminal group) in the form of dicyanomethylene is used because it can lower the LUMO value of the compound, and side chains are selected in the form of alkoxy so that two BDT derivatives are obtained namely BDTQ-6 (hexyloxy) and BDTQ-10 (decyloxy). Based on the results of TGA, BDTQ-6 and BDTQ-10 have decomposition points of l83°C and 203°C, which indicate the compound has excellent thermal stability. From the UV-Vis measurement, the energy (Eg?1) of 1.7 eV. From the DPV measurement, the LUMO value for the two compounds is -4.3 eV, with an energy gap (Eg) of 1.69 eV (BDTQ-6) and 1.70 eV (BDTQ-10). Based on observations of the crystal structure through x-ray diffraction, it was found that the BDTQ-10 crystal has a "brick type" layer arrangement with a distance between layers of 3.55 A. With excellent thermal stability and suitable LUMO values and energy gaps, it is expected that BDTQ-6 and BDTQ-10 compounds have the potential to be n-Type OTFT materials.


Introduction
Organic semiconductor compounds have attracted many researchers because of their wide use, such as for solar panels [1], organic field-effect transistor (OFET) [2], dye-sensitized solar cell (DSSC) [ 3 ], and so forth. Among these uses, OFET attracts much attention and can be used as an alternative to inorganic materials [ 4 ]. This is due to several advantages of organic semiconductors such as lightweight, inexpensive, and can be applied to flexible substrates such as plastic. They can be applied to several things, such as electronic paper, sensors, or radio frequency identification (RFID) [ 5 , 6].
Several ring-shaped compounds with conjugated double bonds have attracted attention in their application as semiconductors, such as pentacene and thiophene [ 7 ]. Thiophene is a candidate that has been widely developed to form conjugated cyclic compounds, due to its photoelectrochemical properties and better stability when compared to pentacene. Not only that, but the crystal structure of thiophene also has a rigid and planar structure. The oligothiophene crystal structure, also known to have interactions between S-S atoms, causes the distance between layers to get shorter, making charged mobility is expected to be better [8].
During its development, p-type OTFT has been developed with significant progress, in contrast to the n-Type OTFT. Based on several n -Type OTFT from oligothiophene, quinoidal compounds with end-cap in the form of dicyanomethylene groups are quite interesting, because they can produce materials with the lowest unoccupied molecular orbitals (LUMO) [ 9 ]. Based on the working principle of the n-Type OFET, the flow of electrons will be affected by the energy barrier between LUMO and the work function of the source-drain electrode, so reducing the LUMO energy level is very important [10]. δ δ δ δ δ δ δ Some researchers have developed an OTFT n-Type based on thiophene or heterocyclic ring quinoids with end -cap cyanomethyl groups. Wu et al [11] developed a heterocyclic semiconductor named QFBPBP and obtained mobility ( p) of 4 cm 2 V -1 s _ 1 with a LUMO of -4.41 eV. Huang et al [12] succeeded in synthesizing Qdichloromethane-DPPTT, a pyropyrrole-based diciofene semiconductor with a LUMO value of -4.5 eV. Vegiraju et al. [ 13 ] has reported a series of DTTQwith a LUMO value of -4.2 eV and the highest mobility of 0.29 cm 2 V _ 1 s _ 1 . hexyl bromide and desylbromide and continued with bromination and formation of quinoid structures to obtain RDTQ-10 and BDTQ-6 target compounds. Stages of reaction are presented in scheme 1.

. Synthesis of benzo [1,2-b: 4,5 -b'] ditiofen-4, 8dion (c)
18.89 mmol) was put into 100 mL three-necked round bottom flasks which had been conditioned in an inert atmosphere (N 2 ). Then, a 40 mL dry tetrahydrofuran solvent was added. After that, 2.5 M n-butyllithium ( 7.53 mL, 17.83 mmol) was slowly added for 30 minutes at o°C and stirred further for 30 minutes at room temperature. Then the mixture was poured into a beaker filled with ice and stirred for 2 hours. The precipitate formed was filtered and washed using a mixture of methanol and water with a volume ratio of 1: 2 to obtain compound (c) in the form of a green solid (2.8 g, 50 % yield). XH NMR DT I Q Figure 1. Chemical structure of QFBPBP, Qdichloromethane-DPPTT, and DTTQ Based on this, in this research, synthesis of potential compounds as n -Type thiophene-based organic semiconductors which has a rigid structure and planar conjugation. Fused-thiophene compounds with rigid and planar structures can improve overlapping orbitals between compounds, thereby increasing the efficiency of load transport [ 13 ]. The side chain is in the form of alkoxy so that two BDTQ-6 target compounds with an oxadexyl side chain and BDTQ-10 with an oxadexyl side chain are obtained. Both compounds are characterized, both thermal, optical, and electrochemical.

Synthesis of BDTQ-b
Sodium hydride 60% ( 117 mg) dispersed in oil was put into a 100 mL three-necked round bottom flask conditioned in an inert atmosphere (N 2 ). Then 15 mL of dried THF and malononitrile ( 96 mg, 1.45 mmol) was put into the flask slowly at o°C, then stirred at room temperature for 30 minutes. After that, compounds ei (200 mg, 0.272 mmol) and Pd(PPh 3 ) 4 ( 50 mg) were added to the flask and refluxed. After 48 hours, a solution of Br2 was added to the mixture. Then, the mixture was extracted using dichloromethane / H20 three times, and the organic layer obtained was dried with Na2S 04 , then the solvent was evaporated. The crude obtained was purified using column chromatography with hexane: dichloromethane (1: 1) eluent to obtain BDTQ . -6 compounds in the form of green solids ( 52 mg). XH NMR ( 500 MHz, CDCI 3 ), 7.445 ppm (s, 2H ), 4.424 ppm (t, 4 H).

. Synthesis of BDTQ-io
The same procedure with subsection 2.8 is performed where 60% sodium hydride is dispersed in oil (111 mg

. Thermal Properties
TGA analysis results show material decomposition for each material tested. Besides using TGA, BDTQ-6 and BDTQ .
-10 are also analyzed using the melting point apparatus to determine each material's melting point. TGA results show the composition point at 5 % weight loss for BDTQ-6 and BDTQ-10 are 297.520 C and 306.5 l°C with melting points of 183°C and 203°C , respectively. The thermal properties of the two compounds are summarized in Table 1. Based on research conducted, thermal stability is an essential trait in developing organic semiconductor compounds to get a useful device. In BDTQ . -6 and BDTQ .
-10, both compounds have excellent thermal stability, as evidenced by their decomposition point above 290°C , as shown in Figure 2. Ogawa et al. [ 14 ] reported that the anthra [2, 3 -b] thieno[2, 3 -d] thiophene (ATT) compound with a thermal decomposition value ( 5 % weight loss) of 290°C , could have mobility amounting to 0.012 cm 2 V _ 1 s _ \ BDTQ 7 I0 has a higher decomposition value because BDTQ . -10 has a higher molecular mass due to a longer alkyl chain. In the decomposition process, when heating reaches the decomposition point, the alkyl on both sides of the compound is first released (decomposed). This is confirmed by the percentage of weight reduction in TGA.

. Electrochemical Properties
In this section, the electrochemical properties of BDTQ-6 and RDTQ-10 are discussed. All data were measured using DPV using ferrocene as an internal standard and tetrabutylammonium phosphate solution at 25°C . HOMO and LUMO levels of compounds were calculated based on the oxidation and reduction energy values of BDTQ-6 and BDTQ-10.
Based on DPV data, the HOMO values for BDTQ-6 and BDTQ-10 are -6.07 eV and -6.08 eV, respectively, while the LUMO value for both is -4.38 eV. The value, Eg for BDTQ-6, is 1.69 eV, while for BDTQ-10 is 1.70 eV. An illustration of the energy levels of the two compounds is presented in Figure 4 . The results of Eg are obtained in accordance with Eg 0 ? 1 data taken from UV-Vis data calculations. This result can be explained by the similarly long benzodithiophene nucleus and the small effect of the length of the alkoxy group on BDTQ-6 and BDTQ-10. Data on the electrochemical properties of the two compounds are summarized in table 3 .

. Optical Properties
In this section, the optical properties of BDTQ-6 and and BDTQ-10 are discussed by investigating the of UV-Vis spectra to obtain the energy gap (Eg 0 ? 1 ) of optical data.  The LUMO value obtained is following the requirements to get good material for n-Type. Based on Facchetti [ 15 ] report, from an energy level perspective, to get an n -Type material with good device performance, the LUMO value of material must be in the range of -4.0 -4.3 eV, to obtain a material with good charge transport. Too high LUMO level will produce material that is easily degraded after exposure to ambient air. Conversely, too low LUMO levels will produce materials with strong air resistance (challenging to degrade the surrounding air) but produce devices with difficult turnoffs.

599
1.71 The optical properties of the two compounds were analyzed using a UV -Vis spectrophotometer using 0dichlorobenzene as a solvent. The UV-Vis graph is presented in Figure 3 . Figure 3 shows that the of the two compounds is 599 nm. This is consistent with the initial predictions that the alkoxy chain will not significantly affect the n-conjugation on the benzodithiophene ring but can be different when BDTQ is measured in its thin-film form. As reported by Vegiraju et.al. [ 13 ], the different alkyl chains in the DTTQ series of compounds do not have a significant effect on when measurements are taken on the solution. However, it gives little influence on the results of when formed in thin-film (in this case, it is not performed on BDTQ compounds). This indicates that, in thin-film formation, the side chains (alkyl in DTTQ) will give the different molecular arrangements. The Eg°p t of this compound calculated from the edge absorbance is 1.71 eV for BDTQ-6 and BDTQ-10. The optical properties of the two compounds are summarized in table 2.

. Crystal Structure
In this study, the BDTQ-10 Crystal structure was obtained from a mixture of methanol and dichloromethane. The results of BDTQ-10 X-ray diffraction are presented in Figure 5 . Figure  on both sides of the nucleus, the CN and BDT nuclei have a C-C bond length of 1.38 A, which indicates that the bond is a C = C double bond. Besides, BDTQ-10 also has interactions between molecules in the form of hydrogen bonds between N atoms and H atoms with a bond length of 2.59 A. The single crystal from RDTQ-10 has a brick type arrangement with a spacing distance of 3.55 A and a slipping angle of 85.4°. With a distance between layers of 3.55 A, it is expected that electrons will flow quickly when applied in utilization as an OTFT, therefore they can get high electron mobility. High mobility can be obtained if the organic material used has a dense crystal structure with a short distance.

. Conclusions
Two compounds from the BDTQ series, RDTQ-6, and BDTQ-10, have been successfully synthesized and characterized. Based on the characterization results, both BDTQ-6 and BDTQ-10 have similar thermal, optical, and electrochemical properties.