Synthesis and Characterization of Pyrazine Derived Compounds as Potential Materials for Hole Transporting Layer ( HTL )

Three simple compounds that have the potential as a hole transporting layer ( HTL ) based on pyrazine derivatives conjugated with electron donor groups in the form of triphenylamine have been successfully synthesized and characterized . The synthesis began with a substitution reaction at high temperatures between 4 bromoaniline and 4 - iodoanisole to produce 4 - bromo - N , N - bis ( methoxyphenyl ) aniline , followed by substitution of bromo atoms with tributylstanum at low temperatures and inert atmosphere ( N 2 ) producing 4 - methoxy - N - ( 4 -( tributylstanyl ) phenyl ) aniline . The conjugation reaction was carried out through a Stille coupling reaction between 1 , 2 - bis ( 4 - bromophenyl ) ethane - i , 2 - dione with 4 - methoxy - N properties . Thus , it is expected that the three compounds have the potential as HTL material , with DNB and bDNB better than DNP .


Introduction
The development of electronic technology has been developing quickly; this is followed by the increasing need for electricity usage. Coal is still the primary energy source used in electricity generation. However, the use of coal causes many losses, such as environmental pollution, air pollution [1], or even respiratory disorders Based on these problems, we need a renewable electricity source that is more environmentally friendly such as hydroelectric energy [ 3 ], geothermal electric energy [ 4 ], or solar electric energy (solar panels). From several renewable energy sources, solar panels are promising renewable sources because of their unlimited sources. However, the use of solar panels as an energy source still has its challenges in its application, such as [2]. λ λ λ Jurnal Kimia Sains dan Aplikasi 23 (6) (2020): 228-233 229 expensive materials or low energy conversion. Based on data compiled by the National Renewable Energy Laboratory (NREL), solar panels made from Si-active crystals still hold the highest conversion efficiency, compared to solar panels with other active ingredients, such as organic solar panels, or perovskite. To that end, researchers in the field of materials to date have tried to find ways to optimize the value of power conversion energy (PCE) from solar panels, both research in the manufacture of devices (devices) or the synthesis of potential compounds for solar panels [ 5 ].
Several solar panels have evolved, such as organic photovoltaic (OPV) with active ingredients in the form of small organic compounds or polymers, Si-crystalline, and Perovskite Solar Cells (PSC). Perovskite is a calcium titanium oxide (CaTi 03 ) mineral that was discovered by Gustave Rose in 1839 . In perovskite solar panels, the term perovskite used does not refer to CaTi 03 crystals, but crystalline compounds that have the same structure as CaTi 03 [6], During its development, one of the perovskites used in solar cells was methylammonium lead halide (CH 3 NH 3 PbI(I 3 )) developed by Miyasaka. CH 3 NH 3 PbI(I 3 ) as an active component in solar cells has advantages such as low exciton binding energy, sharp optical absorption, and good carrier diffusion-length [ PSC is composed of several essential parts, such as transparent conductive layers (IOP, indium tin oxide), the electron transporting layer (ETL), active layer (perovskite), the hole transporting layer (HTL), and metal electrodes. The composition of the PSC can be seen in Figure l( i). HTL is a part that functions to transfer holes (positive charges) to the electrodes. Generally, this section uses Spiro-OMeTAD (Figure 1 (ii)) as its forming material. However, Spiro-OMeTAD is a compound that is difficult to synthesize and purify, so we need a new compound that is easier to make and has a high PCE when used in PSC [8]. the same device. Cui et al [10] could also synthesize simple HTL compounds from hexaphenylbenzene (HPB) derivatives with the donor group triphenylamine. HPB has HOMO and LUMO values of -5.20 eV and -2.10 eV with on this, then, in this study, developed an organic material based on pyrazine derivatives with an electron donor group triphenylamine, which can be used as HTL. (DNP) started from synthesis 4 -bromo-N,N-bis( 4methoxyphenyl)aniline (a) using starting material 4bromoanilin (i) and 4 -iodoanisole ( j) (step i). In stage ii, the transformation of Br group to tributyltin chloride was carried out through a substitution reaction between compound (a) and tributyltin chloride (d) to produce 4methoxy-N-( 4 -(tributylstanyl)phenyl)aniline Furthermore, the Stille coupling reaction (step iii) was carried out between compound (b) and i,2-bis( 4bromophenyl)ethane-i,2-dione (e) so that the compound i,2-bis( 4' -bis( 4 -methoxyphenyl)amino)- From these compounds, an imitation was carried out using 1,2-diaminobenzidine (f ), 2, 3 -diaminopiridin (g), and 3 , 3' -diaminobenzidine (h) to obtain the desired HTL. The complete scheme can be seen in Figure 3 .

. Synthesis of DNB
Compounds c ( 75 mg, 0.09 mmol) and 1,2diaminobenzene (11 mg, 0.101 mmol) were mixed into a 100 mL round bottom flask and dissolved in 30 mL ethanol. The mixture was refluxed for 24 hours. After that, the reaction mixture was filtered and the solids were washed using ethanol, so that a DNB (68 mg, 82.93 % yield ) was obtained as a yellow solids XH NMR

. Synthesis of DNP
Compounds c ( 75 mg, 0.09 mmol) and 2, 3diaminopiridin (11 mg, 0.101 mmol) were mixed into 100 mL round bottom flask and dissolved in 30 mL ethanol. The mixture was refluxed for 24 hours. After that, the reaction mixture was filtered, and the solid was washed using methanol. Then the crude was purified by column chromatography with hexane: ethyl acetate (2: 1) eluent

NMR analysis
NMR is a qualitative analysis method for determining the chemical structure of a compound. In this study, Tf -NMR was used, spectra were recorded with 300 MHz Bruker and 500 MHz using CDC 13 solvent.

. Ultraviolet-visible (UV-Vis) analysis
All final compounds were analyzed using a UV-Vis spectrophotometer. O-dichlorobenzene was used as a solvent and a blank solution. The cuvette used was made of orz with a thickness of 10 mm. The results of the UV analysis were used to calculate the energy gap (Eg) with the formula:

Analysis
Differential Pulse Voltammetry (DPV) analysis is used to determine the HOMO-LUMO level of the final compound. O-dichlorobenzene was used as a solvent. DPV was measured in an inert condition (N 2 ) and was used ferrocene as an internal standard in measurement.
The energy gap (Eg) values of DNB, bDNB, and DNP from optical calculations are 2.65 eV, 2.5 eV, and 2.49 eV.
Eg of DNB is higher when compared to DNP because N atom is more electronegative than C atom (on the benzene DNB ring), so N atom can decrease the LUMO level of DNP. The optical properties of the three compounds are summarized in Table 1.

. Optical properties
The results of the UV-Vis analysis of DNB, bDNB, and DNP compounds are presented in Figure 4 . Based on these spectra results, the values of and DNP are 348.5 nm, 356 nm, and 350 nm. In the bDNB compound, the absorption peak produced was not significantly different even though it was more conjugated than the other two compounds. This can be explained because, in bDNB compounds, the bond between C atoms in one benzene ring and C in another benzene rotates (there is a dihedral angle), so the conjugation of bDNB becomes imperfect [11]. This gives an effect on the value of data, the three compounds have well as the Spiro-OMeTAD uptake (

. Electrochemical properties
DPV analysis is used to obtain the reduction potential and oxidation potential of DNB, bDNB, and DNP, so the HOMO-LUMO value of each compound can be calculated. The values of HOMO, LUMO, reduction potential, oxidation potential, and Eg of the three compounds are summarized in Table 2. The LUMO values for the three compounds are -2.46 eV, -2.76 eV, and -2.87 eV for DNB, bDNB, and DNP. In DNB and bDNB compounds, different LUMO values are obtained, because bDNB has more electron-pushing groups than DNB, so the LUMO level of bDNB is more negative. The LUMO value on DNP is more negative when compared to the analogs, namely DNB. This more negative value is due to the presence of more electronegative N atoms in the pyridine ring. In contrast to LUMO, the HOMO values in the three compounds do not differ significantly. The overall energy level of HOMO-LUMO in the three compounds is illustrated using the diagram in Figure 5 .   257.5°C respectively, which is higher than that of Spiro-OMeTAD ( 240°C ), with a decomposition point of 437.3°C , 445.6°C , and 400.9°C . The decomposition point of the three compounds is above 400°C , so it can be said that the three compounds have good thermal stability. With the appropriate optical, electrochemical, and thermal properties (almost the same as the commonly known HTL, Spiro-OMeTAD), it is expected that DNB, bDNB, and DNP have the potential to be applied as HTL on perovskite solar panels, with DNB and bDNB being better than with DNP.

. Thermal properties
Thermal properties were analyzed using Thermogravimetric analysis (TGA) to determine the heat resistance of the three final compounds synthesized. Besides TGA, the melting point apparatus is also used to determine the melting point of each compound. Based on the melting point apparatus results, the melting points for DNB, bDNB, and DNP were obtained respectively 254.3°C , 290.2°C , and 257.5°C . The melting point of the three compounds is higher than that of Spiro-OMeTAD (commercial HTL), which has a melting point of 240°C .
Based on the TGA results, a decomposition point ( 5 % weight loss) from DNB, bDNB, and DNP were obtained at temperatures of 437.3°C , 445.6°C and 400.9°C respectively, while Spiro-OMeTAD was at 424°C [ 14 ]. Based on these data, it is obtained that bDNB has the highest decomposition point. This is possible because bDNB has a higher molecular weight than the other two. From the TGA results, it can be concluded that the three compounds have good thermal stability to be used as HTL material.

. 4 . 'H NMR ( 500 MHz) DNB
f-ITI T f .   T   T  T  T  T  T  T  T  T   PL1