1,2,4-Triazine-based Materials: Spectroscopic Investigation, DFT, NBO, and TD-DFT Calculations as Well As Dye-sensitized Solar Cells Applications

In this manuscript, we report four series for 1,2,4-triazine derivatives as dye-sensitized solar cells (DSSCs). Density functional theory (DFT) methods via utilizing Becke's three-parameter functional and LeeeYangeParr functional (B3LYP) level with 6-31G (d, p) basis set to investigate their modeling molecular structures. Optimized molecular structures for studied molecular structures are obtained using the DFT/B3LYP/6-31G (d, p) method. In addition, the time-dependant density functional theory (TD-DFT) is used to study the optoelectronic properties and absorption spectra using DFT/CAM-B3LYP/ 6-31G +  + (d, p) level in the Gaussian 09 program. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy gap (Eg), light harvest efficiency (LHE), and open-circuit voltage (Voc) of the studied molecular structures are calculated and illustrated. These properties indicate that these molecular modeling structures as good candidates for utilization in organic DSSCs.


Introduction
Because of the great challenge used in new research on renewable energy sources, solar cells are considered one of the most important renewable energies recently [1][2][3][4]. Photovoltaic technologies have become one of the most important topics in solar cells to convert the sun into electrical energy [5,6]. In addition, the most important challenges are represented in capturing solar energy and converting it into electrical energy at a low cost [7]. It was taken out of the photovoltaic devices that are based on inorganic materials in their manufacture, such as crystalline and amorphous silicon, cadmium telluride (CdTe), gallium arsenide (GaAs), with the knowledge that they give their efficiency from 10 to 32% [8]. However, since these are expensive materials and scarce, in addition to their toxicity, many researchers have resorted to searching for other new, cheap, and more efficient materials. According to what will be said, solar cells based on organic compounds are considered an attractive and appropriate choice due to their flexibility and ease of processing in addition to their low cost, but their efficiency at present is considered less than those that depend on inorganic materials [9]. The efficiency obtained from these types of cells is still not marketable, as the most efficient devices are 4 to 5% [10].
Recently, new organic compounds used in solar cells have been studied and developed. Among these materials are the dyes for sensitive solar cells (DSSCs), as they receive great interest among researchers due to their low cost and high efficiency in converting solar energy into electricity [11]. Moreover, the manufacture of their devices is easy. Also, photovoltaic cells based on the DSSCs have many advantages, including their compatibility with many supporting materials and production under moderate conditions that make them less expensive compared to other DSSCs [12,13]. The first DSSCs were based on titanium dioxide, which was discovered in 1991, and their efficiency was from 7 to 8 percent. [13].
In this manuscript, the results of quantum calculations for four organic compounds based on 1,2,4-triazine derivatives as dye-sensitized solar cells (DSSCs) are presented. In these structures, we used sulfur atom as electron donor unit (D) and the phenyl and triazine groups were used as electron acceptor groups (A) for all compounds.
Time-Dependent DFT (TD-DFT) has been widely used to discuss the properties of organic compounds in their excited state because its high accuracy is reasonable with the abinitio method and less computational time cost. In this paper, the obtained calculated results via utilizing TD-DFT/ CAM-B3LYP/ 6-31 + + G (d, p) like the optoelectronic properties and photovoltaic properties (open-circuit voltage (Voc), oxidation potential energy, and electron injection force) of all molecular structures were investigated and presented.

Computational Investigation
The molecular modeling and photoelectronic properties of the studied compounds (DTT, BDTTB, BDTTMB, and BDTTMP) based on 1,2,4-triazine were obtained and investigated using Density functional theory (DFT) methods via utilizing Becke's three-parameter functional and LeeeYange-Parr functional (B3LYP) [14] level with 6-31G(d, p) [15] basis set. All computational calculations were presented by utilizing the Gaussian 09 program [15]. Upon utilizing the DFT/B3LYP/6-31G (d, p) level, the molecular structures of neutral molecules are optimized, and their molecular electronic properties as HOMO, LUMO levels, and the energy gap (E g ) are obtained. The oscillator strengths (f) and the optical transitions have been illustrated using TD-DFT combined with CAM-B3LYP functional and 6-31G + + (d, p) basis set. Finally, the ultraviolet-visible (UV-Vis.) absorption spectra of the studied molecules were presented utilizing Gauss View software [16].
Since in BDTTB the middle group is a butyl group, and in the BDTTMB compound, the butyl group is replaced by a p-xylenyl group, and finally in the BDTTMP compound, the middle group is 2,6-butidienyl. The four studied molecular structures under study were prepared, characterized and published in 2121 by Sakr et. al. [17].

Molecular Structure Properties
The optimization of the molecular structures under study has been obtained via using the DFT/B3LYP/6-31G (d, p) method in the gas state; the result of optimized molecular structures are presented in Fig. 2. The effect of substituents such as butyl in BDTTB, p-xylenyl in BDTTMB, and 2,6-butidienyl in BDTTMP has been studied on some important selected optimized molecular structure parameters such as (bond length in Å and dihedral angle in o ) for DTT optimized molecular structure; the results are collected in Table 1. Some important comments that can be deduced are as follows; (1) The DDT molecular structure is not planar as one of the two phenyl groups rotate out of the triazine by angle 33.44° to prevent the steric hindrance. (2) all molecular structures (BDTTB, BDTTMB, and BDTTMP) are not planar as one of the phenyl groups rotates by 34.04, 15.13, and 35.49° for BDTTB, BDTTMB, and BDTTMP respectively out of the triazine moiety to decrease steric hindrance. Hence, the π-interactions among sub-systems in each molecule are small. (3) Generally, the stability of molecular structure can be judged via the length of the bond [18], where the more stable molecular structure is combined with the shorter the bond length. As shown in Table 1, the bond lengths of DDT and BDTTMB are less than that of BDTTB and BDTTMP hence, the DDT and BDTTMB are more stable molecular structures compared to BDTTB and BDTTMP. (4) The selected bond angles for all studied molecular structures refer to SP 2 hybridization.

Energy levels of the studied molecular structures
The graphical presentation of the HOMO and LUMO MOs, for DDT, BDTTB, BDTTMB, and BDTTMP compounds in gas at the B3LYB/6-31G (d, p) level of theory is shown in Fig. 3. It is noticeable from Fig. 3 that the HOMO MOs of the compounds under study are localized on certain parts of these compounds (sulfur atom, triazine, and phenyl groups), but not all of them. On the other side, the LUMO MOs are localized on (triazine and phenyl groups). This indicates that the sulfur atom, triazine, and phenyl groups act as a donor, but on the other hand, triazine and phenyl groups act as an acceptor. Furthermore, the calculated energy values of HOMO, HOMO-1, HOMO-2, LUMO, LUMO + 1, and LUMO + 2 and the energy gap between the following; (HOMO and LUMO (E g1 ), HOMO-1 and LUMO + 1 (E g2 ) and HOMO-2 and LUMO + 2(E g3 )) of the compounds under study are written in Tables 2 and 3 The HOMO of the studied molecular structures are ordered as follows; DDT < BDTTMP < BDTTMB < BDTTB. Also, the LUMO energy MOs are arranged in the following order; BDTTMP < BDTTB < DTT < BDTTMB. The calculated E g1 of the studied molecular structures are arranged in the following order BDTTB < BDTTMP < BDTTMB < DTT. This indicates that compound BDTTB has the highest reactivity and compound DTT has the lowest reactivity. Also, the reduction in the E g1 value favors the red-shifted of the electronic absorption maximum peak in the UV-Vis absorption spectrum. Therefore, the electronic UV-Vis absorption spectra for BDTTB, BDTTMP, and BDTTMB are red shifted, in contrast to, the UV-Vis absorption spectrum of DTT. As a whole, the electron density of HOMO MOs is mainly located near the donor and the electron density of LUMO is mainly located near the acceptor. Consequently, the sulfur atom acts     (DTT, BDTTB, BDTTMB, and BDTTMP) can more easily recover electrons from electrolytes (I 3 − ). In addition to, the LUMO energy of the four studied molecules (DTT, BDTTB, BDTTMB, and BDTTMP) is higher than the conduction band energy of semiconductor TiO 2 (− 4.00 eV) as shown in Fig. 4. Indicating, that the electrons can be successfully transferred into TiO 2 from the excited state of all studied dyes. Consequently, all studied molecular structure dyes may be good candidates for application in photovoltaic devices.
The conversion efficiency (Ƞ) of sunlight to electrical energy in solar cell devices is determined by the shortcircuit current density (Jsc), the open-circuit photovoltage (Voc), and the fill factor (FF) and incident solar power (Pinc   From the formula, it can be seen that high V OC and J SC are the basis for producing photoelectric conversion efficiency. The maximum value for Voc is a significant photovoltaic parameter that can be obtained computationally via utilizing the difference between the HOMO of dye and the LUMO of the electron acceptor (conduction band of TiO 2 ). The computational value of Voc has been calculated by utilizing the following (2) [20,21]: Whilst, in DSSCs, Voc can be approximately obtained as the different energy among LUMO of the dye and conduction band (CB) of the semiconductor The theoretically calculated value of Voc for all studied molecular structures is presented in Table 2. The range value of Voc is 1.738 to 1.938 eV for semiconductors (TiO 2 ). These values are positive; thus indicating that the electron transfer will be easy from all studied molecular structures (DTT, BDTTB, BDTTMB, and BDTTMP) to TiO 2 . Furthermore, these values are sufficient to give the best efficient electron injection. Moreover, these triazine derivatives compounds can be utilized as sensitizers of the electron injection process from the excited dye to the conduction band of TiO 2 .
(1) = J sc V oc FF P inc Another parameter denoted (α) was calculated via the difference between the LUMO energy levels of the studied dyes and the LUMO energy level of PCBM (-3.2 eV) [22]. The value can be obtained by using the following Eq. (4): The light-harvesting efficiency (LHE) is obtained using the following equation (LHE = 1-10 f ) [22], where f is the oscillator strength of the dye molecular structure; the calculated LHE is presented in Table 4.
As presented in Table 4, the obtained values of α were in the range of (0.938-1.138 eV). This indicates, that all LUMO levels for all studied compounds are placed higher than the LUMO level of PCBM. So, these triazine derivatives compounds can be utilized as sensitizers of the electron injection process from the excited dye to the conduction band of PCBM. Depending on the values of LHE in Table 4, the best dye that can act as DSSCs is DDT dye compared to the rest compounds under study.

UV-Vis Properties
The optical properties and the electronic transition for the all studied dyes (DTT, BDTTB, BDTTMB and BDTTMP) are investigated using TD-DFT/ CAM-B3LYP / 6-31G + + (d, p) method [23]. The calculated transition energies of studied molecular structures for the absorption wavelength, vertical excitation energy (E ex ), oscillator strength (f), and the transition character of these dyes are listed in Table 4. The UV-Vis absorption spectra of triazine dye derivatives simulated at the TD-DFT/ CAM-B3LYP / 6-31G + + (d, p) method are located from 300 to 450 nm as shown in Fig. 5. The experimental electronic absorption spectra of the DDT, BDTTB, BDTTMB, and BDTTMP molecular structures were obtained [17]. The electronic experimental maximum absorption wavelengths were 400, 350, 350, and   Fig. 5 and Table 4. The three calculated electronic transitions for BDTTB dye appear at 416 nm, 348 nm and 335 nm in methanol where in agreement with practical results. The first, second and third absorption peak arises from the electronic transition HOMO-2-> LUMO + 1, HOMO-> LUMO + 1 and HOMO-3-> LUMO + 2. The same findings were also found for both BDTTMB and BDTTMP but with red shift for DTT one.

Molecular Electrostatic Potentials Map (MEPM)
A molecular electrostatic potential map (MEPM) investigates reveals the electrophilic versus nucleophilic sites of all molecular structures under study, giving for the prediction of reaction areas [25]. The MEPMs of the studied DTT and its derivatives were calculated employing B3LYP/6-31G (d,p) in Fig. 6. The negatively, positively and neutral zones of the MEPM interfaces are represented by the colors red, blue, and green, respectively (Fig. 6). The negative site (red color) is inside terminal benzene rings, suggesting that such rings are electrophilic attacks. The positive area (blue color) encompassed the carbon and hydrogen atoms, suggesting nucleophilic attacks in these sites. The neutral areas ( green color) of the molecular structure are those that are devoid of any charge distribution. The blue area around the H-atoms of the sulfur atom was associated with electron deficiency (i.e., nucleophilic reactivity), rendering it a favorable place for intermolecular H-bonding (nucleophile attack). The electron density (red area) appeared centered within the circle, according to the ESP map. The nucleophile in the phenyl ring has had the highest negative possibility, making it an ideal subject for an electrophilic engagement [26].

Evaluating DTT Dye for DSSC Device
we continue our study of a prototypical dye molecule for DSSC devices from Fig. 7. In an actual device, the DTT end of the dye molecule is bound to a titanium oxide nanostructure. The original researchers [27] Fig. 7B. Interestingly. The titanium complex is also involved in HOMO and LUMO, hence it is becoming part of the donor and acceptor. The 3d orbital from the titanium atom is visible in the LUMO and

Conclusion
In this paper, the optimized molecular structure, and electronic and optical properties of four triazine derivative dyes DTT, BDTTB, BDTTMB, and BDTTMP were investigated via utilizing DFT/TD-DFT. According to the ground state geometry, we note that all stable conformations are not planar. Data Availability All data generated or analyzed during this study are included in this published article.
Code Availability Not applicable.

Declarations
Ethical Approval This article does not contain any studies involving animals performed by any of the authors.

Conflict of Interest The authors have declared no conflict of interest.
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