Hydrazone-based Materials; DFT, TD-DFT, NBO Analysis, Fukui Function, MESP Analysis, and Solar Cell Applications

Due to numerous pharmaceutical and biological activities hydrazone (TC) based materials, it was important to investigate quantum chemical studies such as Density functional theory (DFT) calculations, natural bond orbital (NBO) analysis, molecular electrostatic potential (MESP), and local reactivity usage Fukui function for six TC derivatives compounds. DFT, NBO, MESP, and local reactivity calculations were obtained via utilizing CAM-Becke's three-parameter functional and Leee Yange Parr (CAM-B3LYP) functional and 6-311G +  + (2d, 2p) basis set. Optimized molecular structures for all studied compounds were obtained usage the DFT/CAM-B3LYP/6-311G +  + (2d, 2p) method. In addition, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy gap (Eg), light harvest efficiency (LHE), and open-circuit voltage (Voc) of all studied MSs are calculated and illustrated. These properties indicate that these molecular modeling structures as good candidates for utilization in organic DSSCs. The calculated spectroscopic investigations of hydrazine derivatives have been obtained by applying the TD/CAM-B3LYP/6-311G +  + (2d, 2p) method. the calculated UV–Vis absorption spectra for molecular structures under study show nice correlations with experimental spectra.

Because of the great challenge used in new research on renewable energy sources, solar cells (SCs) are considered one of the most significant renewable energies recently [9][10][11][12]. Photovoltaic (PV) technologies have become one of the most important topics in SCs to convert the sun into electrical energy [13,14]. In addition, the most important challenges are represented in capturing solar energy and converting it into electrical energy at a low cost [15]. It was taken out of the PV devices that are based on inorganic materials in their manufacture, such as crystalline and amorphous silicon (Amorph. Si), cadmium telluride (CdTe), gallium arsenide (GaAs), with the knowledge that they give their efficiency from 10 to 32% [16]. 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, SCs 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 time is considered less than those that depend on inorganic materials [17]. The efficiency obtained from these types of cells is still not marketable, as the most efficient devices are 4 to 5% [18].
Recently, new organic compounds used in SCs 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 [19]. Moreover, the manufacture of their devices is easy. Also, PV 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 [20,21]. The first DSSCs were based on titanium dioxide, which was discovered in 1991, and their efficiency was from 7 to 8 percent [13].
In last the study [22], a series of novel N / -(2-thiouracil-5-oyl) hydrazones were designed, chemically synthesized, and characterized [22]. The anticancer results showed that those compounds exhibit the most prominent effect on breast cancer cells [22]. In addition, molecular docking studies were also performed [22]. Hence, it was important to carry out the following important quantum studies in this manuscript; DFT calculations, NBO analysis, Fukui function analysis, MESP, and solar cell application using CAM-B3LYP/6-311G + + (2d, 2p) level of theory.

DFT Calculations
The optimization of the MSs understudy has been obtained via using DFT/CAM-B3LYP/6-31G + + (2d, 2p) method in a gaseous state; the results of optimized MSs are presented in Fig. 1. Exitance of C = N in all considered MSs, lead to the possibility of much more than one conformer. The possible conformations for each considered MS were investigated and the more stable conformer for each studied MS is presented in Fig. 1. The influence of substituents such as chloride (Cl) in CBTC, bromide (Br) in BBTC, methoxy (CH 3 -O) in MOBTC, methyl (CH 3 -) in MBTC, and furane in FMTC on some important selected optimized MS parameters such as (bond length in Å, bond angle and dihedral angle in degree) for BTC, CBTC, BBTC, MBTC, MOBTC, and FMTC optimized MSs have been investigated; the results are collected in Table 1. Some important comments that can be deduced are as follows; (i) Referring to the listed values of dihedral angles, all studied MSs are planar. (ii) Owing to increasing bond order, the C 22 -C 20 bond length is less than C 19 -C 17 bond lengths in all studied MSs by about (0.016-0.088) Å. (iii) Referring to the listed values of bond angles, the type of hybridization overall studied MSs is SP 2 . (iv) The obtained bond lengths alter because all C-C, ring's C-N bonds are either doubly bonded or partially multiple bonded. (v) Generally, the stability of MS can be judged via the length of the bond [25], where the more stable molecular structure is combined with the shorter the bond length. As shown in Table 1, the bond lengths of CBTC, BBTC, MBTC, and MOBTC MSs are less than that of the BTC compound. Therefore, the CBTC, BBTC, MBTC, and MOBTC MSs are the highest stable comparable BTC compound. (vi) Also, the bond lengths in FMTC MS are shorter than those in BTC MS. Hence, the FMTC MS is more stable than the BTC compound due to the substitution of furan with the phenyl group.
The molecular orbitals (MOs) of all studied MSs (BTC, CBTC, BBTC, MBTC, MOBTC, and FMTC) are investigated; the obtained results such as HOMO (H) / LUMO (L), H-1/L + 1, H-2/L + 2 MOs, and energy gaps between the following graphically; H and L (E g ), H-1 and L + 1 (E g1 ) and H-2 and L + 2 (E g2 ) in the gaseous state via utilizing CAM-B3LYP/6-311G + + (2d, 2p) level of theory are collected in Figs. 2 and 3. The HOMO MOs are localized on phenyl (-C 6 -H 5 ) and its derivatives like (4-Cl-C 6 -H 5 , 4-Br-C 6 -H 5 , 4-CH 3 -C 6 -H 5, and 4-CH 3 -O-C 6 -H 5 ) in CBTC, BBTC, MBTC, and MOBTC MSs respectively. But, upon substitution of the phenyl group with furan, the HOMO MOs become localized on furan substituent instead of phenyl one. On the other hand, the LUMO MOs are localized over the whole studied MSs. The HOMO energy MOs of all studied MSs are ordered as follow; BBTC < BTC < CBTC < MBT C < MOBTC < FMTC. Indicating, that the BBTC MS has the highest stable HOMO MOs while FMTC MS has the highest reactive HOMO MOs. Also, the LUMO energy MOs are arranged in the following order: FMTC < BBTC < CBT C < BTC < MBTC < MOBTC. This refers to the FMTC MS having the highest stable LUMO MOs, but the MOBTC has the highest reactive LUMO MOs. The energy gap (E g ) value is calculated by applying the difference (E L -E H ). The calculated E g values for all studied MSs rise in the following order: FMTC < MOBTC < MBTC < BBTC < CBTC < BTC. These results indicate that the FMTC MS is more reactive compared to the other studied MSs and BTC has the lowest reactivity. Also, the reduction in the Eg value favors the red-shifted of the electronic absorption maximum peak in the UV-Vis absorption spectrum. Therefore, the calculated electronic UV-Vis absorption spectra for FMTC, MOBTC, MBTC, BBTC, and CBTC are red shifted, in contrast to, the UV-Vis absorption spectrum of BTC. The total density of states (TDOS) for BTC, CBTC, BBTC, MBTC, MOBTC, and FMTC MSs using the B3LYP/6-31G* level of theory is investigated; the obtained calculated spectra are collected in Fig. 4. The obtained TDOS for all studied MSs are calculated using Multiwfn software [26]. As presented in Fig. 4, the highest reactive MS is FMTC and the lowest one is BTC.

Quantum Chemical Parameters Calculations
Some important quantum chemical parameters like dipole moment ( ), chemical potential ( ), electronegativity ( ), and chemical hardness ( ) were calculated via using E L and E H values. These quantum parameters are calculated using the following equations ρ = [27]. Generally, when the chemical structure has high dipole moment, it has a large asymmetry in the electric charge distribution, and then it can be highly sensitive due to the change of chemical structure and electronic properties under an external electric field. Thus, as shown in Table 2, the μ value of the compound BTC is highest in contrasting the rest of the other compounds under study. Therefore, this compound is more active compared to the rest of the compounds under study.
As presented in Table 2, the ρ value of BBTC MS is lower compared to the other studied compounds. Indicating, that the escaping electrons from BBTC MS are low compared to   Table 2) leads to the ability of this compound to attract electrons from other compounds [28]. On another side, the value for BTC MS is high compared to the rest of the other compounds (see Table 2). This indicates that the BTC compound is exceedingly difficult to liberate electrons, while the other compounds (CBTC, BBTC, MBTC, MOBTC, and FMTC) are good candidates to give electrons to another acceptor molecule.

Local Reactivity Using Fukui Function
The local reactivity of all atoms in all studied MSs using the Fukui function at the DFT/CAM-B3LYP/6-311G) + + (2d, 2p) were investigated; the calculated results are written in Tables 4, 5, and 6 [30]. The electrophilic (f − ), nucleophilic (f + ) and free radical (f 0 ) attack are calculated via utilizing the following equations;     and 10S respectively. But the free radical attack for MOBTC and MBTC MSs is not the same which is 1C for MOBTC and 16O for MBTC. This is due to the highest ED properties for the CH 3 -O group in the MOBTC MS contrasting CH 3 -group in the MBTC compound. (Note: All the compounds under study are compared to the BTC compound, and therefore the reasons for the highest reactive site for nucleophilic, free radical, and electrophilic attacks depend on this comparison).

Molecular Electrostatic Potential (MESP)
The electron density is an incredibly significant factor for investigating the reactivity of electrophilic (E) and nucleophilic (Nu) sites and the interactions of hydrogen bonds [30], as well as this density, is related to the molecular electrostatic potential (MESP). Therefore, for predicting this reactivity of Nu and E sites attacks for all studied MSs, we obtained the MESP of these MSs usage the CAM-B3LYP /6-311G + + (2d, 2p) level of theory. The numerous colors (red, blue, and green) at the MESP surface indicate different values of the ESP as the regions of highest negative, highest positive, and zero ESP respectively. The negative sites at MESP (red) refer to E reactivity, the positive sites (blue) refer to Nu reactivity, and the green represents regions of zero potential (See Fig. 5). As presented  Fig. 6. Indicating, that the electrons can be successfully transferred into TiO 2 from the excited state of all studied dyes. Consequently, all studied MSs dyes may be good candidates for application in PV devices.  from the formula, it can be seen that high V OC and J SC are the basis for producing photoelectric conversion efficiency.
(1) = J sc V oc FF P inc

MOBTC FMTC
The maximum value for Voc is a significant PV 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 via utilizing Eq. (2) [27,32]: However, in DSSCs, Voc can be obtained as the different energy among LUMO of the dye and conduction band (CB) of the semiconductor [27,32].  The light-harvesting efficiency (LHE) is obtained using the following equation (LHE = 1-10 −f ) [33], where f is the oscillator strength of the dye MS; the calculated LHE is presented in Table 7. The f values for all studied MSs were calculated using the TD-DFT/CAM-B3LYP/6-3111G + + (2d, 2p) method.
As presented in Table 7, the obtained values of α were in the range of (1.014-2.414 eV). Indicating, that all LUMO MO level for all studied MSs is placed higher than the LUMO MO level of PCBM [33]. So, these studied compounds can be utilized as sensitizers being of the electron injection process from the excited dye to the conduction band of PCBM. Since the value of LHE increase in the following order as shown in Table 7; FMTC < BTC < MBTC < CBTC = MOBTC < B BTC therefore the best dye that can act as DSSCs is BBTC dye comparable to the rest compounds under study. The LHE value for BTC was 0.856 eV as shown in Table 7. The presence of chloride (Cl − ), bromide (Br − ), methyl (CH 3 -), and methoxy (CH 3 -O-) substituents at the para position of the phenyl group in CBTC, BBTC, MBTC, and MOBTC MSs respectively enhance the LHE by the range of (0.078-0.095) eV. This is due to the EW properties of Cl − and Br − and ED properties of CH 3 -and CH 3 -O-. On the other hand, comparable FMTC to BTC, the LHE of FMTC is decreased via 0.22 eV due to the substitution of furan with a phenyl group. The LHE values for BTC, CBTC, BBTC, MBTC, and MOBTC were in the close range (0.856-0.951) indicating that those MSs gave similar photocurrent [34].

Spectroscopic Investigations
The experimental absorption spectra for hydrazone-based materials class were discussed in the past [35,36]. Those spectra were within the 300-450 nm range, and the reason for those electronic absorption spectra was π-π* electronic transition [36]. The accurate functional used to calculate the electronic UV-Vis absorption spectra for the hydrazone derivatives family was B3LYP [36]. But the reviewer recommended using CAM-B3LYP instead of B3LYP as the density functional B3LYP is well known to poorly handle dispersion interactions. Hence, the computational electronic UV-Vis absorption spectra for TC-based materials in methanol were calculated using the TD/CAM-B3LYP/6-311G + + (2d, 2p) method; the results calculated spectra are presented in Fig. 7 and the corresponding optical parameters are written in Table 8. The computational UV-Vis absorption spectra for all studied MSs are within 250-450 nm rang as shown in Fig. 7. Comparing the computational results with the experimental one, it was shown that the computational electronic UV-Vis absorption spectra for all studied MSs lie in the same region as the experimental one. Therefore, the collected computational results are nice with the experimental.    Table 8.

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

Consent to participate
This article does not contain any studies involving animals performed by any of the authors.

Consent to publish
All authors mentioned in the manuscript have given consent for submission and subsequent publication of the manuscript.

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