Effect of chlorine and bromine on the nonlinear optical, electronic, optoelectronic and thermodynamic properties on the BEDT-TTF molecule: ab-initio and DFT calculations

In recent years, designing high performance NLO materials is an interesting topic in nonlinear optics. In this work, the influence of hydrogen atoms substitution with chlorine and bromine atoms of BEDT-TTF (C10H8S8\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${C}_{10}{H}_{8}{S}_{8}$$\end{document}) on nonlinear optical, electronic, optoelectronic and thermodynamic properties is investigated by using ab-initio and density functional theory. The proposed materials exhibit good nonlinear optical response. Results obtained with B3LYP/cc-pVDZ method shown a better fit to experimental data than RHF/cc-pVDZ in terms of geometric parameters (bond lengths and bond angles). First and second hyperpolarizabilities (β\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta$$\end{document} and γ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}) values show that the proposed materials have very good optoelectronic and nonlinear optical properties. Energies gap, Eg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{g}$$\end{document} show that the molecules may have semi-conductors properties and hence have applications in the field of optoelectronic devices.


Introduction
In the last few decades, the developing of nonlinear optical (NLO), electronic and optoelectronic materials have become an interesting topic and attracting great interest, because they find many applications in optical communication, optical computing, dynamic image processing, telecommunication, information storage, optical switching, photovoltaic, light emitting diode (LED), field effect transistor (FET) and other laser devices (Shakerzadeh et al. 2016). Research on designing new high performance NLO materials is being an intriguing issue for scientist (Shakerzadeh et al. 2016).
The discovery of bis(ethylenedithio) tetrathiafulvalene abbreviated as BEDT-TTF or simply ET in 1980 (Demiralp et al. 1995) (Fig. 1), we find in the literature others organic conductors, which are designed, studied and synthesized by others authors (Imamura et al. 1999). The works of Demiralp et al. (1997), have been showed that about 30 organic superconductors based on BEDT-TTF have been synthesized with critical temperature Tc up to 12.8 K. Previously in the literature, the detailed of infrared and Raman spectra of BEDT-TTF have been reported by the works of Kozlov et al. (1987), Eldridge et al. (1995) and Liu et al. (1997). In addition, the works of Wallis and Griffiths (2005) revealed others molecules from the chirality of substituted BEDT-TTF derivatives. Moreover, gives opportunities to prepare multifunctional materials. All these works have contributed much to understand this family of molecules. Despite some progress made in our previous work on this BEDT-TTF molecule and its derivatives doping with boron atoms (Olinga et al. 2021). The NLO, electronic, optoelectronic and thermodynamics properties are still not fully understood. In these last decades many works (Sundaraganesan et al. 2007; Moreira et al. 2015;Costa et al. 2016), have shown that the ab-initio and the density functional theory (DFT) have become a powerful tools in order to predict and study some important properties of molecules such as electronic structure, linear and nonlinear optics properties, thermodynamics and optoelectronics properties. The aim of this research article is to propose organic molecules, which can have potential applications in organic electronic. In this article, we study the semi-conductor nature of bis(ethylenedithio)tetrathiafulvalene and some of its derivatives by doping it with some halogen atoms (chlorine and bromine atoms), which may having application in quantum dots and other material. Our main objectives are to use ab-initio and DFT quantum mechanical calculations to decipher the electronic structure, dipole moments, average polarizability, hyperpolarizabilities, first and second hyperpolarizabilities, molecular orbital diagram, HOMO-LUMO energy gap, susceptibility, refractive index, dielectric constant, electrical conductivity, Electron affinity, Ionization potential and Molar refractivity of bis(ethylenedithio)tetrathiafulvalene with its dopedsystems. In this work, we reported the theoretical calculations (ab-initio and DFT) in gas phase using Hartree-Fock (HF) and B3LYP (Becke-3-Lee-Yang-Parr) methods with the same cc-pVDZ basis set in the ground state. This paper is organized in four sections. The calculation method is presented in Sect. 2. In Sect. 3, the results and discussion are presented. The conclusion is given in Sect. 4.

Computational details
Our calculations were performed using Gaussian 09W calculation code (Frisch et al. 2009). Before investigating the electronic structure, the molecules have been constructed using Gauss View 6.0.16 modeling software (Dennington and Keith, 2016). So, our properties were computed with Hartree-Fock (HF) and the hybrid method B3LYP program package, adopting the standard cc-pVDZ basis set. Previous work has shown that Beck's three parameter hybrids function combined with the Lee-Yang-Parr correlation function (B3LYP) Beck's three parameter exact exchange-function (B3) combined with gradient-corrected correlational functional of Lee, Yang, and Parr (LYP) predict the best results for molecules geometry, vibrational frequencies and density of states (Sundaraganesan et al. 2007;Becke 1988;Lee et al. 1988).

Theoretical framework
In order to investigate the relationships among molecular structures and nonlinear optical and optoelectronic properties, the dipole moment, the polarizabilities, first and second hyperpolazablities of BEDT-TTF molecule and its derivatives (chlorinated and brominated structures) are calculated using RHF/cc-pVDZ and B3LYP/cc-pVDZ methods.
In the case of strong fields, the dipole moment is given by the formula below: where is the polarizability, β is the first order hyperpolarizability and is the second molecular hyperpolarizability. (1) The anisotropy of polarizability (Δα) is given by the Eq. 5 below: Refractive index (n) in semiconductor is given by Eq. 6: The total dipole moment μ tot is calculated using the following Eq. 6 (Ejuh et al. 2018a, b): The electronegativity, based on the average of the electron affinity and ionization potential of molecules, energy gap, softness and the global hardness are given by the Eqs. 8-13 (Fankam et al. 2020a, b;Kabe et al. 2020;Midoune and Messaoudi 2020): The ionization potential (IP) and electron affinity (EA) can be expressed by HOMO and LUMO orbital energies as (see Eq. 14) (Anu et al. 2020).
The equations below given in the literature (Anu et al. 2020) are used for the calculation of optoelectronic properties:

Optimized structure
We obtained fully optimized structures of C 10 H 8 S 8 , C 10 H 4 S 8 Cl 4 and C 10 H 4 S 8 Br 4 using B3LYP method by employing the cc-pVDZ basis set. In Fig. 2

Geometric properties
The optimized geometric parameters of bis (ethylenedithio) tetrathiafulvalene undoped molecule (a) such as the bond lengths and bond angles obtained by RHF and B3LYP methods with cc-pVDZ as basis set are listed in Table 1 and compared with the experimental results (Guionneau et al. 2000). Based on our calculations, the result of the hybrid functional B3LYP have shown to better fit with experimental data than the HF method given in literature (Guionneau et al. 2000). Some of the calculated bond lengths (in Å) for BEDT-TTF such as C1-C1 1.354, C2-C2 1.353 and C3-C3 1.519 were approximately equal to experimental values given by Guionneau et al. (2000). Moreover, the largest difference between experimental and computed bond length is about 0.056 Å for B3LYP and 0.042 Å for RHF. On the other hand, the smallest difference is about 0.001 Å for B3LYP and − 0.039 Å for RHF. Those bond lengths varies slightly as we move from the uncor-  correlation makes them longer (Allen 2002). This elongation usually makes a better agreement between theory and experiment. This pattern is also observed here. Most of the bond lengths and angles obtained with B3LYP are very similar to the values reported by Guionneau et al. (experimental data), by Imamura et al. (1999) and by Demiralp et al. (1997), except for the C3-S3 bond length, which is slightly greater. The bond angles (°) of the undoped molecule follow the same pattern. The calculated bond angles were slightly equal to the values listed by Guionneau et al. It is noted that the experimental results and the theoretical calculations have better agreement.

Non linear optical properties (NLO)
The NLO properties of organic semi-conductors attract much attention in current global research, because these materials have wide applications in photonic technologies, modern communication technologies, optical signal processing and data storage (Saji et al. 2020;Spiridon et al. 2015;Bouchouit et al. 2004). It is important to understand the relationship of molecular structure with the NLO properties. The calculation of the first-order hyperpolarizability (β) and second hyperpolarizability ( ) of our molecules are determined theoretically. These both parameters allowed us to measure the NLO activity of the proposed molecules in this work.
The two parameters β and , which are the NLO responses of the BEDT-TTF original molecule and the both doped structures, the chlorinated molecule (Cl4ET) and brominated (Br4ET) are carried out using the both Restricted Hartree-Fock (RHF) and DFT/ B3LYP methods with the same basis set cc-pVDZ in the gas phase. In addition, we notice that, higher values of first and second molecular hyperpolarizabilities, dipole moment and polarizability are very important for more NLO properties. Table 2 show our calculated results from the Eqs.
(2), (3), (4), (5) and (7) given above. In this Table 2, the first molecular and second hyperpolarizabilities (β and ) values of all the considered molecules are listed. Also, we observed that all the values of the dipole moment ( ) are different from zero, which leads to believe that these molecules are polar. In addition, these dipole moment values decrease from the uncorrelated method (HF) to the B3LYP method, which takes into account the correlation of electronic exchanges of the considered systems. We have observed the highest first molecular hyperpolarizability β for C 10 H 4 S 8 Br 4 (β mol = 10,494.1467 × 10 −33 e.s.u), the molecule functionalized by the bromine atom. While, the lowest first molecular hyperpolarizability is observed for C 10 H 8 S 8 (β mol = 973.45 × 10 −33 e.s.u), undoped molecule. In addition, first molecular hyperpolarizability value of C 10 H 4 S 8 Cl 4 is β mol = 4156.79 × 10 −33 e.s.u. In order to compare these β values off all the considered molecules with the Urea (β o = 928 × 10 −33 e.s.u.) (Olinga et al. 2021). Urea is used as reference molecule to study the nonlinear optical properties of the One the other hand, another important NLO parameter have also computed, second molecular hyperpolarizability . This second molecular hyperpolarizability is a microscopic property of the molecule, and is defined as the sum of electronic contributions and a contribution coming from the orientation of the permanent dipole momentum in the electric field. The Eq. 4, which is given in literature (Anu et al. 2020), is used to carry out these second hyperpolarizabilities of all the considered molecules and their values are grouped in Table 2. The results show the negative values of second hyperpolarizabilities.
The theoretical values of the second hyperpolarizabilities for Cl4ET and Br4ET obtained in gas phase using B3LYP/cc-pVDZ are − 5.17474 × 10 −61 C 4 m 4 J −3 ) and − 5.93047 × 10 −61 C 4 m 4 J −3 ) respectively. These negative values of are the fact the contributions to gamma are due to the pi-electrons. With these large values of gamma, and the comparison of these computed results with others recently published (Fankam et al. 2020a, b, Ejuh et al. 2020 allowed us to confirm that these proposed materials are a good candidate for NLO applications. Due to the large values of the hyperpolarizabilities β and of our doped systems, compared to the organic reference molecule Urea, and based on the work of other authors. Such as Speridon et al. (2015), Kulyk et al. (2017) and El Kouari et al. (2015), we believe that these considered materials have potential applications in the field of optoelectronic devices such as data storage, optical communication, optical switching.

Electronic properties
Energy gap is defined as the difference between LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital). This property is very important because it allows us to explain clearly the stability and the reactivity of the molecules (Saji et al. 2020;Muthu and Uma Maheswari 2002;Fonkem et al. 2019;Ejuh et al. 2017). Table 3 gives details of HOMO and LUMO and others related parameters such as Ionization potential abbreviated IP, Electron affinity (AE)…. These parameters which can be obtained from the HOMO and LUMO values are calculated using the Eqs. (8), (9) (10), (11), (12), (13) and (14) given above. The HOMO-LUMO energy gap of bis (ethylenedithio) tetrathiafulvalene (ET) undoped molecule and both doped molecules (Cl 4 ET and Br 4 ET) in gas phase are computed as 3.81 eV, 3.62 eV and 2.88 eV using B3LYP/cc-pVDZ respectively. In literature, energy gap of semi-conductor is less than 3 eV (E gap ˂ 3 eV) (Mveme et al. 2020).Therefore, Br 4 ET brominated molecule is a good semiconductor material. On the other hand, for original molecule ET, its energy gap is more than 3 eV, we can confirm that the molecule is a good dielectric material such as reported in literature (Ejuh et al. 2016Veved et al. 2020;Parthasarathi et al. 2004). Based on our results, we can conclude that our proposed materials are very reactive because they have low values of ionization energy. Moreover, the chemical hardness (η) values obtained describe the chemical stability and reactivity of these compounds. Also, negative chemical potential ( ) values of molecules obtained confirm the stability of the molecules. We can conclude that our new molecules are stable and reactive. The HOMO-LUMO diagram given in Figs. 3 and 4, show a good charge distribution on the double bond in the ethylene group of the undoped molecule as well as for the doped molecules. In addition, there is a strong contribution of chlorine and bromine in the LUMO of doped systems, which increases an electronic displacement and consequently favors the transfer of charge in the molecules. Based on these results, our materials are good candidate to find application in electronic devices.

Optoelectronic properties
Optoelectronic properties of the molecules (ET, Cl4ET and Br4ET), such as the polarization density (P), the electric field (E), electric susceptibility ( ), dielectric constant ( ), the displacement vector magnitude (D) and the refractive index ( ) are calculated in gas phase and listed in Table 4. These parameters values are computed using the Eq. 15 given above. We observed that our computed optoelectronic properties are very different when we move from undoped to doped molecule. Moreover, this difference is slightly between the uncorrelated to the electron-correlated level of theory. Therefore, high values of the electric field

Thermodynamic properties
Thermodynamic properties of the molecules (ET, Cl4ET and Br4ET), presented in Table 5 such as Total electronic energy (Eelec), Zero vibrational point energy (ZPVE), Gibbs free energy (G), Thermal energy (E), Entropy (S), Enthalpy (H), constant volume calorific capacity (Cv) are calculated at room temperature of 298.15 K and a pressure of 1 atm. These results reveal that molecules doped with chlorine and bromine have a lower total energy than the undoped molecule (C 10 H 8 S 8 ). On the other hand, the variation of the thermodynamic properties such as entropy, enthalpy and specific heat depend on the effect of correlation of electrons taking in to account in the both level of theory used to compute these standard thermodynamics parameters. This difference is observed when going from the HF/cc-pVDZ to the B3LYP/cc-pVDZ. Therefore, we are able to conclude that there is an influence of doping with chlorine and bromine on the entropy of molecular structures, which confirms that the charge dynamics of the doped molecules are higher than its original molecule at the same temperature. This result further demonstrates that these doped materials have a high chemical reactivity and a high thermal resistivity. A better agreement with works reported by others authors in literature (Ejuh et al. 2018a, b;Tchangnwa Nya et al. 2017;Mveme et al. 2020).

Conclusion
Theoretical studies of NLO, electronic, optoelectronic and thermodynamic properties were performed in gas phase using ab-initio and DFT methods to decipher some important properties of considered materials. Our goal was to investigate the effect or influence of doping with halogen atoms such as chlorine and bromine atoms on the original molecule of BEDT-TTF. The results obtained from the B3LYP and RHF methods using cc-pVDZ basis set show that BEDT-TTF has a weak nonlinear optical (NLO) behavior. However, substituted hydrogen by chlorine and bromine atoms show more interesting properties. Also, higher first and second molecular hyperpolarizabilities (β and ,) make these doped molecules to find applications in the field of optoelectronic devices as active NLO materials. In fact, we presented for the first time the result of hydrogen atoms substitution with chlorine and bromine atoms on BEDT-TTF ( C 10 H 8 S 8 ). The materials obtained are a good candidate to find application in modern and emerging technologies. The HOMO-LUMO energy gap obtained lead us to believe that BEDT-TTF and its derivatives are good semiconductor materials, which can be used in photovoltaic cells, light emitting diode (LED) and in the field effect transistor (FET). Finally, thermodynamic parameters computed are in agreement with the work of other researchers.