# Structural, Electronic, and Charge Transport Properties of New Materials based on 2-(5-Mercapto-1,3,4-Oxadiazol-2-yl) Phenol for Organic Solar Cells and Light Emitting Diodes by DFT and TD-DFT

Authors: Caryne Isabelle Lekeufack Alongamo; Stanley Numbonui Tasheh; Nyiang Kennet Nkungli; Fritzgerald Kogge Bine; Julius Numbonui Ghogomu
Journal: Journal of Chemistry
 (2022)

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## Abstract

This work reports on the density functional theory (DFT) and its time-dependent extension (TD-DFT) study of the structural, electronic, and charge transport properties of 2-(5-mercapto-1,3,4-oxadiazol-2-yl) phenol (MODP) and some of its transition M2+ complexes (M = Fe, Co, Cu, Ni, Zn, Pd, Pt). Reorganization energy, integral charge transfer, mobility, open circuit voltage, and electronic properties of these compounds have been calculated by employing the global hybrid functional PBE0 in conjunction with the Karlsruhe basis set def2-TZVP. Results show that MODP and its transition metal complexes are good electron donors for organic solar cells (OSC) owing to their relatively higher HOMO and LUMO energies compared to the prototypical (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM). Energy gaps ranging between 2.502 and 4.455 eV, energy driving forces (∆EL-L) ranging between 2.08 and 2.44 eV, and large open circuit voltages (VOC) ranging from 1.12 to 2.05 eV were obtained. The results also revealed that MODP and its Pd(II) and Pt(II) complexes could serve as ambipolar charge transport materials owing to their very small reorganization energies, integral charge transfers, high rate charge transfers, and mobilities. All studied molecules showed OSC donor and hole/electron transport characteristics required by organic light-emitting diodes (OLEDs). Based on these results, new ways for designing charge transport materials for OLEDs as well as donor materials in OSCs are proposed.


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## Body

## 1. Introduction

Organic optoelectronic devices like organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) have attracted tremendous research attention in recent years [1–4]. OLED devices find applications in display and lighting [5, 6], while OSC devices are most promising for electrical energy applications [7]. Organic electronics based on small π-conjugated molecules and polymers are ideal for optoelectronic applications due to their absorption, light emission, and charge transport properties [4–8]. Although enormous advances have been made in the search and development of new materials, the low efficiency and thermal stability of small molecule-based organic electronics materials still remain a problem for their application and has limited their marketing. It is therefore necessary to develop new multifunctional organic materials with high-efficiency for OLEDs and OSCs, which are more stable and capable of functioning as better light emitters, absorbers, and charge transporters. Such materials serve as efficient light emitters in OLEDs or donor materials capable of converting abundant sunlight energy directly into electricity for OSCs and as charge transport materials, or both. OLEDs and OSCs are devices whose components comprise an emitting layer (EL) made out of a film of organic compound, a hole transport layer (HTL), and an electron transport layer (ETL), all located between two electrodes [2, 9]. In OLED devices, N,N-diphenyl-N,N-bis (3-methylphenyl) (1,1-biphenyl)-4,4-diamine (TPD) is often used as a good material for hole transport because of its efficient hole injection and high hole mobility properties [10], whereas tris (8-hydroxyquinolinato) aluminum (Alq3) is commonly used as a material for electron transport thanks to its Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energy levels, as well as its effective hole blocking ability [11].Recently, several studies on compounds with a D-π-A framework have been carried out [1, 8, 12]. The presence of an extended delocalization of π-electrons between a donor (D) and an acceptor (A) is a key element in the design of OLEDs and OSCs [1, 13]. This has stimulated great interest in the synthesis of π-conjugated systems comprising fullerene derivatives [14]. For instance, (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM), a fullerene derivative, plays a crucial role in the synthesis of OSCs because of its stability, capacity to accept electrons, and its proper frontier molecular orbital (FMO) energy levels [8, 14]. In OSC materials, the active layer generally consists of two components (i.e., a donor and an acceptor material), which are mounted in a two-layered structure or in the form of a mixture. It is worth mentioning that such an arrangement is observed in PCBM and its derivatives, which are mostly used as electron acceptor materials [8, 14]. This arrangement is also seen in poly(3-hexylthiophene) (P3HT) and its derivatives, which are generally used as electron donors in OSCs [15, 16]. For the design of OSCs and OLEDs materials, knowledge of the HOMO and LUMO energy levels of the compounds under investigation is of prime importance [1, 17]. These energy levels are also necessary to elucidate the propensity of charge transfer [17, 18].Within the framework of the design of OLEDs and OSCs, 1,3,4-oxadiazole and their derivatives serve as important materials thanks to theirπ-conjugated nature, high quantum efficiency, high photoluminescence, and their chemical and thermal stability [9, 19, 20]. These compounds have proven to be good electron transporters (ET) and important hole blockers (HB) in organic optoelectronic devices [20, 21]. Bhava and co-workers [22] synthesized and characterized Cu(II), Co(II), Ni(II), and Zn(II) complexes based on 2-(5-Mercapto-1,3,4-Oxadiazol-2-yl) Phenol (MODP) (see Figure 1) with the aim of searching for new antimicrobial agents based on the 1,3,4-oxadiazoles skeleton. In spite of the fact that these oxadiazole-based molecules and their metal complexes have been studied both experimentally and theoretically, very little attention has been focused on the structural, electronic, and charge transport properties of MODP and its transition metal complexes. It is clear from Figure 1 that MODP is a heterocyclic compound with promising push-pull potentials necessary for charge transport and photovoltaic properties of interest in OLEDs and OSCs. In addition to the metal ions that have been investigated experimentally in conjunction with MODP, Pd(II) and Pt(II) metal ions are also studied herein, since several works have shown that complexes bearing these ions are good materials for OLEDs [23, 24].Figure 1
Structure of the 2-(5-mercapto-1,3,4-oxadiazol-2-yl) phenol (MODP) (a) and complexes (b).
(a)(b)In order to shed more light on the applicability of the studied molecules as OLED and OSC materials, this work sets out to study the geometric, electronic, and charge transport properties of MODP, as a bidentate ligand, along with its Cu(II), Ni(II), Co(II), Zn(II), Fe(II), Pd(II), and Pt(II) complexes. In order to achieve this goal, the HOMO and LUMO energies,EHOMOandELUMO, the EHOMO−EHOMOgapsEgap, the open circuit voltage Voc, the energy driving force (∆EL-L), the absorption spectra, reorganization energies λ, integral charge transfer V, and the λmax of electronic absorption have been computed and discussed. The density functional theory (DFT) method (and its time-dependent extension (TD-DFT)) has been used in this work since it is good compromise between computational time and accuracy and also takes into account the electron correlation [25, 26].


## 2. Computational Details

All quantum chemical calculations herein were carried out using the ORCA 4.0.1 program [27]. In all calculations, the medium-sized numerical quadrature grid5 and the tight self-consistent field (SCF) convergence criterion were utilized. The initial geometries of the molecules studied were prepared using the Avogadro visualization program [28]. Geometry optimization and frequency calculations were done with the aid of the low-cost composite electronic structure B97-3c method, which is very effective especially for geometry optimization of metal containing systems [29, 30]. The B97-3c approach is used for molecular geometries and to exhibit excellent performance for non-covalent interaction energies of small and large molecules [29, 31]. To confirm all optimized geometries as stable structures (i.e., minima on the potential surface energy), vibrational frequencies were computed at the same level of theory as that used for geometry optimization, and no imaginary frequencies were obtained.Based on the B97-3c optimized geometries, single-point (SP) calculations were carried out using the hybrid-GGA functional PBE0 [30] along with the Karlsruhe basis set def2-TZVP [27, 32]. The PBE0 functional [30] has been used in this study as it provides good energies for transition metals [33, 34]. PBE0 has been combined to def2-TZVP because it is suitable for energy calculations [35]. The Grimme’s atom-pairwise dispersion correction with Becke-Johnson damping dubbed (D3BJ) [36] was used in all calculations to account for long-range dispersion interactions. To speed up the calculations though with a minimal loss in accuracy, the Resolution-of-Identity (RI-J) approximation [37] was employed together with the Chain-of-Spheres (COSX) approximation [38] resulting in the RIJCOSX approximation. With the aid of the TD-DFT method [39], some electronic absorption properties (excited-state energies, maximum absorption wavelengths, λmax, and oscillation strengths f) of the investigated compounds were calculated based on the B97-3c optimized ground state geometries in the gaseous phase and in Dimethylsulfoxide (DMSO) at the CAM-B3LYP/def2-TZVP level of theory. The range-separated functional Coulomb-attenuating method applied to Becke’s 3 Lee-Yang-Parameter (CAM-B3LYP) [39] was used because it is suitable for electronic spectra studies [39]. Implicit solvent effects on the absorption spectra of the compounds studied were included using the conductor-like polarizable model (CPCM) as implemented in the Orca 4.0.1 program [27, 34].Given that the performance of OSCs is influenced by the FMO energy level [40], two major FMO-based performance parameters were computed in this work. These include the energy driving forces (∆EL − L), which correspond to the energy difference between the LUMO energy levels of the donor (the oxadiazoles studied herein) and the acceptor (PCBM) and the open circuit voltage VOC. VOC is a significant parameter for studying organic solar cells [3] (see Figure 2). Theoretically, VOC has been calculated as shown in the following equation [41, 42]:(1)VOC=EHOMODonor−ELUMOAcceptor−0.3,where VOC is the open circuit voltage, EHOMODonor is the HOMO energy of the donor, ELUMOAcceptor is the LUMO energy of the acceptor, and 0.3 is an empirical factor.Figure 2
Energy level diagram for a donor-acceptor in organic solar cells.HOMO energy determines the ability of a molecular species to give out an electron, while the LUMO energy indicates its ability to accept electrons [43]. Studies have shown that the difference in energy between the HOMO of the donor and the LUMO of the acceptor significantly affects the VOC value [40, 41].∆EL − L is the determining factor of an exciton’s dissociation efficiency at the donor/acceptor interface in OSCs materials as shown in Figure 2. ∆EL − L is calculated according to equation (2). It is worth noting that the value of ∆EL − L must be at least 0.3 eV; otherwise, dissociation will be unlikely [15].(2)∆EL−L=LUMODonor−LUMOAcceptor,VOC and ∆EL − L of the investigated molecules were obtained by carrying out Single Point calculations on the B97-3c-optimized geometries at the RIJCOSX-PBE0-D3(BJ)/def2-TZVP level of theory, using equations (1) and (2). LUMOD and LUMOA are the LUMOs of the donor and acceptor, respectively. In a similar manner, HOMOD and HOMOA are the HOMOs of the donor and acceptor, and EgapD and EgapA are the energy gaps of the donor and acceptor, respectively.Marcus’ theory, as summarized in equation (3), has been widely used to describe charge transfer and rate charge transfer speed [44, 45].(3)K=4π2h14πλTkBV2exp−λ4kBT,where kB is the Boltzmann constant, T is the temperature, λ is the reorganization energy, h is the Planck constant, ΔGo is the standard Gibbs free energy for the change in the process, and V is the element of electron coupling matrices between two adjacent species.The reorganization energyλ and the transfer integral V are two key parameters that govern charge transport properties [46]. λ is the parameter, which determines the charge transfer properties of materials and is comprised of the intermolecular reorganization energy λext and the intramolecular reorganization energy λint [47]. Attention is generally focused on λint because environmental relaxations are ignored [48]. In general, the lower the λ values, the higher the charge transfer rate. In this paper, we focus on the internal reorganization energy exclusively because of the low dielectric constant of medium in OSCs materials. λint is the sum of the electron λe and the hole λh reorganization energies, respectively, and evaluated according to equations (4) and (5) [46]:(4)λe=E0−−E−−+E−0−E00,(5)λh=E0+−E+++E+0−E00,where E0+ and E0− are, respectively, the cationic and anionic energies of a given molecule, which are computed using the optimized geometries of an electrically neutral structure; E++ and E−− are the energies of the cationic and anionic optimized geometries; and E+0, E−0 and E00 are energies of the neutral geometries computed using the cationic, anionic, and neutral geometries, respectively. In this study, the reorganization energies were calculated at the RIJCOSX-PBE0-D3(BJ)/def2-TZVP level of theory, based on the Single Point energies of the neutral and ionic structures of the studied molecules.V represents the orbital coupling of neighboring molecules [24]. Dimers comprising monomers were constructed from the optimized geometries of the individual monomers using the B97-3c method. A 4.0 Å distance between the monomer units of each dimer was used here, which corresponds to the minimal distance where the overlap between the molecular wavefunctions of the donor and acceptor systems could be neglected [49, 50]. The simplest way to evaluate the charge transfer integral in a molecular dimer is by using the method of dividing the boundary orbital energy levels [50]. This method was used herein based on Koopmans’ theorem, whereby the HOMO and HOMO+1, LUMO and LUMO+1 energy levels were calculated. Using Marcus’s theory, the charge transfer integrals of the electron Ve and the hole Vh were obtained according to equations (6) and (7) [49].(6)Ve=12EL+1−EL,(7)Vh=12EH−EH−1,where Ve and Vh are the electron transfer and hole integrals, respectively, EH is the HOMO energy, EH−1 is the HOMO-1 energy, EL is the LUMO energy, and EL+1 is the LUMO+1 energy. In order to compute full charge transfer of the electron and hole, the PBE0-D3(BJ)/def2-TZVP method was used.The manufacture of high-performant OLED devices requires materials with good charge transport properties and high chemical stability, as well as a considerable shelf life [51]. Stability is an important factor that often limits the functionality of charge transport and luminescent materials [2]. From molecular orbitals perspectives, the prediction of the stability of the molecules studied was via chemical hardness η, which was calculated according to equation (8) [51, 52].(8)η=12∂μ∂N=12∂2E∂N=IP−EA2,where µ is the chemical potential, N is the number of electrons. IP and EA represent the ionization potential and electronic affinity, respectively. The adiabatic IP of these molecules is defined as the energy difference between the cationic and neutral species, while the EA is the energy difference between the neutral and anionic species, respectively. These values were calculated according to equations (9) and (10):(9)IP=EM+−EM0,(10)EA=EM0−EM−,where EM0,EM+ and EM− are the energies of the neutral, cationic, and anionic forms of the investigated molecules, respectively. In this work, these energies were calculated at the PBE0-D3(BJ)/def2-TZVP level of theory. The stability of transition metal complexes has also been studied based on the interaction energy Eint as described by equation (11).(11)Eint=EML2H2O2−2EMODP+2EH2O+EM2+,where EML2H2O2 is the energy of the complex, EMODP is the energy of the ligand, EM2+ is the energy of metal ion, and EH2O is the energy of the water molecule.


## 3. Results and Discussion

### 3.1. Stability Analysis of Spin States

Transition metal ions with incompletely filled d-orbitals can exhibit high and low possible spin states. With this in mind, the most stable spin states corresponding to the lowest energy geometries of the currently investigated complexes were determined via geometry optimization of the doublet and quartet spin states for Co2+ ions, singlet and triplet spin states for Ni2+ ions, Pt2+ and Pd2+ ions, and the singlet and quintet spin states for Fe2+ ions. For the Cu2+ and Zn2+ ions, spin effects were not investigated because Cu2+ ions have only one unpaired electron in its outermost d-orbital, and Zn2+ ions have a completely filled d-orbital. The calculated electronic energies of the aforementioned spin states at the DFT/B97-3c level of theory are listed in Table 1.Table 1
Energy (in kcal/mol) of the studied complexes at the different spin states computed using the B97-3c method in gas phase.
Metal complexB97-3cH-M complexL-M complex[Pt(MODP)2(H2O)2]−13824.021−13824.003[Co(MODP)2(H2O)2]−21752.935−21752.937[Ni(MODP)2(H2O)2]−22541.035−22540.807[Fe(MODP)2(H2O)2]−21005.722−21005.436[Pd(MODP)2(H2O)2]−13879.301−13879.259H-M⟶ High multiplicity, L-M ⟶ Low multiplicity.The results show that apart from the Co(II) complex, the energies of the high spin complexes are lower than those of the low spin counterparts. As a result, only the triplet state for the Ni(II), Pd(II), and Pt(II) complexes, the doublet state for the Co(II) complex, and the quintet state for the Fe(II) complex have further been considered throughout this work.


### 3.2. Infrared (IR) Vibrational Analysis

In order to confirm the suitability of the DFT/B97-3c level of theory employed in the computation of IR frequencies, computed frequencies have been compared with their experimentally obtained counterparts from [22]. The spectra of MODP and its complexes are presented in Figure S1 of the accompanying electronic supplementary material (ESM). For better comparison of the theoretical and experimental IR frequencies, correlation equations (12) and (13) have been established based on the vibrational modes of the MODP and the complexes, respectively.(12)νcal=1.019νexp+1.707R2=0.998;for MODP,(13)νcal=139.478νexp+8.717R2=0.949;MIIMODP complexes,where νcal and νexp represent the calculated and experimental wavenumbers, respectively.The high correlation coefficients (R2 = 0.998) for the MODP and (R2 = 0.949) depict a good linear correlation between the calculated (unscaled) and experimental IR frequencies. This confirms the suitability of the level of theory employed in these calculations.Presented in Table2 are some experimental and calculated gas phase IR frequencies of MODP and its M(II) complexes along with their probable assignments.Table 2
Calculated vibrational frequencies (cm-1) for MODP and its complexes at B97-3c level and the corresponding experimental IR frequencies.
SpeciesνO−HνC=NνS−HνC=SνH2OνM−NνM−OνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalMODP321932811595156125232641780844——————[Cu(MODP)2H2O] H2O——153415222522263677986734503541564525424440[Zn(MODP)2(H2O)2]——155615182525264377886934333796563514440425[Co(MODP)2(H2O)2]——157615572522263477887034503670564527430458[Ni(MODP)2(H2O)2]——153215172524263877886633603589567572426433νexp represents experimental wavenumbers by [22]. νcal represents wavenumbers calculated in this work.The results show that the IR spectra calculated for MODP and its transition metal complexes showed bands around 2635–2643 cm−1 and 844–870 cm−1 corresponding to the νS−H and ν (C-S) vibrations. The experimental IR frequency assigned to the phenolic O-H vibrational mode of MODP is 3382 cm−1, according to [22]. The calculated frequency for the phenolic O-H νO−H vibration, 3281 cm−1, is closer to the experimental value reported in [22]. Upon complexation, the disappearance of the IR bands corresponding to the O-H vibrations of MODP indicates the loss of its carboxylic protons. The IR spectrum of MODP is more or less modified by the coordination of the transition metals. The calculated IR bands ν (H2O) between 3541–3796 cm−1 are present in the spectra of the complexes due to the vibrations of stretching of the O-H bonds in H2O, which shows the presence of coordinated water. Calculated IR bands due to the vibrations of O-H bonds in H2O are absent in the spectrum of MODP because it does not contain attached H2O molecules.The results also inform that the computed spectra of the complexes show the appearance of new bands due to metal-ligand vibrations (M-N) and (MO) in the regions 514–572 cm−1 and 425–458 cm−1, respectively. This further confirms coordination with the central metal ion by the phenolic oxygen atom and one of the azomethine nitrogen atoms. The metal-ligand vibration frequencies of these complexes are in agreement with the experimental values in [22]. The IR spectra of the complexes indicate that MODP behaves as a bidentate ligand and is bound to the metal ion via the N-atom of the azomethine group (C = N) and the O-atom of the phenolic group (OH) (after deprotonation).


### 3.3. Geometric Parameters

The optimized geometries of MODP and its Cu(II), Ni(II), Co(II), Zn(II), Fe(II), Pd(II), and Pt(II) complexes are displayed in Figure3, and their optimized Cartesian coordinates are provided in Tables S1‒S16 of the accompanying electronic supplementary material (ESM).Figure 3
Optimized geometries of MODP and its complexes studied in the gas phase at the DFT/B97-3c level of theory.Figure3 shows that an intramolecular hydrogen bond (H-bond) is formed between the hydroxyl group and one of the oxadiazole ring nitrogen in MODP, which certainly stabilizes the molecule. This H-bond also hinders free rotation about the C6-C7 bond, thus maintaining the molecule in a somewhat planar structure that promotes π-conjugation.Selected geometric parameters (bond lengths, bond angles, and dihedral angles) are listed in Table3.Table 3
Selected bond lengths (Å), bond angles (°), and dihedral angles (°) of molecules optimized in DFT/B97-3c in the gas phase.
ParameterMODP[Cu (MODP)2H2O] H2O[Co (MODP)2 (H2O)2][Ni (MODP)2 (H2O)2][Zn (MODP)2 (H2O)2][Fe (MODP)2 (H2O)2][Pd (MODP)2 (H2O)2][Pt (MODP)2 (H2O)2]Bond lengtha (Å)N8-H191.791———————M15-O14—1.9521.8992.0122.0121.9642.2522.390M15-N8—1.9701.9072.0342.1272.1232.1422.055M15-O22—1.9591.8982.0122.0091.9632.2532.390M15-N23—1.9621.9062.0332.1292.1242.1422.055M15-O16—3.7362.3772.1792.3092.3192.4062.532M15-O19—2.7252.3892.1792.3062.3162.4062.530Bond anglea (°)O14-M15-N23—91.389.293.494.795.298.998.6N8-M15-O22—93.789.293.494.995.498.998.5N8-M15-O14—89.790.886.685.284.781. 181.4O22-M15-N23—89.790.786.685. 284.681.181.5O16-M15-N8—128.992.689.391.191.289.292.5O16-M15-O22—112.897.880.6100. 596.6109.464.9O19-M15-O14—127.597.580.6100.096. 7109.565.0O19-M15-N23—89.892.589.290.991.189.192.4Dihedral anglea (°)M15-N23-C26-O27—162.6173.8166.4164.5168.4159.3172.9M15-N8-C7-O11—174.7173.8166.5164.4168.5159.3172.8O22-M15-N8-N9—20.815.014. 610.65.311.514.3O14-M15-N23-N24—96.015.414.410.76.111.714.3O19-M15-O14-N23—90.892.480.691.891.792.288.7O19-M15-O22-N8—80.287.691.388.288.387.891.3O16-M15-N23-O14—37.982.299.479.483.170.5114.8O16-M15-N8-O22—123.597.780.679.896.7109.365.1aRefer to Figure 3 for atomic numbering, M = Co(II), Ni(II), Zn(II), Fe(II), Pd(II), and Pt(II).From the dihedral angles, MODP is improved upon metal ions complexation. From the metal-ligand bond lengths, we conclude that there is effective metal-ligand binding in all complexes studied.The results from this table also show that the bond angles around the central metal ion in the complexes investigated are around 90°, with exception of the Cu(II) complex where large discrepancies are observed in some cases. This indicates a high planarity of MODP in its Co(II), Ni(II), Zn(II), Fe(II), Pd(II), and Pt(II) complexes, as well as octahedral geometries around the central metal ions. The Cu(II) complex of MODP apparently adopts a trigonal bipyramidal geometry around the Cu(II) ion, since one of the ancillary H2O ligands is clearly out of the coordination sphere and is rather attached to one of the MODP ligands through an H-bond. This may be attributed to the fact that the Cu(II) ion is not always stable in octahedral geometries, and the complexes of this ion often have a square or tetrahedral planar structure. This arrangement comes from the stretching of the octahedron, along the z-axis (Jahn-Teller effect). The Cu(II) complex is Jahn-Teller distorted because the bonds to the central Cu(II) ion along the z-axis (M15-O16 and M15-O19 with bond lengths 3.736 Å and 2.725 Å, respectively) are much longer than the bonds along the x-axis and y-axis: 1.970 and 1.962 Å for M15-N8 and M15-N23 and 1.952 and 1.959 Å for M15-O14 and M15-O22. This corresponds to an extension along the z-axis and compression along the x- and y-axes.


### 3.4. Photovoltaic Properties

#### 3.4.1. Electronic Parameters

The efficiency of the donor-acceptor charge transfer in organic solar cells is determined by important factors such as the HOMO and LUMO energy levels of the donor and acceptor [18]. For effective charge transfer to occur, the HOMO and LUMO energies of efficient charge transporters must be higher than that of (6, 6)-Phenyl-C61-butyric acid methyl ester (PCBM) (HOMO = −6.1 eV and LUMO = −3.75 eV) and lower than that of Poly(3-hexylthiophene) (P3HT) (HOMO = −4.65 eV and LUMO = −2.13 eV) [16, 42]. P3HT and PCBM are reference electron donor and acceptor materials, respectively. In order to assess the electron transfer possibilities of the molecules studied, their HOMO and LUMO energy levels were compared with those of PCBM (see Table 4). PCBM was chosen in this study as electron acceptor because of its stability, its capacity to accept electrons, and its proper energy level [53] as earlier mentioned.Table 4
The EHOMO, ELUMO, and Egap values of the studied molecules and PCBM calculated at PBE0-D3(BJ)/def2-TZVP theory of level in the gas phase, all in eV.
SpeciesEHOMOELUMOEgap (ELUMO-EHOMO)MODP−6.09−1.644.46[Cu(MODP)2H2O]H2O−5.64−1.314.33[Zn(MODP)2(H2O)2]−5.81−1.394.41[Co(MODP)2(H2O)2]−5.48−1.434.05[Ni(MODP)2(H2O)2]−5.71−1.404.31[Fe(MODP)2(H2O)2]−5.67−1.354.32[Pd(MODP)2(H2O)2]−5.72−1.534.19[Pt(MODP)2(H2O)2]−5.17−1.673.50PCBM−6.10−3.752.35The results reveal that all the studied molecules have higher HOMO and LUMO energies than PCBM. This indicates the ability of molecules to inject electrons into PCBM. The results also show that complexation modifies the HOMO and LUMO energies and the energy gaps. Complexation increases the HOMO energy values by a factor of about 0.50 eV and the LUMO energy values by about a factor of 0.25 eV, except for the LUMO energy of the Pt(II) complex (which is slightly reduced by 0.03 eV). Accordingly, the investigated molecules can be used as donor materials in organic solar cells. Moreover, photo-excited electron transfer from the studied molecules to PCBM may be sufficiently effective for applications in photovoltaic devices [40, 54]. Also, the Egap values of the molecules are in the range 3.50–4.46 eV and decrease in the order: MODP > [Zn(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Fe(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Pd(MODP)2 (H2O)2] > [Co(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2]. Consequently, metal complexation decreases the Egap of MODP. It is also observed that the Pt(II) complex has the smallest Egap (3.50 eV), while that of the Zn(II) complex (4.41 eV) is the highest. This implies that the Pt(II) complex is the most reactive and least stable, while the Zn(II) complex is the most stable and least reactive. Consequently, the HOMO-LUMO electron transport is easier in the Pt (II) complex than in the other complexes.The frontier molecular orbital (FMOs) energy gaps, Egap, of MODP and its complexes are displayed in Figure4, along with the isosurfaces of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), as visualized using Avogadro 1.2.1 [28].Figure 4
Isosurfaces of the FMOs of MODP and its complexes computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.The findings show that the HOMOs of the Cu(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes are localized on the transition metals and on MODP, while those of the Co(II) and Zn(II) complexes are located on MODP and exhibitπ-bonding character. The LUMOs of the Co(II), Cu(II), Zn(II), Pt(II), and Pd(II) complexes are mainly located on MODP, while those of the Fe(II) complex are centered on the transition metal ion and on MODP, and those of the Ni(II) complex on the transition metal ions and the oxygen atoms of MODP around the central metal and have a π antibonding character.


#### 3.4.2. Open Circuit VoltageVOC

One of the parameters that influence the efficiency of solar cells is Voc. In organic solar cells, Voc is a function of the HOMO energy of the donor and the LUMO energy of the acceptor, which are important in determining whether an effective transfer will take place between the donor and the acceptor [40]. The Voc values of the studied molecules (donor), as well as those of the prototypical donor material P3HT and the acceptor PCBM, are presented in Table 5, along with the EL−L (ELUMOD−ELUMOA) values.Table 5
HOMO, LUMO energy values (eV), and Voc/PCBM (eV) of the different molecules calculated at the DFT/PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesEHOMOELUMOVoc/PCBMEL−LP3HT−4.65−2.130.601.62MODP−6.10−1.642.052.11[Cu(MODP)2 H2O]H2O−5.64−1.311.592.44[Zn(MODP)2 (H2O)2]−5.81−1.391.762.36[Co(MODP)2 (H2O)2]−5.48−1.431.432.32[Ni(MODP)2 (H2O)2]−5.71−1.401.662.35[Fe(MODP)2 (H2O)2]−5.67−1.351.622.40[Pd(MODP)2 (H2O)2]−5.72−1.531.672.22[Pt(MODP)2 (H2O)2]−5.17−1.671.122.08PCBM−6.10−3.75——The obtained Voc values are in the range 1.12–2.05 eV and are higher than those of the prototypical material P3HT/PCBM. This indicates that these values are sufficient for the electrons to be injected into the unoccupied orbital of the acceptor ([6, 6]-Phenyl-C61-butyric acid methyl ester (PCBM)). Therefore, all studied molecules can be used as donors, since the process of injecting electrons from the excited molecules to the conduction band of the PCBM is possible. The Voc values decrease in the order: MODP > [Zn(MODP)2(H2O)2] < [Pd(MODP)2(H2O)2] < [Ni(MODP)2(H2O)2]  <  [Fe(MODP)2(H2O)2]  <  [Cu(MODP)2H2O]H2O < [Co(MODP)2(H2O)2] < [Pt(MODP)2(H2O)2]. We also note that complexation reduces the Voc values of the complexes, suggesting that the transfer of electrons is easier from the compounds studied to PCBM.The results in Table5 also show that the differences in the LUMO energy levels of the studied compounds and the acceptor range from 2.08 to 2.44 eV and are all higher than 0.3 eV. This suggests that the transfer of photo-excited electrons from the molecules to PCBM is sufficient to be used in photovoltaic devices [54]. This assures an electron transfer between the donors and the acceptor. For all the molecules studied, the EL−L values obtained with respect to the PCBM acceptors are higher than those obtained with respect to the P3HT/PCBM pair. The EL−L values of all the molecules studied are higher than those of VOC and the binding energy (0.2 to 1.0 eV). Therefore, MODP and its complexes are promising donors with respect to the PCBM acceptor. As a result, these compounds can be used as solar cells.


### 3.5. UV-Visible Absorption Spectra

The electronic transitions within a material can greatly influence its photovoltaic applications. It is desirable that donor materials should have intense electronic absorption compared to those of the acceptor materials. In fact, the maximum absorption wavelengthλmax of a suitable material must satisfy the absorption property requirement, according to which, λmax should not exceed 920 nm [55]. For each compound, 10 excited states were calculated. The calculated absorption wavelengths λcal, oscillator strengths f, electronic transitions, the molecular orbital (MO) contributions to electronic transitions, and excitation energies E of the investigated molecules are presented in Table 6. All the parameters in Table 6 were obtained in the excited state where the oscillation strength is the greatest and corresponds to the most intense band of the UV-Vis spectrum.Table 6
Absorption spectra data calculated by TD-DFT methods for species at CAMB3LYP/def2-TZVP level of theory in the gas phase and in DMSO.
MoleculesGazDMSOTransitionAssignments and MO contributionE (eV)λcal (nm)fTransitionAssignments and MO contributionE (eV)λcal (nm)fMODPS0 ⟶ S2H-2 ⟶ L + 1 (70.4%)5.193238.50.3792S0 ⟶ S8H-2 ⟶ L (37.0%)6.905179.50.5392[Co(MODP)2(H2O)2]S0 ⟶ S10H-2 ⟶ L + 2 (31.5%)2.806441.90.0001S0 ⟶ S10H-2 ⟶ L + 1 (32.0%)2.840436.60.0001[Zn(MODP)2(H2O)2]S0 ⟶ S2H-1 ⟶ L-1 (49.2%)4.093302.90.5942S0 ⟶ S2H-1 ⟶ L (48.3%)4.103302.20.6186[Ni(MODP)2(H2O)2]S0 ⟶ S9H-3 ⟶ L (54.0%)4.031307.60.3089S0 ⟶ S9H-4 ⟶ L + 2 (16.0%)3.656339.20.0003[Cu(MODP)2H2O]H2OS0 ⟶ S8H-2 ⟶ L + 2 (38.9%)3.320373.50.0401S0 ⟶ S6H-2 ⟶ L + 2 (31.0%)2.522491.60.0760[Fe(MODP)2(H2O)2]S0 ⟶ S9H-2 ⟶ L + 3 (36.6%)3.822324.40.0622S0 ⟶ S9H-3 ⟶ L + 3 (30.8%)3.807325.70.0576[Pd(MODP)2(H2O)2]S0 ⟶ S5H-2 ⟶ L + 2 (92.4%)2.519492.20.0514S0 ⟶ S5H-3 ⟶ L + 2 (25.0%)2.518492.30.0416[Pt(MODP)2(H2O)2]S0 ⟶ S10H ⟶ L (52.5%)3.203387.10.0422S0 ⟶ S10H ⟶ L (54.2%)3.279378.10.0513H⟶ HOMO, L ⟶ LUMO, λcal: calculated absorption wavelengths, f: oscillator strength.Interestingly, theλcal of all molecules studied is below 500 nm in both DMSO and the gas phase. λcal of the molecules in DMSO is in the order: MODP < [Zn(MODP)2 (H2O)2]  <  [Fe(MODP)2 (H2O)2]  <  [Pt(MODP)2(H2O)2]  <  [Ni(MODP)2 (H2O)2]  < [Co(MODP)2(H2O)2] < [Cu(MODP)2H2O]H2O < [Pd(MODP)2(H2O)2]. From this trend, we note increases in the wavelengths after complexation in all complexes in both the gas phase and in DMSO solvent. This bathochromic effect can be explained by an increase in the number of double bonds in the complexes compared to MODP, since conjugated systems are important for studying electronic and absorption properties of compounds. Generally, light absorbance of a molecule is improved by higher number of conjugated double bonds [56]. The UV-vis absorption spectra of the ligand and its transition metal complexes as studied in gas phase and in DMSO are shown in Figure 5. The absorption spectra (see Figure 5) give the maximum wavelengths of the MODP and its complexes in DMSO solvent and in the gas phase. The spectra for MODP show a maximum wavelength band at 179.5 nm corresponding to the oscillator strength f = 0.5392 in DMSO solvent and the band of maximum wavelength at 238.8 nm corresponding to oscillator strength f = 0.3792 in the gaseous phase. These values correspond to the highest absorption with the excitation energy 6.905 eV and the MO contribution H-2 ⟶ L in DMSO solvent. In the gas phase, these values correspond to the excitation energy 5.193 eV, of the H-2 ⟶ L + 1 MO contribution. The UV-Vis absorption spectrum of the Co(II) complex consists of a band with a maximum wavelength λ = 436.6 nm and an oscillator strength f = 0.0001. This band corresponds to an energy of 2.840 eV, a charge transfer percentage of 32.01%, and MO contribution H-2 ⟶ L + 1 in DMSO solvent. In the gas phase, the Co(II) complex shows a maximum wavelength band at 238.8 nm, which is greater than that obtained in DMSO solvent.Figure 5
Absorption spectra of MODP and its complexes at the CAMB3LYP/def2-TZVP theory level in (a) gas phase and (b) DMSO solvent.
(a)(b)Concerning the Pd(II) and Pt(II) complexes, their absorption spectra show maximum bands at 492.3 nm and 378.1 nm corresponding to the excitation energies 2.518 eV and 3.279 eV with oscillator strengthsf  = 0.0416 and f = 0.0513, respectively, as shown in Table 5. These values correspond to the MO contribution H-3 ⟶ L + 2 and H ⟶ L, respectively, in DMSO solvent. These values increase in the gas phase. For the Cu(II), Zn(II), Ni(II), and Fe(II) complexes in DMSO solvent, their absorption spectra as illustrated in Figure 5 exhibit maximum wavelength bands as follows: λ = 491.6 nm, λ = 302.2 nm, λ = 339.2 nm, λ = 325.7 nm corresponding to the oscillator strengths: f = 0.0760, f = 0.6186, f = 0.0003, f = 0.0576, respectively. These bands are due to the MO contribution H-2 ⟶ L + 2, H-1 ⟶ L, H-4 ⟶ L + 2 and H-3 ⟶ L + 3, respectively, as shown in Table 5. An increase in these values is observed in the gas phase. The absorption wavelengths obtained in the gas phase and in DMSO show that the MODP and its complexes can be used in organic solar cells.


### 3.6. Charge Transport Properties

#### 3.6.1. Reorganization Energies and Charge Transport

The charge transport properties of the investigated molecules have been predicted from computed reorganization energies of the electronλe and the hole λh. In Table 7, the reorganization energies of the molecules studied are compared with those of the prototypical molecules, N-N′-diphenyl-N-N′-bis (3-methylphenyl-(1,1′-biphyl)-4,4′-diamine (TPD) molecules (λh = 0.290 eV) and Tris(8-hydroxyquinolinato) aluminum (Alq3) (λe = 0.276 eV) [10, 11] for hole and electron transfer, respectively.Table 7
Values ofλe and λh of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesλeeVλheVMODP0.1870.154[Cu(MODP)2H2O] H2O0.9760.107[Zn(MODP)2 (H2O)2]0.1840.711[Co(MODP)2 (H2O)2]1.2020.179[Ni(MODP)2 (H2O)2]0.3720.015[Fe(MODP)2 (H2O)2]0.1601.724[Pd(MODP)2 (H2O)2]0.1420.161[Pt(MODP)2 (H2O)2]0.1210.257The results show that theλe values of MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes are smaller than those of Tris (8-hydroxyquinolato) aluminum (Alq3) (λe = 0.276 eV) indicating that the charge transfer rate of these molecules may be higher than that of Alq3. Accordingly, MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes may serve as good electron transporters in OLEDs owing to their small reorganization energies. It is clear from Table 7 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases its λe, whereas coordination with Cu(II), Co(II), and Ni(II) reduces its λe. It can also be inferred from this table that the λh values of MODP and its Cu (II), Co (II), Ni (II), Pd(II), and Pt(II) complexes are smaller than those of TPD (λh = 0.290 eV), showing that their charge transfer rates could be higher than those of TPD. Therefore, MODP and its Cu (II), Co (II), Ni (II), Pd (II), and Pt (II) complexes are promising hole transporters for use in OLEDs. Again, MODP (λe = 0.187 eV and λh = 0.154 eV), the Pd(II) complex (λe = 0.142 eV and λh  = 0.161 eV), and the Pt(II) (λe = 0,121 eV and λh = 0,257 eV) complexes are excellent hole and electron transporters, respectively. Therefore, these materials are likely to function as ambipolar charge transporters (electron and hole) under the proper operating conditions for OLEDs and OSCs based on the smaller reorganization energies.


#### 3.6.2. Charge Transfer Integral and Charge Transport

The values of charge transfer integral for electronsVe and holes Vh, as calculated at the PBE0/def2-TZVP/D3BJ level theory, are presented in Table 8.Table 8
Charge transfer integralVeandVh values of the different molecules computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.
SpecieseVeVMODP0.0920.005[Cu(MODP)2H2O]H2O0.3560.489[Zn(MODP)2(H2O)2]0.0021.607[Co(MODP)2(H2O)2]0.2891.288[Ni(MODP)2(H2O)2]1.0800.013[Fe(MODP)2(H2O)2]0.7570.037[Pd(MODP)2(H2O)2]1.5850.126[Pt(MODP)2(H2O)2]1.2870.111The results show that theVeandVh values of MODP are smaller than those of its complexes (except for the Ve value of the Zn(II) complex). The smaller values of the VeandVh of MODP are confirmed by the small values of the reorganization energies lower than the prototype (see Table 7). MODP can function as an ambipolar electron/hole transport material. It is clear from Table 8 that, apart from coordination with the Zn(II) ion, metal complexation by MODP increases the charge transfer integral. Ve values vary in the order: [Pd(MODP)2H2O)2] > [Pt(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Fe(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Co(MODP)2(H2O)2] > MODP > [Zn(MODP)2(H2O)2]. Among the molecules studied, the Zn(II), Fe(II), and Pt(II) complexes of MODP are potential electron transporters that may be useful in the fabrication of OLEDs and OSCs, owing to their small Ve and λe values. The results also indicate that the coordination of MODP with all ions increases the values Vh. The Co(II), Cu(II), and Ni(II) complexes of MODP are apparently good hole transporters due to their small Vh and λh values.


#### 3.6.3. Charge Transport Rate and Stability

The calculated values of chemical hardnessη and interaction energies, ΔEint, of molecules studied herein are listed in Table 9.Table 9
Values (in eV) of the chemical hardnessη and of the interaction energy Eint of the different molecules calculated in the gas phase.
SpeciesPBE0-D3(BJ)/def2-TZVPB97-3cIPEAηEintMODP8.1260.1313.997—[Cu(MODP)2H2O] H2O7.8141.3303.222−1.246[Zn(MODP)2 (H2O)2]6.9200.3133.304−2.193[Co(MODP)2 (H2O)2]7.289−0.6203.954−1.422[Ni(MODP)2 (H2O)2]6.7180.0843.316−1.059[Fe(MODP)2 (H2O)2]6.7820.2693.256−2.999[Pd(MODP)2 (H2O)2]6.8550.4563.199−2.563[Pt(MODP)2 (H2O)2]6.1740.6032.785−1.858It is clear from Table9 and Figure 6 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases the values of IP and EA. IP follows the trend: MODP  >  [Cu(MODP)2H2O] H2O > [Co(MODP)2(H2O)2] > [Zn(MODP)2 (H2O)2]  >  [Pd(MODP)2 (H2O)2] > [Fe(MODP)2 (H2O)2]  >  [Ni(MODP)2 (H2O)2]  >  [Pt(MODP)2 (H2O)2]. This classification shows that Cu(II) complex has the highest IP suggesting that it has the highest electron releasing power and Pt(II) complex the lowest. On the other hand, Co(II) complex has the highest electron accepting ability, while Zn(II) has the least accepting power. EA follows the trend: MODP > [Co(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2]  >  [Zn(MODP)2 (H2O)2]  >  [Fe(MODP)2 (H2O)2] > [Cu(MODP)2H2O]H2O > [Pd(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2].Figure 6
IP, EA energies, andη of the molecules studied at the PBE0-D3(BJ)/def2-TZVP level of theory.A careful inspection of same Table9 and Figure 6 clearly shows that the η value of MODP is very close to those of the complexes. It is also clear from Table 9 that the ligand has the largest η, which indicates that MODP is the most kinetically stable and least reactive molecule studied. The complexation does not significantly affect the stability of these molecules. Among the complexes, the Co(II) complex is the most kinetically stable, and this stability is apparently confirmed by its negative Eint value. The relatively high η values of the Cu(II), Zn(II), Co(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes indicate their chemical stabilities. Moreover, the stabilities of these complexes are indicated by their negative Eint values, which show that their formation from the ligand and the various metal ions are energetically favorable. In conclusion, metal coordination causes a slight decrease in the chemical hardness of MODP.


#### 3.6.4. The Charge Transfer Rates and Mobility

According to Marcus’ theory, the rate of charge transport is important for determining the mobility of electrons and holes in a compound. The charge mobility is an important parameter to evaluate the performance of OLED and OSC materials [57, 58]. Mobility can be approximately calculated by Einstein’s relation [44, 49]:(14)µe/h=ed2kelectron/hole2KBT.Withe electronic charge, d is the transport distance from the molecular center to center in a dimer, KB Boltzmann constant ke/h charge transfer rate, and T temperature at 298 k. The µe/hole values and the charge transfer rate of the electron ke and hole kh of the studied molecules are presented in Table 10.Table 10
Charge transfer rate valueske/h and mobility µe/h of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
Speciesdke (s−1)kh (s−1)μe (cm2·V−1·s−1)μh (cm2·V−1·s−1)MODP45.328 × 10132.389 × 10111.65680.0074[Cu(MODP)2H2O] H2O41.634 × 10114.329 × 10150.0051134.6370[Zn(MODP)2 (H2O)2]42.613 × 10105.123 × 10130.00081.5932[Co(MODP)2 (H2O)2]41.079 × 10121.153 × 10160.0336358.7420[Ni(MODP)2 (H2O)2]48.624 × 10141.998 × 101326.81880.6213[Fe(MODP)2 (H2O)2]45.069 × 10159.279 × 105157.65012.885 × 10−8[Pd(MODP)2 (H2O)2]42.810 × 10161.386 × 1014873.86014.3119[Pt(MODP)2 (H2O)2]42.461 × 10163.360 × 1013765.45391.0452It is clear from Table10 that the ke and μe values of the molecules are in the range 1.079 × 1012–2.810 × 1016 s−1 and 0.0008–873.8601 cm2 V−1 s−1. They decrease in the order: [Pd (MODP)2 (H2O)2]  >  [Pt (MODP)2 (H2O)2]  >  [Fe (MODP)2(H2O)2] > [Ni(MODP)2 (H2O)2] > MODP > [Co(MODP)2 (H2O)2] > [Cu(MODP)2 H2O]H2O > [Zn(MODP)2 (H2O)2]. From this trend, we note that complexation with Ni(II), Fe(II), Pd(II), and Pt(II) increases the values of ke and μe, whereas coordination with Cu(II), Co(II), and Zn(II) reduces these values. On this basis, the Pt(II) complex has a very high charge transfer rate and mobility of electrons with values of 2.810 × 1016 s−1 and 873.8601 cm2 V−1 s−1, respectively. These results show that an increase in the charge transfer rate of electron leads to an increase in the mobility of electrons.It can also be seen from Table10 that an increase in the value of the charge transfer rate of the hole leads to an increase in the mobility of the hole. The results also show that complexation increases the values of ku and μu for all complexes studied except for those of the Fe(II) and Cu(II) complexes, which reduces these values. The Co(II) complex has a high charge transfer rate (1.153 × 1016 s−1) and mobility (358.742 cm2 V−1 s−1). Based on these results, the complexes of Pd(II), Pt(II), and Fe(II) have high values of electron charge transfer rate and electron mobility, and those of Co(II) and Cu(II) have high values of charge transfer rate and hole mobility. Thus, these molecules are good candidates for the design of OLED and OSC devices in optoelectronics.


## 3.1. Stability Analysis of Spin States

Transition metal ions with incompletely filled d-orbitals can exhibit high and low possible spin states. With this in mind, the most stable spin states corresponding to the lowest energy geometries of the currently investigated complexes were determined via geometry optimization of the doublet and quartet spin states for Co2+ ions, singlet and triplet spin states for Ni2+ ions, Pt2+ and Pd2+ ions, and the singlet and quintet spin states for Fe2+ ions. For the Cu2+ and Zn2+ ions, spin effects were not investigated because Cu2+ ions have only one unpaired electron in its outermost d-orbital, and Zn2+ ions have a completely filled d-orbital. The calculated electronic energies of the aforementioned spin states at the DFT/B97-3c level of theory are listed in Table 1.Table 1
Energy (in kcal/mol) of the studied complexes at the different spin states computed using the B97-3c method in gas phase.
Metal complexB97-3cH-M complexL-M complex[Pt(MODP)2(H2O)2]−13824.021−13824.003[Co(MODP)2(H2O)2]−21752.935−21752.937[Ni(MODP)2(H2O)2]−22541.035−22540.807[Fe(MODP)2(H2O)2]−21005.722−21005.436[Pd(MODP)2(H2O)2]−13879.301−13879.259H-M⟶ High multiplicity, L-M ⟶ Low multiplicity.The results show that apart from the Co(II) complex, the energies of the high spin complexes are lower than those of the low spin counterparts. As a result, only the triplet state for the Ni(II), Pd(II), and Pt(II) complexes, the doublet state for the Co(II) complex, and the quintet state for the Fe(II) complex have further been considered throughout this work.


## 3.2. Infrared (IR) Vibrational Analysis

In order to confirm the suitability of the DFT/B97-3c level of theory employed in the computation of IR frequencies, computed frequencies have been compared with their experimentally obtained counterparts from [22]. The spectra of MODP and its complexes are presented in Figure S1 of the accompanying electronic supplementary material (ESM). For better comparison of the theoretical and experimental IR frequencies, correlation equations (12) and (13) have been established based on the vibrational modes of the MODP and the complexes, respectively.(12)νcal=1.019νexp+1.707R2=0.998;for MODP,(13)νcal=139.478νexp+8.717R2=0.949;MIIMODP complexes,where νcal and νexp represent the calculated and experimental wavenumbers, respectively.The high correlation coefficients (R2 = 0.998) for the MODP and (R2 = 0.949) depict a good linear correlation between the calculated (unscaled) and experimental IR frequencies. This confirms the suitability of the level of theory employed in these calculations.Presented in Table2 are some experimental and calculated gas phase IR frequencies of MODP and its M(II) complexes along with their probable assignments.Table 2
Calculated vibrational frequencies (cm-1) for MODP and its complexes at B97-3c level and the corresponding experimental IR frequencies.
SpeciesνO−HνC=NνS−HνC=SνH2OνM−NνM−OνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalνexpνcalMODP321932811595156125232641780844——————[Cu(MODP)2H2O] H2O——153415222522263677986734503541564525424440[Zn(MODP)2(H2O)2]——155615182525264377886934333796563514440425[Co(MODP)2(H2O)2]——157615572522263477887034503670564527430458[Ni(MODP)2(H2O)2]——153215172524263877886633603589567572426433νexp represents experimental wavenumbers by [22]. νcal represents wavenumbers calculated in this work.The results show that the IR spectra calculated for MODP and its transition metal complexes showed bands around 2635–2643 cm−1 and 844–870 cm−1 corresponding to the νS−H and ν (C-S) vibrations. The experimental IR frequency assigned to the phenolic O-H vibrational mode of MODP is 3382 cm−1, according to [22]. The calculated frequency for the phenolic O-H νO−H vibration, 3281 cm−1, is closer to the experimental value reported in [22]. Upon complexation, the disappearance of the IR bands corresponding to the O-H vibrations of MODP indicates the loss of its carboxylic protons. The IR spectrum of MODP is more or less modified by the coordination of the transition metals. The calculated IR bands ν (H2O) between 3541–3796 cm−1 are present in the spectra of the complexes due to the vibrations of stretching of the O-H bonds in H2O, which shows the presence of coordinated water. Calculated IR bands due to the vibrations of O-H bonds in H2O are absent in the spectrum of MODP because it does not contain attached H2O molecules.The results also inform that the computed spectra of the complexes show the appearance of new bands due to metal-ligand vibrations (M-N) and (MO) in the regions 514–572 cm−1 and 425–458 cm−1, respectively. This further confirms coordination with the central metal ion by the phenolic oxygen atom and one of the azomethine nitrogen atoms. The metal-ligand vibration frequencies of these complexes are in agreement with the experimental values in [22]. The IR spectra of the complexes indicate that MODP behaves as a bidentate ligand and is bound to the metal ion via the N-atom of the azomethine group (C = N) and the O-atom of the phenolic group (OH) (after deprotonation).


## 3.3. Geometric Parameters

The optimized geometries of MODP and its Cu(II), Ni(II), Co(II), Zn(II), Fe(II), Pd(II), and Pt(II) complexes are displayed in Figure3, and their optimized Cartesian coordinates are provided in Tables S1‒S16 of the accompanying electronic supplementary material (ESM).Figure 3
Optimized geometries of MODP and its complexes studied in the gas phase at the DFT/B97-3c level of theory.Figure3 shows that an intramolecular hydrogen bond (H-bond) is formed between the hydroxyl group and one of the oxadiazole ring nitrogen in MODP, which certainly stabilizes the molecule. This H-bond also hinders free rotation about the C6-C7 bond, thus maintaining the molecule in a somewhat planar structure that promotes π-conjugation.Selected geometric parameters (bond lengths, bond angles, and dihedral angles) are listed in Table3.Table 3
Selected bond lengths (Å), bond angles (°), and dihedral angles (°) of molecules optimized in DFT/B97-3c in the gas phase.
ParameterMODP[Cu (MODP)2H2O] H2O[Co (MODP)2 (H2O)2][Ni (MODP)2 (H2O)2][Zn (MODP)2 (H2O)2][Fe (MODP)2 (H2O)2][Pd (MODP)2 (H2O)2][Pt (MODP)2 (H2O)2]Bond lengtha (Å)N8-H191.791———————M15-O14—1.9521.8992.0122.0121.9642.2522.390M15-N8—1.9701.9072.0342.1272.1232.1422.055M15-O22—1.9591.8982.0122.0091.9632.2532.390M15-N23—1.9621.9062.0332.1292.1242.1422.055M15-O16—3.7362.3772.1792.3092.3192.4062.532M15-O19—2.7252.3892.1792.3062.3162.4062.530Bond anglea (°)O14-M15-N23—91.389.293.494.795.298.998.6N8-M15-O22—93.789.293.494.995.498.998.5N8-M15-O14—89.790.886.685.284.781. 181.4O22-M15-N23—89.790.786.685. 284.681.181.5O16-M15-N8—128.992.689.391.191.289.292.5O16-M15-O22—112.897.880.6100. 596.6109.464.9O19-M15-O14—127.597.580.6100.096. 7109.565.0O19-M15-N23—89.892.589.290.991.189.192.4Dihedral anglea (°)M15-N23-C26-O27—162.6173.8166.4164.5168.4159.3172.9M15-N8-C7-O11—174.7173.8166.5164.4168.5159.3172.8O22-M15-N8-N9—20.815.014. 610.65.311.514.3O14-M15-N23-N24—96.015.414.410.76.111.714.3O19-M15-O14-N23—90.892.480.691.891.792.288.7O19-M15-O22-N8—80.287.691.388.288.387.891.3O16-M15-N23-O14—37.982.299.479.483.170.5114.8O16-M15-N8-O22—123.597.780.679.896.7109.365.1aRefer to Figure 3 for atomic numbering, M = Co(II), Ni(II), Zn(II), Fe(II), Pd(II), and Pt(II).From the dihedral angles, MODP is improved upon metal ions complexation. From the metal-ligand bond lengths, we conclude that there is effective metal-ligand binding in all complexes studied.The results from this table also show that the bond angles around the central metal ion in the complexes investigated are around 90°, with exception of the Cu(II) complex where large discrepancies are observed in some cases. This indicates a high planarity of MODP in its Co(II), Ni(II), Zn(II), Fe(II), Pd(II), and Pt(II) complexes, as well as octahedral geometries around the central metal ions. The Cu(II) complex of MODP apparently adopts a trigonal bipyramidal geometry around the Cu(II) ion, since one of the ancillary H2O ligands is clearly out of the coordination sphere and is rather attached to one of the MODP ligands through an H-bond. This may be attributed to the fact that the Cu(II) ion is not always stable in octahedral geometries, and the complexes of this ion often have a square or tetrahedral planar structure. This arrangement comes from the stretching of the octahedron, along the z-axis (Jahn-Teller effect). The Cu(II) complex is Jahn-Teller distorted because the bonds to the central Cu(II) ion along the z-axis (M15-O16 and M15-O19 with bond lengths 3.736 Å and 2.725 Å, respectively) are much longer than the bonds along the x-axis and y-axis: 1.970 and 1.962 Å for M15-N8 and M15-N23 and 1.952 and 1.959 Å for M15-O14 and M15-O22. This corresponds to an extension along the z-axis and compression along the x- and y-axes.


## 3.4. Photovoltaic Properties

### 3.4.1. Electronic Parameters

The efficiency of the donor-acceptor charge transfer in organic solar cells is determined by important factors such as the HOMO and LUMO energy levels of the donor and acceptor [18]. For effective charge transfer to occur, the HOMO and LUMO energies of efficient charge transporters must be higher than that of (6, 6)-Phenyl-C61-butyric acid methyl ester (PCBM) (HOMO = −6.1 eV and LUMO = −3.75 eV) and lower than that of Poly(3-hexylthiophene) (P3HT) (HOMO = −4.65 eV and LUMO = −2.13 eV) [16, 42]. P3HT and PCBM are reference electron donor and acceptor materials, respectively. In order to assess the electron transfer possibilities of the molecules studied, their HOMO and LUMO energy levels were compared with those of PCBM (see Table 4). PCBM was chosen in this study as electron acceptor because of its stability, its capacity to accept electrons, and its proper energy level [53] as earlier mentioned.Table 4
The EHOMO, ELUMO, and Egap values of the studied molecules and PCBM calculated at PBE0-D3(BJ)/def2-TZVP theory of level in the gas phase, all in eV.
SpeciesEHOMOELUMOEgap (ELUMO-EHOMO)MODP−6.09−1.644.46[Cu(MODP)2H2O]H2O−5.64−1.314.33[Zn(MODP)2(H2O)2]−5.81−1.394.41[Co(MODP)2(H2O)2]−5.48−1.434.05[Ni(MODP)2(H2O)2]−5.71−1.404.31[Fe(MODP)2(H2O)2]−5.67−1.354.32[Pd(MODP)2(H2O)2]−5.72−1.534.19[Pt(MODP)2(H2O)2]−5.17−1.673.50PCBM−6.10−3.752.35The results reveal that all the studied molecules have higher HOMO and LUMO energies than PCBM. This indicates the ability of molecules to inject electrons into PCBM. The results also show that complexation modifies the HOMO and LUMO energies and the energy gaps. Complexation increases the HOMO energy values by a factor of about 0.50 eV and the LUMO energy values by about a factor of 0.25 eV, except for the LUMO energy of the Pt(II) complex (which is slightly reduced by 0.03 eV). Accordingly, the investigated molecules can be used as donor materials in organic solar cells. Moreover, photo-excited electron transfer from the studied molecules to PCBM may be sufficiently effective for applications in photovoltaic devices [40, 54]. Also, the Egap values of the molecules are in the range 3.50–4.46 eV and decrease in the order: MODP > [Zn(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Fe(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Pd(MODP)2 (H2O)2] > [Co(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2]. Consequently, metal complexation decreases the Egap of MODP. It is also observed that the Pt(II) complex has the smallest Egap (3.50 eV), while that of the Zn(II) complex (4.41 eV) is the highest. This implies that the Pt(II) complex is the most reactive and least stable, while the Zn(II) complex is the most stable and least reactive. Consequently, the HOMO-LUMO electron transport is easier in the Pt (II) complex than in the other complexes.The frontier molecular orbital (FMOs) energy gaps, Egap, of MODP and its complexes are displayed in Figure4, along with the isosurfaces of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), as visualized using Avogadro 1.2.1 [28].Figure 4
Isosurfaces of the FMOs of MODP and its complexes computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.The findings show that the HOMOs of the Cu(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes are localized on the transition metals and on MODP, while those of the Co(II) and Zn(II) complexes are located on MODP and exhibitπ-bonding character. The LUMOs of the Co(II), Cu(II), Zn(II), Pt(II), and Pd(II) complexes are mainly located on MODP, while those of the Fe(II) complex are centered on the transition metal ion and on MODP, and those of the Ni(II) complex on the transition metal ions and the oxygen atoms of MODP around the central metal and have a π antibonding character.


### 3.4.2. Open Circuit VoltageVOC

One of the parameters that influence the efficiency of solar cells is Voc. In organic solar cells, Voc is a function of the HOMO energy of the donor and the LUMO energy of the acceptor, which are important in determining whether an effective transfer will take place between the donor and the acceptor [40]. The Voc values of the studied molecules (donor), as well as those of the prototypical donor material P3HT and the acceptor PCBM, are presented in Table 5, along with the EL−L (ELUMOD−ELUMOA) values.Table 5
HOMO, LUMO energy values (eV), and Voc/PCBM (eV) of the different molecules calculated at the DFT/PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesEHOMOELUMOVoc/PCBMEL−LP3HT−4.65−2.130.601.62MODP−6.10−1.642.052.11[Cu(MODP)2 H2O]H2O−5.64−1.311.592.44[Zn(MODP)2 (H2O)2]−5.81−1.391.762.36[Co(MODP)2 (H2O)2]−5.48−1.431.432.32[Ni(MODP)2 (H2O)2]−5.71−1.401.662.35[Fe(MODP)2 (H2O)2]−5.67−1.351.622.40[Pd(MODP)2 (H2O)2]−5.72−1.531.672.22[Pt(MODP)2 (H2O)2]−5.17−1.671.122.08PCBM−6.10−3.75——The obtained Voc values are in the range 1.12–2.05 eV and are higher than those of the prototypical material P3HT/PCBM. This indicates that these values are sufficient for the electrons to be injected into the unoccupied orbital of the acceptor ([6, 6]-Phenyl-C61-butyric acid methyl ester (PCBM)). Therefore, all studied molecules can be used as donors, since the process of injecting electrons from the excited molecules to the conduction band of the PCBM is possible. The Voc values decrease in the order: MODP > [Zn(MODP)2(H2O)2] < [Pd(MODP)2(H2O)2] < [Ni(MODP)2(H2O)2]  <  [Fe(MODP)2(H2O)2]  <  [Cu(MODP)2H2O]H2O < [Co(MODP)2(H2O)2] < [Pt(MODP)2(H2O)2]. We also note that complexation reduces the Voc values of the complexes, suggesting that the transfer of electrons is easier from the compounds studied to PCBM.The results in Table5 also show that the differences in the LUMO energy levels of the studied compounds and the acceptor range from 2.08 to 2.44 eV and are all higher than 0.3 eV. This suggests that the transfer of photo-excited electrons from the molecules to PCBM is sufficient to be used in photovoltaic devices [54]. This assures an electron transfer between the donors and the acceptor. For all the molecules studied, the EL−L values obtained with respect to the PCBM acceptors are higher than those obtained with respect to the P3HT/PCBM pair. The EL−L values of all the molecules studied are higher than those of VOC and the binding energy (0.2 to 1.0 eV). Therefore, MODP and its complexes are promising donors with respect to the PCBM acceptor. As a result, these compounds can be used as solar cells.


## 3.4.1. Electronic Parameters

The efficiency of the donor-acceptor charge transfer in organic solar cells is determined by important factors such as the HOMO and LUMO energy levels of the donor and acceptor [18]. For effective charge transfer to occur, the HOMO and LUMO energies of efficient charge transporters must be higher than that of (6, 6)-Phenyl-C61-butyric acid methyl ester (PCBM) (HOMO = −6.1 eV and LUMO = −3.75 eV) and lower than that of Poly(3-hexylthiophene) (P3HT) (HOMO = −4.65 eV and LUMO = −2.13 eV) [16, 42]. P3HT and PCBM are reference electron donor and acceptor materials, respectively. In order to assess the electron transfer possibilities of the molecules studied, their HOMO and LUMO energy levels were compared with those of PCBM (see Table 4). PCBM was chosen in this study as electron acceptor because of its stability, its capacity to accept electrons, and its proper energy level [53] as earlier mentioned.Table 4
The EHOMO, ELUMO, and Egap values of the studied molecules and PCBM calculated at PBE0-D3(BJ)/def2-TZVP theory of level in the gas phase, all in eV.
SpeciesEHOMOELUMOEgap (ELUMO-EHOMO)MODP−6.09−1.644.46[Cu(MODP)2H2O]H2O−5.64−1.314.33[Zn(MODP)2(H2O)2]−5.81−1.394.41[Co(MODP)2(H2O)2]−5.48−1.434.05[Ni(MODP)2(H2O)2]−5.71−1.404.31[Fe(MODP)2(H2O)2]−5.67−1.354.32[Pd(MODP)2(H2O)2]−5.72−1.534.19[Pt(MODP)2(H2O)2]−5.17−1.673.50PCBM−6.10−3.752.35The results reveal that all the studied molecules have higher HOMO and LUMO energies than PCBM. This indicates the ability of molecules to inject electrons into PCBM. The results also show that complexation modifies the HOMO and LUMO energies and the energy gaps. Complexation increases the HOMO energy values by a factor of about 0.50 eV and the LUMO energy values by about a factor of 0.25 eV, except for the LUMO energy of the Pt(II) complex (which is slightly reduced by 0.03 eV). Accordingly, the investigated molecules can be used as donor materials in organic solar cells. Moreover, photo-excited electron transfer from the studied molecules to PCBM may be sufficiently effective for applications in photovoltaic devices [40, 54]. Also, the Egap values of the molecules are in the range 3.50–4.46 eV and decrease in the order: MODP > [Zn(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Fe(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Pd(MODP)2 (H2O)2] > [Co(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2]. Consequently, metal complexation decreases the Egap of MODP. It is also observed that the Pt(II) complex has the smallest Egap (3.50 eV), while that of the Zn(II) complex (4.41 eV) is the highest. This implies that the Pt(II) complex is the most reactive and least stable, while the Zn(II) complex is the most stable and least reactive. Consequently, the HOMO-LUMO electron transport is easier in the Pt (II) complex than in the other complexes.The frontier molecular orbital (FMOs) energy gaps, Egap, of MODP and its complexes are displayed in Figure4, along with the isosurfaces of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), as visualized using Avogadro 1.2.1 [28].Figure 4
Isosurfaces of the FMOs of MODP and its complexes computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.The findings show that the HOMOs of the Cu(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes are localized on the transition metals and on MODP, while those of the Co(II) and Zn(II) complexes are located on MODP and exhibitπ-bonding character. The LUMOs of the Co(II), Cu(II), Zn(II), Pt(II), and Pd(II) complexes are mainly located on MODP, while those of the Fe(II) complex are centered on the transition metal ion and on MODP, and those of the Ni(II) complex on the transition metal ions and the oxygen atoms of MODP around the central metal and have a π antibonding character.


## 3.4.2. Open Circuit VoltageVOC

One of the parameters that influence the efficiency of solar cells is Voc. In organic solar cells, Voc is a function of the HOMO energy of the donor and the LUMO energy of the acceptor, which are important in determining whether an effective transfer will take place between the donor and the acceptor [40]. The Voc values of the studied molecules (donor), as well as those of the prototypical donor material P3HT and the acceptor PCBM, are presented in Table 5, along with the EL−L (ELUMOD−ELUMOA) values.Table 5
HOMO, LUMO energy values (eV), and Voc/PCBM (eV) of the different molecules calculated at the DFT/PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesEHOMOELUMOVoc/PCBMEL−LP3HT−4.65−2.130.601.62MODP−6.10−1.642.052.11[Cu(MODP)2 H2O]H2O−5.64−1.311.592.44[Zn(MODP)2 (H2O)2]−5.81−1.391.762.36[Co(MODP)2 (H2O)2]−5.48−1.431.432.32[Ni(MODP)2 (H2O)2]−5.71−1.401.662.35[Fe(MODP)2 (H2O)2]−5.67−1.351.622.40[Pd(MODP)2 (H2O)2]−5.72−1.531.672.22[Pt(MODP)2 (H2O)2]−5.17−1.671.122.08PCBM−6.10−3.75——The obtained Voc values are in the range 1.12–2.05 eV and are higher than those of the prototypical material P3HT/PCBM. This indicates that these values are sufficient for the electrons to be injected into the unoccupied orbital of the acceptor ([6, 6]-Phenyl-C61-butyric acid methyl ester (PCBM)). Therefore, all studied molecules can be used as donors, since the process of injecting electrons from the excited molecules to the conduction band of the PCBM is possible. The Voc values decrease in the order: MODP > [Zn(MODP)2(H2O)2] < [Pd(MODP)2(H2O)2] < [Ni(MODP)2(H2O)2]  <  [Fe(MODP)2(H2O)2]  <  [Cu(MODP)2H2O]H2O < [Co(MODP)2(H2O)2] < [Pt(MODP)2(H2O)2]. We also note that complexation reduces the Voc values of the complexes, suggesting that the transfer of electrons is easier from the compounds studied to PCBM.The results in Table5 also show that the differences in the LUMO energy levels of the studied compounds and the acceptor range from 2.08 to 2.44 eV and are all higher than 0.3 eV. This suggests that the transfer of photo-excited electrons from the molecules to PCBM is sufficient to be used in photovoltaic devices [54]. This assures an electron transfer between the donors and the acceptor. For all the molecules studied, the EL−L values obtained with respect to the PCBM acceptors are higher than those obtained with respect to the P3HT/PCBM pair. The EL−L values of all the molecules studied are higher than those of VOC and the binding energy (0.2 to 1.0 eV). Therefore, MODP and its complexes are promising donors with respect to the PCBM acceptor. As a result, these compounds can be used as solar cells.


## 3.5. UV-Visible Absorption Spectra

The electronic transitions within a material can greatly influence its photovoltaic applications. It is desirable that donor materials should have intense electronic absorption compared to those of the acceptor materials. In fact, the maximum absorption wavelengthλmax of a suitable material must satisfy the absorption property requirement, according to which, λmax should not exceed 920 nm [55]. For each compound, 10 excited states were calculated. The calculated absorption wavelengths λcal, oscillator strengths f, electronic transitions, the molecular orbital (MO) contributions to electronic transitions, and excitation energies E of the investigated molecules are presented in Table 6. All the parameters in Table 6 were obtained in the excited state where the oscillation strength is the greatest and corresponds to the most intense band of the UV-Vis spectrum.Table 6
Absorption spectra data calculated by TD-DFT methods for species at CAMB3LYP/def2-TZVP level of theory in the gas phase and in DMSO.
MoleculesGazDMSOTransitionAssignments and MO contributionE (eV)λcal (nm)fTransitionAssignments and MO contributionE (eV)λcal (nm)fMODPS0 ⟶ S2H-2 ⟶ L + 1 (70.4%)5.193238.50.3792S0 ⟶ S8H-2 ⟶ L (37.0%)6.905179.50.5392[Co(MODP)2(H2O)2]S0 ⟶ S10H-2 ⟶ L + 2 (31.5%)2.806441.90.0001S0 ⟶ S10H-2 ⟶ L + 1 (32.0%)2.840436.60.0001[Zn(MODP)2(H2O)2]S0 ⟶ S2H-1 ⟶ L-1 (49.2%)4.093302.90.5942S0 ⟶ S2H-1 ⟶ L (48.3%)4.103302.20.6186[Ni(MODP)2(H2O)2]S0 ⟶ S9H-3 ⟶ L (54.0%)4.031307.60.3089S0 ⟶ S9H-4 ⟶ L + 2 (16.0%)3.656339.20.0003[Cu(MODP)2H2O]H2OS0 ⟶ S8H-2 ⟶ L + 2 (38.9%)3.320373.50.0401S0 ⟶ S6H-2 ⟶ L + 2 (31.0%)2.522491.60.0760[Fe(MODP)2(H2O)2]S0 ⟶ S9H-2 ⟶ L + 3 (36.6%)3.822324.40.0622S0 ⟶ S9H-3 ⟶ L + 3 (30.8%)3.807325.70.0576[Pd(MODP)2(H2O)2]S0 ⟶ S5H-2 ⟶ L + 2 (92.4%)2.519492.20.0514S0 ⟶ S5H-3 ⟶ L + 2 (25.0%)2.518492.30.0416[Pt(MODP)2(H2O)2]S0 ⟶ S10H ⟶ L (52.5%)3.203387.10.0422S0 ⟶ S10H ⟶ L (54.2%)3.279378.10.0513H⟶ HOMO, L ⟶ LUMO, λcal: calculated absorption wavelengths, f: oscillator strength.Interestingly, theλcal of all molecules studied is below 500 nm in both DMSO and the gas phase. λcal of the molecules in DMSO is in the order: MODP < [Zn(MODP)2 (H2O)2]  <  [Fe(MODP)2 (H2O)2]  <  [Pt(MODP)2(H2O)2]  <  [Ni(MODP)2 (H2O)2]  < [Co(MODP)2(H2O)2] < [Cu(MODP)2H2O]H2O < [Pd(MODP)2(H2O)2]. From this trend, we note increases in the wavelengths after complexation in all complexes in both the gas phase and in DMSO solvent. This bathochromic effect can be explained by an increase in the number of double bonds in the complexes compared to MODP, since conjugated systems are important for studying electronic and absorption properties of compounds. Generally, light absorbance of a molecule is improved by higher number of conjugated double bonds [56]. The UV-vis absorption spectra of the ligand and its transition metal complexes as studied in gas phase and in DMSO are shown in Figure 5. The absorption spectra (see Figure 5) give the maximum wavelengths of the MODP and its complexes in DMSO solvent and in the gas phase. The spectra for MODP show a maximum wavelength band at 179.5 nm corresponding to the oscillator strength f = 0.5392 in DMSO solvent and the band of maximum wavelength at 238.8 nm corresponding to oscillator strength f = 0.3792 in the gaseous phase. These values correspond to the highest absorption with the excitation energy 6.905 eV and the MO contribution H-2 ⟶ L in DMSO solvent. In the gas phase, these values correspond to the excitation energy 5.193 eV, of the H-2 ⟶ L + 1 MO contribution. The UV-Vis absorption spectrum of the Co(II) complex consists of a band with a maximum wavelength λ = 436.6 nm and an oscillator strength f = 0.0001. This band corresponds to an energy of 2.840 eV, a charge transfer percentage of 32.01%, and MO contribution H-2 ⟶ L + 1 in DMSO solvent. In the gas phase, the Co(II) complex shows a maximum wavelength band at 238.8 nm, which is greater than that obtained in DMSO solvent.Figure 5
Absorption spectra of MODP and its complexes at the CAMB3LYP/def2-TZVP theory level in (a) gas phase and (b) DMSO solvent.
(a)(b)Concerning the Pd(II) and Pt(II) complexes, their absorption spectra show maximum bands at 492.3 nm and 378.1 nm corresponding to the excitation energies 2.518 eV and 3.279 eV with oscillator strengthsf  = 0.0416 and f = 0.0513, respectively, as shown in Table 5. These values correspond to the MO contribution H-3 ⟶ L + 2 and H ⟶ L, respectively, in DMSO solvent. These values increase in the gas phase. For the Cu(II), Zn(II), Ni(II), and Fe(II) complexes in DMSO solvent, their absorption spectra as illustrated in Figure 5 exhibit maximum wavelength bands as follows: λ = 491.6 nm, λ = 302.2 nm, λ = 339.2 nm, λ = 325.7 nm corresponding to the oscillator strengths: f = 0.0760, f = 0.6186, f = 0.0003, f = 0.0576, respectively. These bands are due to the MO contribution H-2 ⟶ L + 2, H-1 ⟶ L, H-4 ⟶ L + 2 and H-3 ⟶ L + 3, respectively, as shown in Table 5. An increase in these values is observed in the gas phase. The absorption wavelengths obtained in the gas phase and in DMSO show that the MODP and its complexes can be used in organic solar cells.


## 3.6. Charge Transport Properties

### 3.6.1. Reorganization Energies and Charge Transport

The charge transport properties of the investigated molecules have been predicted from computed reorganization energies of the electronλe and the hole λh. In Table 7, the reorganization energies of the molecules studied are compared with those of the prototypical molecules, N-N′-diphenyl-N-N′-bis (3-methylphenyl-(1,1′-biphyl)-4,4′-diamine (TPD) molecules (λh = 0.290 eV) and Tris(8-hydroxyquinolinato) aluminum (Alq3) (λe = 0.276 eV) [10, 11] for hole and electron transfer, respectively.Table 7
Values ofλe and λh of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesλeeVλheVMODP0.1870.154[Cu(MODP)2H2O] H2O0.9760.107[Zn(MODP)2 (H2O)2]0.1840.711[Co(MODP)2 (H2O)2]1.2020.179[Ni(MODP)2 (H2O)2]0.3720.015[Fe(MODP)2 (H2O)2]0.1601.724[Pd(MODP)2 (H2O)2]0.1420.161[Pt(MODP)2 (H2O)2]0.1210.257The results show that theλe values of MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes are smaller than those of Tris (8-hydroxyquinolato) aluminum (Alq3) (λe = 0.276 eV) indicating that the charge transfer rate of these molecules may be higher than that of Alq3. Accordingly, MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes may serve as good electron transporters in OLEDs owing to their small reorganization energies. It is clear from Table 7 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases its λe, whereas coordination with Cu(II), Co(II), and Ni(II) reduces its λe. It can also be inferred from this table that the λh values of MODP and its Cu (II), Co (II), Ni (II), Pd(II), and Pt(II) complexes are smaller than those of TPD (λh = 0.290 eV), showing that their charge transfer rates could be higher than those of TPD. Therefore, MODP and its Cu (II), Co (II), Ni (II), Pd (II), and Pt (II) complexes are promising hole transporters for use in OLEDs. Again, MODP (λe = 0.187 eV and λh = 0.154 eV), the Pd(II) complex (λe = 0.142 eV and λh  = 0.161 eV), and the Pt(II) (λe = 0,121 eV and λh = 0,257 eV) complexes are excellent hole and electron transporters, respectively. Therefore, these materials are likely to function as ambipolar charge transporters (electron and hole) under the proper operating conditions for OLEDs and OSCs based on the smaller reorganization energies.


### 3.6.2. Charge Transfer Integral and Charge Transport

The values of charge transfer integral for electronsVe and holes Vh, as calculated at the PBE0/def2-TZVP/D3BJ level theory, are presented in Table 8.Table 8
Charge transfer integralVeandVh values of the different molecules computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.
SpecieseVeVMODP0.0920.005[Cu(MODP)2H2O]H2O0.3560.489[Zn(MODP)2(H2O)2]0.0021.607[Co(MODP)2(H2O)2]0.2891.288[Ni(MODP)2(H2O)2]1.0800.013[Fe(MODP)2(H2O)2]0.7570.037[Pd(MODP)2(H2O)2]1.5850.126[Pt(MODP)2(H2O)2]1.2870.111The results show that theVeandVh values of MODP are smaller than those of its complexes (except for the Ve value of the Zn(II) complex). The smaller values of the VeandVh of MODP are confirmed by the small values of the reorganization energies lower than the prototype (see Table 7). MODP can function as an ambipolar electron/hole transport material. It is clear from Table 8 that, apart from coordination with the Zn(II) ion, metal complexation by MODP increases the charge transfer integral. Ve values vary in the order: [Pd(MODP)2H2O)2] > [Pt(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Fe(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Co(MODP)2(H2O)2] > MODP > [Zn(MODP)2(H2O)2]. Among the molecules studied, the Zn(II), Fe(II), and Pt(II) complexes of MODP are potential electron transporters that may be useful in the fabrication of OLEDs and OSCs, owing to their small Ve and λe values. The results also indicate that the coordination of MODP with all ions increases the values Vh. The Co(II), Cu(II), and Ni(II) complexes of MODP are apparently good hole transporters due to their small Vh and λh values.


### 3.6.3. Charge Transport Rate and Stability

The calculated values of chemical hardnessη and interaction energies, ΔEint, of molecules studied herein are listed in Table 9.Table 9
Values (in eV) of the chemical hardnessη and of the interaction energy Eint of the different molecules calculated in the gas phase.
SpeciesPBE0-D3(BJ)/def2-TZVPB97-3cIPEAηEintMODP8.1260.1313.997—[Cu(MODP)2H2O] H2O7.8141.3303.222−1.246[Zn(MODP)2 (H2O)2]6.9200.3133.304−2.193[Co(MODP)2 (H2O)2]7.289−0.6203.954−1.422[Ni(MODP)2 (H2O)2]6.7180.0843.316−1.059[Fe(MODP)2 (H2O)2]6.7820.2693.256−2.999[Pd(MODP)2 (H2O)2]6.8550.4563.199−2.563[Pt(MODP)2 (H2O)2]6.1740.6032.785−1.858It is clear from Table9 and Figure 6 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases the values of IP and EA. IP follows the trend: MODP  >  [Cu(MODP)2H2O] H2O > [Co(MODP)2(H2O)2] > [Zn(MODP)2 (H2O)2]  >  [Pd(MODP)2 (H2O)2] > [Fe(MODP)2 (H2O)2]  >  [Ni(MODP)2 (H2O)2]  >  [Pt(MODP)2 (H2O)2]. This classification shows that Cu(II) complex has the highest IP suggesting that it has the highest electron releasing power and Pt(II) complex the lowest. On the other hand, Co(II) complex has the highest electron accepting ability, while Zn(II) has the least accepting power. EA follows the trend: MODP > [Co(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2]  >  [Zn(MODP)2 (H2O)2]  >  [Fe(MODP)2 (H2O)2] > [Cu(MODP)2H2O]H2O > [Pd(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2].Figure 6
IP, EA energies, andη of the molecules studied at the PBE0-D3(BJ)/def2-TZVP level of theory.A careful inspection of same Table9 and Figure 6 clearly shows that the η value of MODP is very close to those of the complexes. It is also clear from Table 9 that the ligand has the largest η, which indicates that MODP is the most kinetically stable and least reactive molecule studied. The complexation does not significantly affect the stability of these molecules. Among the complexes, the Co(II) complex is the most kinetically stable, and this stability is apparently confirmed by its negative Eint value. The relatively high η values of the Cu(II), Zn(II), Co(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes indicate their chemical stabilities. Moreover, the stabilities of these complexes are indicated by their negative Eint values, which show that their formation from the ligand and the various metal ions are energetically favorable. In conclusion, metal coordination causes a slight decrease in the chemical hardness of MODP.


### 3.6.4. The Charge Transfer Rates and Mobility

According to Marcus’ theory, the rate of charge transport is important for determining the mobility of electrons and holes in a compound. The charge mobility is an important parameter to evaluate the performance of OLED and OSC materials [57, 58]. Mobility can be approximately calculated by Einstein’s relation [44, 49]:(14)µe/h=ed2kelectron/hole2KBT.Withe electronic charge, d is the transport distance from the molecular center to center in a dimer, KB Boltzmann constant ke/h charge transfer rate, and T temperature at 298 k. The µe/hole values and the charge transfer rate of the electron ke and hole kh of the studied molecules are presented in Table 10.Table 10
Charge transfer rate valueske/h and mobility µe/h of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
Speciesdke (s−1)kh (s−1)μe (cm2·V−1·s−1)μh (cm2·V−1·s−1)MODP45.328 × 10132.389 × 10111.65680.0074[Cu(MODP)2H2O] H2O41.634 × 10114.329 × 10150.0051134.6370[Zn(MODP)2 (H2O)2]42.613 × 10105.123 × 10130.00081.5932[Co(MODP)2 (H2O)2]41.079 × 10121.153 × 10160.0336358.7420[Ni(MODP)2 (H2O)2]48.624 × 10141.998 × 101326.81880.6213[Fe(MODP)2 (H2O)2]45.069 × 10159.279 × 105157.65012.885 × 10−8[Pd(MODP)2 (H2O)2]42.810 × 10161.386 × 1014873.86014.3119[Pt(MODP)2 (H2O)2]42.461 × 10163.360 × 1013765.45391.0452It is clear from Table10 that the ke and μe values of the molecules are in the range 1.079 × 1012–2.810 × 1016 s−1 and 0.0008–873.8601 cm2 V−1 s−1. They decrease in the order: [Pd (MODP)2 (H2O)2]  >  [Pt (MODP)2 (H2O)2]  >  [Fe (MODP)2(H2O)2] > [Ni(MODP)2 (H2O)2] > MODP > [Co(MODP)2 (H2O)2] > [Cu(MODP)2 H2O]H2O > [Zn(MODP)2 (H2O)2]. From this trend, we note that complexation with Ni(II), Fe(II), Pd(II), and Pt(II) increases the values of ke and μe, whereas coordination with Cu(II), Co(II), and Zn(II) reduces these values. On this basis, the Pt(II) complex has a very high charge transfer rate and mobility of electrons with values of 2.810 × 1016 s−1 and 873.8601 cm2 V−1 s−1, respectively. These results show that an increase in the charge transfer rate of electron leads to an increase in the mobility of electrons.It can also be seen from Table10 that an increase in the value of the charge transfer rate of the hole leads to an increase in the mobility of the hole. The results also show that complexation increases the values of ku and μu for all complexes studied except for those of the Fe(II) and Cu(II) complexes, which reduces these values. The Co(II) complex has a high charge transfer rate (1.153 × 1016 s−1) and mobility (358.742 cm2 V−1 s−1). Based on these results, the complexes of Pd(II), Pt(II), and Fe(II) have high values of electron charge transfer rate and electron mobility, and those of Co(II) and Cu(II) have high values of charge transfer rate and hole mobility. Thus, these molecules are good candidates for the design of OLED and OSC devices in optoelectronics.


## 3.6.1. Reorganization Energies and Charge Transport

The charge transport properties of the investigated molecules have been predicted from computed reorganization energies of the electronλe and the hole λh. In Table 7, the reorganization energies of the molecules studied are compared with those of the prototypical molecules, N-N′-diphenyl-N-N′-bis (3-methylphenyl-(1,1′-biphyl)-4,4′-diamine (TPD) molecules (λh = 0.290 eV) and Tris(8-hydroxyquinolinato) aluminum (Alq3) (λe = 0.276 eV) [10, 11] for hole and electron transfer, respectively.Table 7
Values ofλe and λh of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
SpeciesλeeVλheVMODP0.1870.154[Cu(MODP)2H2O] H2O0.9760.107[Zn(MODP)2 (H2O)2]0.1840.711[Co(MODP)2 (H2O)2]1.2020.179[Ni(MODP)2 (H2O)2]0.3720.015[Fe(MODP)2 (H2O)2]0.1601.724[Pd(MODP)2 (H2O)2]0.1420.161[Pt(MODP)2 (H2O)2]0.1210.257The results show that theλe values of MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes are smaller than those of Tris (8-hydroxyquinolato) aluminum (Alq3) (λe = 0.276 eV) indicating that the charge transfer rate of these molecules may be higher than that of Alq3. Accordingly, MODP and its Zn(II), Fe(II), Pd(II), and Pt(II) complexes may serve as good electron transporters in OLEDs owing to their small reorganization energies. It is clear from Table 7 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases its λe, whereas coordination with Cu(II), Co(II), and Ni(II) reduces its λe. It can also be inferred from this table that the λh values of MODP and its Cu (II), Co (II), Ni (II), Pd(II), and Pt(II) complexes are smaller than those of TPD (λh = 0.290 eV), showing that their charge transfer rates could be higher than those of TPD. Therefore, MODP and its Cu (II), Co (II), Ni (II), Pd (II), and Pt (II) complexes are promising hole transporters for use in OLEDs. Again, MODP (λe = 0.187 eV and λh = 0.154 eV), the Pd(II) complex (λe = 0.142 eV and λh  = 0.161 eV), and the Pt(II) (λe = 0,121 eV and λh = 0,257 eV) complexes are excellent hole and electron transporters, respectively. Therefore, these materials are likely to function as ambipolar charge transporters (electron and hole) under the proper operating conditions for OLEDs and OSCs based on the smaller reorganization energies.


## 3.6.2. Charge Transfer Integral and Charge Transport

The values of charge transfer integral for electronsVe and holes Vh, as calculated at the PBE0/def2-TZVP/D3BJ level theory, are presented in Table 8.Table 8
Charge transfer integralVeandVh values of the different molecules computed at PBE0-D3(BJ)/def2-TZVP level of theory in gas phase.
SpecieseVeVMODP0.0920.005[Cu(MODP)2H2O]H2O0.3560.489[Zn(MODP)2(H2O)2]0.0021.607[Co(MODP)2(H2O)2]0.2891.288[Ni(MODP)2(H2O)2]1.0800.013[Fe(MODP)2(H2O)2]0.7570.037[Pd(MODP)2(H2O)2]1.5850.126[Pt(MODP)2(H2O)2]1.2870.111The results show that theVeandVh values of MODP are smaller than those of its complexes (except for the Ve value of the Zn(II) complex). The smaller values of the VeandVh of MODP are confirmed by the small values of the reorganization energies lower than the prototype (see Table 7). MODP can function as an ambipolar electron/hole transport material. It is clear from Table 8 that, apart from coordination with the Zn(II) ion, metal complexation by MODP increases the charge transfer integral. Ve values vary in the order: [Pd(MODP)2H2O)2] > [Pt(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2] > [Fe(MODP)2(H2O)2] > [Cu(MODP)2H2O]H2O > [Co(MODP)2(H2O)2] > MODP > [Zn(MODP)2(H2O)2]. Among the molecules studied, the Zn(II), Fe(II), and Pt(II) complexes of MODP are potential electron transporters that may be useful in the fabrication of OLEDs and OSCs, owing to their small Ve and λe values. The results also indicate that the coordination of MODP with all ions increases the values Vh. The Co(II), Cu(II), and Ni(II) complexes of MODP are apparently good hole transporters due to their small Vh and λh values.


## 3.6.3. Charge Transport Rate and Stability

The calculated values of chemical hardnessη and interaction energies, ΔEint, of molecules studied herein are listed in Table 9.Table 9
Values (in eV) of the chemical hardnessη and of the interaction energy Eint of the different molecules calculated in the gas phase.
SpeciesPBE0-D3(BJ)/def2-TZVPB97-3cIPEAηEintMODP8.1260.1313.997—[Cu(MODP)2H2O] H2O7.8141.3303.222−1.246[Zn(MODP)2 (H2O)2]6.9200.3133.304−2.193[Co(MODP)2 (H2O)2]7.289−0.6203.954−1.422[Ni(MODP)2 (H2O)2]6.7180.0843.316−1.059[Fe(MODP)2 (H2O)2]6.7820.2693.256−2.999[Pd(MODP)2 (H2O)2]6.8550.4563.199−2.563[Pt(MODP)2 (H2O)2]6.1740.6032.785−1.858It is clear from Table9 and Figure 6 that the coordination of MODP with the Zn(II), Fe(II), Pd(II), and Pt(II) ions increases the values of IP and EA. IP follows the trend: MODP  >  [Cu(MODP)2H2O] H2O > [Co(MODP)2(H2O)2] > [Zn(MODP)2 (H2O)2]  >  [Pd(MODP)2 (H2O)2] > [Fe(MODP)2 (H2O)2]  >  [Ni(MODP)2 (H2O)2]  >  [Pt(MODP)2 (H2O)2]. This classification shows that Cu(II) complex has the highest IP suggesting that it has the highest electron releasing power and Pt(II) complex the lowest. On the other hand, Co(II) complex has the highest electron accepting ability, while Zn(II) has the least accepting power. EA follows the trend: MODP > [Co(MODP)2(H2O)2] > [Ni(MODP)2(H2O)2]  >  [Zn(MODP)2 (H2O)2]  >  [Fe(MODP)2 (H2O)2] > [Cu(MODP)2H2O]H2O > [Pd(MODP)2(H2O)2] > [Pt(MODP)2(H2O)2].Figure 6
IP, EA energies, andη of the molecules studied at the PBE0-D3(BJ)/def2-TZVP level of theory.A careful inspection of same Table9 and Figure 6 clearly shows that the η value of MODP is very close to those of the complexes. It is also clear from Table 9 that the ligand has the largest η, which indicates that MODP is the most kinetically stable and least reactive molecule studied. The complexation does not significantly affect the stability of these molecules. Among the complexes, the Co(II) complex is the most kinetically stable, and this stability is apparently confirmed by its negative Eint value. The relatively high η values of the Cu(II), Zn(II), Co(II), Ni(II), Fe(II), Pd(II), and Pt(II) complexes indicate their chemical stabilities. Moreover, the stabilities of these complexes are indicated by their negative Eint values, which show that their formation from the ligand and the various metal ions are energetically favorable. In conclusion, metal coordination causes a slight decrease in the chemical hardness of MODP.


## 3.6.4. The Charge Transfer Rates and Mobility

According to Marcus’ theory, the rate of charge transport is important for determining the mobility of electrons and holes in a compound. The charge mobility is an important parameter to evaluate the performance of OLED and OSC materials [57, 58]. Mobility can be approximately calculated by Einstein’s relation [44, 49]:(14)µe/h=ed2kelectron/hole2KBT.Withe electronic charge, d is the transport distance from the molecular center to center in a dimer, KB Boltzmann constant ke/h charge transfer rate, and T temperature at 298 k. The µe/hole values and the charge transfer rate of the electron ke and hole kh of the studied molecules are presented in Table 10.Table 10
Charge transfer rate valueske/h and mobility µe/h of the different molecules calculated at the PBE0-D3(BJ)/def2-TZVP level of theory in the gas phase.
Speciesdke (s−1)kh (s−1)μe (cm2·V−1·s−1)μh (cm2·V−1·s−1)MODP45.328 × 10132.389 × 10111.65680.0074[Cu(MODP)2H2O] H2O41.634 × 10114.329 × 10150.0051134.6370[Zn(MODP)2 (H2O)2]42.613 × 10105.123 × 10130.00081.5932[Co(MODP)2 (H2O)2]41.079 × 10121.153 × 10160.0336358.7420[Ni(MODP)2 (H2O)2]48.624 × 10141.998 × 101326.81880.6213[Fe(MODP)2 (H2O)2]45.069 × 10159.279 × 105157.65012.885 × 10−8[Pd(MODP)2 (H2O)2]42.810 × 10161.386 × 1014873.86014.3119[Pt(MODP)2 (H2O)2]42.461 × 10163.360 × 1013765.45391.0452It is clear from Table10 that the ke and μe values of the molecules are in the range 1.079 × 1012–2.810 × 1016 s−1 and 0.0008–873.8601 cm2 V−1 s−1. They decrease in the order: [Pd (MODP)2 (H2O)2]  >  [Pt (MODP)2 (H2O)2]  >  [Fe (MODP)2(H2O)2] > [Ni(MODP)2 (H2O)2] > MODP > [Co(MODP)2 (H2O)2] > [Cu(MODP)2 H2O]H2O > [Zn(MODP)2 (H2O)2]. From this trend, we note that complexation with Ni(II), Fe(II), Pd(II), and Pt(II) increases the values of ke and μe, whereas coordination with Cu(II), Co(II), and Zn(II) reduces these values. On this basis, the Pt(II) complex has a very high charge transfer rate and mobility of electrons with values of 2.810 × 1016 s−1 and 873.8601 cm2 V−1 s−1, respectively. These results show that an increase in the charge transfer rate of electron leads to an increase in the mobility of electrons.It can also be seen from Table10 that an increase in the value of the charge transfer rate of the hole leads to an increase in the mobility of the hole. The results also show that complexation increases the values of ku and μu for all complexes studied except for those of the Fe(II) and Cu(II) complexes, which reduces these values. The Co(II) complex has a high charge transfer rate (1.153 × 1016 s−1) and mobility (358.742 cm2 V−1 s−1). Based on these results, the complexes of Pd(II), Pt(II), and Fe(II) have high values of electron charge transfer rate and electron mobility, and those of Co(II) and Cu(II) have high values of charge transfer rate and hole mobility. Thus, these molecules are good candidates for the design of OLED and OSC devices in optoelectronics.


## 4. Conclusion

In this work, a theoretical study of the geometries, electronic properties, charge transfer, and absorption of the ligand and its metal complexes was carried out using the DFT and TD-DFT methods. The possibility of their application as components of organic solar cells and organic light-emitting diodes has been discussed. All optimized complexes have been found to adopt octahedral geometries around the central metal ion, except the Cu(II) complex, which adopts a trigonal bipyramidal geometry. The calculated absorption maxima are in the range 179–492 nm in the gas phase and 238–492 nm in DMSO. The results also reveal that MODP and the studied complexes can be used as donor materials in OSCs, thanks to their higher HOMO and LUMO energy values than those of PCBM, along with their high energy gaps, high open circuit voltage (Voc), and their intense absorption bands. For all molecules studied, the values of EL−L obtained with PCBM acceptors are higher than 0.3 eV, which suggests that the transfer of photo-excited electrons from the molecules to PCBM is sufficiently efficient to be used in photovoltaic devices. The results also showed that MODP and its Pd(II) and Pt(II) complexes are good electron (0.187, 0.142, and 0.121 eV, respectively) and hole transporters (0.154, 0.161, and 0.257 eV, respectively) due to their very small reorganization energies, integral charge transfers, high charge transfer rates, and mobilities. Therefore, these materials can be good transport materials for the manufacture of OLEDs and OSCs. Furthermore, the Zn(II) and Fe(II) complexes of MODP are found to be very good electron transporters, while the Cu(II), Co(II), and Ni(II) complexes are likely to be good hole carriers. The results of chemical hardness η and interaction energies ΔEint have shown that MODP and its Co(II) complex are amongst the most kinetically stable molecules, while all complexes have been found to be thermodynamically stable materials, and complexation does not significantly affect the stability of these molecules. Therefore, MODP and its complexes are suitable charge transport materials (electron and hole) that can be used in the manufacture of organic light-emitting diodes (OLEDs) and organic solar cells (OSCs).



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