Theoretical Design of Complex Molecule via Combination of Natural Lawsone and Synthetic Indoline D 131 Dyes for Dye Sensitized Solar Cells Application

The dye sensitized solar cells (DSSCs) have been extensively studied due to their low production cost and simple fabrication process. Dye co-sensitization broadens the absorption spectrum of the sensitizer; thus enhances light harvesting efficiency; and contributes to the improvement of the DSSCs overall efficiency. In this study we performed theoretical design of complex molecule (C45H32N2O4) through combination (esterification reaction) of the natural dye lawsone and synthetic metal free indoline dye D131. The excitation energies, vibration spectra, molecular structures, electronic absorption spectra and electron transitions in individual dyes and complex molecule were investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) B3LYP5 methods, with 3-21G, 6-31G and 6-31G(d,p) basis sets. The UV-Vis absorption spectra of the individual dyes and their mixture in chloroform solution were measured using spectrophotometer. For the complex formation reaction, enthalpy, entropy and Gibbs free energy were calculated and the results indicated the reaction was endothermic and non-spontaneous. Electron density distribution of the frontier and adjacent molecular orbitals and energy levels alignment were used for analysis of the electronic spectra and mechanism of transitions. The results indicated that the designed complex molecule satisfied the requirements for good photosensitizer of DSSCs.


Introduction
Due to increase in the environmental pollution and limited resource of fossil fuel, the development of solar energy harvesting is considered as a promising technology to substitute the fossil fuel, eliminate environmental pollution and sustain development of human societies.
DSSCs have attracted substantial attention, not only because of their environmental friendliness, low manufacturing cost, simple fabrication process and tunable aesthetic color, but also due to their lower dependence on the angle of incident light and higher efficiency under low irradiance compared to traditional Si-based solar cells [1]- [7].DSSC is composed of two sandwiched electrodes, working and counter electrode which is made of transparent conducting glass mainly composed of fluorine doped tin oxide (FTO) or indium doped tin oxide.
Working electrode is made of thin layer of semiconducting materials such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), tin dioxide (SnO 2 ) coated with dyes on top of FTO [8] [9].Counter electrode is made of FTO with thin layer of platinum or carbon [10].Electrolyte is placed between counter electrode and photoelectrode to mediate electrons [11].Among of these components of DSSC, the sensitizer (dye) is a crucial element which plays a significant role in light capturing, electron injection and dye regeneration which impact on power conversion efficiency (PCE) as well as the stability of the devices.
Although the Incident Photon to Charge Carrier Efficiency (IPCE) of DSSCs based on ruthenium dyes has reached 12% [23] [24] [25] under simulated AM 1.5 irradiation (100 mW•cm −2 ), ruthenium is rare, expensive, and toxic in nature that limits its potentially wide application.Despite the highest efficiency of 13% [26] has been reported for DSSCs based on free metal dyes (organic dyes), the high price hampers their extensive use.DSSCs based on natural dyes have poor photoelectrochemical properties but the natural dye sensitizers are cheap and environmentally friendly.
To overcome challenges facing DSSCs, theoretical and experimental studies of physical and chemical properties of dyes are required as they contribute to understanding the relationship between structure, properties of dyes and performance of a solar cell and hence facilitate design and synthesis of efficient dyes for DSSCs application [20] [27]- [32].In selecting and designing an efficient dye, electron donating part, unit to adjust the absorption spectrum and the electron-acceptor part of the dye [33] must be taken into design consideration.
Mainly the dye to be an efficient sensitizer should be designed with a target of eliminating dye aggregation, minimizing dye recombination and maximizing the driving force for electron injection from the dye to the semiconductor and driving force for dye regeneration, broaden up the absorption spectra which finally improve the overall efficiency of the DSSCs [34] [35] [36].Combination of dyes with different absorption properties has been investigated intending to enhance light harvesting efficiency as well as general performance of DSSCs [37]- [42].
In this study, the effect of combination of natural dye lawsone and synthetic metal free indoline D131 dye for the purpose of utilization in DSSCs has been investigated.The computations of vibrational and electronic absorption spectra of the indoline D131, lawsone and designed complex molecule as well as mechanisms of electron transitions and energy levels alignment are included in the theoretical part.The UV-Vis spectra of the individual dyes solutions and their mixture are considered in the experimental part.

Computational Details
The initial coordinates of indoline and lawsone dyes were taken from ChemSpider database [43] and NIST [44], respectively.The geometrical parameters were optimized using density function theory DFT/B3LYP5 with the different basis sets, 3-21G, 6-31G and 6-31G(d,p).The optimized coordinates were used as inputs for computation of vibrational and electronic spectra.
The complex dye molecule C 45 H 32 N 2 O 4 was designed through modeling of esterification reaction between indoline D131 and lawsone dyes.The structure of the designed dye was optimized using DFT/B3LYP5 starting from the lowest STO-3G through 3-21G, 6-31G to highest 6-31G(d,p) basis set.The energy, Gibbs free energy and enthalpy of the complex formation reaction were computed at different basis sets with the corresponding optimized coordinates.The energy of the reaction r E ∆ was calculated as a difference between the total energies of the products and reactants: In quantum chemical computations, the total energies of species correspond to temperature 0 K and normal pressure.The enthalpy of the reaction ( ) r 0 H ∆ ° was found using the ∆ r E and zero point vibration energy (ZPVE) Δ r ε ( ) ( ) where h is the Plank's constant, c is the speed of light in free space,  The vertical excitation and electronic absorption spectra of the lawsone dye, indoline D131 dye and the designed complex molecules both in vacuum and solvent (chloroform) were computed at the TD-DFT B3LYP5/6-31G(d,p) level of theory.The solvent effects were accounted by employing the polarized continuum model (PCM).All quantum chemical computations were performed using the Firefly QC package [46] which is partially based on the GAMESS (US) source code [47].The geometrical structures, vibrational and electronic spectra were visualized and analyzed using the Chemcraft [48] and MacMolPlt software [49].

Experimental Details
The indoline D131 dye with a purity of 97% and lawsone dye with purity 97% were bought in powder form from Sigma-Aldrich Chemical Company and were used without further purification for solution preparation at room temperature.
A single solvent both for D131 and lawsone was needed.Chloroform was found to be appropriate solvent [50] [51]; it was bought from Sigma-Aldrich Chemical supplier.
The solutions of 1.15 × 10 −4 M for lawsone and 2.95 × 10 −5 M for indoline D131 were prepared and used for the UV-Vis spectra measurements.The mass 8 mg of lawsone dissolved in 40 ml of chloroform to have 1.15 × 10 −3 M and then 1 ml of the solution was added to 9 ml of chloroform resulting in 1.15 × 10 −4 M.
For the case of indoline D131 dye, 6 mg of the sample was dissolved in 40 ml of chloroform resulting in 2.95 × 10 −4 M, then 2 ml of this solution was added to 18 ml of chloroform to get 2.95 × 10 −5 M indoline D131 solution.The solution of the dyes mixture was prepared by combination of the indoline D131 and lawsone dye solutions in the ratio of 1:1.The UV-Vis spectra of the samples were recorded in the region 190 -820 nm on a single beam 2800 UV-Vis spectrophotometer (Hitachi U2000) with quartz cuvettes of 10 mm path length.

Molecular Structure of Individual Dyes and Complex Molecule
The equilibrium geometrical structures of lawsone, indoline D131 and the designed complex molecules computed at B3LYP5/6-31G(d,p) are given in Figure 1.The selected geometrical parameters of the individual dyes are compared with those in the complex molecule in Table 1 and Table 2; the differences between respective parameters are shown in the far right columns.The bond C1-O2-C37 in the complex is specified with parameters R(C1-O2) = 1.397Å, R(O2-C37) = The thermodynamic characteristics of the complex formation reaction (1): energy ∆ r E, zero point vibration energy ∆ r ε, enthalpies ∆ r H˚(0) and ∆ r H˚(298), Gibbs free energy ∆ r G˚(298) and entropy ∆ r S˚(298) computed at different basis sets are given in Table 3.
As is seen, the extension of the basis set The values of enthalpies ∆ r H˚ are positive that implies the reaction (1) is endothermic.The entropy ∆ r S˚(298) is negative hence depicting an increase of the order of the system.The Gibbs free energy ∆ r G˚(298) being positive shows non-spontaneity of the complex formation through direct combination of the dye molecules.

Vibrational Spectra of the Species
The IR spectra of the individual dyes and complex molecule computed at B3LYP5/6-31G(d,p) are shown in Figures 2(a)-(c).The absence of imaginary frequencies proves that the geometrical structures of the species correspond to minima on the potential energy surfaces.

Electronic Spectra of the Species
The electronic absorption spectra for lawsone, indoline D131 and the designed complex are computed both for vacuum and chloroform solution at the TD-DFT B3LYP5/6-31G(d,p); the PCM is used to account the solvent effect.
The vertical excitation energies, wavelengths, oscillator strengths and electronic configurations of the transitions are listed in Table 4.The singlet-singlet excitations with nonzero oscillator strengths (f > 0.001) are taken into account.
For the lawsone molecule the most probable transitions in vacuum are to the fifth and eighth excited states and attributed to H-4 → L and H-1 → L + 1, while in chloroform solution to the fifth and seventh excited states assigned to H-3 → L and H → L + 1 transitions.For the indoline both in vacuum and solution the highest oscillator strength is observed for the electron transition H → L, to the first excited state.For the complex molecule the most probable transition occurs to the third excited state and assigned to H → L + 1 transition, both for vacuum and solution.Worth to note that in spectra of lawsone the band of the maximum absorption wavelength λ max is in the ultraviolet region while for the D131 and designed complex molecule the values of λ max correspond to visible region.
Moreover for the complex, the additional peak is appeared at 655 (vacuum) and 814 nm (chloroform) but of low probability.From vacuum to chloroform solution, the red shift of λ max is observed for all three species that is mostly common effect mentioned elsewhere e.g.[28] [39] [55].
The theoretical electronic absorption spectra are compared to experimental UV-Vis spectra of dyes in chloroform solution in Figures 3-5.
The theoretical UV-Vis spectra of lawsone (Figure 3

Analysis of Frontier Molecular Orbitals and Energy Levels Alignment
Analysis of molecular orbitals can provide useful information on electronic structure of molecules and mechanisms of electron transitions and chemical reactions [57] [58] [59] [60].However for understanding the mechanism of electron transitions the probabilities of the transitions must be taken into account, therefore the analysis of the whole electronic spectrum is required.
The frontier and adjacent molecular orbitals which mostly participate in the transitions are selected according to the data listed in Table 4 and shown in For the lawsone molecule, the shape of orbitals depicts that H-3 and HOMO are delocalized over both rings, benzenoid and quinoid, whereas LUMO and L + 1 are centered mainly on the quinoid ring.This implies that π → π* transitions involve electron localization to the quinoid ring of lawsone.
In the D131, the shape of the HOMO indicates that the electron density is delocalized on the extended donor unit, while the LUMO is localized on the acceptor groups (cyanoacrylic acid).The HOMO → LUMO transition contributes mostly to the first excited state transition (Table 4).For the H-1 MO, electron density is delocalized over the entire molecule except the cyclopentane group.
For the designed complex molecule, the HOMO has the electron density only on the indoline part, mainly on the benzene rings, indole unit and cynoacrylic group adjacent to the linkage with lawsone.In the LUMO, the electron density is located mostly on the lawsone part with a small contribution from the linkage.
In the L+1 MO, the electrons are concentrated on the indole unit and cynoacrylic part, while in both end parts and cyclopentane no any electron density is ob-   5.As is seen the energy gaps for each dye are systematically bigger compared to respective excitation energies; the maximum difference between E g and E ex is observed for lawsone dye, 0.60 eV and 0.74 eV for vacuum and chloroform solution, respectively.
Thus the ESOPs are lower compared to respective unoccupied orbitals.
The energy levels of the relevant occupied MOs and ESOPs of the molecules under the study are shown in Figure 9; the results are compared with the valence and conduction bands of semiconductors TiO 2 [63] [64] [65], ZnO [6] [65] and redox potential level of the electrolyte [66].According to photosensitizer requirements, HOMO and LUMO levels of a dye must match conduction band (CB) of semiconductor and redox potential of electrolyte; the HOMO level of the  sensitizer has to be lower than the redox potential for efficient dye regeneration, while the LUMO must be slightly higher than the CB edge so that the electron injection to semiconductor is energetically favorable [67].
In our case instead of LUMO we consider the ESOP energy levels (Figure 9).The HOMO and H-1 levels of lawsone are far below the redox potential of the electrolyte, whereas the ESOPs are above the CB edge of TiO 2 but only ESOP = −1.6 eV is above the CB of ZnO.The HOMO and H-1 levels of the indoline and designed molecule are below the electrolyte redox potential, while the ESOPs are above the CB edges of both semiconductors.Therefore regarding the proper energy levels alignment, the designed complex, lawsone and indoline satisfy the requirement to be used as a photosensitizer for TiO 2 or ZnO semiconductors.The designed complex is predicted to be better sensitizer as it possesses smaller excitation energies (3.20, 1.52 and 2.54 eV) compared to the indoline (3.28 and 2.60 eV) and lawsone (4.38, 3.27 and 5.39 eV).Furthermore, the transition from HOMO to the first excited state favours sensitization of TiO 2 as the respective ESOP = −3.63eV is close to the CB edge (−4.0 eV).The transitions from H-1 and from HOMO to the second excited state are of high probability to be involved in sensitizing the ZnO semiconductor, as the two corresponding ESOP levels (−2.69 and −2.61 eV) are slightly above the CB of ZnO (−2.96 eV).

Conclusion
The complex molecule was theoretically designed via combination of natural lawsone dye and synthetic metal free indoline D131 dye.The equilibrium geometry structures, electronic and IR spectra of the individual dyes and complex molecule were computed using DFT and TD-DFT methods; also the UV-Vis of the vibrational frequencies of the products and reactants, respectively.The Gibbs free energy( ) r G T ∆ °of the reaction was obtained by the following equations: , of the participants of the reaction have been calculated with OpenThermo software[45].

1 .
370 Å and ∠C1-O2-C37 = 117.0˚;which are comparable with those of the ester C-O-C linkage, 1.43 Å[52] [53] and 119˚[54].The most noticeable change is observed in the vicinity of the new chemical bond: +0.026 Å (elongation) for C37-O2 bond in the lawsone moiety and −0.022 Å (shortening) for C1=O1 in the indoline part.The biggest change is seen in the bond angles, +3.6˚ (O2-C37-C36) and −4.7˚ (O2-C37-C38) in the lawsone part, and in the dihedral angles, 3.9˚ (C17-C16-C6-C9) and +4.2˚ (C10-C11-C1-O1) in the indoline part.The difference in other respective parameters is not significant; it does not exceed 0.009 Å for bond lengths and 1.5˚ for bond angles.The parameters of the far parts of the complex molecule remain almost the same as in the individual molecules.
from 3-21G to 6-31G results in the decrease of the value of ∆ r E by 15.3 kJ•mol −1 but the polarization functions addition does not influence much on this value.The values of enthalpy and Gibbs free energy follow the trend of the energy ∆ r E as the ZPVE values are not much sensitive to the basis set change.
(a) and Figure 3(b)) depict a red shift of maximum wavelength from 279 (vacuum) to 283 nm (chloroform).The experimental UV-Vis spectrum of lawsone in chloroform solution shows the highest absorbance band in UV region with absorption peak at 292 nm and lower absorbance band at 338 nm.Thus both theoretical and experimental results are in accordance and indicate maximal peaks in UV region with slight differences in the peaks position.In the theoretical UV-Vis spectrum of the indoline D131 dye, the peak of maximum wavelength and highest intensity is observed at 447 (vacuum) and 476 nm (chloroform) hence the red shift is seen from vacuum to solution.The experimental spectrum of the indoline solution (Figure4(c)) has broad bands with maxima located at 294, 334, 354 and 450, 488 nm.Thus the experimental results are comparable with the theoretical and also show similarity with data reported in[51] [56].In the theoretical UV-Vis spectrum of the designed complex molecule (Figure5(a) and Figure5(b)), the highest peak is at 450 (vacuum) and 488 nm (chloroform), thus the red shift is also seen.In the experimental spectrum of the mixture (Figure5(c)), the absorption bands lie in the UV-Vis region with maximum absorption peak at 500 nm.Two peaks in visible region (450 and 500 nm) apparently correspond to those in spectrum of the individual indoline D131 dye (450 and 488 nm) while other peaks correspond to both individual dyes.Generally despite the slight dissimilarities, both the peak intensities and wavelengths in the spectrum of the mixture refer to those of the individual dyes.

Figure 5 .
Figure 5. UV-Vis spectrum: (a) of complex molecule computed for vacuum; (b) of complex molecule computed for chloroform solution; (c) measured experimentally for the mixture of lawsone and indoline in chloroform solution.

Figure 9 .
Figure 9. Energy level diagram of the relevant occupied MOs, transition energies and ESOPs of lawsone, indoline D131 and designed complex molecule computed for chloroform solution.The conduction and valence bands of TiO 2 [63] [64] [65] and ZnO [6] [65] semiconductors and redox potential of the electrolyte 3 I I − − [66] are given for compari-

Table 4 .
The excitation energies E ex , absorption wavelengths λ, oscillator strengths f and electronic transition configurations of the individual dyes and designed complex molecule C 45 H 32 N 2 O 4 computed at the TD-DFT B3LYP5/6-31G(d,p).

Table 5 .
The energies of molecular orbitals ε(MO), excitation energies E ex , energy gaps E g , and excited state oxidation potentials ESOPs; all values in eV.