Theoretical Studies of Electronic Structure and Photophysical Properties of a Series of Indoline Dyes with Triphenylamine Ligand

To design efficient organic sensitizer, a series of D-π-A indoline dyes with different donor parts have been investigated by density functional theory (DFT) and time-dependentDFT (TD-DFT) approach.Themolecular geometries, frontiermolecular orbitals, and absorption spectra of these dyes have been systematically investigated to provide comprehensive understanding of the structureproperty relationships. Compared with D149, our designed dyes have proper HOMO and LUMO energy level, narrowed HOMOLUMO energy gap, and broadened absorption band by introducing the N(CH 3 ) 2 and N(phenyl) 2 groups at the donor part. Furthermore, the dimeric dyes and dye-(TiO 2 ) 6 systems have been optimized by DFT method to simulate the intermolecular interactions, as well as interaction between the dyes dimmers and semiconductor interface, respectively. Through the analyses of absorption energies (Eads), energy levels of the HOMO and LUMO, light harvesting efficiency (LHE), and the driving force of electrons injections (ΔGinject), it is found that the designed dyes should have improved optical properties by importing the N(CH3)2 group. This work is hoped to provide a theoretical guiding role in design of new dyes for dye-sensitized solar cells.


Introduction
Due to increasing energy demands and concerning global warming, scientists have paid tremendous effort on developing cheap and easily accessible renewable energy sources.Dye-sensitized solar cells (DSSCs) have been regarded as one of the most promising next-generation photovoltaic cells due to their potentially low fabrication costs, easy production, and flexibility compared to traditional silicon-based solar cells [1].A typical DSSC is constructed with the dye sensitizer, dye-absorbed nanocrystalline semiconductor oxide (such as TiO 2 or ZnO), electrolyte containing I − /I 3 − redox couples, and closed circuit.Obviously, the sensitizer plays a significant role for improving the efficiency of DSSCs.Therefore, considerable synthetic works have been carried out on Ru-based sensitizers [2][3][4] and metal-free organic dye sensitizers [5][6][7][8][9][10].Among them, indoline dye sensitizers [11] have attracted much attention because of their high molar extinction coefficient, relatively less expensive, and low pollution of environmental.Although the significant progress has been made in the organic dyes, the conversion efficiency of indoline type dyes is still lower than the Ru-based sensitizer [12].Therefore, the investigation on the organic dye is very important for realizing large-scale commercial production.
Except the massive synthesized research, theoretical studies have been accepted as extremely powerful and comparably low-cost tools for revealing the effect of chemical modifications on the optical properties [13][14][15][16][17][18].In spite of outstanding electronic and optical properties of triphenylamine derivatives in solar cell [19][20][21][22], to our knowledge, there is no detailed study on indoline-containing materials coupled with triphenylamine as donor part.Consequently, in order to reveal the role of triphenylamine on the photophysical properties and develop more efficient dyes, the donor part of D149 dye is replaced by triphenylamine unit.Then, triphenylamine-containing indoline dyes are designed by introducing the N(CH 3 ) 2 , N(phenyl) 2 , and CF 3 at the donor part of D149 in this work (see Figure 1).A systematic comparison and analysis of the geometries, frontier molecular orbitals, and absorption spectra of these dyes are carried out.Furthermore, the light harvesting efficiency (LHE) and the driving force of electrons injections (Δ inject ) are calculated to estimate the short-circuit photocurrent density.In addition, the electron coupling of dimeric dyes and dye-(TiO 2 ) 6 systems is simulated by DFT method to testify the electron injection efficiency.This work is expected to reveal the effect of chemical modification on the DSSCs efficiency and provide valuable guidance for developing efficiency dyes.

Computational Details
All calculations were performed with the Gaussian 09 program package [23].The density functional theory DFT (B3LYP) approach had been successfully used to calculate the ground geometries in indoline dye sensitizers [13][14][15]24].Therefore, in this work, the geometries structures of monomer and dimer dyes were fully optimized at the DFT level of B3LYP method [25,26] using the 6-31G(d) basis set.Following the optimized geometry, the vibrational frequencies were calculated and the results confirmed that all the optimized monomer geometries were stable geometric structures.The absorption spectra of these dyes were investigated by TDDFT with the B3LYP approach associated with the polarized continuum model (PCM) [27,28] in methanol media.
To investigate the electronic coupling interaction between these dyes and TiO 2 nanoparticles, the absorption geometries of dyes/(TiO 2 ) 6 clusters were calculated by B3LYP method.The 6-31G(d) basis sets were used for C, H, O, N, S, and F atoms and LANL2DZ for Ti atom.Although (TiO 2 ) 6 cluster is a simplistic model, it was believed to qualitatively reproduce the trend of the charge distribution [29].The absorption energies ( ads ) of dyes/(TiO 2 ) 6 were calculated by the following equation: The  dye ,  (TiO 2 ) 6 , and  [dye/(TiO 2 ) 6 ] were the total energies of dyes, (TiO 2 ) 6 cluster, and dye/(TiO 2 ) 6 , respectively.
The short-circuit current density ( sc ) which is closely connected to the energy conversion efficiency () of the solar cell [30] can be determined by the following equation [31]: where LHE() is the light-harvesting efficiency at a given wavelength [31], Φ inject is the electron injection efficiency to the semiconductor substrate, and  collect is the electron collection efficiency. collect can be assumed as a constant in the same DSSCs with sole different dyes [32].As a result, the enhancement of  sc should focus on improving the LHE() and Φ inject .The Φ inject is related to the driving force Δ inject for the electron injection.According to Preat's method [33], the Δ inject of electron injections from the excited states of dyes to semiconductor surface are calculated.The light-harvesting efficiency LHE() can be calculated with where  is the oscillator strength of the maximum absorption spectra of dyes.

The Molecular Geometries of Dyes
The sketch map and the optimized geometries of D149 and D1-D4 are shown in Figures 1 and 2, respectively.

Frontier Molecular Orbital Analysis.
The energy levels of the HOMO and LUMO, as well as the HOMO-LUMO gaps, are drawn in Figure 3, together with the energy levels of TiO 2 semiconductor conduction band (−4.00 eV) [34] and the redox potential of I − /I 3 − (−4.80 eV) [35].This means that the studied dyes ensure efficient dye regeneration.In addition, the LUMO energy of D1-D4 is higher than the conduction band edge of TiO 2 , which would provide enough driving force for electron injection for ultrafast excited-state electron injection [36].Thus, it is a good approach to enhance the ability of dye regeneration by introducing the N(CH 3 ) 2 and N(phenyl) 2 substitutions at the donor part, especially for CF 3 group.With the dramatically stabilized HOMO energy, the HOMO-LUMO energy gap of these dyes increases in the following order: D1 (2.22 eV) < D2 (2.28 eV) < D3 (2.34 eV) < D4 (2.66 eV).The energy gaps of our designed dyes D1-D3 are much smaller than the famous D149.As reported by previous work [37,38], the sensitizers that have smaller energy gap values may show higher efficiency in the DSSCs.Thus, it can be inferred that our designed molecules may have improved light harvesting ability.
As drawn in Figure 3, the electron densities of the HOMO of D149 is mainly located on the donor part and indoline ring; the electron densities of its LUMO are mainly distributed around the  group.It suggests that the HOMO to LUMO transition is a good charge separated state.The electron distributions of the HOMOs and LUMOs for D1-D4 have the similar features.Both the HOMOs and LUMOs of D1-D4 possess  and  * orbital delocalization.Therefore, it can be confirmed that the intramolecular charge transfer (ICT) may occur when electron excitation occurs from the HOMO to LUMO in these dyes.

The Absorption Spectra.
On the basis of the optimized ground state geometries, the absorption spectra of dyes are calculated by TDDFT/B3LYP method with PCM in methanol media.The simulated electronic absorption spectra are depicted in Figure 4, and the corresponding absorption data are collected in Table 1.
Figure 4 shows the absorption spectrum of D1-D3 displaying three major absorption bands in their UV/Vis spectra, the lowest energy absorption band (in the region of 600.0-800.0nm), lower energy absorption band (in the region of 450.0 to 600.0 nm), and higher energy absorption band (in the region of 300.0 to 450.0 nm).Compared with D1-D3, the absorption spectra of D4 are different in the band locations and intensity, due to importing the electronwithdrawing CF 3 groups.From Figure 4 and Table 1, the lowest lying distinguishable absorption speak ( 1 ) of these dyes exhibits dramatically blue shift tendency: D1 (735.1 nm) > D2 (693.5 nm) > D3 (661.5 nm) > D4 (600.2 nm) > D149 (556.1 nm), which is closely related to the increased HOMO-LUMO energy gaps of these compounds (see Figure 3).Based on the analysis of electron density of HOMO and LUMO (see Section 3.2), all of the absorption behaviors of the  1 can be ascribed to the intramolecular charge transfer (ICT) character.The maximum absorption speak of D1 and D2 is located at 517.5 and 518.7 nm, respectively, which is contributed by HOMO-1 → LUMO transition.The HOMO-1 of D1 and D2 is contributed by 23.1% (D) + 76.8% () and 53.4% (D) + 42.6% (), respectively.Thus it is reasonable to assign the 517.5 and 518.7 nm for D1 and D2 to ICT character.For D3, the absorption speak with the largest oscillator strength is located at 515.7 nm, which is contributed by HOMO-2 → LUMO transition.The HOMO-2 of D3 is focused on 99.9% (D); therefore, the 515.7 nm of D3 still displays ICT character.With respect to D4, the absorption at 491.5 (attributed by HOMO-1 → LUMO) and 371.3 nm (attributed by HOMO → LUMO+4) are assigned to ICT character.The high-energy absorption speak for D1-D4 is located at 398.From the above calculations, it can be seen that all the absorption speaks of D1-D4 are covering the entire visible region, which makes these dyes suitable for the application of DSSCs.The location and intensity of the absorption band of these dyes are greatly affected by introducing the N(CH 3 ) 2 , N(phenyl) 2 , and CF 3 groups, which means the photophysical properties can be well tuned by proper substitutions.All the intense absorption bands of D1-D4 correspond to the ICT transition, which will play an important role in conversion efficiencies.

Photoelectric Conversion Efficiency of Dyes.
Rapid electron injection from the photoinduced excited states of dyes to the TiO 2 is desired for high performance of DSSCs.
According to Preat's method [33], the driving force Δ inject of electrons injections from the excited states of dyes to semiconductor surface is calculated and the results are drawn in Figure 5.
Figure 5 shows that all the Δ inject values are very negative (in the region of −1.55-−1.86eV), which suggests that the dye excited state lies above the TiO 2 conduction band edge.Therefore, it is favorable for electron injection from the excited state dye to the TiO 2 conduction band edge.Compared with D149 (−1.08 eV), D1-D4 have much more negative Δ inject values.As expected, the electron injection ability has been improved by replacing the donor part of D149 by triphenylamine.Furthermore, the Δ inject values of D2 and D3 are −1.74eV and 1.66 eV, respectively, which are much more positive than D1 (−1.86 eV), indicating that the introduction of diphenylamine at the end of triphenylamine unit results in destabilization of the oxidized dye and therefore reduces the electron injection.In addition, Δ inject value of D4 (−1.55 eV) is positive than that of D3, indicating it would not be favorable to enhance the electron injection by importing the CF 3 group at the end of phenylamine.According to (3) in Section 2, the LHE() of studied complexes are calculated and the results are also drawn in Figure 5.As depicted in Figure 5, the D1 has larger LHE() than D2-D4, which means that D1 has more efficient light harvesting capability than others.As discussed in Section 2, the large values of Δ inject and LHE() are possible approaches to maximize the short-circuit ( sc ).Consequently, it is a promising approach to enhance the efficiency by introducing the N(CH 3 ) 2 group at the donor part.

Structures and Properties of Dye Dimers.
The intermolecular interactions of these dyes dimer are investigated by B3LYP method to predict the stabilization upon dye aggregation.Figure 6 illustrates the optimized geometries of D1 dimer.Further survey of the HOMO and LUMO orbitals of D1 dimer is also collected in Figure 6.The properties of D2-D3 dimers are given in Figure S1 in the supporting information (see Supplementary Material available online at http://dx.doi.org/10.1155/2015/605728).As shown in Figure 6 and Figure S1, all these stable molecules have formed intermolecular H⋅ ⋅ ⋅ O hydrogen bonds.Take D1 dimer as an example, the carboxyl and terminal carbonyl group of thiazolidine form three H⋅ ⋅ ⋅ O hydrogen bonds.One H⋅ ⋅ ⋅ O hydrogen bond is 2.568 Å, which is formed by the carboxyl and one terminal H atoms in the ethyl.Another two H⋅ ⋅ ⋅ O hydrogen bonds are 2.623 and 2.448 Å, which are formed by the two O atoms in the carbonyl group of thiazolidines interact with the two H atoms.In addition, the HOMOs of dimers are mainly located on one monomer, while their LUMO are contributed by another monomeric unit.Clearly, the intermolecular charge transfer process between the dyes dimers may occur for all the dyes.Compared with the isolated dye monomer (see Figure 3), the HOMO energy of D1 dimer slightly increases by 0.06 eV, while the LUMO energy decreases by 0.12 eV, respectively.Because of the dramatically decreased LUMO energy, the HOMO-LUMO energy gap of D1 dimer becomes narrowed 0.18 eV compared with monomer.Similarly, the HOMO-LUMO energy gap of D2 dimer and D3 dimer decreases by 0.14 eV and 0.18 eV, respectively, compared with corresponding monomer.Normally, the narrowed HOMO-LUMO energy gap favors red shifted of absorption spectra.
As reported by Pastore and De Angelis [13], the red-shift of absorption spectra for indoline dye are due to the dye aggregation.Therefore dye aggregation is expected to be associated with the HOMO-LUMO energy gap of dye dimer.Since there are small differences of HOMO-LUMO energy gap of these dimers compared with corresponding monomer, introducing the N(phenyl) 2 substitutions at the donor may play a slightl effect on avoiding unfavorable dye aggregation.
3.5.Dye-(TiO 2 ) 6 Complexes.For a better understanding of the effect of the triphenylamine-based donor on the electron coupling, these dyes bound to (TiO 2 ) 6 cluster are optimized by B3LYP method.As well accepted, there are two common absorption models for dyes absorbed on the surface [39,40].One is a single Ti-O monodentate binding mode; another is a bidentate chelating structure mode.In this work, we take D4 as an example to testify which mode is stable absorption configuration.The optimized geometries of D4-(TiO 2 ) 6 in different models are drawn in Figure S2.It can be seen that the total energy of bidentate-chelating configuration is lower 3.75 kcal/mol than single monodentate mode.This agrees with the previous work; that is, the bidentate chelating structure mode is usually predicted to be stable [13,[41][42][43].Therefore, the D149 and D1-D3 absorbed on the surface are optimized under the bidentate chelating structure mode.The optimized geometry, absorption energies ( ads ), and the contour plot of the HOMO and LUMO of dye-(TiO 2 ) 6 complexes are depicted in Figure 7.The energy level of frontier molecule orbital (eV) of the dye-(TiO 2 ) 6 and the energies differences between the dye and dye-(TiO 2 ) 6 are drawn in Figure 8.
As shown in Figure 7, the decreased order of  abs for D149 and D1-D4 is D149 (23.68 kcal/mol) > D1 (20.81 kcal/mol) > D2 (15.66 kcal/mol) > D3 (10.32 kcal/mol) > D4 (2.85 kcal/ mol), indicating the interactions between the dye and TiO 2 surface is decreasing in the same order.In particular, for D4, the  abs is so small that D4 is difficult to be chemisorbed on the cluster.After absorption to the (TiO 2 ) 6 cluster, the calculated distances of D1 between the carboxylic oxygen atoms and the (TiO  have decreased the electron coupling between the dyes and surface, which is consistent with the analyses of  ads .Molecular orbital spatial distribution shows that the HOMO of dye-(TiO 2 ) 6 is mainly localized over the donor part of dyes, while the LUMO is focused on the (TiO 2 ) 6 part.As expected, the interfacial electron injection from the dye to the TiO 2 surface may successively transfer when the dyes bind on the TiO 2 film.Furthermore, compared with the isolated sensitizer, the HOMOs have subtle changes (about 0.1 eV) when the D149 and D1-D4 absorb to (TiO 2 ) 6 cluster.The most noticeable difference between the isolated dyes and dye-(TiO 2 ) 6 is focused on the LUMO energy.The LUMO energy of (D1-D4)-(TiO 2 ) 6 dramatically decreases compared with D149-(TiO 2 ) 6 , suggesting that the LUMOs of all designed dyes create relative strong electronic coupling with TiO 2 film and, as a result, further boost the electron injection.In addition, the LUMO of D1-(TiO 2 ) 6 is lower 1.45 eV than that of D1.While the lower-lying energy LUMO

Conclusions
The electronic structures, absorption spectra, and photophysical properties of triphenylamine-based indoline dyes as photosensitizers for application in DSSCs were investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT).The calculated results demonstrated that the donor parts had great effect on the structure and optical properties.Compared with D149, D1-D3 had proper HOMO and LUMO energy level as well as narrow HOMO-LUMO energy gap by introducing the N(CH 3 ) 2 and N(phenyl) 2 groups at the donor part; therefore D1-D3 may be good DSSC sensitizers.The absorption speaks of D1-D4 covered the entire visible region and the intense absorption bands of them all corresponded to the intramolecular charge transfer transition, which may play an important role in conversion efficiencies.The stable molecule of D1-D3 dimmers were formed by intermolecular H⋅ ⋅ ⋅ O hydrogen bonds.The data of  abs and the LUMO energy level of dye-(TiO 2 ) 6 suggested that D1 had the strongest interactions between the dye and TiO 2 surface of these dyes.Furthermore, D1 had larger Δ inject and LHE() values than D2-D4, indicating that D1 may have improved energy conversion efficiency by introducing the N(CH 3 ) 2 group at the donor part of triphenylamine.We hope that this work will be helpful for designing the organic dyes with high efficiency.

Figure 1 :
Figure 1: Sketch map of the studied dyes.

Figure 2 :
Figure 2: The optimized geometry of the studied dyes, together with the important bond length ( Å) and dihedral angle ( ∘ ).

Figure 3 :
Figure 3: The energy levels and contour plot of HOMO and LUMO, as well as HOMO-LUMO energy gap (eV) of D149 and D1-D4.The redox potential of I − /I 3 − (−4.80 eV) and energy level of conduction band (CB) of the TiO 2 surface (−4.00 eV) are drawn in red line.

Figure 6 :
Figure 6: (a) The optimized ground geometry of D1 dimer and (b) its HOMO and LUMO orbital.The H⋅ ⋅ ⋅ O bond length ( Å) is labeled in the optimized geometry.