An Experimental and Theoretical Study of Dye Properties of Thiophenyl Derivatives of 2-Hydroxy-1,4-naphthoquinone (Lawsone)

A prospective study of the dye properties of non-toxic lawsone thiophenyl derivatives, obtained using a green synthetic methodology allowed for the description of their bathochromic shifts in comparison to those of lawsone, a well-known natural pigment used as a colorant that recently also has aroused interest in dye-sensitized solar cells (DSSCs). These compounds exhibited colors close to red, with absorption bands in visible and UV wavelength range. The colorimetric study showed that these compounds exhibited a darker color than that of lawsone within a range of colors depending on the substituent in the phenyl ring. Computational calculations employing Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT), showed that the derivatives have lower excitation energies than lawsone, while the alignment of their frontier orbitals regarding the conduction bands of TiO2 and ZnO and the redox potential of the electrolyte I−/I3− suggests that they could be employed as sensitizers. The study of the interactions of the lawsone and a derivative with a TiO2 surface model by different anchoring modes, showed that the adsorption is thermodynamically favored. Natural bond orbital (NBO) analysis indicates a two-center bonding (BD) O-Ti as the main interaction of the dyes with TiO2.


Introduction
Since the components of the quinone/hydroquinone redox couple are easily interconvertible and closely balanced energetically [1,2], they serve as important links in electron transport processes during metabolic pathways and many biological reactions [3], also explaining their use in industrial applications [4][5][6][7]. Among naturally occurring quinones, lawsone (2-hydroxy-1,4-naphthoquinone), a red-orange pigment, present in henna, has long been used for the coloration of the skin, hair, and textile materials [8]. Important reasons for the use of lawsone as a dye are the ease of obtaining it and the absence of environmental problems related to its use. It has been demonstrated to not be mutagenic in catalase mutant Escherichia coli, both in the absence and presence of metabolic activation [9]. On the other hand, it has been suggested that its low cytotoxicity is probably mediated, at least in part, by the release of reactive oxygen species [9]. However, extensive research in the field of synthetic dyestuffs has led to a drastic decline in the use of natural dyes because of their limited range of colors among other disadvantages. Despite the advantages that synthetic compounds offer in this area, several synthetic colorants are ultimately environmentally deleterious. On the other hand, besides its use as a textile colorant, lawsone has aroused a recent interest in the fields of protein crystallography [10] and dye-sensitized solar cells Table 1. Compounds studied in this work [26].

Colorimetric Study
The CIE Lab system was used to evaluate the color parameters and the color differences of the thiophenyl derivatives of lawsone; the results are shown in Table 2. According

Colorimetric Study
The CIE Lab system was used to evaluate the color parameters and the color differences of the thiophenyl derivatives of lawsone; the results are shown in Table 2. According to a* and b* values, their colors were present in the yellow-red quadrant. These compounds showed L* values < 40, indicating that they have darker colors than those of lawsone (1) (L* > 60). The color differences (∆E) were significant at 51.8-71.2 ( Figure 1) due to the incorporation of the differently substituted thiophenyl moieties in lawsone structure 1. The bromo derivatives, such as 5 and 6, showed the highest ∆E with values of 71.2 and 70.4, respectively, while fluoro derivatives 7 and 9 showed the lowest values of 65.6 and 67.4, respectively. The L* value of 1 was compared with the published value of the henna dye solution [27].  Figure 1. Three-dimensional chromaticity diagram of the different thio-lawsone derivatives.  Table 3 resume experimental UV/Vis results. Figure 2 shows the absorption bands from 200 to 700 nm for lawsone and its thiophenyl derivatives 2 and 3 (spectra for all compounds are presented in supporting information). Lawsone dyes exhibited peaks at range 208-334 nm. The band at 334 nm is attributed to C=O and OH groups [33], and its long tail in the visible region is responsible for the yellow-orange color of lawsone [34].

UV/Vis Spectroscopy
UV/Vis spectra of lawsone derivatives showed bands at range 205-478 nm. The latter indicates a bathochromic shift from 334 nm (lawsone) to values over 450 nm. This displacement to the visible region corresponds with a color close to red for all derivatives. Lawsone analogs, such as the natural anthraquinones emodin and dermocybin, exhibit differences in color; i.e., emodin dyed polyamide is brownish-orange, while dyed with dermocybin is wine red, but on polyester, the colors are totally different: bright yellow and bright reddish-orange, respectively [28]. Similar results were obtained for different styryl and azo-lawsone dyes of synthetic origin, exhibiting brown (CIELab: 25.057, 13.577, and 17.516) and orange (CIELab: 63.554, 36.699, and 67.159) colors on polyester, respectively, while on nylon, both exhibited darker colors (L* < 40) in the yellow-red quadrant, which agree with our results [29,30]. Moreover, polymers based on lawsone have been obtained with color differences. For example, Mahkam et al. (2014) synthesized a series of polymers derived from lawsone with acryloyl chloride, methacrylic acid, acrylic acid, and acrylonitrile to obtain different polymers using free-radical copolymerization. The resulting polymers presented colors in the yellow-red quadrant [31] in a similar manner to the thiophenyl lawsone derivatives reported here.
Generally, the color of dyes depends on the nature of their coupled components [32]; consequently, this series of lawsone derivatives shows that it is possible to obtain a wide gamma of colors by replacing the substituent in the phenyl ring of this kind of lawsone thiophenyl derivative, thereby suggesting potential industrial uses.  Table 3 resume experimental UV/Vis results. Figure 2 shows the absorption bands from 200 to 700 nm for lawsone and its thiophenyl derivatives 2 and 3 (spectra for all compounds are presented in supporting information). Lawsone dyes exhibited peaks at range 208-334 nm. The band at 334 nm is attributed to C=O and OH groups [33], and its long tail in the visible region is responsible for the yellow-orange color of lawsone [34].   208  270  222  213  212  207  208  208  205  243  459  268  267  266  264  251  210  265  249  273  465  467  478  280  252  277  279  325  463  279  473  334 474 465

Electronic and Spectroscopic Study
To gain insight into the molecular structure and the effect of modifications on the electron distribution and energy levels of lawsone derivatives, we carried out DFT and TD-DFT calculations. To evaluate different functionals and their suitability to this kind of calculation, we chose three DFT functionals, B3LYP, M06-2X, and HSE2PBE, in addition to the 6-311+G(d,p) basis set to perform the calculations. To evaluate the accuracy of the employed functionals, we compared the HOMO energy of lawsone obtained experimentally [35] with values obtained using these functionals. The calculated EHOMO was −7.50 eV for HSE2PBE, −8.80 eV for M06-2x, and −7.36 eV for B3LYP. B3LYP gave a value closer to the experimentally reported value (−5.21 eV) than the other functionals did. Therefore, we continue our study with the results obtained when using this function.
The optimized geometries, as well as HOMO and LUMO distribution for lawsone and its thio derivatives, are presented in Figure 3.  UV/Vis spectra of lawsone derivatives showed bands at range 205-478 nm. The latter indicates a bathochromic shift from 334 nm (lawsone) to values over 450 nm. This displacement to the visible region corresponds with a color close to red for all derivatives.

Electronic and Spectroscopic Study
To gain insight into the molecular structure and the effect of modifications on the electron distribution and energy levels of lawsone derivatives, we carried out DFT and TD-DFT calculations. To evaluate different functionals and their suitability to this kind of calculation, we chose three DFT functionals, B3LYP, M06-2X, and HSE2PBE, in addition to the 6-311+G(d,p) basis set to perform the calculations. To evaluate the accuracy of the employed functionals, we compared the HOMO energy of lawsone obtained experimentally [35] with values obtained using these functionals. The calculated EHOMO was −7.50 eV for HSE2PBE, −8.80 eV for M06-2x, and −7.36 eV for B3LYP. B3LYP gave a value closer to the experimentally reported value (−5.21 eV) than the other functionals did. Therefore, we continue our study with the results obtained when using this function.
The optimized geometries, as well as HOMO and LUMO distribution for lawsone and its thio derivatives, are presented in Figure 3. All derivatives present a CSC angle at about 103°, and dihedral CCSC at about 50°, which leads to the formation of the aromatic ring of thiophenyl moiety, and the quinonic ring does not become coplanar. This structural feature can lead to a decline in the charge transfer between both sides of the molecule and also generates an anti-aggregation effect [36]. It has been widely studied that aggregation of dyes that can interact by π-π interactions tend to negatively affect their performance in DSSCs [37]. Aggregation of dyes molecules in a face-to-face alignment, as occur in p-p interactions, lead to hypsochromic absorption shift (H-aggregate) [37]. The latter avoids that dye can absorb in the longer-wavelength region of the visible and near-infra-red (NIR) spectrum, which are a desirable part of the spectrum. It has been experimentally tested that for organic dyes, aggregation, when they are on TiO2 surface leads to quenching of the excited states of the dyes, resulting in low efficiency of electron injection [38,39]. Besides, aggregation favor charge recombination between the injected electrons in the semiconductor and the oxidized redox mediator in the electrolyte resulting in a reduced Voc value [39][40][41].
In the case of lawsone, its coplanarity favors their potential aggregation by π-π interactions. Therefore, non-planar derivatives which prevent this kind of aggregation can lead to an improvement in their dye sensitizer performance [42].
Electronic distribution of frontier orbitals demonstrates that for all derivatives, the HOMO is localized mainly on the thiophenyl moiety and partially spread on the quinone ring. On the other hand, also for all derivatives, LUMO is fully localized on the quinone ring and sulfur atom with almost no presence on the aromatic ring of the thiophenyl moiety. These results show that overlap between HOMO and LUMO occurs in the lower extension for these derivatives when compared with that for lawsone. All derivatives present a CSC angle at about 103 • , and dihedral CCSC at about 50 • , which leads to the formation of the aromatic ring of thiophenyl moiety, and the quinonic ring does not become coplanar. This structural feature can lead to a decline in the charge transfer between both sides of the molecule and also generates an antiaggregation effect [36]. It has been widely studied that aggregation of dyes that can interact by π-π interactions tend to negatively affect their performance in DSSCs [37]. Aggregation of dyes molecules in a face-to-face alignment, as occur in p-p interactions, lead to hypsochromic absorption shift (H-aggregate) [37]. The latter avoids that dye can absorb in the longer-wavelength region of the visible and near-infra-red (NIR) spectrum, which are a desirable part of the spectrum. It has been experimentally tested that for organic dyes, aggregation, when they are on TiO 2 surface leads to quenching of the excited states of the dyes, resulting in low efficiency of electron injection [38,39]. Besides, aggregation favor charge recombination between the injected electrons in the semiconductor and the oxidized redox mediator in the electrolyte resulting in a reduced Voc value [39][40][41].
In the case of lawsone, its coplanarity favors their potential aggregation by π-π interactions. Therefore, non-planar derivatives which prevent this kind of aggregation can lead to an improvement in their dye sensitizer performance [42].
Electronic distribution of frontier orbitals demonstrates that for all derivatives, the HOMO is localized mainly on the thiophenyl moiety and partially spread on the quinone ring. On the other hand, also for all derivatives, LUMO is fully localized on the quinone ring and sulfur atom with almost no presence on the aromatic ring of the thiophenyl moiety. These results show that overlap between HOMO and LUMO occurs in the lower extension for these derivatives when compared with that for lawsone.
To understand the experimental UV/Vis results, TD-DFT calculations were carried out for all compounds. Table 4 presents the excitation energies (Eex), wavelength (λ), oscillator strengths (f), light-harvesting efficiency (LHE), and the corresponding electronic transition for selected relevant excited state (full data is in Table S1 of SI). As has been previously de-Materials 2021, 14, 5587 7 of 15 scribed for lawsone, the most probable transitions, in both vacuum and solution (methanol), fall in the UV region, and they occur from the ground state to excited states 4 and 5, which can be assigned mostly to the H-2→L and H-3→L MOs [14]. On the other hand, for all these derivatives, the most probable transitions, in both vacuum and solution (methanol), fall in both the visible and UV regions; occur from the ground state to excited states 1 and 8 (7 for Cpd 3), respectively; and can be assigned mostly to the H→L and H→L+1 MOs. The LHEs calculated for lawsone in vacuum are similar to those previously described [14], and they are considered rather low, as in the general case of natural dyes [43,44]. In methanol, we noted that LHE increased more than in dimethyl sulfoxide and dichloromethane according to s previous report [14]. LHEs for thiophenyl derivatives of lawsone decrease slightly, but they demonstrate absorption in the visible range, which benefits their potential performance in DSSCs. Comparisons among theoretical and experimental UV peaks are shown in Table S2 of the Supplementary Material. To study the alignment of energy levels, which define the overall conversion efficiency of DSSCs, we considered the alignment of the frontier Mos energy levels of the compounds with valence and a conduction band edge of the semiconductors TiO 2 and ZnO. Table 5 lists the MOs, excitation energies E ex , energy gaps E gap , and excited-state oxidation potentials (ESOPs). ESOPs have been proposed as appropriate for the representation of excited states, and they were calculated from occupied energy levels and relevant excitation energies [14,45,46]. E ex is the most important factor in the evaluation of the efficiency of DSSCs, as a sensitizer with a low E ex facilitates light absorption in the long-wavelength range [14]. We noted that thiophenol derivatives present lower E ex than lawsone, which indicates that they could be better sensitizers.
Energy diagrams of the most relevant MOs of the species in both the gas phase and solution are shown in Figure 4. E LUMO of a good photosensitizer dye should lie slightly above the conduction band (CB) edge of the semiconductor to ensure electron injection from the excited dye [47], while E HOMO should lie under the redox potential of the electrolyte for dye regeneration by accepting electrons from the electrolyte [23]. We used a TiO 2 (CB = −4.05 eV [48]) ZnO (CB = −4.45 [49]) semiconductor in addition to the electrolyte I − /I 3 − (−4.80 eV [50]) to evaluate our compounds. As displayed in Figure 4, all thiophenyl derivatives of lawsone meet the criteria of energy level alignment to be appropriate photosensitizers for TiO 2 and ZnO semiconductors. In the case of TiO 2 , the gap between CB and E LUMO is lower than that for ZnO, which suggests that these compounds perform well as sensitizers in DSSC with TiO 2 . Besides, E HOMO for all derivatives lie below the redox level of the I − /I 3 − electrolyte, allowing electron transfer to the dye from the electrolyte. (−4.80 eV [50]) to evaluate our compounds. As displayed in Figure 4, all thiophenyl derivatives of lawsone meet the criteria of energy level alignment to be appropriate photosensitizers for TiO2 and ZnO semiconductors. In the case of TiO2, the gap between CB and ELUMO is lower than that for ZnO, which suggests that these compounds perform well as sensitizers in DSSC with TiO2. Besides, EHOMO for all derivatives lie below the redox level of the I − /I3 − electrolyte, allowing electron transfer to the dye from the electrolyte.

Adsorption of Lawsone and 3-Thiophenyl-lawsone on a TiO2 Model
An essential requisite for an organic dye can effectively acts as a sensitizer in DSSCs, is the anchoring to the surface of the semiconductor. The latter can be achieved by incorporating different polar functional groups in the structure of sensitizer (as carboxylic acid, amines, ketones, among others). Prinzisky et al. studied the performance of a hydroxyanthraquinone imine as a dye for DSSCs [51], where the hydroxyl and imine groups acting anchoring the molecule to the TiO2 semiconductor surface. Recently, Sreeja and Pesala

Adsorption of Lawsone and 3-Thiophenyl-lawsone on a TiO 2 Model
An essential requisite for an organic dye can effectively acts as a sensitizer in DSSCs, is the anchoring to the surface of the semiconductor. The latter can be achieved by incorporating different polar functional groups in the structure of sensitizer (as carboxylic acid, amines, ketones, among others). Prinzisky et al. studied the performance of a hydroxyanthraquinone imine as a dye for DSSCs [51], where the hydroxyl and imine groups acting anchoring the molecule to the TiO 2 semiconductor surface. Recently, Sreeja and Pesala studied the performance of natural dyes indigo, lawsone, and betanin as co-sensitizer for solar cells [23]. They associated one carbonyl group of the lawsone as an anchor group to the surface of TiO 2 . However, lawsone possesses a hydroxyl group in the alpha position to one of the carbonyls of the quinone ring, which can additionally contribute to anchoring the molecule to the surface of the semiconductor. Considering this background, we studied the interactions between lawsone and 3-thiophenyl-lawsone with a TiO 2 surface model by DFT calculations. The adsorption of the dyes was simulated with a (TiO 2 ) 9 cluster, based on previous works of Sánchez de Armas et al. [52,53], who showed that a group of (TiO 2 ) 9 is large enough to reproduce adequately the electronic properties of a dye-TiO 2 system. There are different ways to adsorb a dye onto a TiO 2 surface, depending on the conditions used to prepare DSSC [37]. Considering the above, we explored three possible anchoring modes for both molecules, depending on the protonation state of the hydroxyl ( Figure 5). The first mode corresponds to a not dissociate state of hydroxyl (neutral I), the second mode corresponds to a dissociate state, where the hydroxyl proton is transferred to an oxygen of TiO 2 surface (neutral II), and the third mode corresponds to a dissociate state where the hydroxyl proton is transferred to another species with basic character, potentially present in the medium (anionic).

Complex
Lawsone To evaluate the anchoring of the studied dyes on the TiO2 surface quantitatively, we calculated the adsorption energies (Eads) for the complexes as follows [57,58]: where E correspond to the energy of complex, E to the energy of the isolated dye, and E to the energy of isolated (TiO2)9 cluster. Table 6 presents the values of Eads for each complex, as well as their HOMO, LUMO, and gap energies (Egap). All complexes presented negatives Eads, indicating that anchoring of these dyes is termodynamically favorable. Between the two neutral adsorption mode, the second (bidentate) was more favorable for both dyes. Comparing between lawsone and 3thiophenyl-lawsone, the latter presents a more negative value, indicating a stronger interaction with the TiO2 cluster. Anionic complexes present the most negative Eads for For the first mode (neutral I), we found that two interactions are responsible for the anchoring: an intermolecular hydrogen bond (IHB) between the -OH in the dye and an oxygen in the TiO 2 cluster, along with an interaction between the oxygen of a carbonyl group. The distance of O1-H1···O3 was 1.70 Å for both complexes. In the case of O2···Ti3 interaction, the distance was slightly shorter for the 3-thiophenyl-lawsone···(TiO 2 ) 9 complex compared to lawsone···(TiO 2 ) 9 (2.00 vs. 2.03 Å). For the second mode (neutral II), a bidentate interaction between both neighbor oxygen in the dye with one Ti on the surface, along with a potential IHB between oxygen protonated in the cluster and one of the oxy-gen in the dye, are responsible for the adsorption. The latter mode is similar to it found when carboxylic acid groups binding to TiO 2 surface [54][55][56]. Distances for intermolecular interactions O1···T3, O2···T3, and O2···H4 were shorter for the complex 3-thiophenyl-lawsone···(TiO 2 ) 9 compared to those for complex lawsone···(TiO 2 ) 9 (1.89 versus 1.92 Å, 2.01 versus 2.28 Å, and 2.02 versus 2.17 Å, respectively). The third mode ( Figure 5C,F), with anionic deprotonated species, showed a similar configuration to the second mode, with a Ti of cluster interacting with two neighbor carbonyl in the dyes. For lawsone···(TiO 2 ) 9 complex the O1···Ti3 distance was shorter compared to those of 3-thiophenyl-lawsone···(TiO 2 ) 9 complex (1.94 versus 2.47 Å, respectively). On the other hand, the O2···Ti3 distance was shorter 3-thiophenyl-lawsone···(TiO 2 ) 9 compared to those of lawsone···(TiO 2 ) 9 complex (2.43 versus 2.47 Å, respectively).
To evaluate the anchoring of the studied dyes on the TiO 2 surface quantitatively, we calculated the adsorption energies (E ads ) for the complexes as follows [57,58]: where E dye−(TiO 2 ) 9 correspond to the energy of complex, E dye to the energy of the isolated dye, and E (TiO 2 ) 9 to the energy of isolated (TiO 2 ) 9 cluster. Table 6 presents the values of E ads for each complex, as well as their HOMO, LUMO, and gap energies (E gap ). All complexes presented negatives E ads , indicating that anchoring of these dyes is termodynamically favorable. Between the two neutral adsorption mode, the second (bidentate) was more favorable for both dyes. Comparing between lawsone and 3-thiophenyl-lawsone, the latter presents a more negative value, indicating a stronger interaction with the TiO 2 cluster. Anionic complexes present the most negative E ads for both dyes (C and F), indicating that they interacts stronger with TiO 2 cluster than the neutral forms. Unlike the second mode of adsorption (the most favorable between neutrals), lawsone presented a complex more stable than 3-thiophenyl-lawsone. The value of E ads obtained for complexes (−17.79 to −97.39 kcal/mol) indicates in all cases a strong enough interactions that allow one to suggest that these dyes are chemisorbed on the surface of TiO 2 [57,59]. To analyze the nature of the interaction between anchoring atoms in the dyes with the atoms on the surface of the TiO 2 cluster, we carried out a natural bond orbital (NBO) analysis for the most stable neutral complexes B and E, and for anionic C and F [60]. The results showed that all complexes studied present a two-center bond of type BD for O1-Ti 3 , which explains the strong E ads . The results also showed a less strong donor-acceptor interaction between a LP of O 2 with Ti 3 atoms in the cluster for C and F. Additionally, an interaction LP of O 2 with H4 in the TiO 2 cluster for B and E, which is characteristic of hydrogen bonds [60].

Synthesis
Compounds 1-7 were obtained using an on-water methodology previously described [26]. 1 H and 13 C NMR spectra were obtained from a spectrometer operating at either 400. 13 MHz ( 1 H) or 100.61 MHz ( 13 C) at 300 K. All melting points were uncorrected and were determined using Electrothermal 9100 apparatus (Cole-Parmer, Stone, UK). Silica gel 60 (230-400 mesh ASTM) and TLC aluminum sheets silica gel 60 F254 were used for flashcolumn chromatography and analytical TLC, respectively.

Color Evaluation by Nix Pro Sensor
CIELab coordinates (L*, a*, b*, C*, and h • , where L* denotes lightness, a the red/green value, b* the yellow/blue value, C* the saturation value, and h • is the hue) [45] were calculated from the reflectance data of the 10 • observer and daylight.

Measurement of the UV/Vis Spectrum
Methanol (analytical grade) from PanReac AppliChem (Castellar del Vallès, Barcelona, Spain) and lawsone from Acros Organics (Beel, Antwerp, Belgium ) were used without further purification procedures. The absorption spectra of compounds in the methanolic solution were recorded using a Double Beam UV/VIS spectrophotometer Rayleigh-UV2601 (Beijing Beifen-Ruili Analytical Instrument, Beijing, China). A solvent blank was measured before each recorded spectrum. All measurements were performed at ambient temperature in standard quartz cuvettes of a 1 cm optical path.

Computational Calculations
The geometries of the studied compounds were optimized at the B3LYP, M06-2X, and HSE2PBE in addition to the 6-311+g(d,p) basis set. The electronic absorption spectra of the molecules in vacuum and methanol (MeOH) were simulated using time-dependent density functional theory (TD-DFT) at the B3LYP/6-311+G(d,p) level. The solvent effect was evaluated based on the polarizable continuum model (PCM) [46]. Calculations of the dyes attached to a cluster of (TiO 2 ) 9 were carried out following a known methodology [58,61] at DFT B3LYP with basis set 6-311G(d,p) for C, H, S, and O atoms, and LanL2DZ for Ti atoms. No imaginary vibrational frequencies were found at the optimized geometries, indicating that they are the true minima of the potential energy surface.

Conclusions
The results of this study showed that, by linking lawsone with substituted benzenethiols, it is possible to obtain compounds exhibiting a gamma of colors with bathochromic shifts to a color close to red when compared with that of the parent compound. The inclusion of thiophenyl moiety in lawsone allows for absorption in a wide range of visible and UV wavelength range, compared with not substituted lawsone. Computational calculations employing Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) showed that the compounds presented lower excitation energies than lawsone, while the alignment of their frontier orbitals regarding the conduction bands of TiO 2 and ZnO and the redox potential of the electrolyte I − /I 3 − suggests that they may be employed as sensitizers, like lawsone. Geometrical analysis showed that the incorporation of thiophenyl substituents on lawsone structure led to a loss of coplanarity, which favor an anti-aggregation behavior, which is a factor that improves the open-circuit photovoltage (Voc) when dyes tend to form H-aggregation, as occur commonly in organic dyes with aromatic rings. The study of the interactions of the lawsone and 3-thiophenyl-lawsone with a TiO 2 surface model by different anchoring modes, showed that the absorption is thermodynamically favored. Natural bond orbital (NBO) analysis indicates a two-center bond (BD) O-Ti as the main interaction of the dyes with TiO 2 , both in neutral and anionic state.