International Journal of Materials Science and Applications
Volume 4, Issue 5, September 2015, Pages: 314-324

Vibrational and Electronic Spectra of Natural Dyes Constituents for Solar Cell Application: DFT and TDDFT Study

Joseph Makuraza*, Tatiana Pogrebnaya, Alexander Pogrebnoi

Dept. of Materials, Energy Science and Engineering, The Nelson Mandela African Institution of Science and Technology, Arusha, Tanzania

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(J. Makuraza)
(T. Pogrebnaya)
(A. Pogrebnoi)
(A. Pogrebnoi)

To cite this article:

Joseph Makuraza, Tatiana Pogrebnaya, Alexander Pogrebnoi. Vibrational and Electronic Spectra of Natural Dyes Constituents for Solar Cell Application: DFT and TDDFT Study.International Journal of Materials Science and Applications.Vol.4, No. 5, 2015, pp. 314-324. doi: 10.11648/j.ijmsa.20150405.16


Abstract: Selected constituents of natural dyes, phenol, 1,2-benzoquinone, 1,4-benzoquinone, 1,4-naphthoquinone, and 9,10-anthraquinone have been studied theoretically using the density functional theory and time-dependent density functional theory. The vibrational and electronic spectra have been computed with 6-311++G(d,p) basis set. It was found that 1,2-benzoquinone, 1,4-naphthoquinone, and 9,10-anthraquinone may satisfy some criteria to become photosensitizer in DSSCs; the absorption bands computed for molecules in vacuum appeared at 396, 348, and 326 nm, respectively. When computed for molecules in solutions using the polarized continuum model, the bands were red-shifted: 446 (1,2-benzoquinone in water), 355 (1,4-naphthoquinone in heptane), and 329 nm (9,10-anthraquinone in heptane). Our results have shown that 1,2-benzoquinone among the others would exhibit better photovoltaic properties in terms of light absorption and energy level alignment.

Keywords: Dye Sensitized Solar Cell, Sensitizer,Phenol, Benzoquinone, Naphthoquinone, Antraquinone, Geometrical Parameters, Vibrational Spe, Electronic Spectra


1. Introduction

Alternative energy sources and electric energy storage are becoming important for solving issues on pending energy crisis and sustainable use. Photovoltaic solar cell systems are among the alternative energy systems that provide green electrical energy. Solar energy is essentially almost infinite, and freely available; and is directly converted into electrical energy without emitting greenhouse gases. Crystalline silicon solar cells have been extensively studied and used for practical applications, but the materials are expensive and manufacturing costs are high that eventually have resulted in long energy payback times of solar panels.

This makes the development of new molecular materials and nanostructures using organic heterocyclic highly desirable for solar cell applications [1]. A dye-sensitized solar cell (DSSC) was first fabricated by O’Regan and Grätzel [2]. DSSCs are working thanks to organic and inorganic sensitizers. The DSSCs have a high potential to be an alternative to more expensive silicon solar cells because of their reasonable production costs, ease of fabrication, and promising conversion efficiency of sunlight into electricity [3]. Ruthenium-based complexes have shown to be the best sensitizers of high conversion efficiency [4], but the metal is toxic and not abundant, and the synthesis of dyes based on ruthenium is complicated and costly. For these factors more studies are needed to develop organic sensitizers for DSSC. The role of a sensitizer is to absorb light and inject the photo-generated electrons to the conduction band of the nanocrystalline semiconductor (TiO2, ZnO, SnO2 and others).

To be sensitizers in DSSCs, constituents of natural dyes have to satisfy the following arguments: (i) their intense absorption band in the visible and near-UV region, (ii) the presence of the anchoring groups allowing the bonding to the TiO2 and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte [2, 4, 5].

Recently, Mphande and Pogrebnoi [6] found that the crude natural dyes contain different constituents such as phenols, flavonoids, quinones, coumarins and carotenoids. These compounds have several desirable characteristics related to their use in different applications. Quinones are used in production of hydrogen peroxide [7], biochemistry, medicinal [8], dyes, reagents in organic chemistry [9] and battery charge carrier [10]. Theoretical and experimental studies on structural and vibrational spectra of some quinones and its derivatives have been carried out [11-13]. But a few theoretical studies have been done on electronic spectra, and also objectives of authors were not addressing to photosensitizers [14, 15].

The objective of this work is to compute vibrational and electronic spectra of phenol, 1,2-benzoquinone (1,2-BQ), 1,4-benzoquinone (1,4-BQ), 1,4-naphthoquinone (1,4-NQ), and 9,10-antraquinone (9,10-AQ) molecules using density functional theory (DFT) and time-dependent density function theory (TDDFT).

2. Computational Details

The structural parameters have been optimized for all species. The vibrational spectra have been calculated and all structures were confirmed as true energy minima by the absence of imaginary frequencies. Electronic structure and spectra were computed for species both in vacuum and in solutions using the polarized continuum model (PCM). All computations were carried out using Firefly 8.1.0 [16] which is partially based on the GAMESS (US) source code [17]. The DFT method, the level of B3LYP5/6-31G(d) and B3LYP5/6-311++G(d,p) [18] was applied to compute the geometrical parameters and vibrational spectra. Electronic spectra were calculated using the TD-DFT [19] at the B3LYP/6-311++G(d,p) level. Thermodynamic functions have been calculated using OpenThermo software [20]. The HyperChem [21] software was used to calculate initial coordinates of atoms. The ChemCraft program [22] was applied for visualization of the results of the Firefly output files.

3. Results and Discussion

3.1. Optimized Geometrical Parameters

The optimization was done for all species using two basis sets: B3LYP5/6-31G(d) and B3LYP5/6-311++G(d,p). There were no contradiction between the parameters found with both basis sets; moreover the difference was not essential. The results obtained with the basis set B3LYP5/6-311++G(d,p) are given hereafter. The optimized geometrical structures of phenol, 1,2-BQ, 1,4-BQ, 1,4-NQ, and 9,10-AQ are shown in Fig. 1. The selected geometrical parameters are listed in Tables 1-3.

Phenol. The optimized bond lengths and bond angles are summarized in Table 1. The bond lengths O1-C2, O1-H8, C-H and aromatic C-C, are equal to 1.371 Å, 0.963 Å, (1.082-1.085) Å, (1.390-1.395) Å, and valence angle C2-O1-H8 is 109.6° respectively. Bond angles inside and outside benzene ring are all equal to 120.0°, only one angle C2-O1-H8 differs from others and equal to 109.5°.

(a)

(b)

(c)

(d)

(e)

Figure 1. Optimized geometrical structures of the species: (a) phenol; (b) 1,2-benzoquinone; (c) 1,4-benzoquinone; (d) 1,4- naphtoquinone; (e) 9,10-antraquinone.

Bond angles inside and outside benzene ring are all equal to 120.0°, only one angle C2-O1-H8 differs from others and equal to 109.5°. The microwave experimental geometrical parameters reported in [23] are O1-C2 1.374 Å, O1-H8 0.957 Å, the C-H 1.081-1.086 Å and C-C 1.391-1.395 Å, the bond angle is C-O-H 108.8°. The calculated geometrical parameters are in agreement with experimental data.

1,2-BQ and 1,4-BQ. The molecular structures of 1,2-BQ and 1,4-BQ differ only in the position of the oxygen group attached to a benzene ring (Figs. 1b, 1c). The results on the selected geometrical parameters are presented in Table 2.

The selected calculated bond lengths and bond angles of 1,2-BQ are C1-C2, C2-C3, C3=C6, C1=O7 and C2=O8 which are equal to 1.568, 1.473, 1.345, 1.210, and 1.210 Å. The optimize angles are C2-C1-O7 equal to 120.6° and C-C-C ranged from ~119-121°. The computed selected structural parameters are comparable to experimental results obtained from single-crystal X-ray diffraction [11, 24]. The bond lengths are equal to 1.541, 1.463, 1.366, 1.216 and 1.216 Å, respectively; the valence angles are in the range 117-122° [11, 24]. It is distinctly found out that our geometrical parameters calculated are in the closest agreement with experimental results. The geometrical parameters of this molecule has been also computed in [11] using different methods of DFT including BLYP/6-31G(d) and B3LYP/6-31G(d), the calculated results are in good agreement with our data.

The selected computed geometrical parameters of the 1,4-BQ molecule are equal to 1.339 (C1=C2), 1.486 (C1-C3), 1.220 (C=O), 1.085 Å (C5-H12) and 117.2° (C2-C6-C5). The accompanying data in Table 2 are some selected experimental geometrical parameters determined by electron diffraction in gaseous phase [25]: 1.344 (C1=C2), 1.481 (C1-C3), is, 1.225 (C=O), 1.089 Å (C-H) and 118.1° (C2-C6-C5). It is seen that there is no big difference between computed and experimental results reported in [25].

Table 1. Geometrical parameters of phenol.

Parameter B3LYP5/6-311++G(d,p) Expt [23]
Bond lengths, Å
O1-C2 1.371 1.374
C2-C3 1.395 1.391
C2-C5 1.395 1.391
C3-C4 1.390 1.392
C4-C7 1.395 1.395
C5-C6 1.393 1.394
C6-C7 1.392 1.395
O1-H8 0.963 0.957
C3-H9 1.084 1.081
C4-H10 1.085 1.083
C5-H11 1.082 1.086
C6-H12 1.085 1.083
Bond angles, deg
C3-C4-C7 120.0 120.7
C3-C2-O1 117.3 117.0
C5-C2-O1 122.5 122.1
C2-C5-H11 120.0 120.0
C2-O1-H8 109.6 108.8

Our data for 1,4-BQ are also in agreement with the theoretical results calculated earlier [26]. It may be also observed that some bond lengths like C=O and C-H of two species, 1,2-BQ and 1,4-BQ, are almost the same, respectively.

Table 2. Geometrical parameters of 1,2-benzoquinone and 1,4-benzoquinone.

1,2-Benzoquinone 1,4-Benzoquinone
Parameter B3LYP5/6-311++G(d,p) Expt [11, 24] Parameter B3LYP5/6-311++G(d,p) Expt [25]
Bond lengths, Å
C1-C2 1.568 1.541 C1=C2 1.339 1.344
C2-C3 1.473 1.463 C1-C3 1.486 1.481
C3=C6 1.345 1.366 C3-C4 1.486  
C5-C6 1.462 1.445 C4-C5 1.486  
C4=C5 1.345 1.336 C5-C6 1.486 1.481
C1-C4 1.473 1.463 C2-C6 1.486  
C1=O7 1.210 1.216 C3=O8 1.220 1.225
C2=O8 1.210 1.216 C6=O7 1.220 1.225
C3-H9 1.084   C4-H11 1.085  
C4-H10 1.085   C5-H12 1.085 1.089
Bond angles, deg
C2- C1-O7 120.6 119.9 C1-C3-O8 121.4  
C2-C1-C4 118.6 117.2 C2-C6-C5 117.2  
C1-C4-C5 120.0 120.5 C2-C6-O7 121.4  
C6-C5-C4 121.3 122.3 C1-C2-H10 122.7  
C4-C5-H11 120.0   C2-C6-C5 117.3 118.1

Table 3. Selected geometrical parameters of 1,4-naphthoquinone and 9,10-antraquinone.

1,4-naphthoquinone 9,10-antraquinone
Parameter B3LYP5/ 6-311++G(d,p) Expt [27] Parameter B3LYP5/ 6-311++G(d,p) Expt [28] Expt [29]
Bond lengths, Å  
C1-C2 1.394 1.43 O1=C8 1.229 1.220 1.220
C1-C6 1.398 1.36 C2-C5 1.399 1.383 1.400
C2-C3 1.400 1.41 C2-C3 1.392 1.390 1.400
C4-C5 1.398 1.39 C3-C4 1.399 1.381 1.400
C5-C6 1.409 1.39 C6-C7 1.400 1.380 1.400
C6-C7 1.492 1.48 C6-C11 1.491 1.495  
C1-H13 1.086   C8-C9 1.491 1.495 1.499
C3-H15 1.087   C9-C12 1.399 1.380  
C5-C10 1.492 1.43 C10-C11 1.491 1.472  
C7-C8 1.485 1.45 C12-C13 1.392 1.380  
C8=C9 1.344 1.31 C13-C14 1.399 1.388  
C7=O12 1.225 1.22 C14-C15 1.392 1.388  
C10=O11 1.225 1.21 C2-H17 1.085 1.080 1.087
C8-H17 1.087   C3-H18 1.086 1.080 1.087
C9-H18 1.087          
Bond angles, deg  
C2-C1-C6 121.3 121.5 C2-C5-C6 120.2   120.1
C6-C7-O12 118.9 118.0 C2-C3-C4 120.0    
C6-C7-C8 120.1   C3-C2-C5 119.7   117.4
C7-C8-C9 119.7   C2-C5-C6 120.2    

1,4-NQ. The 1,4-NQ molecule comprises of two parts one is enedione structure and benzenoid structure (Fig. 1d). The geometrical parameters are presented in Table 3. The calculated aromatic bond lengths C-C are (1.340-1.460) Å, the single bond lengths C-C are (1.355-1.462) Å, the double C8=C9 is 1.344 Å, the two C=O are equal to 1.225 Å and C-H are all equal to 1.087 Å. The crystal phase X-ray diffraction data [27] are given in Table 3 for comparison. The experimental values C1-C2, C1-C6, C10=O11 and C7=O12 of the 1,4-NQ are found to be 1.43 Å, 1.36 Å, 1.21 Å and 1.22 Å, respectively [27]. Our computed results in general do not contradict with the experimental results. But for C1-C2, C5-C10, C7-C8 the deviation is rather big approaching 0.04-0.06 Å. The optimized geometrical parameters of 1,4-NQ molecule have been also found theoretically in [13]; our data are in agreement with these results.

9,10-AQ. The calculated parameters listed in Table 3 show that the carbon-carbon bond lengths are ranged between 1.392-1.491 Å, the C=O is 1.229 Å and the C-H is 1.085-1.086 Å. The bond angles C-C-C are in the range 119.7-120.2°, C-C-H is 120.0°. Earlier the theoretical geometrical parameters of 9,10-AQ have been obtained using semiempirical methods like ZINDO/1 and AM1 [14]. The experimental data have been also reported for anthraquinone: X-ray data for crystal [28] and electron diffraction for gas phase [29]. The experimental results are as follows: 1.380-1.400 Å (C-C aromatic bonds), 1.472-1.499 Å (C-C, single bonds), 1.220 Å (C=O), and 1.080-1.087 Å (C-H). Our results are in a very good agreement with data of electron diffraction gas phase [29] and do not contradict to the X-ray data for crystal structure [28]. The maximum deviation of 0.018 Å between our parameters and X-ray values is observed for C2-C5 and C4-C3 bonds.

3.2. Vibrational Spectra Analysis

The vibrational spectra of molecules under investigation were determined theoretically for optimized geometrical structures. The analysis of computed vibrational spectra was made relative to the most significant group of frequencies for the most common functional groups and structural components found in organic compounds.

Phenol, 1,2-BQ and 1,4-B. Theoretical spectra of phenol (Fig. 2 a), shows two different regions of vibrations. It is seen that intensive vibrations are found in the first low frequency region of spectra start from 300 cm-1 to 1650 cm-1, that correspond to H-O bend vibrations with peak at 305 cm-1, C-H wagging with peaks at 763 cm-1, C-H rocking and bending vibrations with peak at 1194 cm-1, C-C-O stretching and C-H bend vibrations with peak at 1273 cm-1, and C-C stretching vibrations with peak at 1638 cm-1. The C-H and H-O stretching vibrations are found in the second region with peaks at 3162 cm-1 and 3864 cm-1 respectively. The experimental vibrational frequencies of phenol measured in vapour by Evans [30] and Green [31] are located in the range of (300-3800 cm-1) and (408-3623 cm-1) respectively. The IR spectra measured in vacuum and reported by NIST (National Institute of standards and Technology) [32] are found in the range of (400-4000 cm-1) as shown in Fig.2b. It is distinctly seen that our computed IR spectra are in good accordance with the experimental spectra.

The theoretical IR spectra of 1,2-BQ is shown in Fig. 3.

(a)

(b)

Figure 2. The IR spectrum of phenol: (a) theoretical spectrum for vacuum computed by B3LYP5/6-311++G(d,p); here and hereafter in Figs. 3, 4a, 5a, and 6a, the unit of IR intensity is D2/(amu×Å2), (b) experimental spectrum measured in gas phase [32].

Figure 3. Theoretical IR spectrum of 1,2-benzoquinone computed for vacuum by B3LYP/6-311++G(d,p).

As may be observed, the most intensive vibrations correspond to the C-H wagging, C-C stretching, C-H bending and C=O stretching vibrations with peak at 739, 1262, 1734 and 3153 cm-1. The less intensive and broad vibrations are found in the second region of spectra at high frequencies which relate to C-H stretching vibrations. From the best of our knowledge, there is no experimental IR spectra of 1,2-benzoquinone up to now.

The theoretical infrared spectra of 1,4-benzoquinone (Fig. 4a) has a most intensive peak at 1723 cm-1 which relates to the stretching mode of the C-C=O fragment. The C-H stretching modes of very low intensity are found in the second region ranged from 3190 to 3230 cm-1. In the experimental spectra measured in gas phase [32] the characteristic peak of high intensity is seen at 1700 cm-1 (Fig. 4b). The vibrational bands data reported in [33] are ranged from 105 up to 3062 cm-1, so there is a good correlation to our computed results.

1,4-NQ and 9,10-AQ. Theoretical spectra of the species are shown in Figs. 5a and 6a. Similarly to other quinone derivatives, 1.2-BQ and 1,4-BQ, the spectra of 1,4-NQ the most intensive vibration frequencies correspond to C-H wagging, C-H bending, C-C=O stretching vibrations with peaks at 759, 1340 and 1710 cm-1 respectively. The bands of very low intensities relate to the C-H stretching modes. Experimental data measured in gas phase [32] show the characteristic peak at 1700 cm-1 (Fig. 5b), that is the theoretical value 1710 cm-1 is in agreement with the experimental one. For other bands, the agreement between our theoretical spectrum and experimental one [32] also is observed. Worth to mention that our results are in accordance with the theoretical vibrational spectrum of 1,4-NQ calculated in [13], in which the maximum peak is located at 1729 cm-1 and the most intensive vibration bands are observed in region from ~700 to 1800 cm-1.

In the theoretical IR spectrum of 9,10-antraquinone (Fig. 6a), the most intensive bands are found in the region from ~400 up to ~1810 cm-1; the intensive vibrations correspond to C-H wagging, C-H bending, C-C stretching, C-C=O stretching vibrations with peaks at 690, 1353, 1545, and 1806 cm-1 respectively.

(a)

(b)

Figure 4. The IR spectrum of 1,4-benzoquinone: (a) theoretical spectrum for vacuum computed by B3LYP5/6-311++G(d,p); (b) experimental spectrum of measured in gas phase [32].

The bands of low intensities are observed at ~3800 cm-1, which correspond to C-H stretching vibrations. The first two intensive peaks (690 and 1353 cm-1) seen in our spectrum correspond to experimental ones, ~740 and ~1300 cm-1 [32]. Nevertheless the position and relative intensities of other peaks in our spectrum for the range 1500-2000 cm-1, differ from those in experimental one. But the last two intensive peaks (~1700 and 1800 cm-1), being considered as merged together, may fit the most intensive peak of ~1700 cm-1 in the experimental spectrum.

Based on the calculated vibrational frequencies and optimized geometrical parameters of the species the thermodynamic functions of species have been calculated; the results are given in Appendix.

(a)

(b)

Figure 5. The IR spectrum of 1,4-naphthoquinone: (a) theoretical spectrum computed for vacuum by B3LYP5/6-311++G(d,p), (b) experimental spectrum of measured in gas phase [32].

(a)

(b)

Figure 6. (a) Theoretical IR spectrum of 9,10-antraquinone computed for vacuum by B3LYP/6-311++G(d,p); (b) experimental IR spectra of 9,10-antraquinone measured in gas phase [32].

3.3. Electronic Absorption Spectra Analysis

Electronic absorption spectra of molecules under study were computed for vacuum and solutions using the polarized continuum model. The computed energy of excitation (Eex), absorption wavelength (λ), oscillator strength (f), electronic transition configurations and experimental reported wavelength (λexp) are summarized in Table 4. Only singlet–singlet (S0Sex) transitions have been considered, no ST excitations taken into account. The choice of solvents, water for phenol and 1,2-BQ and heptane for 1,4-BQ, 1,4-NQ and 9,10-AQ, related to the available experimental data [15, 34].

Phenol. The computed absorption spectrum of phenol in vacuum (Fig. 7a) shows that phenol has three bands of moderate oscillator strength at 240, 208 and 179 nm. As may be observed in Table 4 excitation energy, oscillator strength and electronic transition configurations at a maximum absorption wavelength (240 nm) are almost the same in vacuum and water solution. When compared the computed and reference data, an agreement is observed for the band at 208 nm (vacuum) and 207 nm (water solution) with 205 nm (gas), and 210 nm (water solution) from [32, 34]. Nevertheless, mismatch is seen for the theoretical peak at 240 nm, as no band occurs in the experimental spectrum at this wavelength. At the same time, a broadened band is observed between ~260 and 280 nm (Fig. 7b).

As for DSSC application, phenol cannot be appropriate component as the absorption bands are located in ultraviolet C range (280-100 nm). In this part of electromagnetic radiations all light is completely absorbed by the ozone layer in atmosphere.

Table 4. Electronic absorption spectra computed using TDDFT, B3LYP/6-311++G(d,p).

Vacuum Solution
Molecule Excited state No. Eex, eV λ, nm f Electronic transition configurations Eex, eV λ, nm f Electronic transition configurations λexpa, nm
Phenol 1 5.16 240 0.029 H→ L (78%) 5.16 240 0.027 H→L (78%)  
3 5.76 215 0.002 H→L+3 (99%) 5.95 208 0.041 H→L+1 (69%)  
4 5.96 208 0.040 H→L+2 (69%) 5.99 207 0.002 H→L+3 (99%) 210 [34]
5 6.06 204 0.001 H-1→L+1 (96%) 6.76 184 0.002 H-1→L+3(97%)  
1,2-BQ 2 3.13 396 0.048 H-1→ L (99%) 2.78 446 0.033 H→ L (99%)  
5 5.45 228 0.053 H-3→ L (78%) 5.16 240 0.066 H-3→ L (81%)  
8 6.16 201 0.003 H→ L+3 (99%) 6.40 194 0.171 H→ L+1 (55%)  
9 6.31 196 0.111 H→ L+3 (99%)          
1,4-BQ 4 4.94 251 0.328 H-3→L (91%) 4.85 256 0.327 H-3→L (91%) 277[15]
1,4-NQ 3 3.57 348 0.011 H-2→L (94%) 3.49 355 0.010 H-2→L (95%)  
  4 3.60 344 0.052 H-1→L (97%) 3.55 349 0.053 H-1→L (97%) 328 [15]
  5 4.89 254 0.233 H-4→L (86%) 4.86 255 0.231 H+4→L (87%) 246 [15]
  8 5.37 231 0.077 H-2→L+1(62%) 5.37 231 0.086 H-2→L+1(63%) 240 [15]
  10 5.69 218 0.003 H-1→L+1(84%)          
9,10-AQ 4 3.80 326 0.109 H-3→L (94%) 3.77 329 0.110 H-3→L (91%) 332 [15]
7 4.46 278 0.174 H-5→ L (94%) 3.79 327 0.002 H-1→L (92%)  
9 5.00 248 0.009 H-1→L+1(96%) 4.44 279 0.171 H-5→L (94%) 269 [15]
10 5.03 247 0.467 H-2→L+1(81%) 5.00 248 0.008 H-1→L+1(96%)  
          5.02 247 0.491 H-2→L+1(82%) 249 [15]

Note: aThe solvents used were water for phenol and heptane for 1,4-BQ, 1,4-NQ and 9,10-AQ.

1,2-BQ. As shown in Fig. 8, this molecule has three bands of moderate oscillator strength at 396, 228 and 196 nm. The experimental spectrum reported in [35] shows also a peak at ~394 nm. When computed in solution (water), the electron transitions were red-shifted to the absorption bands with peaks at 446 and 240 nm. Our results are in good agreement with experimental [35]. Excitation energy at a maximum absorption wavelength was reduced from 3.13 eV in vacuum up to 2.78 eV in water solution.

(a)

(b)

Figure 7. The UV-Vis absorption spectrum of phenol: (a) theoretical spectrum computed for vacuum using TDDFT, B3LYP/6-311++G(d,p); (b) experimental spectrum [37, 38].

(a)

(b)

Figure 8. The UV-Vis absorption spectra of 1,2-benzoquinone: (a) theoretical spectrum computed for vacuum using TDDFT, B3LYP/6-311++G(d,p); (b) experimental spectrum [35, 37].

(a)

(b)

Figure 9. The UV-Vis absorption spectra of 1,4-benzoquinone: (a) theoretical spectrum computed for vacuum using TDDFT, B3LYP/6-311++G(d,p); (b) experimental spectrum [37].

1,4-BQ. The values of Eex, λ, and f computed for vacuum are almost the same as the results obtained for heptane solution (Table 4). As may be seen in Fig. 9a, there is the intensive peak at 251 nm that is in agreement with the most intensive experimental band shown in Fig. 9b. Meanwhile in heptane solution, the agreement is not so good as the maximum experimental wavelength observed is 277 nm [15] compared to our result of 256 nm.

1,4-NQ. The theoretical UV-Vis spectrum (Fig. 10) computed for vacuum shows four absorption bands at 348, 344, 254 and 231 nm. When computed for heptane solution, the first two bands were red-shifted to 349 and 355 nm while the position of other two bands has not changed. The experimental data measured in heptane [15] are 328.5, 245.5, and 239.5 nm. A fair correspondence between our computed results and reference data may be noted.

9,10-AQ. The theoretical UV-Vis spectrum for vacuum and experimental one for gas phase are compared on in Figs. 11a and 11b. A very good accordance is observed. In the computed spectrum for vacuum, the maximum absorption wavelength is 326 nm, while for heptane solution it is red-shifted slightly, by 3 nm. Our computed λmax is in the close agreement with the experimental one measured in heptane solution, 331.8 nm [15].

Figure 10. Theoretical UV-Vis absorption spectra of 1,4-naphthoquinone computed for vacuum using TDDFT, B3LYP/6-311++G(d,p).

(a)

(b)

Figure 11. The UV-Vis absorption spectra of 9,10-antraquinone: (a) theoretical spectrum computed for vacuum using TDDFT, B3LYP/6-311++G(d,p); (b) experimental spectrum [37, 39].

3.4. Energy Level Alignment

We performed calculations of highest occupied molecular orbitals (HOMOs), and lowest unoccupied molecular orbitals (LUMOs) of molecules under investigation. Calculations were performed both for vacuum and solutions. The values of HOMOs, LUMOs energies and energy gaps are summarized in Table 5.

Table 5. Molecular orbital energies (in eV) computed at B3LYP/6-311++G(d,p) level.

Vacuum Solution
Molecule HOMO LUMO Eg HOMO LUMO Eg
Phenol -6.32 -0.37 5.95 -6.48 -0.52 5.96
1,2-BQ -7.17 -3.96 3.21 -7.49 -4.08 3.41
1,4-BQ -7.78 -3.92 3.87 -7.65 -3.74 3.91
1,4-NQ -7.56 -3.53 4.03 -7.47 -3.23 4.23
9,10-AQ -7.37 -3.18 4.19 -7.63 -3.56 4.07

When proceed from vacuum to solutions, the energies ε(HOMO) change differently for different species: decrease for phenol, 1,2-BQ, and 9,10-AQ and increase for two other species. As for the energy gap the change is small, the largest being 0.20 eV for 1,2-BQ and 1,4-NQ. Comparing the energy gap Eg with the first excitation energy Eex (Table 5), the values of Eg are bigger than Eex, and the difference varies between 3% (1,2-BQ) and 28% (1,4-BQ).

The energy level alignment between a semiconductor, electrolyte and sensitizer is fundamental for DSSCs. The simplest way to consider this alignment is to apply the energy levels of the frontier orbitals. But the more appropriate way is to use the energy levels calculated by TDDFT [4]. To reveal the ability of a sensitizer to satisfy this alignment, the quantity of the excited state oxidation-potential (ESOP) is used. According to definition, the ESOP is the difference of free energy between neutral and oxidized species in the excited state [5]. In this work the ESOPs were calculated as the sum of ground state HOMO energy and the singlet-singlet transition excitation energy Eex: ESOP = ε(HOMO) + Eex. To carry out ESOPs calculations, the minimum excitation energy Eex with non-zero oscillator strength was used.

Figs. 12a and 12b represent the energy levels of the HOMOs and the ESOPs for phenol and quinone derivatives, a nanocrystalline TiO2 electrode [5] and I-/I3- redox electrolyte [4, 40]. As it is noted in the introduction, for a dye to be operational and ensuring that there is enough driving force for the dye generation reaction, its HOMO energy level must be located within the band gap of the semiconductor and below electrolyte (I-/I3-) redox level, and ESOP to lie a bit above the conduction band of semiconductor [4, 5, 36].

As may be observed in Fig. 12a, the only HOMO levels of phenol and 1,2-benzoquinone are located within the band gap of TiO2 metal oxide and below the I-/I3- redox level, this implies that there may be a sufficient driving force for dye generation reaction. The calculated HOMO levels of 1,4-BQ, 1,4-NQ, and 9,10-AQ are found to be out of the band gap of TiO2, this means that these three species do not have enough driving force of the dye generation reaction. The ESOPs computed both for vacuum and solutions of the molecules under study are located above the TiO2 CB level, except 1,2-benzoquinone shown in Fig. 12b. But the ESOP of phenol is not laid nearby; it is located far above from CB of TiO2. The rest species, 1,4-naphthoquinone, 9,10-antraquinone, 1,2 and 1,4-benzoquinone, may have sufficient driving force for electron injection to TiO2. The calculated absorption wavelengths show that all molecules under study, absorb in the near UV region, the only 1,2-benzoquinone absorb light up to the visible region of the electromagnetic spectrum. From analysis stated above 1,2-benzoquinone molecules show better electronic properties to be a photosensitizer in DSSCs.

4. Conclusion

In this work we have studied vibrational and electronic absorption spectra of phenol, 1,2-benzoquinone, 1,4-benzoquinone 1,4-naphthoquinone and 9,10-antraquinone. The calculated theoretical spectra, as well as geometrical parameters, are in an agreement with available experimental data. Based on the ability to absorb light in the visible region, appropriate energy level alignment with the conduction band edge of the semiconductor substrate and the redox level of the electrolyte, 1,2-benzoquinone seems to be a better candidate among the considered molecules for DSSCs.

(a)

(b)

Figure 12. Schematic energy level representation showing the HOMOs and ESOPs for phenol and quinone derivatives, calculated using TDDFT, B3LYP/ 6-311++G(d,p): (a) vacuum; (b) solutions (water for phenol and 1,2-BQ, heptane for 1,4-BQ, 9,10-AQ and1 1,4-NQ). The valence and conduction bands of a nanocrystalline TiO2 were taken from [4, 5] and redox level of the I-/I3- electrolyte from [5, 40].

Appendix

The thermodynamic functions of phenol, 1,2-benzoquinone, 1,4-benzoquinone, 1,4-naphthoquinone and 9,10-antraquinone in gaseous phase were calculated using Openthermo software [16]. For temperature interval between 100-1000 K, the molar heat capacity cp°(T); the Gibbs reduced free energy Ф°; the entropy S°(T); and the enthalpy increment H°(T) – H°(0) are listed in Table A; the units are J×mol-1×K-1, J×mol-1×K-1, J×mol-1×K-1 and kJ×mol-1, respectively. The Gibbs reduced free energy was calculated using the following formula: Ф°(T) = -[(H°(T)–H°(0)) - TS°(T)]/T. The experimental data for cp°(T) were taken from [41] (phenol), [42] (1,4-BQ), [43] (1,4-NQ and 9-10-AQ). Our computed results for cp° are in good accordance with experimental results; the biggest deviation of ~12% (at 1000 K) is observed for 9,10-antraquinone.

Table A. Thermodynamic functions of phenol, 1,2-benzoquinone, 1,4- benzoquinone, 1,4-naphthoquinone, and 9,10-antraquinone.

T, K cp°(T) S°(T) H°(T)–H°(0) Ф°(T)
This work Expt [41-43]      
Phenol          
100 40.209 41.38 243.645 3.484 208.805
200 67.189 69.65 279.145 8.760 235.345
298.15 99.576 94.61 311.955 16.931 255.168
300 100.190 103.86 312.573 17.116 255.519
400 131.600 135.79 345.796 28.740 273.946
500 157.950 161.91 378.095 43.263 291.569
600 179.100 182.48 408.833 60.155 308.574
700 196.100 198.84 437.764 78.944 324.986
800 210.010 212.14 464.887 99.272 340.797
900 221.590 223.19 490.310 120.869 356.011
1000 231.380 232.49 514.178 143.530 370.648
1,2-BQ          
100 47.359   243.536 3.857 204.966
200 75.503   284.578 9.940 234.878
298.15 105.990   327.878 18.977 264.228
300 106.210   321.029 19.033 257.585
400 134.070   355.502 31.083 277.794
500 157.250   387.998 45.689 296.620
600 175.920   418.381 62.382 314.411
700 190.960   446.670 80.753 331.308
800 203.200   472.995 100.482 347.392
900 213.310   497.531 121.323 362.727
1000 221.750   520.456 143.088 377.368
1,4-BQ          
100 47.359 47.65 243.536 3.857 204.966
200 75.503 76.51 284.578 9.940 234.878
298.15 105.660 107.39 320.374 18.837 257.194
300 106.210 107.95 321.029 19.033 257.585
400 134.070 136.31 355.502 31.083 277.794
500 157.250 159.73 387.998 45.689 296.620
600 175.920 178.51 418.381 62.382 314.411
700 190.960 193.57 446.670 80.753 331.308
800 203.200 205.80 472.995 100.482 347.392
900 213.310 215.86 497.531 121.323 362.727
1000 221.750 224.22 520.456 143.088 377.368
1,4-NQ          
100 60.716 62.71 272.432 4.385 228.582
200 104.56 100.82 327.449 12.573 264.584
298.15 151.980 148.53 378.033 25.163 293.635
300 152.86 150.22 378.976 25.445 294.159
400 197.66 196.51 429.236 43.023 321.678
500 235.17 236.28 477.517 64.729 348.059
600 265.3 268.51 523.16 89.809 373.478
700 289.42 301.08 565.936 117.590 397.950
800 308.93 315.22 605.901 147.541 421.474
900 324.93 332.26 643.241 179.260 444.063
1000 338.21 345.98 678.184 212.436 465.748
9,10-AQ          
100 74.642 77.91 289.260 5.060 238.660
200 133.880 134.75 358.554 15.396 281.574
298.15 198.340 197.78 423.979 31.689 317.693
300 199.550 199.03 425.209 32.057 318.352
400 261.300 260.18 491.268 55.168 353.348
500 313.170 313.10 555.347 83.981 387.385
600 354.780 355.77 616.267 117.458 420.503
700 387.980 388.79 673.545 154.657 452.606
800 414.730 417.60 727.161 194.839 483.612
900 436.590 440.28 777.313 237.441 513.489
1000 454.690 516.16 824.279 282.033 542.246

Acknowledgment

The authors are thankful to The Nelson Mandela African Institution of Science and Technology (NM-AIST) for sponsorship and the School of Computational and Communicational Science and Engineering at NM-AIST for providing necessary facilities to carry out this work.


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