A Combined Experimental and Computational Investigation of

Oct 14, 2009 - These sensitizers were tested in DSSCs using 0.05 M I2, 0.5 M 1 ..... onto the TiO 2 anatase (1 0 1) surface via heterocyclic anchoring...
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J. Phys. Chem. C 2009, 113, 20117–20126

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A Combined Experimental and Computational Investigation of Anthracene Based Sensitizers for DSSC: Comparison of Cyanoacrylic and Malonic Acid Electron Withdrawing Groups Binding onto the TiO2 Anatase (101) Surface Kola Srinivas,†,‡ K. Yesudas,† K. Bhanuprakash,*,† V. Jayathirtha Rao,*,‡ and L. Giribabu*,§ Inorganic Chemistry DiVision, Organic Chemistry DiVision, and Nanomaterials Laboratory, Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad-500 607 India ReceiVed: August 4, 2009; ReVised Manuscript ReceiVed: September 23, 2009

Four anthracene based sensitizers, 3-(anthracene-9-yl)-2-cyanoacrylic acid (M1), 2-cyano-3-(10-methoxyanthracene-9-yl)acrylic acid (M2), 2-(anthracene-9-ylmethylene) malonic acid (M3), and 2-((10-methoxyanthracene-9-yl)methylene)malonic acid (M4) were designed and synthesized to understand the binding modes of anchoring groups ( and ) on the nanocrystalline TiO2(101) surface and on the efficiency of dye-sensitized solar cells (DSSCs). All four sensitizers have been fully characterized using ATR-FTIR, UV-vis, and CV. These sensitizers were tested in DSSCs using 0.05 M I2, 0.5 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), and 0.5 M lithium iodide (LiI) in methoxypropionitrile (MPN) redox electrolyte. The sensitizers having a monocarboxylic acid group, i.e., M1 and M2, have shown marginally higher IPCE and efficiency than M3 and M4 having dicarboxylic acid groups. To have a detailed understanding of this behavior, the adsorption and binding energies to the TiO2 surface of these dyes have been investigated using computational techniques (periodic DFT). The studies show that the cyanoacrylic acid anchoring group has a stronger binding to the TiO2 surface compared to the malonic acid anchoring group. Introduction The design and synthesis of functional dyes have become a focus of current research in view of their potential applications as sensitizers in dye-sensitized solar cell (DSSC) technologies.1 Molecules with a wide range of absorption in the visible region and containing an anchoring group like carboxylic or phosphonic acids are ideal candidates as sensitizers.2 The DSSC process requires that these dyes absorb sunlight and go to the excited state. This excited electron is then injected into the conduction band of TiO2 in a femtosecond lifetime by the anchoring group, and in this process, the dye gets oxidized. The oxidized dye is then neutralized to ground state by the I3-/I- redox system.3 The efficiency of the cell is affected by the energy difference between the excited state dye and the conduction band of TiO2, the binding between the dye and the semiconductor, and the properties of the redox couple in the electrolyte.2 In addition, the electron density should be localized near the injecting group in the excited state.4 It has also been shown that the redox potential of adsorbed dye depends on the pH of the electrolyte and on the potential applied to the semiconductor.5 By choosing an appropriate sensitizer, the tuning of the photoresponse of the semiconductor can be carried out.6 The most successful charge transfer metal complexes of organic molecules employed so far in DSSC are cis-dithiocyanato bis-(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (red dye) and trithiocyanato 4,4′,4′′-tricarboxy2,2′,6′,2′′-terpyridine ruthenium(II) (black dye), which yields overall conversion efficiencies of up to 10-11% under AM1.5 irradiation.6-8 However, ruthenium dyes are very expensive due to the lack * Corresponding authors. E-mail: [email protected] (K.B.); [email protected] (J.R.); [email protected] (L.G.). † Inorganic Chemistry Division. ‡ Organic Chemistry Division. § Inorganic and Physical Chemistry Division.

of natural abundance of ruthenium. Due to this, metal free sensitizers such as organic dyes and natural dyes are being investigated as alternative sensitizers for DSSC applications.9 Recent literature reports indicate achievement of efficiency of ∼8% using organic dyes.10 Several organic dyes such as coumarins,11-14 merocyanine,15,16 hemicyanine,17-19 porphyrin,20-24 phthalocyanine,25,26 indoline,27,28 and squaraine29-31 dyes and other donor-acceptor organic dyes32-42 have been reported in the literature. Acceptor groups of the dye play a key role in improving the efficiency of the cell.17 Many studies have reported cyanoacrylic acid as the acceptor.43 In a recent study, Gra¨tzel and co-workers have reported that malonic acid shows a considerable improvement in the efficiency of the cell, which they attribute to the stronger binding of the malonic acid on the semiconductor surface.44 On the other hand, Hara et al. have carried out detailed studies of coumarin dyes with cyanocarboxylic acid groups as acceptors and in one case also with the corresponding malonic acid as an acceptor, and from their reports, it is observed that malonic acid has not improved the efficiency.45 On the basis of the literature reports discussed above and our own interest46 in a deeper understanding of the role of dyes in general and anchoring groups in particular, we have taken up this work with an aim to compare the cynoacrylic acid and malonic acid anchoring groups binding on the TiO2(101) surface. Here in this study, we report the synthesis and characterization of anthracene based sensitizers, i.e., 3-(anthracene-9-yl)-2cyanoacrylic acid (M1), 2-cyano-3-(10-methoxyanthracene-9yl)acrylic acid (M2), 2-(anthracene-9-ylmethylene) malonic acid (M3), and 2-((10-methoxyanthracene-9-yl)methylene)malonic acid (M4) (Scheme 1). These have either the malonic acid or the corresponding cyanoacrylic acid as anchoring groups. The adsorption behaviors of the dyes M1 and M3 have been compared by using attenuated total reflection-Fourier transform

10.1021/jp907498e CCC: $40.75  2009 American Chemical Society Published on Web 10/14/2009

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SCHEME 1: Structures of the Anthracene Dyes Synthesized in This Study

infrared (ATR-FTIR) and computational techniques (periodic density functional theory (DFT) calculations). Experimental Section Materials and Instruments. All of the reagents were Analytical Reagent grade and used without further purification. 9-Methoxyanthraldehyde was prepared according to the reported procedure.47 1H NMR spectra were recorded on a Gemini (200 MHz) or Varian Inova (400 MHz) spectrometer in DMSO-d6 and 13C NMR spectra on a Bruker Avance (300 MHz) spectrometer with TMS as a standard in both cases. Mass spectra were obtained by using electronspray ionization ion trap mass spectrometry (Thermofinnigan, Sanzox, CA), electron ionization mass recorded on a VG70-70H instrument, fast atom bombardment (FAB) mass spectra recorded on a VG-AUTOSPEC spectrometer, and high resolution mass spectra (HRMS) were carried out by using electrospray ionization quadruple time-offlight (ESI Q TOF) mass spectrometry (QSTAR XL., Applied biosystems/MSD seiex, Fostercity, CA). UV-visible absorption spectra were measured on a Jasco V-550 spectrophotometer. Thermo Nicolet Nexus 670 spectrometer was used to obtain IR spectra of the sensitizer and the sensitizer-sodium salt at a resolution of 4 cm-1. ATR data were taken with a Thermo Nicolet 5700 spectrometer with a ZnSe window using 512 scans at a resolution of 4 cm-1. Differential pulse voltammetric measurements were performed on a PC-controlled CH instruments model CHI 620C electrochemical analyzer, using 1 mM dye solution in acetonitrile at a scan rate of 100 mV/s using 0.1 M tetrabutyl ammoniumperchlorate (TBAP) as supporting electrolyte. The glassy carbon, standard calomel electrode (SCE), and platinum wire were used as working, reference, and auxiliary electrodes, respectively. Preparation of Dye-Sensitized Nanocrystalline TiO2 Thin Films. TiO2 photoelectrode (area: ca. 0.74 cm2) was prepared by the method reported in the literature.3 A paste consisting of 19-nm-sized TiO2 colloid was obtained from SOLARONIX. The TiO2 paste was first screen-printed on fluorine doped SnO2 conducting glass (transmission >85% in the visible, sheet resistance 10 Ω/square obtained from ASAHI, Japan) to form a transparent layer. Subsequently, a second scattering layer made

Srinivas et al. SCHEME 2: Reagents and Conditions: (i) Cyanoacetic Acid, Piperidine, CHCl3, Reflux, 4-5 h; (ii) Malonic Acid, Piperidine, CHCl3, Reflux, 4-5 h

up of a paste containing 400 nm anatase TiO2 particles. Finally, the screen-printed double-layer film is heated to 450 °C in an oxygen atmosphere and calcinated for 45 min. The dye was dissolved in ethanol at a concentration of 0.1 × 10-3 M. The TiO2 thin films were soaked in the dye solution and then kept at room temperature for 16 h so that the dye gets adsorbed onto the TiO2 films. The electrode was dipped into the dye solution while it was still hot; i.e., its temperature was ca. 80 °C. The dye coating was made immediately after the high temperature annealing in order to avoid rehydration of the TiO2 surface or capillary condensation of water vapors from ambient air inside the nanopores of the film. The presence of water in the pores decreases the injection efficiency of the dye. After completion of the dye adsorption, the photoelectrode was withdrawn from the solution and washed thoroughly with ethanol to remove nonadsorbed dye under a stream of dry air or argon. Dye-Sensitized Solar Cell Fabrication. A sandwich cell was prepared using the photoanode and platinum coated conducting glass electrode as the counter electrode.48 The latter was prepared by chemical deposition of platinum from 0.05 M hexachloroplatinic acid. The two electrodes were placed on top of each other using a thin polyethylene film (50 µm thick) as a spacer to form the electrolyte space. The empty cell was tightly held, and edges were heated to 130 °C to seal the two electrodes together. The active surface area of the TiO2 film electrode was ca. 0.74 cm2. The composition of the electrolyte is 0.05 M I2, 0.5 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), and 0.5 M lithium iodide (LiI) in methoxypropionitrile (MPN). The electrolyte was introduced into the cell by a predrilled hole in the counter electrode and later closed by cover glass to avoid the leakage of the electrolyte solution. Photo-electrochemical Measurements. The photovoltaic performance of the dye-sensitized nanocrystalline TiO2 cells was determined using the instrument SOLARONIX SA SR-IV, unit type 312. The spectral response was determined by measuring the wavelength dependence of the incident photon-to-current conversion efficiency (IPCE) using light from a 100 W xenon lamp that was focused onto the cell through a double monochromator. The whole experiment was run automatically using PV measurement software. For I-V measurements, a 1000 W xenon light source was used as the irradiation source. The spectral output of the lamp matched the AM 1.5 solar spectrum in the region 350-750 nm (mismatch 1.9%). Incident light intensities were adjusted with neutral wire mesh attenuators. The current-voltage characteristics were determined by applying an external potential bias to the cell and measuring the photocurrent using a Keithley model 2000 digital source meter. Synthesis and Characterization. General Procedure for the Preparation of Anthracene DeriWatiWes (M1-M4) (Scheme 2). A mixture of corresponding aldehyde, cyanoacetic acid or malonic acid, and piperidine ([aldehyde]/[cyanoacetic acid]/ [piperidine] ) 1/1/0.01 mol ratio) in chloroform was taken and

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TABLE 1: Short-Circuit Photocurrent Density (Jsc), Open-Circuit Photovoltage (Voc), Fill Factor (ff), Overall Conversion Efficiency (η), IPCE at 430 nm, Oxidation (Eox), Reduction (Ered), Excited State Oxidation (Eox*) Potentials, and Band Gap (E0-0) of the Dyes Used in DSSC Jsca/mA cm–2 Voca/mV a

ff (%) η (%) IPCEa at 430 nm Eoxb/V (vs SCE) Eredb/V (vs SCE) E0–0c/eV Eox*d/V (vs SCE)

M1

M2

M3

M4

0.543 291 0.540 0.085 6.40 1.40

0.546 322 0.564 0.099 2.91 1.21 –1.47 2.49 –1.28

0.498 292 0.530 0.076 3.77 1.20 –1.70 2.79 –1.59

0.237 264 0.535 0.033 1.05 1.08 –1.27 2.71 –1.63

2.58 –1.18

a Error limits: Jsc, (0.1 mA cm-2; Voc, IPCE, (0.1%. b Error limits: E1/2 ) (0.1 V; SCE, glassy carbon, Pt-wire. c Calculated absorption. Error limits: E0-0 ) (0.05 eV. equation Eox* ) Eox - E0-0.

(30 mV; ff, (0.03; MeCN, 0.1 M TBAP, from the onset of d Calculated from the

TABLE 2: Optimized Geometrical Parameters of the Dyes in the Most Stable Conformation Obtained at the B3LYP/ 6-31G(d,p) Level (Bond Lengths in Å and Angles in deg)

Figure 1. Experimental absorption spectra of the dyes M1-M4 in THF.

TABLE 3: Experimental and Calculated Absorption Maxima (nm) of the Anthracene Dyesa B3LYPb molecule Exp M1 M2 M3 M4 b

geometrical parameters C1-C2/C1-C2′ C2-C3/C2′-C3′ C3-C4/C3′-C4′ C4-C5/C4′-C5′ C5-C6/C5′-C6′ C6-C7/C6′-C7′ C7-C8/C7′-C8′ C8-C9 C9-C10 C10-C11 C10-C12 C7′-C8-C9-C10 (twist angle)

406 417 387 400

BHandHLYPb

vacuo

THF

vacuo

THF

466 (0.167) 470 (0.213) 483 (0.216) 486 (0.174)

492 (0.206) 494 (0.256) 504 (0.161) 508 (0.210)

401 (0.108) 409 (0.285) 405 (0.199) 415 (0.252)

419 (0.289) 426 (0.341) 421 (0.247) 430 (0.306)

a Values given in parentheses correspond to oscillator strengths. Obtained at the 6-31G(d,p) basis set.

TABLE 4: Calculated Transition Dipole Moments (µge in D), Main Orbital Transitions, and Coefficients of Wave Functions for Dyes M1-M4 M1

M2

M3

M4

1.397/1.398 1.430/1.429 1.368/1.369 1.422/1.422 1.371/1.372 1.430/1.430 1.423/1.424 1.466 1.360 1.492 1.433 53.2

1.405/1.407 1.429/1.428 1.369/1.370 1.421/1.420 1.371/1.372 1.429/1.430 1.424/1.425 1.462 1.362 1.490 1.432 51.2

1.398/1.398 1.430/1.429 1.368/1.369 1.423/1.423 1.371/1.372 1.430/1.430 1.420/1.421 1.469 1.357 1.526 1.480 57.4

1.406/1.406 1.429/1.428 1.369/1.369 1.421/1.421 1.371/1.372 1.430/1.430 1.422/1.423 1.465 1.359 1.525 1.478 54.0

refluxed for 4-5 h, and the cooled mixture was extracted with sodium carbonate solution, which left a residue containing unreacted aldehyde.49 Acidification of the carbonate extract gave a pale yellow precipitate. Recrystallization of the crude product from Ethylacetate-hexane afforded the desired product in quantitative yield. The characterization data is given below for all of the compounds (M1-M4). 3-Anthracen-9-yl-2-cyano-acrylic acid (M1). Mp: 263-266 °C.50 δH (400 MHz; (CD3)2SO; Me4Si): 9.28 (1H, s); 8.82 (1H, s); 8.21 (2H, d, J ) 7.70); 8.01 (2H, d, J ) 8.63); 7.59-7.69 (4H, m). δC (300 MHz; (CD3)2SO; Me4Si): 162.1, 154.5, 130.5, 129.6, 128.9, 128.1, 127.2, 125.9, 125.8, 124.8, 114.8, 114.7. MS (EI) m/z 273 (M+). IR (KBr) 1699, 2233 cm-1. HRMSESIMS (m/z): [M-] calcd for C18H10NO2, 272.0711; found, 272.0709. 2-Cyano-3-(10-methoxy-anthracen-9-yl)-acrylic acid (M2). Mp: 243-245 °C. δH (400 MHz; (CD3)2SO; Me4Si): 9.22(1H, s); 8.38 (2H, m); 8.04 (2H, m); 7.64-7.71 (4H, m); 4.16 (3H,

functional B3LYP

BHandHLYP

molecule

µge

M1 M2 M3 M4 M1 M2 M3 M4

4.08 4.61 3.60 4.24 4.48 4.98 4.14 4.72

coefficients of wave function 0.637|HfL〉 - 0.125|HfL+1〉 0.632|HfL〉 + 0.110|HfL+1〉 0.638|HfL〉 + 0.173|HfL+1〉 0.632|HfL〉 - 0.158|HfL+1〉 0.660|HfL〉 + 0.109|H-1fL+2〉 0.658|HfL〉 + 0.107|H-1fL+2〉 0.661|HfL〉 0.658|HfL〉

s). δC (300 MHz; (CD3)2SO; Me4Si): 162.3, 154.4, 154.2, 129.3, 127.5, 126.0, 125.4, 123.5, 122.7, 121.9, 115.0, 114.4, 63.7. MS (FAB) m/z 303 (M+). IR (KBr) 1693, 2228 cm-1. HRMSESIMS (m/z): [M+Na] calcd for C19H13NO3Na, 326.0793; found, 326.0790. 2-Anthracen-9-ylmethylene-malonic acid (M3). Mp: 242-245 °C.49 δH (400 MHz; (CD3)2SO; Me4Si): 8.39 (1H, s); 8.64 (1H, s); 8.13 (2H, m); 7.98 (2H, m); 7.52-7.59 (4H, m). δC (300 MHz; (CD3)2SO; Me4Si): 166.1, 164.8, 139.4, 135.2, 130.6, 128.5, 128.0, 127.4, 126.1, 125.5, 125.4. MS (EI) m/z 292 (M+). IR (KBr) 1721 cm-1. HRMS-ESIMS (m/z): [M-] calcd for C18H11O4, 291.0657; found, 291.0666. 2-(10-Methoxy-anthracen-9-ylmethylene)-malonic acid (M4). Mp: 210-215 °C. δH (200 MHz; (CD3)2SO; Me4Si): 8.5(1H, s); 8.28(2H, m); 8.08(2H, m); 7.43-7.56 (4H, m); 4.16 (3H, s). δC (300 MHz; (CD3)2SO; Me4Si): 166.2, 164.9, 152.4, 139.3, 134.4, 130.2, 128.9, 127.5, 126.3, 126.0, 125.6, 63.3. MS (FAB) m/z 322 (M+). IR (KBr) 1724 cm-1. HRMS-ESIMS (m/z): [M-] calcd for C19H13O5, 321.0762; found, 321.0759. Computational Details. The results of calculations reported in this work have been obtained using the Gaussian 03 ab initio/

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Figure 4. ATR-FTIR spectra of the M1 and M3 dyes adsorbed on TiO2.

Figure 2. Computed isodensity (0.02) surfaces of the HOMO and LUMO of the dyes M1 and M3.

To model the adsorption modes of the dyes on the TiO2 anatase (101) surface, periodic density functional studies have been carried out using PWscf code as implemented in quantum espresso.57 For these calculations, we used the ultrasoft pseudopotentials proposed by Vanderbilt58 and generated with the Perdew-Wang 1991 (PW91)59 exchange correlation functional with 18 electrons which include valence states of 2s and 2p shells for oxygen (six electrons) and 3s, 3p, 3d, and 4s shells for titanium (12 electrons). A kinetic energy cutoff of 25 Ry is chosen to describe the smooth part of the wave function in plane waves, while the augmented density cutoff is expanded up to 200 Ry. The Brillouin zone was sampled with 1 × 2 × 2 Monkhorst-Pack k-points60 mesh to ensure convergence. For the relaxation of the isolated TiO2 anatase (101) surface, single molecules, and the adsorbed complexes, the Broyden-FletcherGoldfrab-Shanno (BFGS)61 algorithm has been used with the default convergence criteria. Results and Discussion

Figure 3. Schematic energy level diagram for M1 and M3 with respect to the nanocrystalline TiO2 conduction band and redox system.

DFT quantum chemical package.51 The gas phase relaxations of atomic positions of all the molecules have been carried out using the hybrid density functional-B3LYP and Pople’s splitvalence double-ζ basis set augmented with p and d polarization functions, i.e., 6-31G(d,p) at the default integration grid as implemented in Gaussian 03. The theoretical singlet equilibrium structures were obtained when the maximum internal forces acting on all of the atoms and the stress were less than 4.5 × 10-4 eV/Å and 1.01 × 10-3 kbar, respectively. The minima were further confirmed by vibrational analysis. These minimized geometries were then subjected to time-dependent density functional theory (TDDFT) studies to get the lowest 10 singlet-singlet transitions.52-55 We also carried out the TDDFT calculations using the “half and half” (BHandHLYP) functional which is known to give the correct asymptotic behavior of the charge transfer (CT) states.56 The calculations have been carried out both in vacuum and in solvent (PCM model).

Photoelectrochemical Properties. Photocurrent action spectra of the dye-sensitized solar cells are shown in Figure A of the Supporting Information, where the monochromator was incremented through the visible spectrum to generate the IPCE (λ) curve as defined below.

( )

IPCE(λ) ) 1240

Jsc λΦ

(1)

where λ is the wavelength (nm), Jsc is the photocurrent density under short-circuit conditions (mA/cm2), and Φ is the incident radiative flux (mW/cm2). The incident photon-to-current conversion efficiency (IPCE) is dependent on the anchoring group.17 IPCE’s for all of these dyes are in the range 1-7% at 430 nm (Table 1) which is slightly lower than expected for such a class of dyes. The lack of coplanarity of the cyanoacrylic or malonic acid group with respect to the anthracene units (vide infra) could result in such a decrease in IPCE. From Table 1, we find that the IPCE at 430 nm for M1 when compared to M3 is greater. This trend is also seen in the case

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Figure 5. Possible adsorption configurations of M1 and M3.

of M2 and M4 molecules, where M2 has a larger IPCE. In the same table, the photovoltaic performance of M1-M4 dyes in DSSCs is also tabulated. The overall conversion efficiency η of the photovoltaic cell is calculated from the integral photocurrent density (Jsc), the open-circuit photovoltage (Voc), the fill factor of the cell (ff), and the intensity of the incident light (Iph), using the equation given below.

η ) Jsc × Voc ×

ff Iph

(2)

The fill factor is defined by the following equation.

ff )

Jph(max) × Vph(max) Jsc × Voc

(3)

where Jph(max) and Vph(max) are the photocurrent and photovoltage for maximum power output and Jsc and Voc are the short-circuit photocurrent density and open-circuit photovoltage. From the table, it is clear that the overall efficiencies of the malonic acid derivatives are smaller than those of the corresponding cyanoacrylic acid derivatives. Geometries and Photophysical Properties of Anthracene Dyes. The optimized geometrical parameters for all of the molecules in their most stable conformation obtained using B3LYP/6-31G(d,p) are given in Table 2. We report only the bond lengths and twist angle which is of importance. From the table, it is observed that the geometrical changes caused by substituting cyanoacrylic with a malonic acid group are very small. In the case of the twist angle, the corresponding malonic acid derivative has a slightly larger angle (around 3° difference). Figure 1 shows the absorption spectra of dyes M1-M4 in THF solution. Table 3 gives the electronic absorption properties of these dyes. All of these dyes absorb in the visible region in the range 387-417 nm in THF solution, and the molar extinction coefficients (ε) range from 5400 to 7300 M-1 cm-1. The appearance of fine structure in the absorption spectra for M3 and M4, compared to M1 and M2, may be due to the intramolecular H-bonding in M3 and M4 which is absent in M1 and M2.

To have a deeper understanding of the observed spectra, we have carried out TDDFT studies for these molecules by including the solvent effect. The values obtained along with the oscillator strengths are tabulated in Table 3. From the table, it is observed that the transition energies obtained using the B3LYP functional are not in good agreement both for gas phase and solvent phase calculations. These errors are reduced by making use of the asymptotic “half and half” (BHandHLYP) functional. The major transitions at the orbital level in all of the molecules are observed to be mixture of two contributions (Table 4). One is in between the frontier orbitals with a major contribution and the other from the HOMO, HOMO-1 to LUMO+1, or LUMO+2 with a minor contribution. The computed HOMO and LUMO orbitals are shown in Figure 2 for the dye molecules M1 and M3. As indicated from the figure, the HOMO is of π-character and is delocalized over the entire molecule. In the LUMO orbital which also has π-character, the electron density has shifted toward the acceptor end of the molecule. This indicates that the dipole moment should be considerably larger in the first excited state compared to the ground states. The computed oscillator strength is also large in all of these molecules, which is reflected from the µge value which is around 4.0 D. Clearly, this shows that these transitions are purely due to CT occurring in these dyes through a π-bridge. The red shift in absorption of M2 and M4 compared to M1 and M3 can be explained by the stronger donor group (-OCH3) in the former molecules. Electrochemical Properties. We have carried out the electrochemical investigations (see Figure B in the Supporting Information and Table 1) by using the differential pulse voltammetric technique. Anthracene has an oxidation potential at +1.04 V62 and reduction potential of -2.2 V vs SCE.63 All of these four dyes undergo irreversible oxidations between +1.08 and +1.40 V. We did not observe the reduction of M1 within our experimental conditions. The remaining three compounds have reduction potentials between -1.27 and -1.70 V. Both oxidation and reduction potentials of these dyes are more positive than anthracene. The ground state oxidation potentials (Eox) (Table 1) which correspond to the HOMO levels of the dyes (M1-M4) are well below those of the I-/I3- redox couple (0.24 V vs SCE64). The

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excited state oxidation potentials (Eox*) of the dye, which correspond to the LUMO, can be calculated by subtracting the band gap (E0-0) from the Eox value of the dye, where E0-0 can be estimated from the onset of the absorption.65 Eox* calculated from these values is given in Table 1. The Eox* values of these dyes (M1-M4) are negative enough for the TiO2 conduction band (-0.8 V vs SCE64) to inject the electron for a thermodynamically favorable process to occur (Figure 3 for M1 and M3 is shown). Adsorption of Dyes on the TiO2 Anatase (101) Surface (M1 and M3). FTIR and ATR-FTIR Studies. For clarity, only M1 and M3 adsorption has been studied and compared. FTIR spectra of M1 show a 1699 cm-1 peak, and those of M3 show a 1721 cm-1 peak which is attributed to νCdO stretching frequencies (Figure C, Supporting Information). FTIR spectra of the M1 and M3 in their carboxylate sodium salt form show peaks at 1370 and 1634 cm-1 for M1 and 1373 and 1645 cm-1 for M3 which is attributed to the characteristic symmetric (νsym) and asymmetric (νasym) stretching frequencies of the carboxylate group (Figure D, Supporting Information). ATR-FTIR spectra (shown in Figure 4) of the dyes adsorbed on TiO2 have peaks at 1382 and 1587 cm-1 for M1 and 1395, 1586, and 1730 cm-1 for M3. These characteristic differences between the vibrational frequencies of the dye, dye-sodium salt and dye-TiO2 systems are due to the alteration of the bond order between C and O. A carboxylic acid can bind to the TiO2 in several fashions such as physisorption (mainly due to the H-bonding nature between acidic proton and oxygen atom of TiO2) and chemisorption (monodentate, bidentate chelating, bidentate bridging, as shown in Figure 5). From the ATR-FTIR spectra, the disappearance of νCdO and the presence of new peaks of νsym and νasym for M1 and M3, one can rule out physisorption.66 In order to find out the nature of the binding mode from vibrational frequency analysis, the difference between νsym and νasym of CdO stretching frequencies of the dye-sodium salt (∆DS) and dye-TiO2 (∆DT) must be known. This difference and the nature of the binding mode can be correlated as67,68

∆DS ) νasym - νsym ∆DT ) νasym - νsym for CdO stretching frquencies if ∆DT > ∆DS ⇒ monodenate coordination if ∆DT ∆DT, one can infer that M1 will bind to the TiO2 either through bidentate chelation or bidentate bridging.68 Vittadini69 et al. suggested that the bidentate chelating mode is an unstable binding mode for sodium formate and formic acid on TiO2 anatase through their theoretical studies. Hence, one can suggest that M1 will bind to the TiO2 through the bidentate-bridging mode. This is also supported by our computational studies which we discuss in the next section. For M3, a slightly different behavior was observed due to the two carboxylic acid groups on the same R-carbon. We rule out both carboxylic acid groups binding to the TiO2 at the same time due to the highly strained binding mode which would result from it. Here, we have found 1395 cm-1 for νsym, 1586 cm-1 for νasym, and another intense peak at 1730 cm-1. This shows the presence of a non-H-bonded carboxylic group present in the adsorbed dye (M3). ∆DS and ∆DT are found to be 272 and

Figure 6. TiO2 vacuum slab used for the adsorption studies of the molecules.

191 cm-1, respectively, for M3. As ∆DS > ∆DT, one can infer that one of the anchored carboxylic acids will bind to the TiO2 in a bidentate-bridging mode. Computational Studies of Adsorption Configurations. To study the different possible adsorption configurations of these dyes on the metal oxide surface, the anatase surface model used in the present calculations consists of a periodically repeated slab of 3 × 1 × 2 slabs with 24 [TiO2] units in the XY-plane and perpendicular to the Z-axis of the super cell. A vacuum space of 15 Å is considered in the Z-direction to ensure the sufficient separation between the lowest layer and the upper layer of the slab. To prevent the surface deformation, the lowest layer of the super cell is fixed in the calculation. Thus, the overall dimension of the super cell generated from the vacuum slab will have dimensions of a ) 10.24, b ) 11.35, and c ) 20.86, which is shown in Figure 6. The adsorption energy, Eads, is calculated using the expression

Eads ) Eslab ) Emolecule - (Eslab+molecule)

(5)

where Eslab represents the energy of the clean slab, Emolecule is the energy of the adsorbate in the gas phase, and E(slab+molecule) is the total energy of the slab with adsorbate. A positive value of Eads > 0 indicates stable adsorption. To check the consistency and reliability of our calculations, we compared the molecular adsorption energy of water on fivecoordinated surface titanium (Ti5c) calculated here with the earlier results in Table A (see the Supporting Information). The calculated molecular adsorption energy obtained here for water is 17.94 kcal/mol which is in good agreement with the experimental estimate of 11.53-16.14 kcal/mol.70 Figure E (see the Supporting Information) shows the optimized structure of the water adsorption on the TiO2 surface. Here, we studied only the dissociative adsorption modes of the M1 and M3 dyes based on the ATR-FTIR data.71 The possible dissociative adsorption modes for the acrylic acid derivative are shown in Figure 7. These modes are generated by optimizing the structures in which the separated H+ cation and carboxylate anion (An-COO-) moieties were placed on suitable sites of the surface. The coordination may be monodentate ester-type (MET), bidentate chelating (BC), or bidentate bridging (BB) depending on the number of oxygens used by the anion to coordinate the surface Ti5c acidic sites. The values

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Figure 7. Different adsorption modes of M1 dye on the 3 × 1 × 2 TiO2 surface.

TABLE 5: Important Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for the Different Adsorption Configurations of M1 and M3 configuration

C1-O2/ C1′-O2′

C1-O3

MET BC BB

1.340 1.329 1.285

M1 1.226 1.240 1.287

MMET MBC MBB CBB1 CBB2

1.369 1.320 1.285 1.294 1.348/1.352

M3 1.210 1.251 1.307 1.306 1.221

Ti5c-O2/ Ti5c-O2′ 1.888 2.209 2.052 1.902 2.289 2.081 1.999 1.855/1.889

Ti5c-O3

Eads

2.357 2.153

2.22 –5.81 22.18

2.265 2.174 2.036

2.56 –4.65 15.91 5.39 –1.66

of the adsorption energies for the three modes of M1 are summarized in Table 5, along with important structural parameters. In all of these calculations, a single molecule per super cell is considered. From the table, it is observed that the bonds formed with the Ti5c are within the range of van der Waals radius. The BB configuration is preferred over the MET configuration by 19.96 kcal/mol, while the BC configuration yields negative binding energy which indicates the unpreferable mode of dissociative adsorption of this dye. The most favorable BB configuration from the theory is also consistent with the ATR-FTIR studies. Similarly, the possible dissociative adsorption modes for the malonic acid derivative M3 are also shown in Figure 8. In this case, because of the two carboxylic groups, there exists two types of dissociations, viz., mono and complete dissociations by the loss of hydrogens from the respective acidic groups; therefore, two more additional configurations are obtained. The first three coordinations are similar to the mono carboxylic group, i.e., acrylic acid derivative and are MMET, MBC, and MBB, where M denotes mono dissociation and the additional configurations due to the complete dissociations are CBB1 and CBB2, respectively (the optimized structures are shown in Figure 8). The adsorption energies for these configurations are shown in Table 5. The values show that the MBB configuration (mono dissociation) is most stable. The CBB1 configuration is less stable than the MBB configuration which is also supported by ATR-FTIR studies. This indicates that, in the case of dicarboxylic acid, one of the acid groups is free and the dye anchors with the oxygens belonging to a single carboxylic acid group rather than oxygens from each -COOH group. Thus, in both of the cases, i.e., dye with mono and dicarboxylic acids, the preferred adsorption configurations are the bidendate bridging (BB and MBB).

Electronic Structure of Adsorbed Complexes. To understand the electronic structure, density of states (DOS) for the clean and the adsorbed complexes are presented in Figure 9. The DOS contains broad surface valence and conduction bands separated by a wide band gap. After adsorption, the dyes introduce sharp occupied molecular energy levels in the band gap. The HOMO for these adsorbed systems is a anthracene π-orbital which is uncoupled with the valence bands of the surface. However, in the case of the M3 adsorption complex, the HOMO is slightly destabilized as compared with the HOMO of M1. Similarly, the LUMO is stabilized in M1 and destabilized in M3. From the DOS figure, it is clear that the dyes have a strong overlap with the valence band and the conduction band of the semiconductor over a wide range of energies. We also estimated the effect of the dye adsorption on the open-circuit voltage (Voc) by DFT methods. Voc is given by the difference between the redox potential and the Fermi level in the TiO2.72-78 Increase in Voc value is correlated to the shift of the semiconductor band edge toward negative potentials.78 This shift would also decrease the short-circuit photocurrent density (Jsc).78 From Figure 9, it is clear that, when compared to the bare TiO2 DOS, the DOS of M1 adsorbed and M3 adsorbed TiO2 point out a shift of the conduction band edge of TiO2 to more negative potentials, though the bandgap remains almost the same. It is also seen that the shifts in the Fermi level of M1 (0.884 eV) and M3 (0.822 eV) are almost the same. This is in good agreement with the Voc obtained by experimental methods, as shown in Table 1. Conclusions All four anthracene based sensitizers show reasonable conversion efficiency. From the UV-vis spectrum, it is seen that these have a CT type of absorption. The monocarboxylic acid based dyes absorb slightly in the lower energy region (∼15 nm) which could be attributed to the presence of the -CN electron withdrawing group in the R-position. DFT and TDDFT studies assign the major transitions as HOMO-LUMO. The transition dipole moment calculated is reasonably high and does not vary much with change in substitution. The HOMO and LUMO pictures clearly point out to the electron density being localized on the acceptor groups in the excited state. This should facilitate easy injection into the TiO2 surface. Electrochemical studies and the estimated Eox and Eox* show that the dyes M1 and M2 have more positive HOMO and LUMO when compared to M3 and M4. The FTIR and ATR-FTIR studies reveal that the binding type is dissociative and bidentate bridging. Calculations

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Figure 8. Different adsorption modes of M3 dye on the 3 × 1 × 2 TiO2 surface.

has a lower binding to the TiO2(101) surface compared to the cyanoacrylic acid anchoring group. Acknowledgment. We are very much thankful to The Director, IICT, and to The Head, Inorganic Chemistry Division, IICT, for their constant support in this work. We also thank Dr. B. Jagadeesh and Mr. Sarma for carrying out the ATRFTIR studies. K.S. and K.Y. thank CSIR for the fellowship.

Figure 9. Total DOS (black) and the adsorbate-projected DOS (red) for (a) TiO2, (b) TiO2-M1BB, and (c) TiO2-M3MBB.

Supporting Information Available: The photocurrent action spectra of the dye-sensitized solar cells, electrochemical investigations by using the differential pulse voltammetric technique, νCdO stretching frequencies, symmetric (νsym) and asymmetric (νasym) stretching frequencies of the carboxylate group, optimized structure of the water adsorption on the TiO2 surface, and comparison of the molecular adsorption energy of water on fivecoordinated surface titanium (Ti5c) with earlier results. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

using periodic DFT methods show that the binding of the group to the TiO2 surface is as much as 6 kcal/mol more stable than the group. This larger binding in M1 and M2 seems to be the main reason for the higher efficiency of the sensitizers. It is concluded that the malonic acid group in general

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