Tetrazole as a New Anchoring Group for the Functionalization of TiO2

Article pubs.acs.org/JPCC

Tetrazole as a New Anchoring Group for the Functionalization of TiO2 Nanoparticles: A Joint Experimental and Theoretical Study Julien Massin, Laurent Ducasse, Thierry Toupance,* and Céline Olivier* Université de Bordeaux, UMR-CNRS 5255 Institut des Sciences Moléculaires, 351 cours de la Libération, 33405 Talence, France S Supporting Information *

ABSTRACT: Functional hybrid materials are an important tool for generating original architectures featuring desirable properties for multiple applications. The success in creating innovative materials with valuable functionalities relies on the close interaction between the organic and inorganic parts of the hybrid system. Herein, we report the use of tetrazole as an anchoring group for the photosensitization of TiO2 nanoparticles by an organic chromophore and the related performance in dye-sensitized solar cells. The interaction mode between the tetrazole motif and TiO2 was thoroughly investigated by various techniques. The overall study reveals that the optoelectronic and photovoltaic properties of the dye featuring tetrazole rival those of an analogue bearing a carboxylic acid function, even leading to significantly enhanced photovoltage in the device. These results demonstrate the effectiveness of the tetrazole functional group as a serious alternative anchoring group for organic photosensitizers in hybrid materials for energy.

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INTRODUCTION Functional hybrid architectures combining active organic and inorganic components at the nanometer scale play a crucial role in the development of next-generation materials.1,2 In such systems, the control of organic−inorganic interfaces and the creation of strong linkages between the different elements through covalent or iono-covalent bonds can lead to remarkable properties due to synergistic effects. This has successfully been achieved for silicates both by direct softchemistry routes based on hydrolysis−condensation of alkoxyorganosilanes and silsesquioxanes and by postfunctionalization methods involving surface modification of preformed nanoobjects by functional organosilanes.3,4 Moreover, in the field of transition-metal-based hybrid nanocomposites, coordination chemistry represents a powerful tool for tailoring robust hybrid systems through metal−ligand interactions, thereby creating innovative networks that cover a broad spectrum of properties and find application in extremely diverse fields.5 Among other remarkably stable hybrid nanostructures, coordination polymers6 and metal−organic frameworks (MOFs),7 as examples, have been widely used for gas-uptake,8,9 photocatalysis,10,11 and energy12−14 applications. Coordination chemistry has also been applied to postsynthesis functionalization of transition-metal oxide nanostructures to produce functional hybrid materials for energy. In this scope, dye-sensitized solar cells (DSCs) have lately emerged as an original and promising technology for converting sunlight into electricity.15,16 Within this photovoltaic device, the working electrode comprises interconnected dye-coated titanium dioxide nanoparticles (TiO2 NPs) organized in a porous framework forming a thin film that is able to harvest light effectively and to convey photogenerated electrons. For a DSC device to achieve long-lasting salient power conversion © 2014 American Chemical Society

efficiency, a strong link is compulsory between the main actors of the thin film, that is, the hybrid pair formed by the dye and the semiconductor. In most cases, dyes are grafted onto the metal oxide surface through one or more carboxylic acid functions.17 However, irradiation over long periods can provoke a release of the dyes bound to TiO2 through COOH anchors.18−20 Therefore, alternative functional groups for fastening dyes onto TiO2 have been examined. One first family includes those that, similarly to carboxylate, involve only oxygen atoms for interaction with titania. For instance, acetylacetonate,21 cathecolate,22 and nitro groups23 have been described as robust anchors for the functionalization of TiO2 NPs. A second type is composed of mixed oxygen- and nitrogen-containing groups, such as 8-hydroxylquinoline20 and 2-(1,1-dicyanomethylene)rhodanine.24 The last class, consisting of nitrogen-containing groups exclusively, recently provided examples of dyes with a pyridine ring as an electron-withdrawing and anchoring function.25 Finally, except for phosphinic acid26 and trialkoxysilane27 groups, grafting functions bearing different heteroatoms are rarely used in DSCs. The above-mentioned grafting modes are derived from coordination chemistry; nevertheless, the abundance of chelating functions available to build stable metal−organic hybrid architectures leaves room for further investigations. This, in turn, prompted us to explore the use of an alternative anchoring group to functionalize TiO2 NPs, the tetrazole ring, which is a five-membered heterocyle containing four nitrogen atoms and thus as many potential coordination sites.28 Received: March 12, 2014 Revised: April 23, 2014 Published: April 23, 2014 10677

dx.doi.org/10.1021/jp502488g | J. Phys. Chem. C 2014, 118, 10677−10685

The Journal of Physical Chemistry C

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Chart 1. Molecular Structures of the Dyes Used in This Study Bearing Tetrazole (D1), Carboxylic Acid (D2), and Pyridine (D3) as Anchoring Groups

Scheme 1. Preparation Route to the Dyes D1−D3a

(i) Pd(OAc)2, P(tBu)3, K2CO3, toluene, 90 °C, 24 h; (ii) MePPh3I, tBuOK, tetrahydrofuran (THF), room temperature, 24 h; (iii) pbromobenzaldehyde, cataCXium, 2,6-di-tert-butylcresol, dimethylacetamide (DMAc), 130 °C, 18 h; (iv) nitrile [i.e., 2-(2H-tetrazol-5-yl)acetonitrile, cyanoacetic acid, or 4-pyridylacetonitrile], piperidine, CHCl3, 90 °C, 16 h.

a

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RESULTS AND DISCUSSION Synthesis. Dyes D1−D3 (Scheme 1) were prepared by a straightforward synthetic route. The three dyes were obtained from a common precursor following a four-step procedure, with the desired anchoring function introduced in the last step. The overall procedure involved a Buchwald−Hartwig cross-coupling reaction to provide 4-[bis(4-methoxyphenyl)amino]-benzaldehyde (A). Methylenation of the aldehyde in a Wittig reaction led to 4-ethenyl-bis(4-methoxyphenyl)-benzenamine (B), which was subsequently coupled with p-bromobenzaldehyde by a Heck-type reaction to afford 4-[2-[4-[bis(4methoxyphenyl)amino]phenyl]ethenyl]-benzaldehyde (C). Finally, Knoevenagel condensation of C with the appropriate nitrile derivative, namely, 2-(2H-tetrazol-5-yl)acetonitrile, cyanoacetic acid, and 4-pyridylacetonitrile, in the presence of piperidine led to target dyes D1−D3, respectively, in good yields. Optical and Electrochemical Properties in Solution. Combined spectroscopic and electrochemical studies in solution allowed experimental estimation of the frontier molecular orbital energy levels. UV−vis absorption spectra of D1−D3 in dichloromethane are shown in Figures S1−S3

Although tetrazole is known to provide remarkable coordination networks with transition metals,8,29 this strong chelator has never been employed for surface modification of metal oxides nor introduced within organic sensitizers for DSCs. In this domain, the most efficient dyes are designed according to a donor−π−linker−acceptor (D−π−A) structural pattern, where the acceptor part simultaneously plays the role of electronwithdrawing and anchoring function.30,31 Such a push−pull layout is crucial to achieving strong visible-light absorption; it also provides good directionality of the charge-transfer process and favors electron injection into the semiconductor. Herein, we report a tetrazole-based motif as a new anchoring group for photosensitizers in DSCs. Indeed, the (2H-tetrazol-5-yl)acrylonitrile moiety, which can be easily obtained from malononitrile and sodium azide, not only grafts effectively onto TiO2 but also affords a proper D−π−A structure to the dye molecule. The optoelectronic and photovoltaic properties of dye D1 (Chart 1) were further explored and compared to those of analogues bearing carboxylic acid (D2)32 and pyridine (D3) as grafting functions. 10678



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(Supporting Information); the corresponding data are listed in Table 1. The dyes show two major absorption bands: a shortTable 1. Optical and Electrochemical Properties of Dyes D1−D3 dye

λmax absa (nm) (ε [M−1 cm−1])

λmax ema (nm)

λmax abs at TiO2b (nm)

Eoxc (V)

E0−0d (V)

Eox − E0−0 (V)

D1 D2 D3

465 (24500) 475 (27600) 457 (41100)

607 597 549

420 423 449

0.80 0.84 0.86

2.36 2.31 2.46

−1.56 −1.47 −1.60

Figure 1. Normalized electronic absorption spectra of D1−D3 adsorbed on 3-μm transparent TiO2 film (solid) and in solution in CH2Cl2 (dashed). Note: The cutoff of the curves on TiO2 is due to UV-light absorption by the metal oxide. (red for D1, black for D2, blue for D3).

a Absorption (abs) and emission (em) spectra in CH2Cl2. bAbsorption on 3-μm-thick transparent TiO2 film. cOxidation potential measured in CH2Cl2 including 0.1 M Bu4NPF6 as a salt support at a scan rate of 100 mV s−1 (working electrode, Pt disk; reference electrode, Ag/AgCl, calibrated with ferrocenium/ferrocene as an internal reference, potentials referred to NHE reference by addition of 630 mV;33 counter electrode, Pt). dE0−0 estimated from the intersection between absorption and emission spectra.

presence of a broad absorption band in the visible region indicated effective photosensitization of the oxide layer by the chromophores. A similar trend was observed for the tetrazole (D1) and carboxylic acid (D2) derivatives upon grafting onto TiO2. In both cases, a blue shift of λmax was observed (∼50 nm), attributable to the deprotonation of the dye molecules and to their interaction with the metal oxide.34 However, very little variation of the absorption spectrum was observed for D3 upon adsorption (only 8 nm), suggesting a different interaction mode with TiO2 compared to D1 and D2. As expected, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of D2 (Figure S4, Supporting Information) showed the vanishing of the carboxylic acid stretching peak (ν = 1697 cm−1) upon grafting, with the concomitant appearance of asymmetric and symmetric stretching modes of the carboxylate group (at 1587 and 1390 cm−1, respectively).35 This indicates that D2 is anchored on the metal oxide surface through the carboxylate group through a bidentate chelation or bridging coordination mode rather than through an ester-type linkage. The calculated infrared spectrum of D1 (Figures S5 and S6, Supporting Information) allowed a vibrational stretching for the tetrazole NN bonds to be located around 1050 cm−1. A shift of the corresponding peak observed in the experimental spectrum (from 955 to 895 cm−1, before and after adsorption on TiO2) confirmed that the interaction of D1 with the oxide surface occurs through the conjugated heterocycle. In contrast, only very few changes were observed in the vibrations corresponding to the pyridine ring of D3. This again implies a different type of interaction between pyridine and TiO2 compared to carboxylic acid and tetrazole. Finally, for the three dyes, the stretching peak at around 2220 cm −1 corresponding to the ν CN vibration remained unchanged upon adsorption, suggesting that the nitrile group is not involved in the anchoring process. Quantum Chemical Calculations. Theoretical calculations were performed to gain deeper insight into the coordination mode between the dye molecules and the TiO2 surface. Optimized geometries of the hybrid systems are shown in Figure 2. As expected for D2, a strong interaction between TiO2 and the carboxylate group was evidenced by two short TiO distances (2.09 Å) corresponding to a bridging coordination mode. In the case of D1, the optimized geometry at the minimum energy indicated that the interaction of the tetrazole with the surface occurs through one short TiN distance (2.53 Å) and an additional longer one (3.48 Å). Thus, similarly to the

wavelength one (λ < 400 nm) corresponding to π → π* transitions and an intense band in the visible part, with its maximum wavelength centered around 460−470 nm, attributed to an intramolecular charge transfer (ICT) from the triarylamine electron-rich part to the cyanoacrylic electron-withdrawing part of the molecule. Although slightly lower, the molar extinction coefficient of D1 endowed with a tetrazole anchor (24.5 × 103 M−1 cm−1) was close to that of the carboxylic acid derivative D2 (27.6 × 103 M−1 cm−1). In general, the absorption coefficients of the dyes confirm the good light-harvesting properties of the series. Cyclic voltammograms recorded for D1−D3 showed a reversible monoelectronic process corresponding to their first oxidation potential. As expected from their common triphenylamine electron-donating core, little difference was observed in the Eox values of the dyes (0.80−0.86 V vs NHE). The optical band-gap energy (E0−0) was estimated from the intersection between the normalized absorption and emission spectra of the dyes (Figure S2, Supporting Information); the lowest unoccupied molecular orbital (LUMO) level was deduced from the difference between the highest occupied molecular orbital (HOMO) level, assimilated to Eox, and the lowest transition energy (E0−0). Because of the systematic presence of a nitrile group in the electron-withdrawing part of the dyes, their LUMO levels were also close in energy. The overall experimental data set confirmed a good matching of the dyes energy levels with the DSC device requirements. The HOMO level was more positive than the standard potential of the iodine/iodide redox couple (0.45 V vs NHE), guaranteeing sufficient driving force for the regeneration of the dye by the electrolyte, and the LUMO level was far above the conductionband edge of TiO2 (ca. −0.50 V vs NHE), thus ensuring effective electron injection from the dye to the semiconductor. As a result, on the basis of the HOMO and LUMO energy level experimental estimation, device operation of DSCs combining D1−D3, a TiO2 porous layer, and iodine/iodide as a redox shuttle should be energetically favorable. Interaction with the TiO2 Surface. The functionalization of a nanoparticulate TiO2 thin film with D1−D3 was evidenced by UV/visible absorption and infrared spectroscopies. Absorption spectra of the dyes adsorbed onto TiO2 are shown in Figure 1, and the corresponding λmax values are reported in Table 1. Regardless of the anchoring mode, the 10679



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Figure 2. Calculated interactions of dyes D1−D3 with the TiO2 surface optimized at the self-consistent-charge density-functional tight-binding (SCC-DFTB) level, with enlargement of the anchoring part. Numbers are interatomic distances (in Å).

carboxylic acid derivative D2, the tetrazole-end-capped D1 adopts a strong chemisorption-like coordination mode to attach onto TiO2. Conversely, the pyridine ring in D3 led to longer TiN distances (3.48 Å), resulting in a much weaker interaction with the surface. More interestingly, whereas the minimum energies calculated for the D1 and D2 hybrid systems corresponded to a vertical geometry with regard to TiO2, with the molecules standing on the surface, the preferred geometry found for D3 corresponded to a parallel arrangement with the molecules lying on the surface (Figure S7, Supporting Information). This result was consistent with the quasi-absence of variation in the experimental visible and IR absorption spectra of D3 upon adsorption. Such a feature is associated with physisorption. Because the charge-transfer (CT) character of the lowestenergy transition is a key factor influencing the electron injection efficiency rate into the TiO2 conduction band, relevant computational parameters relative to the CT process occurring upon photoexcitation of the dyes were considered (Table S1, Supporting Information). Time-dependent density functional theory (TD-DFT) calculations were performed for the dyes isolated and adsorbed on TiO2. Similarly to previous reports on triarylamine-based dyes,36,37 the calculations showed good spatial separation of the frontier molecular orbitals (MOs) of D1−D3. Isodensity surface plots of the MOs of the three dyes D1−D3 are depicted in Figure 3. Note that the shapes of the frontier MOs are quite independent of the basis and of the functional. In the ground state, the electron density is distributed over the electron-donating triphenylamine core with important contribution of the penylethenyl π-conjugated linker, whereas at the LUMO level, excited electrons are transferred toward the cyano-vinyl remote part of the molecule, still with a sizable contribution of the π-bridge. In accordance with the experiments, calculations related to the isolated dyes indicated that the energies of the main lightinduced electronic transition are similar for the three compounds, although with a slight red shift for D2 (Figure S8, Supporting Information). The first allowed transition S0 →

Figure 3. Isodensity surface plots of the (left) HOMOs and (right) LUMOs of D1−D3 calculated at the B3LYP/6-311+g(d,p) level.

S1 has a strong HOMO → LUMO character, as reported for other organic dyes with a D−π−A structure.35−37 The quite significant distance of the CT process (DCT > 6 Å) and the high quantity of charge transferred (qCT > 0.9) confirmed the strong push−pull character of the isolated dyes. Calculations relative to the CT process undergone by dyes chemisorbed on a titanium oxide cluster were focused on D1 and D2, as both showed a strong interaction mode with the oxide surface. Again, as observed experimentally, the energies of the main transition calculated for the D1 and D2 hybrid systems were very similar (2.79 eV). The blue shift of the main transition observed upon chemisorption of the dyes might arise from a significant change in the total electron density of the involved MOs, as the first allowed transition of the dye−TiO2 hybrid systems shows a HOMO → LUMO + 2 predominant character (see the Supporting Information for details). 10680



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Photovoltaic Characterization. The photovoltaic performances of dyes D1−D3 were investigated under standard conditions (see Experimental Section) in the presence of iodine-based liquid electrolyte; the results are gathered in Table 2. Current−voltage curves and IPCE profiles are shown in Figure 4.

PCE of 5.53%. This result was slightly higher than that already reported for the same dye.32 The higher short-circuit current density (Jsc) observed for D1 was consistent with a higher molar absorption coefficient (Table 1). Furthermore, as the amount of dye present per unit of TiO2 strongly influences the light-harvesting properties of a hybrid material and, therefore, the quantity of charge injected into the semiconductor and the current output, the dye loading was determined for D1 and D2 (Table 2). Hence, the slightly lower Jsc value measured for D1 could be also interpreted in terms of a lower dye loading, 1.52 × 10−7 mol cm−2, compared to 2.10 × 10−7 mol cm−2 for D2. This result was presumably ascribed to the above-described dye interaction mode with TiO2, with D2 leading to shorter molecule-to-surface distances. As shown in Figure 4b, the incident photon-to-current conversion efficiency (IPCE) maxima of the two dyes were comparable, reaching more than 80% in the 400−500-nm region, however with an increased red response for D2. Interestingly, however, the tetrazole dye D1 afforded an open-circuit photovoltage (Voc) that was 100 mV higher than that of D2. Further insight into the influence of the dyes on Voc was thus obtained by calculating their permanent dipole moments in the ground state (μGS). It was previously demonstrated that surface modification of TiO2 by positive molecular dipoles induced an upward shift of the conduction band (CB) edge energy and, therefore, enhanced the DSC photovoltage.38−41 Here, we calculated dipole moments of μGS = 10.75 and 12.28 D for D1 and D2, respectively (Figure S12, Supporting Information). According to these data, one would expect a larger Voc value for D2, whereas the experimental results showed the opposite. This interesting feature was therefore correlated to the amount of dye loaded. Knowing that the presence of positive charges near the surface provokes a downward shift of the CB,42 the higher dye loading of D2 implies that more protons are released in the vicinity of the TiO2 surface during dye adsorption, and this has negative effect on Voc. Furthermore, the carboxylate chelate in D2, being more basic than the tetrazolate in D1, is likely to result in stronger interactions with cations contained in the electrolyte such as Li+, thus inducing an increased concentration of positive charges along the surface. As a result, the interaction mode of D1 with TiO2 through a tetrazole anchor has a beneficial effect on Voc compared to the carboxylic acid function. Additionally, as illustrated in Figure 4a, the onset of dark current occurred at a higher potential for D1. It is generally admitted that the dark current in DSCs is closely related to electron recombination phenomena between the semiconducting oxide and the iodine-based electrolyte.43−45 Here, it appears that the tetrazole-based dye D1 reduces charge recombinations at the TiO2/electrolyte interface compared to D2. In addition, the performance of a DSC incorporating D1 and the volatile electrolyte Z960 remained stable over a period of several weeks under ambient conditions (Figure S13, Supporting Information), again testifying to a strong linkage between D1 and TiO2 through the tetrazole motif. Note that, under the same conditions, a similar trend was observed for D2 (Figure S14, Supporting Information).

Table 2. Photovoltaic Performance of D1−D3 under Standard Conditionsa and Dye Loadings on TiO2 dye

Jsc (mA cm−2)

Voc (mV)

ff (%)

η (%)

D1c D2c D3d

10.1 12.7 4.1

722 625 506

71.8 69.7 66.8

5.27 5.53 1.40

dye loadingb (×107 mol cm−2) 1.52 2.10 e

a

Photovoltaic data measured at full sunlight (AM1.5G, 100 mW cm−2); active area, 0.159 cm2; Jsc, short-circuit current density; Voc, open-circuit photovoltage; ff, fill factor; η, power conversion efficiency. b Measured on a 1-cm2 TiO2 electrode. cIn the presence of chenodeoxycholic acid (2 mM) as a coadsorbent; electrolyte, Z960. d Without coadsorbent or tert-butylpyridine (TBP). eNot relevant.

Figure 4. (a) Current−voltage (dotted, in the dark; solid, under illumination) and (b) IPCE profiles of DSCs using D1 (red) and D2 (black).

As can be immediately noticed from Table 2, D1 and D2 afforded similar performances in DSCs, whereas the pyridinebased D3 again showed completely different behavior. Indeed, the latter was completely incompatible with use of coadsorbent (chenodeoxycholic acid) as a deaggregating agent, which strongly competed with the dye for adsorption onto TiO2. Moreover the presence of tert-butylpyridine (TBP) in the electrolyte was to be avoided because this additive provoked fast desorption of the dye molecules. As a consequence, the photovoltaic parameters measured with devices employing D3 were quite low (Table 2), although comparable to those previously reported for dyes bearing pyridine as an anchoring group.25 Still, under standard conditions, the tetrazole derivative D1 reached an overall power conversion efficiency (PCE) of 5.27%, closely trailing the carboxylic acid dye D2, which afforded a

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CONCLUSIONS The development of robust organic−inorganic networks remains a great challenge to create innovative hybrid materials with valuable functionalities, in particular for energy conversion applications. In this work, an organic triarylamine-based 10681



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cool to room temperature, the mixture was diluted with CHCl3 and washed with 1 N aqueous HCl and water. After removal of the solvent, the crude product was purified by precipitation from a CH2Cl2/cyclohexane mixture. Finally, the precipitate was filtered, washed with pentane, and dried under a vacuum. (E)-3-(4-(4-(Bis(4-methoxyphenyl)amino)styryl)phenyl)-2(2H-tetrazol-5-yl)acrylonitrile (D1). D1 was obtained according to the general protocol from C (200 mg, 0.46 mmol), 2(2H-tetrazol-5-yl)acetonitrile (501 mg, 4.6 mmol), and piperidine (0.91 mL, 9.2 mmol) in dry CHCl3 (40 mL). The crude product was purified by precipitation to afford 1 as a red powder in 81% yield (196 mg, 0.37 mmol). 1H NMR (400 MHz, acetone): δ 8.49 (s, 1H), 8.10 (d, J = 8.4 Hz, 2H), 7.82− 7.72 (m, 2H), 7.47 (t, J = 9.0 Hz, 2H), 7.41 (d, J = 16.3 Hz, 1H), 7.17 (d, J = 16.3 Hz, 1H), 7.09 (dd, J = 12.3, 5.6 Hz, 4H), 6.94 (d, J = 8.9 Hz, 4H), 6.84 (d, J = 8.7 Hz, 2H), 3.81 (s, 6H). 13 C NMR [100 MHz, dimethyl sulfoxide (DMSO)]: δ 156.76, 156.10, 149.31, 148.20, 142.41, 140.20, 131.99, 131.05, 128.64, 128.51, 127.58, 127.17, 124.64, 119.09, 116.41, 115.47, 95.46, 55.34. MS (ESI): calcd for C32H25N6O2, 525.2044; found, 525.2026 [M − H]−. (E)-3-(4-(4-(Bis(4-methoxyphenyl)amino)styryl)phenyl)-2cyanoacrylic Acid (D2). D2 was obtained according to the general protocol from C (70 mg, 0.16 mmol), cyanoacetic acid (137 mg, 1.6 mmol), and piperidine (0.32 mL, 3.2 mmol) in dry CHCl3 (14 mL). The crude product was purified by precipitation to afford D2 as a red powder in 81% yield (65 mg, 0.13 mmol). 1H NMR (300 MHz, acetone): δ 8.11 (s, 1H), 7.93 (d, J = 8.3 Hz, 2H), 7.61 (d, J = 8.3 Hz, 2H), 7.33 (d, J = 8.5 Hz, 2H), 7.28 (d, J = 15.9 Hz, 1H), 7.02 (d, J = 15.9 Hz, 1H), 6.96 (d, J = 8.8 Hz, 4H), 6.80 (d, J = 8.8 Hz, 4H), 6.70 (d, J = 8.5 Hz, 2H), 3.76 (s, 6H). (Z)-3-(4-(4-(Bis(4-methoxyphenyl)amino)styryl)phenyl)-2(pyridin-4-yl)acrylonitrile (D3). D3 was obtained according to the general protocol from C (200 mg, 0.46 mmol), 4pyridylacetonitrile monohydrochloride (711 mg, 4.6 mmol), and piperidine (0.91 mL, 9.2 mmol) in dry CHCl3 (40 mL). The crude product was purified by precipitation to afford D3 as a red powder in 82% yield (203 mg, 0.38 mmol). 1H NMR (400 MHz, DMSO): δ 8.70 (d, J = 6.0 Hz, 2H), 8.30 (s, 1H), 8.01 (d, J = 8.4 Hz, 2H), 7.74 (m, 4H), 7.45 (d, J = 8.7 Hz, 2H), 7.35 (d, J = 16.3 Hz, 1H), 7.08 (m, 5H), 6.94 (d, J = 8.9 Hz, 4H), 6.76 (d, J = 8.7 Hz, 2H), 3.75 (s, 6H). 13C NMR (100 MHz, DMSO): δ 156.75, 151.03, 149.24, 146.27, 142.08, 141.70, 140.23, 131.95, 131.57, 130.84, 128.74, 128.43, 127.56, 127.05, 124.77, 120.24, 119.14, 117.83, 115.48, 106.72, 55.34. MS (FD+): calcd for C36H29N3O2, 535.2260; found, 535.2273 [M]+ •. Photophysical Characterization. UV−visible absorption and emission fluorescence spectra were recorded on a Shimadzu UV-1650PC spectrophotometer and a Horiba FluoroMax-4 spectrofluorometer, respectively. Molecular extinction coefficients were determined from solutions of the dyes at different concentrations (2 × 10−5, 8 × 10−6, and 4 × 10−6 M). Determination of Dye Loadings. A solution of dye (0.3 mM) and chenodeoxycholic acid (2 mM) in dichloromethane was prepared and utilized to sensitize a nanoparticulate TiO2 thin film (thickness = 6.8 μm, surface area = 1 cm−2) as described below. UV−visible absorption spectra of each dye solution were recorded prior to and after sensitization. The amount of dye loaded onto TiO2 was deduced from the difference between the two sets of data.

chromophore was endowed with tetrazole to be effectively attached onto nanoparticulate TiO2 thin films. The photophysical properties of the new chromophore, along with its photovoltaic performance in dye-sensitized solar cells, were explored and compared to two analogues bearing carboxylic acid and a pyridine ring as grafting functions. This joint experimental and theoretical study evidenced strong anchoring of the tetrazole-equipped dye onto the oxide surface. The photovoltaic results further obtained with the tetrazole derivative outperformed those of the pyridine analogue and closely competed with those of the carboxylic acid, even showing a significant increase in photovoltage for a device using the tetrazole dye. This feature was correlated with the interaction mode of the new anchor with TiO2 surface. Overall, the results demonstrate the effectiveness of the tetrazole functional group as an alternative anchoring function of organic photosensitizers in hybrid materials for energy applications. We therefore anticipate a new generation of efficient tetrazolebased dyes for DSCs. The design and preparation of novel chromophores showing increased red-light absorption properties, in particular through introduction of polythiophene and/or benzothiadiazole motifs, and tetrazole as an anchoring function is currently under development in our group.

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EXPERIMENTAL SECTION General Information. All reagents were obtained from commercially available sources and used without further purification. Solvents were dried from appropriate drying agents (sodium for toluene and tetrahydrofuran; calcium hydride for dichloromethane, chloroform, and methanol) and freshly distilled before use. Chemical reagents were purchased from Sigma-Aldrich and used as received. Dye D2 was synthesized according to the literature procedure.32 1H NMR and 13C NMR analyses were performed on Bruker Avance III 200, Avance 300, and Avance II 400 spectrometers. Chemical shift values are given in ppm with reference to solvent residual signals. High-resolution MS analyses were performed on Qstar [electrospray ionization (ESI) ionization mode] and AccuTOF Jeol [field desorption (FD) ionization mode] spectrometers. FTIR spectra were recorded on a Perkin-Elmer Spectrum100 spectrometer using KBr pellets. The attenuated total reflectance (ATR-FTIR) spectrum of TiO2 particles was subtracted from the spectra of TiO2-grafted dyes, and all reported spectra were normalized with respect to the CN stretching peak. 2-(2H-Tetrazol-5-yl)acetonitrile.46 Malononitrile (11 g, 0.167 mol), sodium azide (10.8 g, 0.166 mol), and ammonium chloride (8.9 g, 0.167 mol) were suspended in dimethylformamide (DMF; 50 mL) and heated at 80 °C for 16 h. The mixture was poured into water, acidified with concentrated HCl, and extracted with ethyl acetate. The combined organic layers were dried over magnesium sulfate and evaporated to give an oily brown residue that slowly crystallized upon drying under a vacuum. The crude product was finally crushed and washed with CH2Cl2 to afford 2-(2H-tetrazol-5-yl)acetonitrile as a light brown crystalline solid (2.92 g, 16%). 1H NMR (300 MHz, MeOD): δ 4.85 (s, 1H), 4.33 (s, 1H). 13C NMR (100 MHz, MeOD): δ 152.74, 115.68, 14.49. FTIR (KBr): νCN = 2266 cm−1; νNH = 1901 cm−1. General Protocol for the Preparation of Dyes D1−D3. To a solution of C32 (1 equiv) and appropriate nitrile (10 equiv) in dry CHCl3, under an inert atmosphere and protected from the sunlight, was added piperidine (20 equiv). The reaction mixture was refluxed overnight. After being allowed to 10682



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Electrochemical Measurements. Cyclic voltammetry analyses were performed using an Autolab PGSTAT100 potentiostat/galvanostat and a three-electrode system (working electrode, Pt disk; reference electrode, Ag/AgCl, calibrated with ferrocenium/ferrocene as an internal reference; counter electrode, Pt) with 0.1 M Bu4NPF6 as the salt support at a scan rate of 100 mV s−1. Theoretical Calculations. A previously described calculation procedure34 was used to assess the changes in the UV− vis spectrum observed experimentally upon chemisorption of the dyes onto the TiO2 surface. The geometries of the isolated dyes and of the hybrid dye@TiO2 systems were obtained using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method with periodic boundary conditions.47 The TiO2 periodic slab used in SCC-DFTB calculations had the formula (TiO2)144 and included six layers in thickness and a vacuum gap of 41 Å to avoid interactions between two slabs along the direction perpendicular to the interface (z axis). The lattice parameters were a = 22.656 Å, b = 20.420 Å, and c = 50.000 Å with angles equal to 90°. Absorption spectra were calculated using the TD-DFT method with the 6-31g(d) basis set and the MPW1K exchange-correlation (XC) functional (containing 42.8% HF exchange).48 Finally, to mimic the experimental conditions, solvent (dichloromethane) effects were included using the polarizable continuum model in its integral equation formalism (IEF-PCM).49,50 The cavity of the solute was built up using atomic radii from the UFF force field, putting individual spheres around each heavy and hydrogen atom. SCC-DFTB and TD-DFT calculations were performed using the DFTB+51 and Gaussian 0952 packages, respectively. Electrode Preparation and Device Fabrication. Fluorine-doped tin oxide- (FTO-) coated glass substrates (NSG10, 10 Ω/square, thickness 3.2 mm, XOPFisica) were cleaned by mean of ultrasonic treatment in a detergent and then subjected to UV−O3 treatment for 20 min to remove carbon residues. The substrate was then treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min to create a blocking layer. The photoanode was prepared by screen printing using commercially available titania pastes. A transparent layer, composed of 20-nm anatase TiO2 particles (Dyesol DSL90-T), was deposited until a thickness of 10 μm was reached. A second layer composed of 250−400-nm TiO2 particles (Dyesol WER2O) was then printed until a thickness of 5 μm was reached. The role of this second layer was to backscatter the unabsorbed photons toward the transparent layer. Ethyl cellulose and terpineol contained in the titania pastes were removed by gradual thermal treatment under an air flow at 325 °C (5 min), 375 °C (5 min), 450 °C (15 min), and 500 °C (15 min). The as-obtained films were again treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min and heated at 500 °C for 30 min. After being allowed to cool to ∼40 °C, the electrodes were immersed in dye solutions (0.3 mM in CH2Cl2) containing, in the case of D1 and D2, an optimized concentration of chenodeoxycholic acid as a coadsorbent and deaggregating agent (2 mM). The sensitization time was optimized to 5 h in the dark. Counter electrodes were prepared by drop casting a solution of H2PtCl6 (5 mM in ethanol) on FTO glass (TCO227, 7 Ω/square, thickness 2.2 mm, Solaronix). Thermal decomposition of the complex at 500 °C for 30 min left a sufficient quantity of platinum. The photoanode and counter electrode were assembled using a hot-melt polymer gasket (25 μm, Surlyn, Dupont). The devices included homemade electrolyte Z960 composed of iodine (0.03 M), lithium iodide

(0.05 M), dimethylimidazolium iodide (1.0 M), tert-butylpyridine (0.5 M), and guanidinium thiocyanate (0.1 M) in 85/15 (v/v) acetonitrile/valeronitrile. Photovoltaic Characterization. The photovoltaic performances of the DSCs were measured by using a mask with an aperture area of 0.159 cm2. Solar cells were illuminated by AM1.5G solar simulator calibrated with a radiometer (IL 1400BL) to provide an incident irradiance of 100 mW cm−2 at the surface of the device. J−V measurements were performed using a Keithley model 2400 digital source meter by independently applying an external voltage to the cell and measuring the photogenerated current out from the cell. Action spectra of the incident photon-to-current conversion efficiency (IPCE) were realized using a Xe lamp associated with a monochromator (Triax 180, Jobin Yvon) to select and increment wavelength irradiation to the cell. No bias light was employed to illuminate the cell. The current produced was measured by steps of 5 nm after 2 s of radiation exposure with a Keithley 6487 picoammeter to be in steady-state conditions. The incident photon flux was measured with a 6-in.-diameter calibrated integrated sphere (Labsphere) and a silicon detector.

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ASSOCIATED CONTENT

S Supporting Information *

Spectroscopic characterization of the dyes (UV−visible absorption, fluorescence emission, and ATR-FTIR spectra). Additional theoretical calculations. Device stability over time. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +33 (0)5 40 00 25 23. *E-mail: [email protected]. Tel.: +33 (0)5 40 00 24 25. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received financial support from the ANR (“FMOCSOLE” ANR-BLAN-2010-93801) and was carried out within the framework of the EU COST HINT Action MP1202. The authors thank Dr. Lionel Hirsh (IMS, UMR 5218 CNRS) for providing access to photovoltaic characterization facilities.

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REFERENCES

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