Cucurbituril Complexes of Viologens Bound to TiO2 Films - Langmuir

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Cucurbituril Complexes of Viologens Bound to TiO2 Films Marina Freitag and Elena Galoppini* Chemistry Department, Rutgers University, 73 Warren Street, Newark, New Jersey 07102 Received December 11, 2009. Revised Manuscript Received January 14, 2010 Methylviologen (1,10 -dimethyl-4,40 -bipyridinium dichloride, MV2þ, 1) and a newly synthesized viologen derivative (1-methyl-10 -p-tolyl-4,40 -bipyridinium dichloride, MTV2þ, 2) were encapsulated in a macrocyclic host, cucur[7]bituril, CB[7]. The complexes MV2þ@CB[7] and MTV2þ@CB[7] were physisorbed on the surface of TiO2 nanoparticles films. Viologens 1 or 2, which do not have anchoring group substituents, did not bind to the films in the absence of CB[7]. The complexation into CB[7] was monitored by 1H NMR spectra in D2O solutions, which showed an upfield shift of the viologen protons upon encapsulation. TiO2 films functionalized with the complexes were studied by FT-IR-ATR and UV-vis absorption. The electrochemical and spectroelectrochemical properties of MV2þ@CB[7] and MTV2þ@CB[7] were studied in solution and in electrochromic windows, where the complexes were bound to TiO2 films cast on FTO. The windows prepared from MV2þ@CB[7]/TiO2/FTO and MTV2þ@CB[7]/TiO2/FTO electrodes showed reversible, sharp (colorless to purple), and fast color switching upon application of -0.8 V. Electrochromic behavior was not observed in control windows prepared in the absence of CB[7].

1. Introduction In electrochromic molecules the loss or gain of an electron obtained by applying an external voltage is associated with changes of the optical absorption spectrum.1-3 Electrochromism is useful to develop displays and other devices, including those based on metal oxide films4-8 or polymeric materials,9-12 particularly when the redox states are reversible and the color change is sharp. Methylviologen (1,10 -dimethyl-4,40 -bipyridinium dichloride) and many viologen derivatives are excellent electrochromic materials because possess fast switching times, and the singly reduced state is long-lived, stable, and intensely colored.13,14 The one-electron reduction of methylviologen (MV2þ) forms a purple radical cation (MV•þ) (Scheme 1). The bleached and colored state can be reversibly switched upon application of a potential (-0.704 V vs SCE). Further reduction of the radical cation generates a yellow-brown neutral compound (MV), which is a highly powerful reducing agent and which has been used for hydrogen generation.13 *Corresponding author. E-mail: [email protected]. (1) Platt, J. R. J. J. Chem. Phys. 1961, 34, 862. (2) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism and Electrochromic Devices; Cambridge University Press: Cambridge, UK, 2007. (3) Granqvist, G. C. Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, 1995. (4) Berger, S.; Ghicov, A.; Nah, Y.-C.; Schmuki, P. Langmuir 2009, 25, 4841. (5) Vlachopoulos, N.; Liska, P.; Augustynsky, J.; Gr€atzel, M. J. Am. Chem. Soc. 1988, 110, 1216. (6) Cinnsealach, R.; Boschloo, G.; Rao, S. N.; Fitzmaurice, D. Sol. Energy Mater. Sol. Cells 1998, 55, 215. (7) Cummins, D.; Boschloo, G.; Ryan, M.; Corr, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 11449. (8) Campus, F.; Bonh^ote, P.; Gr€atzel, M.; Heinen, S.; Walder, L. Sol. Energy Mater. Sol. Cells 1999, 56, 281. (9) Unur, E.; Beaujuge, P. M.; Ellinger, S.; Jung, J.-H.; Reynolds, J. R. Chem. Mater. 2009, 21, 5145. (10) Dyer, A. L.; Reynolds, J. R. Electrochromism and conjugated conducting polymers. In Handbook of Conducting Polymers, 3rd ed.; Reynolds, T. A., Skotheim, J. R., Eds.; CRC Press: Boca Raton, FL, 2007; Vol. 1, Chapter 20. (11) De Filpo, G.; Nicoletta, F. P.; Chidichimo, G. Chem. Mater. 2006, 18, 4662. (12) Unur, E.; Jung, J.-H.; Mortimer, R. J.; Reynolds, J. R. Chem. Mater. 2008, 20, 2328. (13) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,40 -Bipyridine; Wiley: New York, 1998. (14) Mortimer, R. J.; Reynolds, J. R. Displays 2008, 29, 424.

8262 DOI: 10.1021/la904671w

Viologens can be bound to TiO2 nanoparticle films through substituents, for instance carboxylic or phosphonic acid.7,8,15 An anchor group is necessary to form a strong bond with the semiconductor surface, ensure an effective contact, and to avoid desorption. The functionalized films, cast onto transparent conductive electrodes, are used as components of displays and electrochromic windows.7,8,15,16 One of the challenges associated with this approach is the need for synthetic modifications of the viologen framework to introduce the anchor group. Furthermore, dimerization of MV•þ, aggregation of MV, and other side reactions can make the switching process irreversible or lead to degradation. In this paper we describe a new approach to anchor the redoxactive compound to the semiconductor surface: encapsulation of unsubstituted methylviologens into a macrocyclic host, specifically cucur[7]bituril, and anchoring (or, more properly, physisorption) of the MV2þ@host onto the surface of the semiconductor. There are only very few examples of dyes or redox active molecules encapsulated in molecular containers and bound to the surface of semiconductors through the host, rather than directly.17,18 This approach is potentially useful as a new general method to develop dye/semiconductor materials, which have found applications in a variety of fields, ranging from sensors to dye-sensitized solar cells. We show here that it is possible to further develop this strategy to cucurbiturils and their complexes. Cucurbiturils (CBs) are macrocycles consisting of repeating glycoluril units, typically five to eight, that are prepared by the condensation reaction of glycoluril and formaldehyde.19-21 (15) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H. J.; Kim, K. Acc. Chem. Res. 2003, 36, 621. (16) Kim, H. J.; Jeon, W. S.; Ko, Y. H.; Kim, K. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5007. (17) Pagba, C.; Zordan, G.; Galoppini, E.; Deshayes, K.; Piatnitski, E. L.; Hore, S.; Piotrowiak, P. J. Am. Chem. Soc. 2004, 126, 9888. (18) Choi, H.; Kang, S. O.; Ko, J.; Gao, G.; Kang, H. S.; Kang, M. S. Angew. Chem., Int. Ed. 2009, 48, 5938. (19) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, 2nd ed.; John Wiley and Sons: New York, 2009; Vol. 2. (20) Cintas, P. J. Inclusion Phenom. Macrocyclic Chem. 1994, 17, 205. (21) Mock, W. L. In Comprehensive Supramolecular Chemistry; V€ogtle, F., Ed.; Elsevier: New York, 1996; Vol. 2, pp 477-493.

Published on Web 01/29/2010

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Article Scheme 1. Key Electrochemical Processes of Methylviologen and Associated Color Changes13,14

Cucurbiturils encapsulate small, positively charged, or nitrogencontaining molecules such as azastilbenes,22 cinnamic acid,23 fluorophore 2,3-diazabicyclo[2.2.2]oct-2-ene DBO,24 nitroxide radicals,25 ferrocene,26 or the anticancer drug oxaliplatin.27 The complexation process involves charge-dipole as well as hydrogenbonding interactions. Cucur[7]bituril, CB[7], with an internal cavity of 7.3 A˚ in diameter and a height of 9.1 A˚, and an interior which is quite negative,28 forms a 1:1 inclusion complex with methylviologen (Figure 1).15,16,29 The viologens@CB[7] complexes shows thermodynamic and kinetic stability in aqueous solutions (pH 7.3, phosphate buffer). Typical binding constants for viologens@CB[7] complexes are K ∼ 2  105 M-1.16 The complexation of MV2þ and other viologen derivatives has been studied extensively, particularly by Kim15,16,28,29 and Kaifer,26,30-33 as a way to suppress side reactions (dimerization) of viologen during redox processes in solution and, more generally, to develop and study new supramolecular host-guest complexes materials, cyclization reactions, and CT complexes.22,23,28 Here we report the physisorption of CB[7] and of viologens@ CB[7] to nanocrystalline TiO2 films. Two viologens were studied: methylviologen, 1, and a newly synthesized derivative, 1-methyl10 -p-tolyl-4,40 -bipyridinium dichloride, MTV2þ, 2 (Figure 2). The electrochemical and spectroscopic properties of MV2þ@[CB7] and MTV2þ@[CB7] complexes were studied in solution and in electrochromic windows. We explored this strategy as it may offer two advantages compared to the direct binding of viologens on the surface of semiconductors: (i) the ability to anchor nonfunctionalized viologens to the surface of semiconductors and, possibly, (ii) decreased side reactions (aggregation, dimerization) of the viologens. Additionally, it opens the possibility of anchoring to TiO2 other inclusion complexes of cucurbiturils, such as fluorophores or biologically important compounds, for possible sensing applications.

Figure 1. Viologens and their complexes with CB[7] host studied in this work. Only one of the possible encapsulation modes is shown for MTV2þ@CB[7].

2. Experimental Section Synthesis. 1H NMR (499.90 MHz) and

13

C (124.98 MHz) spectra were recorded on a Varian INOVA 500 spectrometer. The (22) Maddlipata, M. V. S. N.; Kaanumalle, L. S.; Natarajan, A.; Pattabiraman, M.; Ramamurthy, V. Langmuir 2007, 23, 7545. (23) Pattabiraman, M.; Kaanumalle, L. S.; Natarajan, A.; Ramamurthy, V. Langmuir 2006, 22, 7605. (24) Marquez, C.; Huang, F.; Nau, W. M. IEEE Trans. NanoBiosci. 2004, 3, 39. (25) Mileo, E.; Mezzina, E.; Grepioni, F.; Pedulli, G. F.; Lucarini, M. Chem.; Eur. J. 2009, 15, 7859. (26) Jeon, W. S.; Moon, K.; Park, S. H.; Chun, H.; Ko, Y. H.; Lee, J. Y.; Lee, E. S.; Samal, S.; Selvapalam, N.; Rekharsky, M. V.; Sindelar, V.; Sobransingh, D.; Inoue, Y.; Kaifer, A.; Kim, K. J. Am. Chem. Soc. 2005, 127, 12984. (27) Kim, K.; Jeon, Y. J.; Kim, S.-Y.; Ko, Y. H. Inclusion Compound Comprising Cucurbituril Derivatives as Host Molecule and Pharmaceutical Composition Comprising the Same. PCT Int. Appl. WO 0324978 A1 20030327, 2003. (28) Ko, Y. H.; Kim, E.; Hwang, I.; Kim, K. Chem. Commun. 2007, 1305. (29) Kim, K.; Selvapalam, N.; Ko, Y. H.; Park, K. M.; Kim, D.; Kim, K. J. Chem. Soc. Rev. 2007, 36, 267. (30) Ong, W.; Kaifer, A. E. Angew. Chem., Int. Ed. 2003, 5, 2164. (31) Ong, W.; Comez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791. (32) Moon, K.; Kaifer, A. E. Org. Lett. 2004, 6, 185. (33) Sindelar, V.; Moon, K.; Kaifer, A. E. Org. Lett. 2004, 6, 2665.

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Figure 2. Proposed physisorption of viologen@CB[7] complexes. Only one of the two possible orientations (tolyl up/down) of 2 in the bound MTV2þ@CB[7] is shown, and it is based on the 1H NMR data in Figure 4. 1 H and 13C NMR chemical shifts δ are given in ppm and are referenced to the central line of the solvent. NMR spectra were recorded at room temperature in the solvents indicated. Coupling constants (J) are reported in hertz. Mass spectra (ESI) were recorded on the Bruker Daltonics FTMS-TOF departmental facility. Column chromatography was performed using silica gel from Sorbent Technologies (standard grade, 32-63 μm particle size). TLC was performed on silica gel plates from Sorbent Technologies using UV light as the developing agent. Acetone, methylviologen (1), CB[7], 4,40 -bipyridine, 1-chloro-2,4-dinitrobenzene, p-toluidine, ferrocenecarboxylic acid, and methyl iodide were purchased from Aldrich or Sigma-Aldrich and used without

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Figure 3. Construction of electrochromic windows. further purification. The following syntheses were performed under a nitrogen atmosphere in anhydrous solvents.

1-(2,4-Dinitrophenyl)-4-(pyridin-4-yl)pyridinium Chloride (3). 4,40 -Bipyridine (1.55 g, 10 mmol) and 2,4-dinitrochlorobenzene (2.03 g, 10 mmol) were dissolved in acetone (15 mL). The solution was refluxed for 12 h. The brown precipitate was collected by filtration and washed twice with n-pentane. The crude product was dried in vacuo. Yield: 1.56 g (45% yield). 1H (D2O): δ 9.47 (d, J = 2.4, 1 H); 9.32 (d, J = 6.7, 2 H); 9.01 (dd, J1 = 2.4, J2 = 8.6, 1 H); 8.92 (d, J = 6.0, 2 H); 8.76 (d, J = 6.7, 2 H); 8.35 (d, J = 8.6, 1 H); 8.11 ppm (d, J = 6.0, 2 H). 13C NMR (D2O): δ 159.4, 152.7, 152.2, 148.3, 145.4, 144.5, 140.9, 133.7, 133.2, 128.6, 125.2 ppm. 4-(Pyridin-4-yl)-1-p-tolylpyridinium Chloride (4). p-Toluidine (0.55 g, 4 mmol) was dissolved with 3 (0.72 g, 2 mmol) in ethanol (3 mL) under a nitrogen atmosphere. The resulting solution was refluxed for 16 h. The precipitate formed was filtered and discarded. The filtrate was dried in vacuo, and the yellow crude product was triturated with acetone (250 mL), filtered, and dried in vacuo. Yield: 0.38 g (70%). 1H (D2O): δ 9.20 (d, J = 6.9, 2 H); 8.78 (d, J = 6.2, 2 H); 8.53 (d, J = 6.9, 2 H); 7.96 (d, J = 6.2, 2 H); 7.68 (d, J = 8.4, 2 H); 7.58 (d, J = 8.4, 2 H); 2.50 ppm (s, 3 H). 13C NMR (D2O): 156.4, 152.5, 146.9, 145.3, 144.5, 142.2, 133.5, 128.4, 126.0, 125.0, 22.8 ppm. 1-Methyl-10 -p-tolyl-4,40 -bipyridinium Chloride (2). Compound 4 (0.15 g, 0.5 mmol) was dissolved in 10 mL of ethanol, and iodomethane (0.09 g, 0.6 mmol) was added with a syringe into the solution. The reaction mixture was stirred at 43 °C for 24 h, and in the meantime an orange precipitate formed. The precipitate was collected by filtration and washed with acetone. The crude product was dried in vacuo. Yield: 0.20 g (99%). 1H (D2O) MTV2þ: δ 9.37 (d, J = 6.8, 2 H); 9.11 (d, J = 6.4, 2 H); 8.71 (d, J = 6.8, 2 H); 8.62 (d, J = 6.4, 2 H); 7.72 (d, J = 8.4, 2 H); 7.61 (d, J = 8.4, 2 H); 4.54 (s, 3 H); 2.52 ppm (s, 3 H). 13C NMR (D2O): 152.9, 152.1, 148.9, 147.8, 145.6, 142.6, 142.4, 133.6, 129.6, 129.4, 129.4, 126.4, 51.1, 23.0 ppm. HRMS (ESI-TOF) calcd for C18H18N2 m/z: 262.1469; found: 262.1486. According to MS (MALDI) data ([M þ Na]þ 353.20) the product does not seem to contain iodide as counterion, although iodomethane is used in the final step. The samples of 2 that we isolated contained traces of an impurity. Complexation. The inclusion of viologens 1 and 2 in CB[7] was done following reported procedures.15,16,28,29 Briefly, equimolar amounts of 1 (or 2) and CB[7] were dissolved in distilled water (100 mL) to form a 0.05 mM solution of the guest@host complex and stirred overnight. This solution was used as the binding solution for the TiO2 films. Evaporation of water led to the solid which was used for the FT-IR-ATR spectra. Formation of the complexes in solution was monitored by 1H NMR in D2O. It was observed that the complexation is fast (minutes). 8264 DOI: 10.1021/la904671w

MV2þ@CB[7]. 1H NMR (500 MHz, D2O): (a) MV2þ δ 4.54 (s, 3 H); δ 8.55 (d, 2 H, J = 15.5), δ 9.04 (d, 2 H, J = 15.5); (b) CB[7] 4.25 (d, 14 H, J = 15.5), 5.56 (s, 14 H), 5.81 ppm (d, 14 H, J = 15.5). The spectrum of this complex matched the results reported by Kim and co-workers (see Supporting Information).16 MTV2þ@CB[7]. 1H NMR (500 Hz, D2O): (a) MTV2þ: δ 9.31 (d, J = 6.0, 2 H); 8.91 (d, J = 5.5, 2 H); 8.63 (d, J = 5.5, 2 H); 8.49 (d, J = 5.5, 2 H); 7.03 (d, J = 7.5, 2 H); 6.63 (d, J = 7.5, 2 H); 3.95 (s, 3 H); 1.97 (s, 3 H); (b) CB[7]: δ 5.80 (d, J = 15.3, 14 H); 5.54 (s, 14 H); 4.24 ppm (d, J = 15.3, 14 H). Determination of the Binding Constant for MTV2þ@CB[7]. The binding constant was determined following a procedure

described by Kaifer and co-workers.30,31 To 1000 μL of an aqueous solution 30 μM in MTV2þ were added 50 μL aliquots of a 100 μM aqueous solution of CB[7] using a micropipet. Both solutions were buffered with a 0.1 M phosphate buffer (pH 7.32). The UV-vis absorption spectra were measured after each each addition. The absorbance at λmax = 218 nm was fitted against the concentration of CB[7] (1:1 complexation model) to obtain the equilibrium constant (K = (1.06 ( 0.7)  105 L/mol) (see Supporting Information, Figures S11 and S12.) Preparation of Nanostructured TiO2 Films.34 Transparent, mesoporous TiO2 films were prepared as previously described34 and were cast on conducting glass (FTO, TEC 7 by Pilkington, 2.2 mm thickness, with a sheet resistance of 8-10 Ω/ sq, ∼80% visible transmittance). Briefly, a 15 wt % colloidal dispersion of TiO2 was prepared by acidic hydrolysis of titanium isopropoxide and autoclaved at 200 °C for 8 h. Poly(ethylene glycol) (PEG 2,000; amount: 6 g/L) was added to the colloid to yield a white viscous paste. The paste was spread using a glass test tube on the precut conductive glass, followed by sintering at 450 °C for 30 min under oxygen flow. The films were allowed to cool down before immediate use or were stored in a desiccator and in the dark. No difference was observed when using films stored for weeks as indicated. Binding. Chemisorption (or physisorption) of MV2þ@CB[7] or MTV2þ@CB[7] was done by immersing the films in an aqueous solution with the complex (0.05 mM) for 24 h. Afterward, the films were rinsed with DIUF water and dried at 105 °C for 20 min. Transparent TiO2 films/ITO or TiO2 films/ITO modified with ferrocenecarboxylic acid (Fc-COOH) were used as counter electrodes for the electrochromic windows. Fc-COOH was selected because the windows prepared using the anchored Fc-COOH exhibited higher stability and reversibility compared to windows prepared using Fc in the electrolyte solution. One of the concerns about using ferrocene is the potential inclusion into the CB[7] host,26 although this process may be minimized for the (34) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655.

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Figure 4. Top: comparison between relevant regions of the 1H NMR spectra of MTV2þ (inset) and MTV2þ@CB[7]. Bottom: chemical shift differences upon complexation. Scheme 2

anchored Fc-COOH. Work with alternative redox couples is in progress.35 The Fc-COOH-modified electrodes were prepared by immersing the nanostructured TiO2/FTO films into an ethanolic 0.05 mM solution of Fc-COOH for 1 h, rinsing with ethanol, and drying at 100 °C for 20 min prior use. Preparation of Electrochromic Windows. The electrochromic windows were assembled following procedures similar to those used by others.8 The steps, as illustrated in Figure 3, are as follows: (1) The FTO glass was cut (3.0  2.5 cm) and cleaned with ethanol in a sonicator for several hours. (2) The glass was masked using a self-adhesive vinyl label with a square cutout (1.8 cm side). (3) The TiO2 colloidal paste was cast on the glass using a glass rod and dried in the air for 10 min, and then the vinyl mask was removed. The film was sintered at 450 °C for 30 min under (35) Bignozzi, Caramori, Freitag, Galoppini, unpublished results.

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oxygen flow and then cooled slowly to room temperature. (4) The TiO2/FTO films were immersed into the binding solution for 24 h. (5) The counter electrode was prepared by immersing the TiO2/ FTO films into a 0.05 mM ethanol solution of Fc-COOH. (6) The electrodes were bound together using a thermoplastic polymer (Surlyn). The thermoplastic was cut to form a frame leaving a small side opening for filling the window with the electrolyte (see below), deposited around the functionalized TiO2 film, and heated to 60 °C. The counter electrode was simultaneously heated, and then the two electrodes were pressed together to form the window. The window was placed in a desiccator, in vacuo, and in the dark for 24 to 48 h. (7) A drop of electrolyte (0.05 M LiClO4 in anhydrous, freshly distilled γ-butyrolactone) was placed on the side opening. The window was filled by placing the cell in a desiccator in vacuo for a few seconds and then admitting air. Finally, the side opening of the window was sealed using epoxy. DOI: 10.1021/la904671w

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Spectroscopic Measurements. FT-IR ATR spectra were acquired on a Thermo Electron. Corp. Nicolet 6700 FT-IR. UV-vis absorbance spectra were collected on an Ocean Optics USB4000þ miniature fiber-optic spectrometer in combination with a PX-2 pulsed xenon light source (150 ms for integration time and 20 scans to average). Electrochemistry. a. In Solution. The electrochemical properties of 1, 2, and their CB[7] complexes in solution were studied by cyclic voltammetry on a BAS CV27 potentiostat. The experiments were conducted in aqueous 0.1 M phosphate buffer (pH 7.3) at room temperature. The solutions were deaerated before and during the measurements by bubbling nitrogen in a standard three-electrode arrangement with glassy carbon (2 mm diameter), Pt gauze counter electrode, and Ag/AgCl (1.0 M KCl) as the reference electrode. Each cycle was measured between ( 1.0 V at a scan rate of 50 and 100 mV/s with a sensitivity of 10 mA/V. Spectroelectrochemical measurements (Figure 11) were obtained in a quartz cell (ALS Japan, distributed by CH instruments, 011240 SEC-C thin-layer quartz glass spectroelectrochemical cell kit, Pt gauze working electrode) and using Ag/AgCl (1.0 M KCl) for aqueous solutions as the reference electrode. b. Electrochromic Windows. The electrochromic windows were assembled as described. Each CV was recorded between (1.0 V with a sensitivity of 1 mA/V at a scan rate of 100 mV/s. A potential of -0.8 V was applied to form the colored state (MV•þ or MTV•þ) and a potential of þ0.1 V to oxidize the species back to the bleached state (MV2þ or MTV2þ).

3. Results and Discussion 1. Synthesis. The binding of unsubstituted viologens encapsulated in CB[7] was tested using commercially available methylviologen as a reference and viologen derivative MTV2þ, 2. Compound 2 was selected because it contains an aryl group that, suitably substituted, is useful to tune the redox properties of the viologen derivative. It is the first in a series of aryl viologens that we are investigating. In addition, fluorescence emission was observed in solutions of 2, and since fluorescence emission is enhanced by complexation,13,24 steady-state and time-resolved studies of 2 could be used as an additional complexation probe. The fluorescent properties of 2, however, are still under investigation because, based on literature precedents, at present we cannot rule out that they are due to trace amounts of a highly fluorescent impurity.36 MTV2þ was synthesized as shown in Scheme 2, using an adaptation of the Zincke reaction by Yamaguchi and coworkers37 to obtain asymmetric viologen derivatives. The Zincke reaction converts pyridine derivatives into pyridinium salts by reaction with 1-chloro-2,4-dinitrobenzene and a primary amine in aqueous conditions.38 The mechanism of this reaction proceeds through a ring-opening step, promoted by nucleophilic attack of the primary amine (in our case p-toluidine).39-42 The pyridinium salt N-2,4-dinitrophenyl-4-pyridylpyridinium chloride (3) was obtained in 45% yield by reaction of 2,4dinitrochlorobenzene with 4,40 -bipyridyl. Reaction of p-toluidine (36) It has been published, for instance, that when fluorescence properties of MV2þ were reported, the observed fluorescence was due to very small trace amounts of oxobipyridinium derivatives, which are highly fluorescent, and that can be formed upon exposure to bases, heat, and oxygen. Mau, A. W.-H.; Overbeek, J.; Loder, J. W.; Sasse, W. H. F. J. Chem. Soc., Faraday Trans. 2 1986, 82, 869. See also ref 13, pp 10 and 212. The NMR and MALDI analyses of samples of 2 show small trace amounts of impurities, and until we obtain a highly purified sample of 2, we cannot exclude that the fluorescence observed is due such impurities . (37) Yamaguchi, I.; Higashi, H.; Shigesue, S.; Shingai, S. Tetrahedron Lett. 2007, 48, 7778. (38) Zincke, T. H.; Weisspfenning, G. J. Liebigs Ann. Chem. 1913, 396, 103. (39) Genisson, Y.; Marazano, C.; Mehmandoust, M.; Gnecco, D.; Das, B. C. Synlett 1992, 431. (40) Kost, A. N.; Gromov, S. P.; Sagitullin, R. S. Tetrahedron 1981, 37, 3423. (41) Becher, J. Synthesis 1980, 589. (42) Kunugi, S.; Okubo, T.; Ise, N. J. Am. Chem. Soc. 1976, 98, 2282.

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Figure 5. FT-IR-ATR of (a) MV2þ (blue solid line) and

MV2þ@CB[7] (black solid line) and (b) MTV2þ (blue solid line) and MTV2þ@CB[7] (black solid line).

Figure 6. FT-IR-ATR of adsorbed complexes MV2þ@CB[7] (black solid line), MTV2þ@CB[7] (red dash line), and CB[7] (blue dotted line) on TiO2/FTO.

with the Zincke salt, 3, yielded 4 in 70% yield. Reaction of 4 with iodomethane produced 2, an orange solid, in quantitative yields (Scheme 2). 2. NMR Spectra of Complexation of MV2þ and MTV2þ into CB[7]. The formation of the complexes MV2þ@CB[7] and MTV2þ@CB[7] was monitored by 1H NMR in D2O solutions. The spectrum of the MV2þ@CB[7] complex (see Supporting Information) matched that reported by Kim and co-workers,16 and the same method was used to study MTV2þ@CB[7]. MTV2þ is an asymmetric compound, and to study which part of the molecule is encapsulated in CB[7], we studied the spectral changes in the 1H NMR in D2O (Figure 4). Upon complexation of MTV2þ in CB[7], significant upfield shifts of some of the MTV2þ protons occurred, while the chemical shift of the CB[7] protons remained Langmuir 2010, 26(11), 8262–8269

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Table 1. Voltammetric Parameters for Free and Encapsulated MTV2þ and MV2þ compound

E11/2, Va (ΔEp, mV)b

E21/2, Va (ΔEp, mV)b

MV2þ -0.661 (72) -0.975 (142) -0.681 (63) -0.992 (122) MV2þ@CB[7] -0.704 MTV2þ -0.767 MTV2þ@CB[7] a Half-wave potentials, in V, for the first reduction process vs saturated Ag/AgCl reference electrode. b In parentheses is reported the potential difference, in mV, between cathodic and anodic peaks at 0.1 V s-1 scan rate.

Figure 8. Visible region of the absorption spectra of one-electron reduced species (radical cations) of (a) MV2þ and MV2þ@CB[7] and (b) MTV2þ and MTV2þ@CB[7] in phosphate buffer solution (pH 7.0) in a spectroelectrochemical cell. Solid and dotted lines show spectra in the absence and presence of equimolar amounts of CB[7], respectively.

Figure 7. Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of 0.05 mM solutions of (a) MV2þ in the absence (black solid line) and in the presence (red dotted line) of CB[7] (b) MTV2þ in the absence (black solid line) and in the presence (red dotted line) of CB[7].

unchanged. The upfield shift can be explained by the shielding environment experienced by the guest’s protons in the cavity of a CB[7] host.16,43 The chemical shift difference (Δδ) of the MTV2þ protons are shown in Figure 4, with the largest difference (Δδ = -0.69 and -0.98) observed for the benzene ring protons of the tolyl unit. This result suggests that, in the complex, part of the bipyridinium moiety remains outside of the CB[7] ring, and the tolyl group is encapsulated. Considering the negative surface potential of the interior of CB[7],15 we expected that the (43) Yongqiang, S.; Yongqiang, S.; Saifeng, X.; Yunjie, Z.; Qianjiang, Z.; Zhu, T. Chin. Sci. Bull. 2003, 48, 2694.

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bipyridinium moiety would be fully encapsulated in the host, but the results are consistent with work reported by Kaifer and co-workers,30-33 and we cannot exclude the occurrence of other complexation modes, depending on experimental conditions (presence of salts, pH, etc.). In addition, the complexation mode in solution is not necessarily representative of what happens on the surface of TiO2. 3. FT-IR-ATR Spectra. FT-IR-ATR spectra of solid samples of 1 and 2 and of their respective complexes showed aromatic CdC and CdN stretching bands of the pyridine and phenyl rings in the 1600-1430 cm-1 range. Upon encapsulation of the guests in CB[7] the FT-IR-ATR spectra of the bound complexes were not additive, rather the spectra of the complexes were mostly similar to the FT-IR-ATR spectrum of CB[7], with a strong band assigned to the carbonyl νas(CdO) stretch at 1725 cm-1 (Figure 5). While CB[7] does not have anchoring groups that can form a strong covalent bond with the surface hydroxyl groups of TiO2 (such as COOH or P(O)(OH)2), it is clear that it physisorbs to the surface, presumably by hydrogen-bonding interactions of the carbonyl group. Upon chemisorption to TiO2 the νas(CdO) carbonyl band broadened and shifted at lower energy by about 10 wavenumbers (1735 cm-1) compared to that of neat CB[7] (Figure 6). While the shift of the νas(CdO) carbonyl band to lower energies seem to be consistent with this picture, the quality of the DOI: 10.1021/la904671w

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data (broad, low intensity) does not allow to draw any conclusion other than the presence of the CB[7] macrocycle on the semiconductor films. 4. Electrochemistry. a. In Solution. The cyclic voltammograms (CVs) of 1 and 2 and their respective CB[7] complexes were collected in 0.1 M phosphate buffer at room temperature.

Figure 9. Cyclic voltammograms of electrochromic windows prepared from the CB[7] complexes of 1 (solid line) and 2 (dotted line).

Figure 10. Absorption spectra of MV2þ@CB[7]/TiO2 and

MTV2þ@CB[7]/TiO2 measured in an electrochromic window after application of -0.8 V.

Freitag and Galoppini

Both free MV2þ and the complex (MV2þ@CB[7]) in solution exhibited the characteristic two reversible one-electron reduction waves, consistent with published data.13,16 The complex exhibited a shift to more negative potentials for the formation of the radical cation MV•þ and the fully reduced MV (Table 1 and Figure 7a). Negative shifts were also observed by Kim15,16,28,29 and Kaifer26,30-33 and were attributed to the smaller complexation affinities of MV•þ and MV, compared to MV2þ. In our experiment, however, the shift (-20 mV) was the same for both reduction processes, suggesting that in our experimental conditions the binding of MV•þ is still preferred over MV but the differential binding affinity for MV•þ over MV is not very pronounced. In addition, the second reduction process was quasi-reversible, suggesting decreased solubility of the reduction product or the occurrence of other processes at the electrode upon reduction. The relatively slow scan rates (0.1 V s-1) were selected so as to avoid a competition between complexation equilibria and the time scale of the measurements. In the CVs of MTV2þ and MTV2þ@CB[7], a similar negative shift occurs upon complexation. However, the reduction process for 2 (alone or in the presence of CB[7]) was irreversible, as no cathodic wave and a single reduction were observed (Figure 7b). One of the possible explanations for the observed irreversibility of 2 is the occurrence of reactions at the electrode, such as insoluble salts deposition or possible electropolymerizations involving the tolyl group in 2. Absorption spectra of the one-electron-reduced species were measured in solution at -0.8 V for 1 and 2 in the presence and in the absence of 1 equiv of CB[7] (Figure 8). In all cases the reduction processes resulted in a broad absorption band centered at 600 nm, which is consistent with data reported for viologen radical cations.7,16 The spectra of the complexes were essentially identical to those of the free compounds. b. In Electrochromic Windows. The CVs of electrochromic windows prepared from MV2þ@CB[7] and MTV2þ@CB[7] show, in both complexes, a semireversible two-electron reduction process (Figure 9). The absorption spectra of electrochromic windows were measured at -0.8 V (Figure 10). A broad band centered at 600 nm was observed for windows prepared from both complexes MV2þ@CB[7] and MTV2þ@CB[7], and it is consistent with the absorption spectrum of the corresponding radical cations. The color change and the redox process were not observed for windows prepared using solutions of 1 or 2, suggesting that the complex formation and the physisorption to TiO2 are necessary to bring the unsubstituted viologens close to the semiconductor surface. The coloration was reversible for several (over 20) switching cycles between bleached and colored state (Figure 11) (see Supporting Information). The reversibility observed in the electrochromic displays is probably the result of

Figure 11. Picture of color changes of an electrochromic window prepared from MTV2þ@CB[7]/TiO2/FTO (a) before and (b) after application of a -0.8 V potential. 8268 DOI: 10.1021/la904671w

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“insulation” (or protection) of the one-electron reduced species (MV•þ and MTV•þ) by the encapsulating host, which prevents dimerization reactions of the reduced species.16 However, this conclusion needs to be proved, and experiments aimed to determine to what extent the presence of the CB[7] host influences reversibility and stability of the electrochromic windows are in progress in our laboratory. A long-term stability study was beyond the scope of this work, but we will also conduct experiments in this direction to determine whether encapsulation in the host leads to improved stability.

4. Conclusions The encapsulation of redox-active or photoactive compounds in hosts on the surface of semiconductor nanoparticles (TiO2) is a new strategy to control molecule/semiconductor interfaces,17,18 and that could lead to an “insulation effect” from the heterogeneity and complexity of such surfaces. Of particular interest is the reversibility and stability that can be gained as dimerization and aggregation processes are prevented. In addition, the approach is potentially useful to avoid the need for synthetic modifications of molecules with binding groups, to control aggregation, and to tune other useful properties that are influenced by encapsulation (i.e., fluorescence lifetime).17,24 Here we describe a proof-of-concept experiment demonstrating the electrochromic properties of two viologen guests encapsulated inside a cucur[7]bituril host and where the host, not the guest, was bound to the surface of the semiconductor. Methylviologen

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(MV2þ, 1) and 1-methyl-10 -p-tolyl-4,40 -bipyridinium dichloride (MTV2þ, 2) were encapsulated in a molecular host, CB[7], and the complexes physisorbed on the surface of TiO2 nanoparticle films. Viologens 1 and 2 did not have anchoring groups that bind to the surface of TiO2 nanoparticle films, and in the absence of CB[7] no binding was observed. Viologen@CB[7]-modified TiO2 films cast on FTO electrodes were used to prepare electrochromic windows that exhibited reversible color switching upon application of -0.8 V, corresponding to the formation of the intensely blue radical cations. Work is in progress to further study the binding of CB[7], to explore photochemical applications of the described encapsulation, and to further develop this binding approach with other dyes and viologen derivatives. Acknowledgment. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for support of this research (ACS PRF #46663-AC10). The authors are grateful to Mr. Keyur Chitre for help with the MS analyses and to Professor Carlo Alberto Bignozzi for insightful discussions. Supporting Information Available: UV-vis of 1 and 2 in solution in the absence and presence of CB[7], 1H NMR of 1, 3, 4, CB[7], MV2þ@CB[7], 13C NMR of 3, 4, FT-IR-ATR of CB[7], movie of color changes in the electrochromic window of Figure 11, and plots for the determination of the binding constant of MTV2þ. This material is available free of charge via the Internet at http://pubs.acs.org.

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