Nondestructive Monitoring of the Photochromic State of

The use of surface plasmon resonance (SPR) as a nondestructive, ... Analytical Chemistry 2009 81 (17), 7141-7148 ... Analytical Chemistry 0 (proofing)...
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Langmuir 2005, 21, 7413-7420

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Nondestructive Monitoring of the Photochromic State of Dithienylethene Monolayers by Surface Plasmon Resonance Jean-Francois Masson, Paul A. Liddell, Soame Banerji, Tina M. Battaglia, Devens Gust, and Karl S. Booksh* Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604 Received April 13, 2005. In Final Form: June 7, 2005 The use of surface plasmon resonance (SPR) as a nondestructive, nonerasing readout of the isomerization state of a photochromic dithienylethene covalently linked to a chemically modified gold surface was investigated. Four different binding layers were examined: 11-mercaptoundecanol (MUO), an aminemodified 11-mercaptoundecanol (MUO-NH2), dextran, and an amine-modified dextran. The binding of dithienylethene to the modified gold surface and photoisomerization of the photochrome in the bound state were established by FTIR. Solvent effects were measured for every layer tested using ethanol and hexanes. In general, large, easily measurable SPR signal changes could be detected under conditions where photoisomerization of the dithienylethene photochrome was not quenched by the gold plasmon, establishing SPR as a viable form of readout for potential dithienylethene-based optical data storage or processing devices. Dextran-bound photochrome in ethanol exhibited the largest SPR response upon photoisomerization, but is more prone to time-dependent fluctuations resulting from swelling of the dextran layer (caused by slow diffusion of the solvent) than the other layers. Large responses are also provided by MUO-NH2 and MUO, and the signal is much more stable than that for dextran.

Introduction Photochromes in general and dithienylethenes in particular have been much investigated for optical and optoelectronic data storage, processing, and transmission applications.1-4 Dithienylethenes such as 1 (Scheme 1) exist in two isomeric forms (Figure 1). One of these, the open form, absorbs light only in the UV spectral region and is only weakly fluorescent. Upon irradiation with UV light (e.g., ∼350 nm) this form photoisomerizes to a ringclosed isomer, which absorbs in the visible and has a significant fluorescence quantum yield. Irradiation of the closed form with visible light drives photoisomerization back to the open form. Both isomers are thermally stable and require light for isomerization. The dithienylethenes demonstrate exceptional photochemical stability; at least 10000 photoisomerization cycles are possible in some cases.2 The high thermal- and photostabilities make the dithienylethenes especially promising candidates for technological applications. Data storage, photoswitching, and most other applications require a readout mechanism to determine the isomeric state of the molecule (open or closed). Fluorescence (induced by one or two photons), infrared (IR) absorption, and gated photochemical activity have been proposed.5-8 Unfortunately, these methods are * Corresponding author. Tel: 480-965-3058. Fax: 480-965-2747. E-mail: [email protected]. (1) Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 2002, 124, 7668-7669. (2) Irie, M. Chem. Rev. 2000, 100, 1685-1716. (3) Engler, E. M. Adv. Mater. 1990, 2, 166-173. (4) Emmelius, M.; Pawloski, G.; Vollman, H. W. Angew. Chem., Int. Ed. 1989, 28, 1445-1471. (5) Yokoyama, Y. Chem. Rev. 2000, 100, 1717-1739. (6) Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992, 114, 8715-8716. (7) Moerner, W. E. Jpn. J. Appl. Phys. 1989, 28, 221-227. (8) Yokoyama, Y.; Yamane, T.; Kurita, Y. J. Chem. Soc., Chem. Commun. 1991, 1722-1724.

either relatively insensitive (e.g., IR absorbance) or involve electronic excitation of the molecule to induce fluorescence or another response. Electronic excitation invariably leads to some photoisomerization, and therefore such a readout process is destructive, reducing and eventually eliminating the population difference between the two isomers. Two different types of gating methods have been proposed for dithienylethenes.2 Conformational gating relies on the relative stability of the two forms of the open isomer. The parallel isomer does not photoisomerize, but the antiparallel conformation will.2 Some solvents stabilize the parallel conformation and inhibit the photoisomerization process. Although useful, this gating process is complicated for long-term data storage since it involves changing the solvent in which the dithienylethene is solubilized. Another gating method is electrochemical. Some dithienylethenes can be reversibly oxidized.2 The oxidized form is not photochromically active. Although gating is an interesting option for long-term data storage, a method that will sensitively readout the refractive index of these photochromes would eliminate the need for gating. Surface plasmon resonance (SPR) spectroscopy offers an attractive readout mechanism for dithienylethenes and other photochromes. SPR is sensitive to the change in refractive index of material within ∼200 nm of a thin gold or silver film. The change in the value of the resonance wavelength (λSPR) or the resonance angle (ΘSPR) is characteristic of the change of the refractive index at the surface. In the special case of fiber optic SPR, used in this work, a white light is passed into an optical fiber attached to a gold-bearing surface, and the light illuminates the gold surface only at a fixed range of angles, which depends on the numerical aperture of the fiber. When the refractive index near the gold surface changes, the λSPR has to change because the corresponding range of angles is invariant. Therefore, fiber optic based SPR provides a sensitive way to measure changes in λSPR.

10.1021/la0509899 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/09/2005

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Scheme 1. Chemical Structures of the Compounds Synthesized

Thus, if dithienylethenes were located within a few hundred nanometers of the gold surface of an SPR probe, photoisomerization would lead to a change in the index of refraction near the surface and induce a change in λSPR or ΘSPR. Other things being equal, the closer the population of photochromes lies to the gold surface, the larger the SPR signal; a monolayer of photochromes on gold would result in the largest response for the fewest number of dithienylethenes, yielding small devices and dense data storage and processing media. However, this leads to a conundrum. It is well-known that when chromophores are located near a gold surface, the gold plasmon can quench the excited states of the molecule.9,10 If this quenching is severe, the quantum yield of photoprocesses such as isomerization may be reduced to the point that they are not observed. The conditions that are most conductive to strong SPR signals are also most likely to reduce photoisomerization. To investigate the interplay of these factors, we have undertaken a study of dithienylethenes bound to gold SPR probes via a number of different linkages and immersed in two different solvents. As discussed below, by proper choice of conditions, it is possible to both achieve significant photoisomerization of the dithienylethene and detect the isomeric state of the molecule using fiber optic SPR. Others have previously reported SPR studies of photochromism in thiol-based self-assembled monolayers using the Kretschmann SPR configuration.11-14 Dihydroindolizine,15 azobenzene,16,17 spiroxazine,12-14,18-20 (9) Chance, R. R.; Prock, A.; Silbey, R. Adv. Chem. Phys. 1978, 37, 1-65. (10) Yang, T. J.; Lessard, G. A.; Quake, S. R. Appl. Phys. Lett. 2000, 76, 378-380. (11) Tamada, K.; Akiyama, H.; Wei, T. X. Langmuir 2002, 18, 52395246. (12) Kim, S. H.; Ock, K. S.; Im, J. H.; Kim, J. H.; Koh, K. N.; Kang, S. W. Dyes Pigments 2000, 46, 55-62. (13) Kim, S. H.; Ock, K. S.; Kim, J. H.; Koh, K. N.; Kang, S. W. Dyes Pigments 2001, 48, 1-6. (14) Kim, S. H.; Choi, S. W.; Kim, J. H.; Jin, S. H.; Gal, Y. S.; Ryu, J. H.; Cui, J. Z.; Koh, K. Dyes Pigments 2001, 50, 109-115. (15) Masson, J. F.; Hartmann, T.; Durr, H.; Booksh, K. Opt. Mater. 2004, 27, 435-439. (16) Gu, J.; Liang, B.; Tian, Y.; Chen, Y.; Lu, B.; Lu, Z. Thin Solid Films 1998, 327-329, 427-430. (17) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436-6440.

spironaphthoxazine,21 and spiropyran22 have been investigated. The observed changes in λSPR were interpreted in terms of photoisomerization on the gold surface. Dithienylethenes are especially interesting photochromes for data storage applications. They retain their photoisomerization properties when incorporated into polymer matrixes. Polymers impregnated with 1,2-bis(3-thienyl)cyclopentene,23 1-(6′-vinyl-2′-methylbenzo[b]thiophene-3′yl)-2-(2′′-methylbenzo-[b]thiophene-3′′ -yl)hexafluorocyclopentene,24 or 1-[6′-(4′′′-vinylbenzoyl)-2′-methylbenzo[b]thiophene-3′-yl]-2-(2′′-methylbenzo[b]thiophene-3′′-yl)hexafluorocyclopentene24 have been reported. However, these polymers were not studied using SPR. Dithienylethenes exhibit a significant refractive index change upon photoisomerization. The refractive index changes of 1,2bis(2-methylbenzo[b]thiophen-3-yl) perfluorocyclopentene,25 1-(2-methyl-3-benzothienyl)-2-(1′,2′-dimethyl-3′indolyl) perfluorocyclopentene,25 3,3-bis(2,4,5-trimethyl3-thienyl) maleic anhydride,25 and 1-[6′-(methacryoyloxyethyloxycarbonyl)-2′-methylbenzo[b]thiophen-3′-yl]-2-(2′′methylbenzo[b]thiophen-3′′-yl) hexafluorocyclopentene26 polymers have been evaluated by the prism coupling method. The refractive index change upon photoisomerization, high cycling capabilities, and retention of the photochromic properties in polymer matrixes make dithienylethene an excellent candidate to monitor photoisomerization using SPR. A dithienylethene monolayer on gold nanoparticles was previously observed to retain the photoisomerization properties.27 To date, no SPR (18) Ock, K. S.; Jin, S. H.; Gal, Y. S.; Kim, J. H.; Kim, S. H.; Koh, K. Mol. Cryst. Liq. Cryst. 2002, 377, 233-236. (19) Choi, S.; Jin, S. H.; Gal, Y. S.; Kim, J. H.; Kim, S. H.; Koh, K. Mol. Cryst. Liq. Cryst. 2002, 377, 229-232. (20) Hur, Y.; Lee, M.; Jin, S. H.; Gal, Y. S.; Kim, J. H.; Kim, S. H.; Koh, K. J. Appl. Polym. Sci. 2003, 90, 3459-3465. (21) Kim, S. H.; Suh, H. J.; Koh, K.; Suck, S. A.; Choi, H. J.; Kim, H. S. Dyes Pigments 2004, 62, 93-97. (22) Sasaki, K.; Nagamura, T. J. Appl. Phys. 1998, 83, 2894-2900. (23) Myles, A. J.; Brenda, N. R. Macromolecules 2003, 36, 298-303. (24) Cho, S. Y.; Yoo, M.; Shin, H. W.; Ahn, K. H.; Kim, Y. R.; Kim, E. Opt.l Mater. 2002, 21, 279-284. (25) Hoshino, M.; Ebizawa, F.; Takashi, Y.; Sukegawa, K. J. Photochem. Photobiol. A-Chem. 1997, 105, 75-81. (26) Kim, E.; Choi, E. Y.; Lee, M. H. Macromolecules 1999, 32, 48554860. (27) Matsuda, K.; Ikeda, M.; Irie, M. Chem. Lett. 2004, 33, 456-457.

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Figure 1. UV-Vis spectra of the open form (solid) and the closed form (dotted) of 1 in dichloromethane. The structures of the open form (above) and the closed form (below) of a dithienylethene are also shown. The open form can be converted to the closed form using UV light, and the open form may be regenerated using visible light around 600 nm.

studies have been reported for dithienylethene monolayers. The nature of the medium constraining the photochrome near the gold surface is expected to be important in determining the magnitude of the SPR response. For example, most of the compounds previously studied were present as self-assembled monolayers attached to gold via short chain organothiols. A dihydroindolizine was studied using a modified dextran layer.15 The dextran film was believed to enhance the signal compared to thiolbased SAM attachment of the photochrome.15 Unlike the SAMs dextran offers a three-dimensional network of binding sites and therefore a potentially enhanced concentration of photochrome. However, dextran binding sites are located farther away from the surface, and the increased distance will lead to a less-intense signal than would otherwise be obtained. Thus, we have investigated several binding motifs for attaching dithienylethenes to SPR probes. The SPR spectral region can be easily tuned not only by modifying the shape of the fiber optic28 but also by changing the solvent. Thus, the choice of solvent is important for determination of the magnitude of the SPR response. If the refractive index of the solvent is higher than the refractive index of the fiber optic, the light striking the gold surface will not be in total internal reflection, and therefore the SP excitation will not occur. Therefore, useful solvents have refractive indices lower than that of the fiber optic (nD ) 1.457 for the experiments described here). We have investigated two solvents of widely different polarities: ethanol and hexanes. Experimental Section Sensor Preparation and SPR Instrumentation. The preparation of the fiber-optic SPR sensors and the SPR system used in this study has been described previously.28,29 The length of the active area on the sensor, which uses a 400 µm optical fiber, is 5 mm. Sensors as small as 50 µm diameter can be manufactured and have a sensing area of about 1 mm. SPR is a surface effect, not affected by the size of the sensor until the propagation length of the surface plasmon wave is reached. With visible light, the propagation length is approximately 20-50 µm depending on the wavelength. Smaller sensors will be as sensitive as the ones presented here. Synthesis. 2-(Trimethylsilylethyl)-4-iodophenylacetate (2). To a flask containing 1.0 g (3.82 mmol) of 4-iodophenylacetic acid, 0.45 g (3.82 mmol) of 2-(trimethylsilyl)ethanol, 0.47 g (3.82 mmol) of 4-(dimethylamino)pyridine, and 25 mL of dichloromethane was added 1.18 g (5.72 mmol) of N,N′-dicyclohexylcarbodiimide. (28) Obando, L. A.; Booksh, K. S. Anal. Chem. 1999, 71, 5116-5122. (29) Masson, J.-F.; Battaglia, T. M.; Kim, Y.-C.; Prakash, A. M. C.; Beaudoin, S.; Booksh, K. S. Talanta 2004, 64, 716-725.

The solution was stirred in the dark under a nitrogen atmosphere for 24 h. During this time a white precipitate formed, and analysis of the reaction mixture by TLC indicated that the reaction was complete. The reaction mixture was diluted with dichloromethane (75 mL), washed with aqueous sodium bicarbonate, dried over anhydrous sodium sulfate, and then concentrated by evaporation of the solvent at reduced pressure. The residue was chromatographed on silica gel (flash column, hexanes/ethyl acetate) to give 1.15 g (83% yield) of compound 2 as a clear oil: 1H NMR (300 MHz): δ 0.02 (9H, s, -CH3), 0.96 (2H, m, -CH2-), 3.51 (2H, s, -CH2-), 4.16 (2H, m, -CH2-), 7.01 (2H, d, J ) 7 Hz, Ar-H), 7.63 (2H, d, J ) 7 Hz, Ar-H); MALDI-TOF-MS calcd for C13H19O2ISi, 362; obsd, 483 (M - I + terthiophene matrix). Dithienylethene (4). To a heavy wall glass tube was added 300 mg (0.52 mmol) of dithienylethene (3),1 182 mg (0.52 mmol) of 2-(trimethylsilylethyl)-4-iodophenylacetate (2), 256 mg (0.84 mmol) of triphenylarsine, and 25 mL of triethylamine. The solution was flushed with argon for 15 min, 96 mg (0.10 mmol) of tris(dibenzylideneacetone)dipalladium (0) was added, and argon flushing was continued for a further 10 min. The tube was sealed with a Teflon screw plug and the contents were warmed at 40 °C for 24 h. The solvent was then evaporated at reduced pressure and the residue was chromatographed on silica gel (flash column, hexanes/dichloromethane 1:1 to 2:3) to give 372 mg (88% yield) of ester (4): 1H NMR (300 MHz): δ 0.03 (9H, s, -CH3), 0.95-1.01 (2H, m, -CH2-), 1.96 (3H, s, thioph-CH3), 1.98 (3H, s, thioph-CH3), 3.61 (2H, s, -CH2COO-), 3.84 (3H, s, -OCH3), 4.16 (2H, m, -CH2-), 6.91 (2H, d, J ) 9 Hz, Ar-H), 7.15 (1H, s, thioph-H), 7.28 (2H, d, J ) 8 Hz, Ar-H), 7.31 (1H, s, thiophH), 7.46 (2H, d, J ) 9 Hz, Ar-H), 7.49 (2H, d, J ) 8 Hz, Ar-H), 7.52 (4H, s, Ar-H). MALDI-TOF-MS: calcd for C43H36F6O3S2Si1 808.2, obsd 808.2. Dithienylethene Acid (1). To a flask containing a solution of 0.40 g (0.49 mmol) of ester (4) and 15 mL of tetrahydrofuran at 0 °C was added 0.2 mL of 1 M tetrabutylammonium fluoride. The solution was stirred for 30 min before an additional 0.2 mL was added. After a total of 2 h the reaction mixture was diluted with dichloromethane (60 mL) and then washed with 2 M citric acid followed by two washings with water. The solvent was evaporated and the residue was chromatographed on silica gel (flash column, chloroform/6-10% methanol) to give 262 mg of 1 (75% yield). 1H NMR (300 MHz): δ 1.95 (3H, s, thioph-CH3), 1.97 (3H, s, thioph-CH3), 3.66 (2H, s, -CH2COO-), 3.83 (3H, s, -OCH3), 6.91 (2H, d, J ) 9 Hz, Ar-H), 7.15 (1H, s, thioph-H), 7.28 (2H, d, J ) 8 Hz, Ar-H), 7.31 (1H, s, thioph-H), 7.46 (2H, d, J ) 9 Hz, Ar-H), 7.48 (6H, m, Ar-H). MALDI-TOF-MS: calcd for C38H26F6O3S2 708.12, obsd 708.12. UV/Vis (CH2Cl2): (open form) 330 nm, (closed form) 276, 353, 608 nm. Layer Preparation. The synthesis of the photochromic layers used a modified version of the carboxymethylated dextran chemistry employed in the immobilization of protein on a SPR surface.30 All reactions were done without any stirring or shaking. The bare gold surface was immersed for 12 h in a 0.005 M solution (30) Johnsson, B.; Lofas, S.; Lindquist, G. Anal. Biochem. 1991, 198, 268-277.

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of 11-mercaptoundecanol (MUO) in 80:20 ethanol:water to form a self-assembled MUO monolayer (layer B in Figure 3). For an amine-modified MUO layer (MUO-NH2), a MUO SAM was first allowed to react with 0.6 M epichlorohydrin in a 1:1 mixture of diglyme and 0.4 M NaOH for 4 h. The resulting layer was then washed with water, washed with ethanol and water again, and treated with 1.0 M ethanolamine at pH 8.5 for 20 h (layer A). The GATR-FTIR spectrum of a monolayer of MUONH2 on a gold slide confirmed that the OH group of ethanolamine reacts with epichlorohydrin, leaving the free amine group at the surface. The characteristic band for the NH2 deformation at 1670 cm-1 was observed. For a dextran layer, the MUO surface was reacted with epichlorohydrin and washed as described for the MUO-NH2 layer. Next, it was allowed to react for 20 h with an aqueous solution containing 0.3 g/mL dextran (500 kDa) and 0.1 M NaOH. Stopping at this stage produces a dextran layer (layer C). To obtain the amine-modified dextran layer (dextran-amine), the dextran polymer (C) was first modified to a carboxymethylated dextran by reaction with 1 M bromoacetic acid in 2 M NaOH for 16 h. The SAM was washed in an HBS buffer for 24 h. HBS was composed of 150 mM NaCl, 10 mM HEPES, 3.4 mM EDTA, and 0.005% Tween 20 (surfactant) in 18 MΩ deionized water. The resulting surface was activated by immersion in a 1:1 mixture of aqueous solutions of 0.4 M N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride and 0.01 M N-hydroxysuccinimide for 10 min, and an amine coupling was performed by treatment with 1.0 M ethylenediamine at pH 5.5 for 20 min. Cross-linking reactions between the carboxymethylated dextran chains occurred, and in addition, free amine groups were produced on the polymer (layer D). The MUO, MUO-NH2, dextran, and dextran-amine layers were coupled with dithienylethene 1 in the following manner to obtain the photochromic layer. The surface was treated with a 1:1 mixture of a 10 mg/mL N,N′-dicyclohexylcarbodiimide solution in dichloromethane and a 1 mg/mL solution of dithienylethene 1 in dichloromethane. A catalytic amount of 4-(dimethylamino)pyridine was added to the mixture. The SPR sensor in the resulting solution was heated for 20 h in the dark in a 50 °C water bath. The reaction vessel was sealed to avoid evaporation of dichloromethane. Ge Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (GATR-FTIR) System. The photoisomerization on the gold surface was monitored using GATRFTIR. Precleaned glass slides were washed with acetone. A 5 nm layer of Cr and a 50 nm layer of Au were deposited on the slides. The slides were modified chemically as described above to attach the photochrome. Upon completion of the reactions, the polymercoated gold slides were washed with dichloromethane and dried with compressed air. Analysis of the slides was performed using a Bruker IFS66v/s FTIR with an MCT detector cooled by liquid nitrogen. A Harrick GATR attachment was also employed. The germanium crystal was washed with methyl ethyl ketone and the coated glass slides were placed face down on the crystal. The GATR attachment was placed in the FTIR and the compartment was evacuated to 1 mbar. All transmission spectra were comprised of the average of 1024 scans with the background subtracted. UV-Visible Spectra. The UV-Visible spectra of the open and closed forms of the dithienylethene 1 were acquired using an HP 8453 UV-Visible spectrometer. A 0.1 mg/mL solution of 1 in the open form, produced by visible light irradiation, was prepared in dichloromethane and the spectrum was acquired. The photochrome was isomerized to a photostationary state containing mainly the closed form using a UV lamp, λ ) 365 nm (1 mW/cm2), for 45 s. The spectrum of the closed form was acquired immediately after the irradiation. Photoexcitation of the Surface. The solvent was bubbled with nitrogen for 30 min to remove dissolved oxygen, and the SPR sensor with the photochrome in the open, colorless form was placed in the solvent and equilibrated for at least 15 min. Next, the SPR signal was continuously acquired at a rate of 1 spectrum every 3 s. After data was acquired for 5 min, the sample was irradiated with UV light (λ ) 365 nm) for 45 s while acquiring a second data set. Data collection was continued for another 5 min, and then the sample was irradiated with visible light (Oriel

Masson et al. light source # 66088, 450 < λ < 800 nm, 108 mW/cm2) for 45 s while acquiring data. The area of the sensor was 0.063 cm2. This cycle was repeated 4 times for each sample. The lamp was turned during irradiation so as to ensure approximately equal exposure of all parts of the gold surface.

Results Characterization of Dithienylethene 1 and Layer Formation. The UV-Vis spectra of 1 in dichloromethane (Figure 1) illustrates the photochromism of the compound, which is characteristic of dithienylethenes in general. The open form shows one absorption band at 330 nm, while the closed form has three bands, at 276, 353, and 608 nm, respectively. Irradiation of the 608 nm band of the closed form initiates photoisomerization to the open form, whereas irradiation in the 300 nm region generates a mixture of closed and open forms characteristic of a photostationary state. The UV-Vis spectra could not be obtained for dithienylethene 1 on the gold surface. UVVis is not sensitive enough to measure the absorption spectrum of a monolayer. Every reaction used to derivatize the gold surfaces of the SPR sensors was monitored using the SPR signal to ensure the completion. The MUO, MUO-NH2, dextran, and dextran-amine layer preparations were previously characterized and reported.15,29 The final coupling reaction for attaching 1 was performed at 50 °C in dichloromethane, and the reaction vessel was sealed to avoid evaporation of the solvent. The reaction could also be performed at room temperature if longer reaction times were used. In this case, the vials were not sealed. After each reaction for attachment of 1 to the linker layer, a red shift indicating an increase of the refractive index of the surface was observed for every type of linker, thus verifying attachment. The extent of the reaction cannot be measured knowing only the change in λSPR observed. The refractive index of the solid film is unknown and must be known to calculate the extent of reaction. From the red shift observed, a coverage of 25-50% of the possible attachment sites is approximated. Larger extent of reaction should improve the SPR signal. GATR-FTIR of MUO Layers. GATR-FTIR was performed on glass slides coated with the derivatized MUO film in order to monitor the presence and photoisomerization of dithienylethene on the surface. The gold slides were chemically treated exactly as were the SPR sensors, and the IR spectra were measured. The bands characteristic of the open form appear at 1637, 1592, and 1544 cm-1 (Figure 2A) as reported by Irie.2 The arrows in the expanded spectrum in Figure 2C indicate two of the relevant bands. The CdC bond of perfluoropentene corresponds to the band at 1637 cm-1 (Band 1, Figure 2C) and the band at 1544 cm-1 corresponds to the double bonds of the thiophene ring (Band 2, Figure 2C). 2 The closed form displays new bands at 1612, 1572, and 1503 cm-1 (Figure 2B). The bands at 1606 cm-1 (Band 3, Figure 2D) and 1570 cm-1 (Band 4, Figure 2D) correspond to the CdC bands of the newly formed conjugated system as mentioned by Irie.2 The arrows in Figure 2D point out these bands. Most of the IR bands characteristic of either the open or the closed form are weak and can be hard to assign reliably when using GATR-FTIR. A more sensitive indicator of the closed ring is the band at 1495 cm-1 (Band 5, Figure 2D). This band is missing in the spectrum of the open ring isomer (Figure 2C). SPR Analysis. When differentiating between closed and open forms of the dithienylethene on the gold surface, SPR is more sensitive to processes occurring close to the surface than to those far away. Therefore, locating the

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Figure 2. GATR-FTIR spectra of the open (A) and closed (B) forms of dithienylethene attached to an MUO layer. Parts C and D show expansions of the spectra from 1750 to 1400 cm-1 for the open form and from 1750 to 1400 cm-1 for the closed form, respectively.

Figure 3. Representation of the different layers produced by reaction with 1. A is MUO-NH2, B MUO, C dextran, and D dextran-amine. In A and D the photochrome is attached via an amide bond, whereas in B and C it is attached by an ester bond. R represents dithienylethene 1.

molecule close to the surface can in principle improve the SPR signal resulting from photoisomerization. The solvent in which the probe is placed can influence both the spectral properties of the isomers of the photochrome (solvatochromism) and the properties of the monolayer. The layer may expand or contract after photoisomerization, depending on solvation changes. All these phenomena must be taken into account when evaluating photochromism in a molecule attached to a surface. Multiple layers with different properties have been investigated here in order to understand the role of these phenomena in controlling signal amplitude, and also to identify factors leading to large changes in λSPR following photoisomerization. Figure 3 shows a cartoon depicting the chemical structures of the different layers. Some representative SPR data for 1 attached to gold via various linkers are shown in Figure 4. Data for all linkers are summarized in Table 1. For several reasons, principal component analysis (PCA) was used to determine the λSPR behavior for each probe. Reference probes, not reacted with 1, were used to create baselines and remove some thermal drift. However, they respond in a different λSPR range from that of the sample. This difference makes it difficult to use the more traditional polynomial curve fitting to the ratioed SPR spectra.31 Also, using organic

solvents rather than water often produces slightly asymmetric SPR spectra, rendering the polynomial curve fitting noisy and inaccurate when employed to calculate the minima of the normalized reflectance spectra. The signals shown in Figure 4 and reported in Table 1 were calculated from the average change in the second principal component (second PC, also named score) using PCA. In general, the second PC values are unitless, and the amplitudes on the vertical axes in Figure 4 are arbitrary. However, because the entire collection of SPR spectra was simultaneously analyzed by PCA, the amplitudes have relative meaning within the data set. The average changes reported in Table 1 were calculated from three measurements obtained from three consecutive cycles. Following UV irradiation, the dithienylethene isomerizes from the open form to the closed form, and the SPR maximum red shifts. With visible light irradiation, the open ring is regenerated, and the SPR maximum blue shifts. The results reported in Table 1 were obtained by subtracting from the signal coming from the sensor with attached dithienylethene the signal of an otherwise chemically similar reference sensor lacking dithienylethene. The use of a reference signal helps remove longterm drift and further confirms that the SPR signal is actually due to dithienylethene isomerization rather than some other phenomenon. MUO. Using MUO (Layer B, Figure 3) as the linker to bind the dithienylethene to the surface is synthetically facile and constrains the dithienylethene to be close to the gold surface. The MUO-tethered photochromic molecules might therefore be expected to exhibit the largest SPR signal, compared to the other monolayers, because the SPR signal decreases exponentially with distance of the photochrome from the gold. However, tethering the dithienylethene so close to the surface might also result in excited-state quenching due to interactions with the gold plasmon. We began our SPR studies using this linker. (31) Gentleman, D. J.; Obando, L.; Masson, J. F.; Holloway, J. R.; Booksh, K. Anal. Chim. Acta 2004, 515, 291-302.

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Figure 4. (A) Raw signal for a sensor with 1 attached to a MUO layer. (B) Reference signal from a MUO layer. (C) Processed signal obtained by subtracting the reference signal (B) from (A). (D) The processed signal obtained using a dextran layer with 1 bound to the surface. Table 1. SPR Signal Change upon Photoisomerization for Different Dithienylethene Layers in Ethanol and Hexanesa ethanol MUO MUO-NH2 dextran dextran-amine a

hexanes

UV

Vis

UV

Vis

0.019 ( 0.002 no signal 0.038 ( 0.021 0.005 ( 0.002

-0.011 ( 0.003 no signal -0.039 ( 0.009 -0.006 ( 0.002

0.021 ( 0.002 0.024 ( 0.004 0.006 ( 0.003 no signal

-0.020 ( 0.002 -0.023 ( 0.001 -0.002 ( 0.003 no signal

Errors are one standard deviation.

SPR results for MUO appear in Figure 4 and Table 1. The photochrome was found to photoisomerize readily and repetitively in both ethanol and hexanes. When the layer is photochemically cycled in hexanes, the change in the second PC following photoisomerization is statistically of the same absolute magnitude for both the ring-opening and ring-closing reactions. A change of 0.021 ( 0.002 was obtained when the sample was irradiated with UV light, and a change of -0.020 ( 0.002 was obtained upon visible irradiation. (The error reported is for one standard deviation.) The signal, after subtraction of that of the reference sensor, is very stable and reproducible. In ethanol, the signal change upon UV irradiation to close the photochrome ring was 0.019 ( 0.002 while the reaction to open the ring led to a signal change of -0.011 ( 0.003 (Table 1). MUO-NH2. MUO-NH2 is a slightly longer molecule than MUO and expected to form a thicker monolayer. It also links to the dithienylethene via an amide bond, rather than an ester group. Figure 5 shows the stability of the SPR signal between irradiations in hexane, suggesting that the readout by SPR is nondestructive photochemically and does not cause photoisomerization. This conclusion is confirmed by Figure 6, which shows the results of measurements over four photoisomerization cycles. SPR

Figure 5. SPR results for one photoisomerization cycle of 1 bound to MUO-NH2 and submerged in hexane solvent.

measurements are nondestructive due to the fact that the light used to excite the surface plasmon never directly excites dithienylethene and the light is usually in a different region of the visible spectrum than the excitation wavelength of dithienylethene. Dithienylethene was analyzed using SPR at 640 nm in ethanol and at 700 nm in

Nondestructive Readout of Dithienylethene Monolayers

Figure 6. Dithienylethene cycling using MUO-NH2 as a support.

hexanes. The signal obtained for MUO-NH2 is slightly larger than that for MUO in hexanes, but the difference is very small. The molecule did not switch repetitively in ethanol; therefore, no signal is reported for MUO-NH2. Dextran. The dextran allowed to react with the surface had a polymer chain weight of ∼500 kDa. It produces a three-dimensional network of binding sites that extends ∼100 nm away from the surface. The SPR effect this far from the gold is of smaller magnitude than that for the monolayers described above, but the number of potential photochrome binding sites is much greater. Dextran can also expand or contract in response to photoisomerization more than the densely packed MUO or MUO-NH2 monolayers can. Dextran is a hydrogel, the thickness for which varies greatly depending on the chemical environment, e.g., salvation, substitutions on the dextran chain. Dextran was observed by the authors to vary by as much as 50 nm when changing functionality with identical molecular weight dextran. In ethanol, the change in SPR signal as a result of dithienylethene photoisomerization is the largest reported in this study (0.039). It is almost twice as large as that obtained for MUO in ethanol (Table 1). The signal is of the same amplitude for the ring-opening and -closing reactions. However, the signal is very irreproducible (Figure 4D). There is a large variation in the signal amplitude, especially for UV irradiation (Table 1). The variation in the signal observed in Figure 4D was not believed to be fatigue. The signal varied randomly, which indicates that the signal is less reproducible than for the other layers. When monitored in hexanes, the signal is very weak, near the noise level of the measurements. Dextran-Amine. The dextran-amine layer also had a polymer chain weight of 500 kDa. Because it is prepared by reacting dextran with bromoacetic acid to carboxymethylate, and then reacting with ethylenediamine to obtain the dextran-amine, this layer offers a different environment for photoisomerization. Dextran has free hydroxyl groups on its surface, whereas dextran-amine has amine groups on its surface. Moreover, the reaction with ethylenediamine will produce some cross-linking, rendering the dextran-amine stiffer than dextran, and altering solvent access. This leads to a large difference in the SPR behavior of the photochrome. Using dextranamine yielded the smallest changes in the signal following irradiation. In ethanol, a small signal slightly larger than the noise level was obtained. It was almost an order of magnitude smaller than that of dextran (Table 1). In hexanes, no signal was obtained. The signal was also much smaller with dextran itself when the photoisomerization reactions were done in hexanes. Discussion Our studies show that dithienylethene 1 can be successfully bound to gold surfaces via a number of different

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chemical linkers. Attachment to the gold surface of a SPR sensor was confirmed by both GATR-FTIR and SPR. For most of the linkers studied, photoisomerization of the dithienylethene occurred at a reasonable rate and was not significantly quenched by the proximity of the photochrome to the gold plasmon. This result is consistent with recent fluorescence and SPM studies of photoisomerization of dithienylethenes bound directly to gold via thiol linkages.32,33 In the references studies, the quantum yield of photoisomerization was significantly reduced, and the photochromes were held closer to the gold surface than those employed in our work. A previous study in our group demonstrated that antibodies attached to a SAM were more active when using a 16 carbon SAM rather than an 11 carbon SAM, demonstrating that effects other than excited-state quenching by the gold plasmon can also affect the dependence of sensitivity on distance from the surface. The different layers used to link the dithienylethene to the gold surface strongly affected the magnitude of the SPR response. MUO (B) exhibited a reproducible change in SPR for both isomerization processes when placed in hexanes. However, the same layer solubilized in ethanol showed a difference in SPR response following the two isomerization processes. The difference in SPR signal change was caused by an unstable baseline. The instability of the baseline was most likely due to the layer solubilization in ethanol being different for the reference surface compared to the photochrome surface. The solubilization processes occur when solvent migrates into the layer, causing the layer to swell. Since the reference probe does not have the photochrome at the surface, the solubilization can occur at different rates, causing a baseline drift that produces a different apparent SPR signal for each isomerization reaction. The photochrome attached to a MUO-NH2 layer (A) did not exhibit a reproducible SPR signal following photoisomerization in ethanol, but showed a response similar to that of the MUO layer in hexanes. Photoisomerization is observed in solution in both solvents. Thus, the lack of response in ethanol must be due to changes in the monolayer resulting from solvation with the polar ethanol, compared with hexanes. The dextran layer (C) also exhibited much different behavior in ethanol than in hexanes. In this case, in ethanol, the SPR signal was large, with some irreproducibility. However, in hexanes, the signal is much smaller. The solvation of the layers using dextran as a support is likely to be very different when using hexanes and ethanol. Dextran is a very loose structure that extends around 100 nm from the surface when in aqueous solution, and a similar structure is likely in ethanol, which hydrogen bonds to the dextran. In such a loose structure, the photoisomerization process could occur readily. However, in hexanes, the dextran layer is practically insoluble. The monolayer collapses, and steric interactions may hinder the photoisomerization of the dithienylethene, decreasing the quantum yield. The SPR signal following photoisomerization of dextranamine monolayers in both solvents was much smaller than that of any other monolayer investigated. The reaction to form dextran-amine produces cross-links between the dextran chains by the reaction of ethylenediamine with two chains. The cross-links make the dextran-amine (32) He, J.; Chen, F.; Liddell, P. A.; Andre´asson, J.; Straight, S. D.; Gust, D.; Moore, T. A.; Moore, A. L.; Li, J.; Sankey, O. F.; Lindsay, S. M. Nanotechnology 2005, 16, 695-702. (33) Dulic, C.; van der Molen, S. J.; Kudernak, T.; Jonkman, H. T.; De Jong, J. J. D.; Bowden, T. N.; van Esch, J.; Feringa, B. L.; van Wees, B. J. Phys. Rev. Lett. 2003, 91, art. no. 207402.

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structure much stiffer and reduce the number of possible reaction sites for dithienylethene binding. The reduction of the number of sites would reduce the concentration of photochrome in the layer, and this would contribute to the small signals obtained with this material. In addition, the stiffer structure may limit the quantum yield of the photoisomerization process as discussed above. In conclusion, SPR was demonstrated to be an excellent technique for investigating the photoisomerization of dithienylethene using different layers and in different solvents. The photoisomerization reactions occur in a reasonable time on the gold surface and are not signifi-

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cantly quenched by the gold. The SPR signal is large enough to detect with a good signal-to-noise ratio. The dithienylethene films were reproducibly cycled multiple times. Acknowledgment. This work was supported by the NSF (Grant No. CHE-0352599 to D.G.). The authors would like to thank Shawn Whaley from the Physics and Astronomy Department at ASU for his help acquiring the GATR-FTIR. LA0509899